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MAINTENANCE ENGINEERING
UNIT I PRINCIPLES AND PRACTICES OF MAINTENANCE PLANNING
Basic Principles of maintenance planning – Objectives and principles of planned maintenance
activity – Importance and benefits of sound Maintenance systems – Reliability and machine
availability – MTBF, MTTR and MWT – Factors of availability – Maintenance organization –
Maintenance economics.
INTRODUCTION
Maintenance Engineering is the discipline and profession of applying engineering
concepts to the optimization of equipment, procedures, and departmental budgets to achieve
better maintainability, reliability, and availability of equipment.
Maintenance, and hence maintenance engineering, is increasing in importance due to rising
amounts of equipment, systems, machineries and infrastructure.
Since the Industrial Revolution, devices, equipment, machinery and structures have grown
increasingly complex, requiring a host of personnel, vocations and related systems needed to
maintain them.
A person practicing Maintenance Engineering is known as a Maintenance Engineer.
OBJECTIVES AND PRINCIPLES:
Analysis of repetitive equipment failures.
Estimation of maintenance costs and evaluation of alternatives.
Forecasting of spare parts.
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Assessing the needs for equipment replacements and establish replacement programs
when due application of scheduling and project management principles to replacement
programs.
Assessing required maintenance tools and skills required for efficient maintenance of
equipment.
Assessing required skills required for maintenance personnel.
Reviewing personnel transfers to and from maintenance organizations assessing and
reporting safety hazards associated with maintenance of equipment.
Reliability may be defined in several ways:
The idea that an item is fit for a purpose with respect to time.
In the most discrete and practical sense: "Items that do not fail in use are reliable" and
"Items that do fail in use are not reliable".
The capacity of a designed, produced or maintained item to perform as required over
time.
The capacity of a population of designed, produced or maintained items to perform as
required over time.
The resistance to failure of an item over time.
The probability of an item to perform a required function under stated conditions for a
specified period of time.
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In line with the creation of safety cases for safety, the goal is to provide a robust set of
qualitative and quantitative evidence that an item or system will not contain unacceptable
risk.
The basic sorts of steps to take are to:
First thoroughly identify as many as possible reliability hazards (e.g. relevant System
Failure Scenarios item Failure modes, the basic Failure mechanisms and root causes) by
specific analysis or tests.
Assess the Risk associated with them by analysis and testing.
Propose mitigations by which the risks may be lowered and controlled to an acceptable
level.
Select the best mitigations and get agreement on final (accepted) Risk Levels, possible
based on cost-benefit analysis.
AVAILABILITY
A Reliability Program Plan may also be used to evaluate and improve Availability of a
system by the strategy on focusing on increasing testability & maintainability and not on
reliability.
Improving maintainability is generally easier than reliability. Maintainability estimates
(Repair rates) are also generally more accurate.
However, because the uncertainties in the reliability estimates are in most cases very
large, it is likely to dominate the availability (prediction uncertainty) problem; even in the
case maintainability levels are very high.
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When reliability is not under control more complicated issues may arise, like manpower
(maintainers / customer service capability) shortage, spare part availability, logistic
delays, lack of repair facilities, extensive retro-fit and complex configuration
management costs and others.
The problem of unreliability may be increased also due to the "Domino effect" of
maintenance induced failures after repairs.
Only focusing on maintainability is therefore not enough. If failures are prevented, none
of the others are of any importance and therefore reliability is generally regarded as the
most important part of availability.
One of the most important design techniques is redundancy.
RELIABILITY THEORY
Reliability is defined as the probability that a device will perform its intended function
during a specified period of time under stated conditions.
ACCELERATED TESTING:
The purpose of accelerated life testing is to induce field failure in the laboratory at a
much faster rate by providing a harsher, but nonetheless representative, environment.
In such a test, the product is expected to fail in the lab just as it would have failed in the field—
but in much less time.
The main objective of an accelerated test is either of the following:
To discover failure modes.
To predict the normal field life from the high stress lab life.
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Software reliability is a special aspect of reliability engineering. System reliability, by
definition, includes all parts of the system, including hardware, software, supporting
infrastructure (including critical external interfaces), operators and procedures. Traditionally,
reliability engineering focuses on critical hardware parts of the system. Since the widespread use
of digital integrated circuit technology, software has become an increasingly critical part of most
electronics and, hence, nearly all present day systems. Despite this difference in the source of
failure between software and hardware, several software reliability models based on statistics
have been proposed to quantify what we experience with software: the longer software is run, the
higher the probability that it will eventually be used in an untested manner and exhibit a latent
defect that results in a failure (Shooman 1987), (Musa 2005), (Denney 2005). As with hardware,
software reliability depends on good requirements, design and implementation. Software
reliability engineering relies heavily on a disciplined software engineering process to anticipate
and design against unintended consequences. There is more overlap between software quality
engineering and software reliability engineering than between hardware quality and reliability. A
good software development plan is a key aspect of the software reliability program. The software
development plan describes the design and coding standards, peer reviews, unit tests,
configuration management, software metrics and software models to be used during software
development.
Define objective and scope of the test
Collect required information about the product
Identify the stresses
Determine level of stresses
Conduct the accelerated test and analyze the collected data.
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MEAN TIME BETWEEN FAILURES
Mean time between failures (MTBF) is the predicted elapsed time between inherent
failures of a system during operation. MTBF can be calculated as the arithmetic mean (average)
time between failures of a system.
FORMAL DEFINITION OF MTBF
By referring to the figure above, the MTBF is the sum of the operational periods divided
by the number of observed failures.
If the "Down time" (with space) refers to the start of "downtime" (without space) and "up time"
(with space) refers to the start of "uptime" (without spMean time betMean time between
failuresween failuresace), the formula will be:
The MTBF is often denoted by the Greek letter θ, or
The MTBF can be defined in terms of the expected value of the density function ƒ(t)
where ƒ is the density function of time until failure – satisfying the standard requirement of
density functions –
The Overview
For each observation, downtime is the instantaneous time it went down, which is after
(i.e. greater than) the moment it went up, uptime. The difference (downtime minus uptime) is the
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amount of time it was operating between these two events. MTBF value prediction is an
important element in the development of products. Reliability engineers / design engineers, often
utilize Reliability Software to calculate products' MTBF according to various methods/standards.
However, these "prediction" methods are not intended to reflect fielded MTBF as is commonly
believed. The intent of these tools is to focus design efforts on the weak links in the design
MTTR
MTTR is an abbreviation that has several different expansions, with greatly differing
meanings. It is wise to spell out exactly what is meant by the use of this abbreviation, rather than
assuming the reader will know which is being assumed. The M can stand for any of minimum,
mean or maximum, and the R can stand for any of recovery, repair, respond, or restore. The most
common, mean, is also subject to interpretation, as there are many different ways in which a
mean can be calculated.
Mean time to repair
Mean time to recovery/Mean time to restore
Mean time to respond
Mean time to replace
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In an engineering context with no explicit definition, the engineering figure of merit,
mean time to repair would be the most probable intent by virtue of seniority of usage.
It is also similar in meaning to the others above (more in the case of recovery, less in the case of
respond, the latter being more properly styled mean "response time").
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UNIT II
MAINTENANCE POLICIES – PREVENTIVE MAINTENANCE
Maintenance categories – Comparative merits of each category – Preventive maintenance,
maintenance schedules, repairs cycle - Principles and methods of lubrication – TPM.
The maintenance is defined as follows: “the work of keeping something in proper
condition; upkeep.” This would imply that maintenance should be actions taken to prevent a
device or component from failing or to repair normal equipment degradation experienced with
the operation of the device to keep it in proper working order. For example, equipment may be
designed to operate at full design load for 5,000 hours and may be designed to go through 15,000
start and stop cycles. The wear-out period is characterized by a rapid increasing failure rate with
time. In most cases this period encompasses the normal distribution of design life failures.
The design life of most equipment requires periodic maintenance. Belts need adjustment,
alignment needs to be maintained, proper lubrication on rotating equipment is required, and so
on. In some cases, certain components need replacement, (e.g., a wheel bearing on a motor
vehicle) to ensure the main piece of equipment (in this case a car) last for its design life. Anytime
we fail to perform maintenance activities intended by the equipment’s designer, we shorten the
operating life of the equipment. But what options do we have? Over the last 30 years, different
approaches to how maintenance can be performed to ensure equipment reaches or exceeds its
design life have been developed in the United States. In addition to waiting for a piece of
equipment to fail (reactive maintenance), we can utilize preventive maintenance, predictive
maintenance, or reliability centered maintenance.
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Reactive Maintenance
Reactive maintenance is basically the “run it till it breaks” maintenance mode. No actions or
efforts are taken to maintain the equipment as the designer originally intended to ensure design
life is reached. Studies as recent as the winter of 2000 indicate this is still the predominant mode
of maintenance in the United States. The referenced study breaks down the average maintenance
program as follows:
>55% Reactive
31% Preventive
12% Predictive
2% Other.
Note that more than 55% of maintenance resources and activities of an average facility are
still reactive.
Advantages to reactive maintenance can be viewed as a double-edged sword. If we are
dealing with new equipment, we can expect minimal incidents of failure. If our maintenance
program is purely reactive, we will not expend manpower dollars or incur capital cost until
something breaks. Since we do not see any associated maintenance cost, we could view this
period as saving money. Our labour cost associated with repair will probably be higher than
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normal because the failure will most likely require more extensive repairs than would have been
required if the piece of equipment had not been run to failure. Chances are the piece of
equipment will fail during off hours or close to the end of the normal workday. If it is a critical
piece of equipment that needs to be back on-line quickly, we will have to pay maintenance
overtime cost. Since we expect to run equipment to failure, we will require a large material
inventory of repair parts. This is a cost we could minimize under a different maintenance
strategy.
Advantages
• Low cost.
• Less staff
Disadvantages
• Increased cost due to unplanned downtime of equipment.
• Increased labour cost, especially if overtime is needed.
• Cost involved with repair or replacement of equipment.
• Possible secondary equipment or process damage from equipment failure.
• Inefficient use of staff resources.
Preventive Maintenance
Preventive maintenance can be defined as follows: Actions performed on a time- or
machine-run-based schedule that detect, preclude, or mitigate degradation of a component or
system with the aim of sustaining or extending its useful life through controlling degradation to
an acceptable level.
While preventive maintenance is not the optimum maintenance program, it does have
several advantages over that of a purely reactive program. By performing the preventive
maintenance as the equipment designer envisioned, we will extend the life of the equipment
closer to design. This translates into dollar savings. Preventive maintenance (lubrication, filter
change, etc.) will generally run the equipment more efficiently resulting in dollar savings. While
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we will not prevent equipment catastrophic failures, we will decrease the number of failures.
Minimizing failures translate into maintenance and capital cost savings.
Advantages
• Cost effective in many capital-intensive processes
• Flexibility allows for the adjustment of maintenance periodicity.
• Increased component life cycle.
• Energy savings
• Reduced equipment or process failure
• Estimated 12% to 18% cost savings over reactive maintenance program.
Disadvantages
• Catastrophic failures still likely to occur.
• Labour intensive.
• Includes performance of unneeded maintenance.
• Potential for incidental damage to components
Predictive Maintenance
Predictive maintenance can be defined as follows: Measurements that detect the onset of
system degradation (lower functional state), thereby allowing causal stressors to be eliminated or
controlled prior to any significant deterioration in the component physical state. Results indicate
current and future functional capability.
Basically, predictive maintenance differs from preventive maintenance by basing
maintenance need on the actual condition of the machine rather than on some preset schedule.
You will recall that preventive maintenance is time-based. Activities such as changing lubricant
are based on time, like calendar time or equipment run time. For example, most people change
the oil in their vehicles every 3,000 to 5,000 miles travelled.
The advantages of predictive maintenance are many. A well-orchestrated predictive
maintenance program will all but eliminate catastrophic equipment failures. We will be able to
schedule maintenance activities to minimize or delete overtime cost. We will be able to minimize
inventory and order parts, as required, well ahead of time to support the downstream
maintenance needs. We can optimize the operation of the equipment, saving energy cost and
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increasing plant reliability. Past studies have estimated that a properly functioning predictive
maintenance program can provide a savings of 8% to 12% over a program utilizing preventive
maintenance alone. Depending on a facility’s reliance on reactive maintenance and material
condition, it could easily recognize savings opportunities exceeding 30% to 40%. In fact,
independent surveys indicate the following industrial average savings resultant from initiation of
a functional predictive maintenance program:
• Return on investment: 10 times
• Reduction in maintenance costs: 25% to 30%
• Elimination of breakdowns: 70% to 75%
• Reduction in downtime: 35% to 45%
• Increase in production: 20% to 25%.
Advantages
• Increased component operational life/availability.
• Allows for pre-emptive corrective actions.
• Decrease in equipment or process downtime.
• Decrease in costs for parts and labour.
• Better product quality.
• Improved worker and environmental safety.
• Improved worker morale.
• Energy savings.
• Estimated 8% to 12% cost savings over preventive maintenance program.
Disadvantages
• Increased investment in diagnostic equipment.
• Increased investment in staff training.
• Savings potential not readily seen by management.
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Reliability Centred Maintenance
Reliability centred maintenance (RCM) magazine provides the following definition of
RCM: “a process used to determine the maintenance requirements of any physical asset in its
operating context.”
The following maintenance program breakdowns of continually top-performing facilities
would echo the RCM approach to utilize all available maintenance approaches with the
predominant methodology being predictive.
• <10% Reactive
• 25% to 35% Preventive
• 45% to 55% Predictive.
Because RCM is so heavily weighted in utilization of predictive maintenance technologies,
its program advantages and disadvantages mirror those of predictive maintenance. In addition to
these advantages, RCM will allow a facility to more closely match resources to needs while
improving reliability and decreasing cost.
Advantages
• Can be the most efficient maintenance program.
• Lower costs by eliminating unnecessary maintenance or overhauls.
• Minimize frequency of overhauls.
• Reduced probability of sudden equipment failures
• Increased component reliability.
Disadvantages
• Can have significant start-up cost, training, equipment, etc.
• Savings potential not readily seen by management.
PLANNED PREVENTIVE MAINTENANCE
Planned Preventive Maintenance ('PPM') or more usual just simple Planned
Maintenance (PM) or Scheduled Maintenance is any variety of scheduled maintenance to an
object or item of equipment. Specifically, Planned Maintenance is a scheduled service visit
carried out by a competent and suitable agent, to ensure that an item of equipment is operating
correctly and to therefore avoid any unscheduled breakdown and downtime.
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Together with Condition Based Maintenance, Planned maintenance comprises preventive
maintenance, in which the maintenance event is preplanned, and all future maintenance is pre-
programmed. Planned maintenance is created for every item separately according to
manufacturer’s recommendation or legislation. Plan can be based on equipment running hours,
date based, or for vehicles distance travelled. A good example of a planned maintenance program
is car maintenance, where time and distance determine fluid change requirements. A good
example of Condition Based Maintenance is the oil pressure warning light that provides
notification that you should stop the vehicle because failure will occur because engine
lubrication has stopped.
Planned maintenance has some advantages over Condition Based Maintenance such as:
• easier planning of maintenance and ordering spares,
• costs are distributed more evenly,
• no initial costs for instruments for supervision of equipment.
Disadvantages are:
• less reliable than equipment with fault reporting associated with CBM
• more expensive due to more frequent parts change.
• requires training investment and ongoing labour costs
Parts that have scheduled maintenance at fixed intervals, usually due to wearout or a fixed shelf
life, are sometimes known as time-change interval, or TCI items
Preventive Maintenance (Time-Based Maintenance)
Basic philosophy
• Schedule maintenance activities at predetermined time intervals.
• Repair or replace damaged equipment before obvious problems occur
This philosophy entails the scheduling of maintenance activities at predetermined time intervals,
where damaged equipment is repaired or replaced before obvious problems occur.. The
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advantages of this approach are that it works well for equipment that does not run continuously,
and with personnel who have enough knowledge, skills, and time to perform the preventive
maintenance work.
Predictive Maintenance (Condition-Based Maintenance)
Basic philosophy
• Schedule maintenance activities when mechanical or operational conditions warrant.
• Repair or replace damaged equipment before obvious problems occur.
This philosophy consists of scheduling maintenance activities only if and when mechanical or
operational conditions warrant-by periodically monitoring the machinery for excessive vibration,
temperature and/or lubrication degradation, or by observing any other unhealthy trends that
occur over time. When the condition gets to a predetermined unacceptable level, the equipment
is shut down to repair or replace damaged components so as to prevent a more costly failure
from occurring. In other words, “Don’t fix what is not broke.” Studies have shown that when it is
done correctly, the costs to operate in this fashion are about $9 per hp per year. Advantages of
this approach are that it works very well if personnel have adequate knowledge, skills, and time
to perform the predictive maintenance work, and that it allows equipment repairs to be scheduled
in an orderly fashion. It also provides some lead-time to purchase materials for the necessary
repairs, reducing the need for a high parts inventory. Since maintenance work is only performed
when it is needed, there is likely to be an increase in production capacity.
LUBRICATION
Lubrication is the process, or technique employed to reduce wear of one or both surfaces
in close proximity, and moving relative to each other, by interposing a substance called lubricant
between the surfaces to carry or to help carry the load (pressure generated) between the opposing
surfaces. The interposed lubricant film can be a solid, (e.g. graphite, MoS2) a solid/liquid
dispersion, a liquid, a liquid-liquid dispersion (a grease) or, exceptionally, a gas.
In the most common cases the applied load is carried by pressure generated within the
fluid due to the frictional viscous resistance to motion of the lubricating fluid between the
surfaces.
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Lubrication can also describe the phenomenon such reduction of wear occurs without
human intervention (hydroplaning on a road).
The science of friction, lubrication and wear is called tribology.
The regimes of lubrication
As the load increases on the contacting surfaces three distinct situations can be observed with
respect to the mode of lubrication, which are called regimes of lubrication:
Fluid film lubrication is the lubrication regime in which through viscous forces the load is fully
supported by the lubricant within the space or gap between the parts in motion relative to one
another (the lubricated conjunction) and solid–solid contact is avoided.
Hydrostatic lubrication is when an external pressure is applied to the lubricant in the
bearing, to maintain the fluid lubricant film where it would otherwise be squeezed out.
Hydrodynamic lubrication is where the motion of the contacting surfaces, and the exact
design of the bearing is used to pump lubricant around the bearing to maintain the
lubricating film. This design of bearing may wear when started, stopped or reversed, as
the lubricant film breaks down.
Elastohydrodynamic lubrication: The opposing surfaces are separated, but there occurs
some interaction between the raised solid features called asperities, and there is an elastic
deformation on the contacting surface enlarging the load-bearing area whereby the
viscous resistance of the lubricant becomes capable of supporting the load.
Boundary lubrication (also called boundary film lubrication): The bodies come into
closer contact at their asperities; the heat developed by the local pressures causes a
condition which is called stick-slip and some asperities break off. At the elevated
temperature and pressure conditions chemically reactive constituents of the lubricant
react with the contact surface forming a highly resistant tenacious layer, or film on the
moving solid surfaces (boundary film) which is capable of supporting the load and major
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wear or breakdown is avoided. Boundary lubrication is also defined as that regime in
which the load is carried by the surface asperities rather than by the lubricant.
Besides supporting the load the lubricant may have to perform other functions as well, for
instance it may cool the contact areas and remove wear products. While carrying out these
functions the lubricant is constantly replaced from the contact areas either by the relative
movement (hydrodynamics) or by externally induced forces.
Lubrication is required for correct operation of mechanical systems pistons, pumps, cams,
bearings, turbines, cutting tools etc. where without lubrication the pressure between the surfaces
in close proximity would generate enough heat for rapid surface damage which in a coarsened
condition may literally weld the surfaces together, causing seizure.
In some applications, such as piston engines, the film between the piston and the cylinder wall
also seals the combustion chamber, preventing combustion gases from escaping into the
crankcase.
If rolling bearings are to operate reliably they must be adequately lubricated to prevent
direct metal-to-metal contact between the rolling elements, raceways and cages. The lubricant
also inhibits wear and protects the bearing surfaces against corrosion. The choice of a suitable
lubricant and method of lubrication for each individual bearing application is therefore
important, as is correct maintenance.
A wide selection of greases and oils is available for the lubrication of rolling bearings and there
are also solid lubricants, e.g. for extreme temperature conditions. The actual choice of a lubricant
depends primarily on the operating conditions, i.e. the temperature range and speeds as well as
the influence of the surroundings. The most favourable operating temperatures will be obtained
when the minimum amount of lubricant needed for reliable bearing lubrication is provided.
However, when the lubricant has additional functions, such as sealing or the removal of heat,
additional amounts of lubricant may be required. The lubricant in a bearing arrangement
gradually loses its lubricating properties as a result of mechanical work, ageing and the build-up
of contamination. It is therefore necessary for grease to be replenished or renewed and for oil to
be filtered and changed at regular intervals. The information and recommendations in this section
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relate to bearings without integral seals or shields. SKF bearings and bearing units with integral
seals and shields on both sides are supplied greased. Information about the greases used by SKF
as standard for these products can be found in the relevant product sections together with a brief
description of the performance data.
Methods of oil lubrication
Oil bath
The simplest method of oil lubrication is the oil bath (fig.1). The oil, which is picked up by the
rotating components of the bearing, is distributed within the bearing and then flows back to the
oil bath. The oil level should be such that it almost reaches the centre of the lowest rolling
element when the bearing is stationary. The use of oil levellers such as the SKF LAHD 500 is
recommended to provide the correct oil level. When operating at high speed the oil level can
drop significantly and the housing can become overfilled by the oil leveller, under these
conditions, please consult the SKF application engineering service
fig 1
Oil pick-up ring
For bearing applications where speeds and operating temperature are such that oil lubrication is
necessary and high reliability is required, the oil pick-up ring lubrication method is
recommended (fig.2). The pick-up ring serves to bring about oil circulation. The ring hangs
loosely on a sleeve on the shaft on one side of the bearing and dips into the oil in the lower half
of the housing. As the shaft rotates, the ring follows and transports oil from the bottom to a
collecting trough. The oil then flows through the bearing back into the reservoir at the bottom.
SKF plummer block housings in the SONL series are designed for the oil pick-up ring
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lubrication method. For additional information please consult the SKF application engineering
service.
fig 2
Circulating oil
Operation at high speeds will cause the operating temperature to increase and will accelerate
ageing of the oil. To avoid frequent oil changes and to achieve a fully flooded condition, the
circulating oil lubrication method is generally preferred (fig.3). Circulation is usually produced
with the aid of a pump. After the oil has passed through the bearing, it generally settles in a tank
where it is filtered and, if required, cooled before being returned to the bearing. Proper filtering
leads to high values for the factor ηc and thus to long bearing service life, see section SKF rating
life.
fig 3
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Oil jet
For very high-speed operation a sufficient but not excessive amount of oil must be supplied to
the bearing to provide adequate lubrication, without increasing the operating temperature more
than necessary. One particularly efficient method of achieving this is the oil jet method (fig.4)
where a jet of oil under high pressure is directed at the side of the bearing. The velocity of the oil
jet must be high enough (at least 15 m/s) to penetrate the turbulence surrounding the rotating
bearing.
fig 4
Oil mist
Oil mist lubrication has not been recommended for some time due to possible negative
environmental effects.
A new generation of oil mist generators permits to produce oil mist with 5 ppm oil. New designs
of special seals also limit the amount of stray mist to a minimum. In case synthetic non-toxic oil
is used, the environmental effects are even further reduced. Oli mist lubrication today is used in
very specific applications, like the petroleum industry.
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TOTAL PRODUCTIVE MAINTANENCE
Total productive maintenance (TPM) originated in Japan in 1971 as a method for
improved machine availability through better utilization of maintenance and production
resources.
TPM is a critical adjunct to lean manufacturing. If machine uptime is not predictable and
if process capability is not sustained, the process must keep extra stocks to buffer against this
uncertainty and flow through the process will be interrupted. Unreliable uptime is caused by
breakdowns or badly performed maintenance. Correct maintenance will allow uptime to improve
and speed production through a given area allowing a machine to run at its designed capacity of
production.
One way to think of TPM is "deterioration prevention": deterioration is what happens
naturally to anything that is not "taken care of". For this reason many people refer to TPM as
"total productive manufacturing" or "total process management". TPM is a proactive approach
that essentially aims to identify issues as soon as possible and plan to prevent any issues before
occurrence. One motto is "zero error, zero work-related accident, and zero loss"
TPM is a management process developed for improving productivity by making
processes more reliable and less wasteful.TPM is an extension of TQM(Total Quality
Management). The objective of TPM is to maintain the plant or equipment in good condition
without interfering with the daily process. To achieve this objective, preventive and predictive
maintenance is required. By following the philosophy of TPM we can minimize the unexpected
failure of the equipment.
To implement TPM the production unit and maintenance unit should work jointly.
Original goal of total productive management:
“Continuously improve all operational conditions, within a production system; by stimulating
the daily awareness of all employees” (by Seiichi Nakajima, Japan, JIPM)
TPM focuses primarily on manufacturing (although its benefits are applicable to virtually
any "process") and is the first methodology Toyota used to improve its global position (1950s).
After TPM, the focus was stretched, and also suppliers and customers were involved (Supply
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Chain), this next methodology was called lean manufacturing. This sheet gives an overview of
TPM in its original form.
An accurate and practical implementation of TPM, will increase productivity within the total
organization, where:
a clear business culture is designed to continuously improve the efficiency of the total
production system.
a standardized and systematic approach is used, where all losses are prevented and/or
known.
all departments, influencing productivity, will be involved to move from a reactive- to a
predictive mindset.
a transparent multidisciplinary organization in reaching zero losses.
steps are taken as a journey, not as a quick menu.
Finally TPM will provide practical and transparent ingredients to reach operational excellence.
PM has basically 3 goals - Zero Product Defects, Zero Equipment Unplanned Failures
and Zero Accidents. It sets out to achieve these goals by Gap Analysis of previous historical
records of Product Defects, Equipment Failures and Accidents. Then through a clear
understanding of this Gap Analysis (Fishbone Cause-Effect Analysis, Why-Why Cause-Effect
Analysis, and P-M Analysis) plan a physical investigation to discover new latent fuguai (slight
deterioration) during the first step in TPM Autonomous Maintenance called "Initial Cleaning".
.
TPM identifies the 7 losses (types of waste) (muda), namely set-up and initial adjustment
time, equipment breakdown time, idling and minor losses, speed (cycle time) losses, start-up
quality losses, and in process quality losses, and then works systematically to eliminate them by
making improvements (kaizen). TPM has 8 pillars of activity,[2] each being set to achieve a
“zero” target. These 8 pillars are the following: focussed improvement (Kobetsu Kaizen);
autonomous maintenance (Jishu Hozen); planned maintenance; training and education; early-
phase management; quality maintenance (Hinshitsu Hozen); office TPM; and safety, health, and
environment. Few organisation also add Pillars according to their Work Place like: Tools
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Management; Information Technology & more. The Base for the TPM Activity is 5S; Seiri
(Sorting out the required or not required items); Seition (Systematic Arrangement of the required
items); Seiso (Cleaniness); Seiketsu (Standardisation); Shitsuke (Self Discipline).
The Pillars & their details
a) Efficient Equipment Utilisation
b) Efficient Worker Utilisation
c) Efficient Material & Energy Utilisation
1. Focussed improvement (Kobetsu Kaizen) - Continuously even small steps of
improvement.
2. Planned Maintenance - It focusses on Increasing Availability of Equipments & reducing
Breakdown of Machines.
3. Initial Control - To establish the system to launch the production of new product & new
equipment in a minimum run up time.
4. Education & Training - Formation of Autonomous workers who have skill & technique
for autonomous maintenance.
5. Autonomous Maintenance (Jishu Hozen) - It means "Maintaining one's equipment by
oneself". There are 7 Steps in & Activities of Jishu Hozen.
6. Quality Maintenance (Hinshitsu Hozen) - Quality Maintenance is establishment of
machine conditions that will not allow the occurrence of defects & control of such
conditions is required to sustain Zero Defect.
7. Office TPM - To make an efficient working office that eliminate losses.
8. Safety, Hygiene & Environment - The main role of SHE (Safety, Hygiene &
Environment) is to create Safe & healthy work place where accidents do not occur,
uncover & improve hazardous areas & do activities that preserve environment.
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Other Pillars Like: Tools Management - To increase the availability of Equipment by reducing
Tool Resetting Time, To reduce Tool Consumption Cost & to increase the tool life.
TPM success measurement - A set of performance metrics which is considered to fit well
in a lean manufacturing/TPM environment is overall equipment effectiveness, or OEE. For
advanced TPM world class practitioners, the OEE cannot be converted to costs using Target
Costing Management (TCM) OEE measurements are used as a guide to the potential
improvement that can be made to equipment and by identifying which of the 6 losses is the
greater, then the techniques applicable to that type of loss. Consistent application of the
applicable improvement techniques to the sources of major losses will positively impact the
performance of that equipment.
Using a criticality analysis across the factory should identify which equipments should be
improved first, also to gain the quickest overall factory performance.
The use of Cost Deployment is quite rare, but can be very useful in identifying the
priority for selective TPM deployment.
REPAIRABLE
Repairable parts are parts that are deemed worthy of repair, usually by virtue of economic
consideration of their repair cost.
Rather than bear the cost of completely replacing a finished product, repairable typically
are designed to enable more affordable maintenance by being more modular.
This allows components to be more easily removed, repaired, and replaced, enabling
cheaper replacement.
Spare parts that are needed to support condemnation of repairable parts are known as
replenishment spares.
A rotable pool is a pool of repairable spare parts inventory set aside to allow for multiple
repairs to be accomplished simultaneously.
This can be used to minimize stockout conditions for repairable items.
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REPAIR CYCLE
From the perspective of logistics, a model of the life cycle of parts in a supply chain can
be developed.
This model, called the repair cycle, consists of functioning parts in use by equipment
operators, and the entire sequence of suppliers or repair providers that replenish
functional part inventories, either by production or repair, when they have failed.
Ultimately, this sequence ends with the manufacturer.
This type of model allows demands on a supply system to ultimately be traced to their
operational reliability, allowing for analysis of the dynamics of the supply system, in
particular, spare parts.
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UNIT III
CONDITION MONITORING
Condition Monitoring – Cost comparison with and without CM – On-load testing and off-load
testing – Methods and instruments for CM – Temperature sensitive tapes – Pistol thermometers –
wear-debris analysis
CONDITION MONITORING
Condition monitoring is the process of monitoring a parameter of condition in machinery,
such that a significant change is indicative of a developing failure.
It is a major component of predictive maintenance. The use of conditional monitoring
allows maintenance to be scheduled, or other actions to be taken to avoid the
consequences of failure, before the failure occurs.
Nevertheless, a deviation from a reference value (e.g. temperature or vibration behaviour)
must occur to identify impeding damages
Predictive Maintenance does not predict failure.
Machines with defects are more at risk of failure than defect free machines. Once a defect
has been identified, the failure process has already commenced and CM systems can only
measure the deterioration of the condition.
Intervention in the early stages of deterioration is usually much more cost effective than
allowing the machinery to fail. Condition monitoring has a unique benefit in that the
actual load, and subsequent heat dissipation that represents normal service can be seen
and conditions that would shorten normal lifespan can be addressed before repeated
failures occur.
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Serviceable machinery includes rotating equipment and stationary plant such as boilers
and heat exchangers.
METHODS OF CM
Screen monitoring records video or static images detailing the contents, or screen capture,
of the entire [video display] or the content of the screen activity within a particular
program or computer application. Monitoring tools may collect real time video,
accelerated or [time-lapse] video or screen shots, or may take video or still image
captures at regular intervals (e.g., once every 4 minutes). They may collect images
constantly or only collect information while the user is interacting with the equipment
(e.g., capturing screens when the mouse or keyboard is active).
Data monitoring tracks the content of and changes to files stored on the local [hard drive]
or in the user's "private" network share.
Keystroke monitoring (e.g., number of keystrokes per minute) may track the performance
of keyboard-intensive work such as word processing or data entry. Keystroke logging
captures all keyboard input to enable the employer to monitor anything typed into the
monitored machine.
Idle time monitoring keeps track of time when the employee is away from the computer
or the computer is not being actively used.
BENEFITS
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Screen monitoring records video or static images detailing the contents, or screen capture,
of the entire [video display] or the content of the screen activity within a particular
program or computer application.
Monitoring tools may collect real time video, accelerated or [time-lapse] video or screen
shots, or may take video or still image captures at regular intervals (e.g., once every 4
minutes).
They may collect images constantly or only collect information while the user is
interacting with the equipment (e.g., capturing screens when the mouse or keyboard is
active).
Data monitoring tracks the content of and changes to files stored on the local [hard drive]
or in the user's "private" network share.
Keystroke monitoring (e.g., number of keystrokes per minute) may track the performance
of keyboard-intensive work such as word processing or data entry. Keystroke logging
captures all keyboard input to enable the employer to monitor anything typed into the
monitored machine.
Idle time monitoring keeps track of time when the employee is away from the computer
or the computer is not being actively used .
LOAD TESTING
Load testing is the process of putting demand on a system or device and measuring its
response.
Load testing is performed to determine a system’s behavior under both normal and
anticipated peak load conditions.
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It helps to identify the maximum operating capacity of an application as well as any
bottlenecks and determine which element is causing degradation.
When the load placed on the system is raised beyond normal usage patterns, in order to
test the system's response at unusually high or peak loads, it is known as stress testing.
The load is usually so great that error conditions are the expected result, although no clear
boundary exists when an activity ceases to be a load test and becomes a stress test.
There is little agreement on what the specific goals of load testing are.
The term is often used synonymously with concurrency testing, software performance
testing, reliability testing, and volume testing.
Load testing is a type of non-functional testing.
TEMPERATURE-SENSITIVE TAPE:
It obtains only your body surface temperature and does not indicate the core temperature.
The tape is applied to the skin, forehead and abdomen. Inside the tape is liquid crystals that
change colour according to temperature.
INFRARED THERMOMETER:
An infrared thermometer is a thermometer which infers temperature from a portion of
the thermal radiation sometimes called blackbody radiation emitted by the object being
measured. They are sometimes called laser thermometers if a laser is used to help aim the
thermometer, or non-contact thermometers to describe the device's ability to measure
temperature from a distance. By knowing the amount of infrared energy emitted by the object
and its emissivity, the object's temperature can often be determined. Infrared thermometers are a
subset of devices known as "thermal radiation thermometers".
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Sometimes, especially near ambient temperatures, false readings will be obtained indicating
incorrect temperature. This is most often due to other thermal radiation reflected from the object
being measured, but having its source elsewhere, like a hotter wall or other object nearby - even
the person holding the thermometer can be an error source in some cases. It can also be due to an
incorrect emissivity on the emissivity control or a combination of the two possibilities.
The most basic design consists of a lens to focus the infrared thermal radiation on to a detector,
which converts the radiant power to an electrical signal that can be displayed in units of
temperature after being compensated for ambient temperature. This configuration facilitates
temperature measurement from a distance without contact with the object to be measured. As
such, the infrared thermometer is useful for measuring temperature under circumstances
where thermocouples or other probe type sensors cannot be used or do not produce accurate data
for a variety of reasons.
Some typical circumstances are where the object to be measured is moving; where the object is
surrounded by an electromagnetic field, as in induction heating; where the object is contained in
a vacuum or other controlled atmosphere; or in applications where a fast response is required, an
accurate surface temperature is desired or the object temperature is above the recommended use
point of a contact sensors, or contact with a sensor would mar the object or the sensor, or
introduce a significant temperature gradient on the object's surface.
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Infrared thermometers can be used to serve a wide variety of temperature monitoring
functions. A few examples provided to this article include:
Detecting clouds for remote telescope operation
Checking mechanical equipment or electrical circuit breaker boxes or outlets for hot spots
Checking heater or oven temperature, for calibration and control purposes
Detecting hot spots / performing diagnostics in electrical circuit board manufacturing
Checking for hot spots in fire fighting situations
Monitoring materials in process of heating and cooling, for research and development or
manufacturing quality control situations
There are many varieties of infrared temperature sensing devices available today, including
configurations designed for flexible and portable handheld use, as well many designed for
mounting in a fixed position to serve a dedicated purpose for long periods.
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Specifications of portable handheld sensors available to the home user will include ratings of
temperature accuracy (usually with measurement uncertainty of ±2 °C/±4 °F) and other
parameters.
The distance-to-spot ratio (D:S) is the ratio of the distance to the object and the diameter of the
temperature measurement area. For instance if the D:S ratio is 12:1, measurement of an object 12
inches (30 cm) away will average the temperature over a 1-inch-diameter (25 mm) area. The
sensor may have an adjustable emissivity setting, which can be set to measure the temperature of
reflective (shiny) and non-reflective surfaces.
A non-adjustable thermometer sometimes can be used to measure the temperature of a shiny
surface by applying a non-shiny paint or tape to the surface, if the allowed measurement error is
acceptable.
The most common infrared thermometers are the:
Spot Infrared Thermometer or Infrared Pyrometer, which measures the temperature
at a spot on a surface (actually a relatively small area determined by the D:S ratio).
Related equipment, although not strictly thermometers, includes:
Infrared Scanning Systems scan a larger area, typically by using what is essentially a
spot thermometer pointed at a rotating mirror. These devices are widely used in
manufacturing involving conveyors or "web" processes, such as large sheets of glass or
metal exiting an oven, fabric and paper, or continuous piles of material along a conveyor
belt.
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WEAR DEBRIS ANALYSIS
Using a Scanning Electron Microscope of a carefully taken sample of debris suspended in
lubricating oil (taken from filters or magnetic chip detectors).
Instruments then reveal the elements contained their proportions, size and morphology.
Using this method, the site, the mechanical failure mechanism and the time to eventual failure
may be determined. This is called WDA - Wear Debris Analysis
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UNIT IV
REPAIR METHODS FOR BASIC MACHINE ELEMENTS
Repair methods for beds, slideways, spindles, gears, lead screws and bearings – Failure
analysis – Failures and their development – Logical fault location methods – Sequential
fault location.
SPINDLE REPAIR - How to Properly Repair Precision Spindles.
Did your spindle fail prematurely after having it repaired? A major contributor to
premature spindle failure is caused directly by the spindle repair process itself.
It is important that the spindle repair facility you choose to repair your Spindle, understands and
recognizes the need for maintaining exacting tolerances, attention to detail and the necessity for
controlling the proper cleanliness levels for all facets of the spindle repair process.
Especially critical is the prepping and Ultrasonic Cleaning of all components utilizing 3
micron “Absolute” filtration for all of the Cleaning Solvents, blowing off of parts with
Compressed Air that is dried to a minimum dew point of -40 degree F and filtered with .08
Micron “Absolute Filtration” just prior to assembly. The assembly must be done in a Certified
“Class 10,000 Clean Room” and when practical, in a “Certified Class 1,000 Laminar Flow
Bench”.
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This is particularly important for “Open” bearings where they have been Grease Packed
for life. If the precision “ABEC 7 or 9” bearings are exposed to dust and dirt particles that are 3 –
5 microns in size and considering the human eye with 20/20 vision can only see 40 micron size
particles, you cannot see the dirt that will damage your bearings. This invisible dirt is always
present and can dramatically reduce the overall Service Life of a bearing and may cause
premature failure of your Spindle.
Even when dealing with bearings that are “Air-Oil” or "Air Oil Mist" lubricated, it is still
important to keep the whole process “Clean”, as the bearings can ingest dirt and dust during
handling and assembly and ultimately sustain undetected damage during the initial startup of the
Spindle. The bearings won’t necessarily fail immediately, but can and will gradually deteriorate
over a period of time. This is known as “McPherson’s Curve”, the chain reaction of wear. If
bearings start out clean and are kept clean, providing the lubricants and the compressed air
delivering the lubrication, where applicable, is “Clean and Dry”, the bearings theoretically, can
last forever, unless “Crashed” or mistreated.
If your spindle repair source does not have the type of facility that incorporates and
“maintains” the Cleanliness levels that are absolutely imperative to the long term health of your
Spindle, you should look elsewhere. You are paying for that level of service and should demand
that anyone repairing your production machine Spindles comply with the cleanliness levels
needed. If this process control is not embraced and adhered to, ultimately, you could receive a
Spindle back from repair that will fail prematurely. How important is it to your manufacturing
facility and your production schedule that your Spindles run and produce quality parts for an
extended period of time? What is the cost of a “Premature” Spindle failure or “Downtime” to
your business?
You need to ask questions about the spindle repair process and the level of expertise for
the personnel repairing your precision Spindle. The Spindle is the “Heart” of your production
machinery.
Atlanta Precision Spindle’s key people have spent years doing the right things. We have
set many of the industry standards for “Clean Dry Oil and Clean Dry Compressed Air” as it
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relates to Spindles. We filter our cleaning solvents that process all of the various components
with 3 micron absolute filtration prior to the final assembly. All of the assembly is done in a
“Certified Class 10,000 Clean Room”, and whenever practical, in a “Certified Class 100 Laminar
Flow Bench”. If Spindles require Air Oil or Air Oil Mist Lubrication, we use only “Purified
Lube Oil” that meet or exceed an ISO Cleanliness level of 14/13/10. You can be assured that
when your Spindle leaves our facility, you have not purchased a dirty uncontrolled Repair
process or dirty wet lube oil or compressed air. We do it right!
Our Written Repair Certification includes running your Spindle at its maximum rated
speed and also at the speed where you historically run the Spindle. Our detailed Spectrum
Analysis is performed utilizing the latest equipment and technology from Schenck-Trebel, the
world leader in balancing and diagnostic testing equipment for rotating assemblies. We are
continually looking for changes and advancements in Spindle testing technology, so those
advancements can be passed along to you. “We want to be Partners in your success”.
FAILURE ANALYSIS
Failure analysis is the process of collecting and analyzing data to determine the cause of
a failure. It is an important discipline in many branches of manufacturing industry, such as the
electronics industry, where it is a vital tool used in the development of new products and for the
improvement of existing products. It relies on collecting failed components for subsequent
examination of the cause or causes of failure using a wide array of methods,
especially microscopy and spectroscopy. The NDT or non-destructive testing methods are
valuable because the failed products are unaffected by analysis, so inspection always starts using
these methods.
A failure analysis engineer often plays a lead role in the analysis of failures, whether a
component or product fails in service or if failure occurs in manufacturing or during production
processing. In any case, one must determine the cause of failure to prevent future occurrence,
and/or to improve the performance of the device, component or structure
Method of Analysis
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The failure analysis of many different products involves the use of the following tools and
techniques:
Microscopes
Optical microscope
Liquid crystal
Scanning acoustic microscope (SAM)
Scanning acoustic tomography (SCAT)
Atomic force microscope (AFM)
Stereomicroscope
Photo emission microscope (PEM)
X-ray microscope
Infra-red microscope
Scanning SQUID microscope
Sample preparation
Jet-etcher
Plasma etcher
Back side thinning tools
Mechanical back-side thinning
Laser chemical back-side etching
Spectroscopic analysis
Transmission line pulse spectroscopy (TLPS)
Auger electron spectroscopy
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Deep-level transient spectroscopy (DLTS)
Device modification
Focused ion beam etching (FIB)
Surface analysis
Dye penetrant inspection
Other Surface analysis tools
Scanning electron microscopy
Scanning electron microscope (SEM)
Electron beam induced current (EBIC) in SEM
Charge-induced voltage alteration (CIVA) in SEM
Voltage contrast in SEM
Electron backscatter diffraction (EBSD) in SEM
Energy-dispersive X-ray spectroscopy (EDS) in SEM
Transmission electron microscope (TEM)
Laser signal injection microscopy (LSIM)
Photo carrier stimulation
Static
Optical beam induced current (OBIC)
Light-induced voltage alteration (LIVA)
Dynamic
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Laser-assisted device alteration (LADA)
Thermal laser stimulation (TLS)
Static
Optical-beam-induced resistance change (OBIRCH)
Thermally induced voltage alteration (TIVA)
External induced voltage alteration (XIVA)
Seebeck effect imaging (SEI)
Dynamic
Soft defect localization (SDL)
Semiconductor probing
Mechanical probe station
Electron beam prober
Laser voltage prober
Time-resolved photon emission prober (TRPE)
FAULT DIAGNOSIS
Basic Concepts
A unit under test (UUT) fails when its observed behaviour is different from its expected
behaviour. Diagnosis consists of locating the physical fault(s) in a structural model of the UUT.
The degree of accuracy to which faults can be located is called diagnostic
resolution. Functionally equivalent faults (FEF) cannot be distinguished. The partition of all
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faults into distinct subsets of FEF defines the maximal fault resolution. A test that achieves the
maximal fault resolution is said to be a complete fault-location test.
Repairing the UUT often consists of substituting one of its replaceable units (RU)
referred as a faulty RU, rather than in an accurate identification of the real fault inside an RU.
We characterize this process by RU resolution. Suppose that the results of the test do not allow to
distinguish between two suspected RUs U1 and U2. We could replace now one of these RUs, say
U1 with a good RU, and return to the test experiment. If the new results are correct, the faulty
RU was the replaced one; otherwise, it is the remaining one U2. This type of procedure we
call sequential diagnosis procedure.
The diagnosis process is often hierarchical, carried out as a top-down process (with a
system operating in the field) or bottom-up process (during the fabrication of the system).
In the top-down approach (system boards ICs) first-level diagnosis may deal with
"large" RUs like boards called also field-replaceable units. The faulty board is then tested in a
maintenance centre to locate the faulty component (IC) on the board. Accurate location of faults
inside a faulty IC may be also useful for improving its manufacturing process.
In the bottom-up approach (ICs boards system) a higher level is assembled only
from components already tested at a lower level. This is done to minimize the cost of diagnosis
and repair, which increases significantly with the level at which the faults are detected.
The rule of 10: if it costs $1 to test an IC, the cost of locating the same defective IC when
mounted on a board and of repairing the board is about $10; when the defective board is plugged
into a system, the cost of finding the fault and repairing the system is $100.
In manufacturing, the most likely faults are fabrication errors affecting the interconnections
between components; in the field the most likely faults are physical failures internal to
components (because every UUT has been successfully tested in the past). Knowing the most
likely class of faults helps in fault location.
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COMBINATIONAL FAULT DIAGNOSIS METHODS
This approach does most of the work before the testing experiment. It uses fault simulation to
determine the possible responses to a given test in the presence of faults. The database
constructed in this step is called a fault table or a fault dictionary. To locate faults, one tries to
match the actual results of test experiments with one of the precomputed expected results stored
in the database. The result of the test experiment represents a combination of effects of the fault
to each test pattern. That's why we call this approach combinational fault diagnosis method. If
this look-up process is successful, the fault table (dictionary) indicates the corresponding fault(s).
On the applet, select Fault Diagnosis Mode, after that a circuit layout, insert needed
vectors and simulate faults. A fault table will be produced.
1. Fault tables
2. Fault dictionaries
3. Minimization of diagnostic data
4. Fault location by structural analysis
1. Fault Table
In general, a fault table is a matrix where columns Fj represent faults, rows
Ti represent test patterns, and aij = 1 if the test pattern Ti detects the fault Fj, otherwise if the test
pattern Ti does not detect the fault Fj, aij = 0.
Denote the actual result of a given test pattern by 1 if it differs from the precomputed expected
one, otherwise denote it by 0. The result of a test experiment is represented by a vector
where ei = 1 if the actual result of the test patterns does not match with the expected result,
otherwise ei = 0. Each column vector fj corresponding to a fault Fj represents a possible result of
the test experiment in the case of the fault Fj.
Three cases are now possible depending on the quality of the test patterns used for carrying out
the test experiment:
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1. The test result E matches with a single column vector fj in FT. This result corresponds to
the case where a single fault Fj has been located. In other words, the maximum diagnostic
resolution has been obtained.
2. The test result E matches with a subset of column vectors {fi, fj … fk} in FT. This result
corresponds to the case where a subset of indistinguishable faults {Fi, Fj … Fk} has been
located.
3. No match for E with column vectors in FT is obtained. This result corresponds to the case
where the given set of vectors does not allow to carry out fault diagnosis. The set of faults
described in the fault table must be incomplete (in other words, the real existing fault is
missing in the fault list considered in FT).
Example:
In the example the results of three test experiments E1, E2, E3 are demonstrated. E1 corresponds to
the first case where a single fault is located, E2 corresponds to the second case where a subset of
two indistinguishable faults is located, and E3 corresponds to the third case where no fault can be
located because of the mismatch of E3 with the column vectors in the fault table.
2. Fault Dictionary
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Fault dictionaries (FD) contain the same data as the fault tables with the difference that the data
is reorganized. In FD a mapping between the potential results of test experiments and the faults
is represented in a more compressed and ordered form. For example, the column bit vectors can
be represented by ordered decimal codes (see the example) or by some kind of compressed
signature.
Example:
3. Minimization of Diagnostic Data
To reduce large computational effort involved in building a fault dictionary, in fault simulation
the detected faults are dropped from the set of simulated faults. Hence, all the faults detected for
the first time by the same vector will produce the same column vector (signature) in the fault
table, and will be included in the same equivalence class of faults. In this case the testing
experiment can stop after the first failing test, because the information provided by the following
tests is not used. Such a testing experiment achieves a lower diagnostic resolution. A trade off
between computing time and diagnostic resolution can be achieved by dropping faults after k>1
detections.
Example:
In the fault table produced by fault simulation with fault dropping, only 19 faults need to be
simulated compared to the case of 42 faults when simulation without fault dropping is carried out
(the simulated faults in the fault table are shown in shadowed boxes). As the result of the fault
dropping, however, the following faults remain not distinguishable: {F2, F3},{F1, F4},{F2, F6}.
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4. Fault Location by Structural Analysis
Assume a single fault in the circuit. Then there should exist a path from the site of the fault to
each of the outputs where errors have been detected. Hence the fault site should belong to the
intersection of cones of all failing outputs. A simple structural analysis can help to find faults
that can explain all the observed errors.
SEQUENTIAL FAULT DIAGNOSIS METHODS
In sequential fault diagnosis the process of fault location is carried out step by step, where
each step depends on the result of the diagnostic experiment at the previous step. Such a test
experiment is called adaptive testing. Sequential experiments can be carried out either by
observing only output responses of the UUT or by pinpointing by a special probe also internal
control points of the UUT (guided probing). Sequential diagnosis procedure can be
1. Fault location by edge-pin testing
2. Generating tests to distinguish faults
3. Guided-probe testing
4. Fault location by UUT reduction
1. Fault Location by Edge-Pin Testing
In fault diagnosis test patterns are applied to the UUT step by step. In each step, only output
signals at edge-pins of the UUT are observed and their values are compared to the expected ones.
The next test pattern to be applied in adaptive testing depends on the result of the previous step.
The diagnostic tree of this process consists of the fault nodes FN (rectangles) and test nodes TN
46
(circles). A FN is labeled by a set of not yet distinguished faults. The starting fault node is
labeled by the set of all faults. To each FN k a TN is linked labeled by a test pattern Tk to be
applied as the next. Every test pattern distinguishes between the faults it detects and the ones it
does not. The task of the test pattern Tk is to divide the faults in FN k into two groups - detected
and not detected by Tk faults. Each test node has two outgoing edges corresponding to the results
of the experiment of this test pattern. The results are indicated as passed (P) or failed (F). The set
of faults shown in a current fault node (rectangle) are equivalent (not distinguished) under the
currently applied test set.
Example:
The diagnostic tree in the Figure below corresponds to the example considered in 3.2.1. We can
see that most of the faults are uniquely identified, two faults F1,F4 remain indistinguishable. Not
all test patterns used in the fault table are needed. Different faults need for identifying test
sequences with different lengths. The shortest test contains two patterns the longest four patterns.
Rather than applying the entire test sequence in a fixed order as in combinational fault diagnosis,
adaptive testing determines the next vector to be applied based on the results obtained by the
preceding vectors. In our example, if T1 fails, the possible faults are {F2,F3}. At this point
applying T2 would be wasteful, because T2 does not distinguish among these faults. The use of
adaptive testing may substantially decrease the average number of tests required to locate a fault.
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2. Generating Tests to Distinguish Faults
To improve the fault resolution of a given test set T, it is necessary to generate tests to
distinguish among faults equivalent under T.
Consider the problem of generating a test to distinguish between faults F1 and F2. Such a test
must detect one of these faults but not the other, or vice versa. The following cases are possible.
1. F1 and F2 do not influence the same set of outputs. Let OUT(Fk) be the set of outputs
influenced by the fault Fk. A test should be generated for F1 using only the circuit
feeding the outputs OUT(F1), or for F2 using only the circuit feeding the
outputs OUT(F2).
2. F1 and F2 influence the same set of outputs. A test should be generated for F1 without
activating F2, or vice versa, for F2 without activating F1.
Three possibilities can be mentioned to keep a fault F2: xk e not activated, where xk denotes a
line in the circuit, and e {0,1}:
1. The value e should be assigned to the line xk.
2. If this is not possible then the activated path from F2 should be blocked, so that the
fault F2 could not propagate and influence the activated path from F1.
3. If the 2nd case is also not possible then the values propagated from the
sites F1 and F2 and reaching the same gate G should be opposite on the inputs of G.
Example:
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1. There are two faults in the circuit: F1: x3,1 0, and F2: x4 1. The fault F1 may influence
both outputs, the fault F2 may influence only the output x8. A test pattern 0010
activatesF1 up to the both outputs, and F2 only to x8. If both outputs will be wrong, F1 is
present, and if only the output x8 will be wrong, F2 is present.
2. There are two faults in the circuit: F1: x3,2 0, and F2: x5,2 1. Both of them influence the
same output of the circuit. A test pattern 0100 activates the fault F2. The fault F1 is not
activated, because the line x3,2 has the same value as it would have had if F1 were
present.
3. There are the same two faults in the circuit: F1: x3,2 0, and F2: x5,2 1. Both of them
influence the same output of the circuit. A test pattern 0110 activates the fault F2. The
faultF1 is activated at its site but not propagated through the AND gate, because of the
value x4 = 0 at its input.
4. There are two faults in the circuit: F1: x3,1 1, and F2: x3,2 1. A test pattern 1001 consists
the value x1 1 which creates the condition where both of the faults may influence only
the same output x8. On the other hand, the test pattern 1001 activates both of the faults to
the same OR gate (i.e. none of them is blocked). However, the faults produce different
values at the inputs of the gate, hence they are distinguished. If the output value on x8 will
be 0, F1 is present. Otherwise, if the output value on x8 will be 1, either F2 is present or
none of the faults F1 and F2 are present.
3. Guided-Probe Testing
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Guided-probe testing extends edge-pin testing process by monitoring internal signals in the UUT
via a probe which is moved (usually by an operator) following the guidance provided by the test
equipment. The principle of guided-probe testing is to backtrace an error from the primary output
where it has been observed during edge-pin testing to its physical location in the UUT. Probing
is carried out step-by-step. In each step an internal signal is probed and compared to the expected
value. The next probing depends on the result of the previous step.
A diagnostic tree can be created for the given test pattern to control the process of probing. The
tree consists of internal nodes (circles) to mark the internal lines to be probed, and of terminal
nodes (rectangles) to show the possible result of diagnosis. The results of probing are indicated
as passed (P) or failed (F).
Typical faults located are opens and defective components. An open between two points A and B
in a connection line is identified by a mismatch between the error observed at B and the correct
value measured at A. A faulty device is identified by detecting an error at one of its outputs,
while only correct values are measured at its inputs.
The most time-consuming part of guided-probe testing is moving the probe. To speed-up the
fault location process, we need to reduce the number of probed lines. A lot of methods to
minimize the number of probings are available.
Example:
Let have a test pattern 1010 applied to the inputs of the circuit. The diagnostic tree created for
this particular test pattern is shown. On the output x8 , instead of the expected value 0, an
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erroneous signal 1 is detected. By back tracing (indicated by bold arrows in the diagnostic tree)
the faulty component NOR- x5 is located.
Diagnostic tree allows to carry out optimization of the fault location procedure, for example to
generate a procedure with minimum average number of probes.
4. Fault Location by UUT Reduction
Initially the UUT is the entire circuit and the process starts when its test fails. While the failing
UUT can be partitioned, half of the UUT is disabled and the remaining half is tested. If the test
passes, the fault must be in the disabled part, which then becomes the UUT. If the test fails, the
tested part becomes the UUT.
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UNIT V
REPAIR METHODS FOR MATERIAL HANDLING EQUIPMENT
Repair methods for Material handling equipment - Equipment records –Job order
systems -Use of computers in maintenance.
MATERIAL HANDLING EQUIPMENT
Material handling equipment is equipment that relate to the movement, storage, control
and protection of materials, goods and products throughout the process of manufacturing,
distribution, consumption and disposal. Material handling equipment is the mechanical
equipment involved in the complete system. Material handling equipment is generally separated
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into four main categories: storage and handling equipment, engineered systems, industrial trucks,
and bulk material handling.
Categories of Material Handling Equipment
The four main categories of material handling equipment include:
Storage
Engineered systems
Industrial trucks
Bulk material handling
Storage and Handling Equipment
Storage equipment is usually limited to non-automated examples, which are grouped in
with engineered systems. Storage equipment is used to hold or buffer materials during
“downtimes,” or times when they are not being transported. These periods could refer to
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temporary pauses during long-term transportation or long-term storage designed to allow the
buildup of stock. The majority of storage equipment refers to pallets, shelves or racks onto which
materials may be stacked in an orderly manner to await transportation or consumption. Many
companies have investigated increased efficiency possibilities in storage equipment by designing
proprietary packaging that allows materials or products of a certain type to conserve space while
in inventory.
Examples of storage and handling equipment include:
Racks, such as pallet racks, drive-through or drive-in racks, push-back racks, and sliding
racks
Stacking frames
Shelves, bins and drawers
Mezzanines
Engineered Systems
Engineered systems cover a variety of units that work cohesively to enable storage and
transportation. They are often automated. A good example of an engineered system is an
Automated Storage and Retrieval System, often abbreviated AS/RS, which is a large automated
organizational structure involving racks, aisles and shelves accessible by a “shuttle” system of
retrieval. The shuttle system is a mechanized cherry picker that can be used by a worker or can
perform fully automated functions to quickly locate a storage item’s location and quickly retrieve
it for other uses.
Other types of engineered systems include:
Conveyor systems
Robotic delivery systems
Automatic guided vehicles (AGV)
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Industrial Trucks
Industrial trucks refer to the different kinds of transportation items and vehicles used to
move materials and products in materials handling. These transportation devices can include
small hand-operated trucks, pallet-jacks, and various kinds of forklifts. These trucks have a
variety of characteristics to make them suitable for different operations. Some trucks have forks,
as in a forklift, or a flat surface with which to lift items, while some trucks require a separate
piece of equipment for loading. Trucks can also be manual or powered lift and operation can be
walk or ride, requiring a user to manually push them or to ride along on the truck. A stack truck
can be used to stack items, while a non-stack truck is typically used for transportation and not for
loading.
There are many types of industrial trucks:
* Hand trucks
* Pallet jacks
* Pallet trucks
* Walkie stackers
* Platform trucks
* Order picker
* Sideloader
Bulk Material Handling Equipment
Bulk material handling refers to the storing, transportation and control of materials in
loose bulk form. These materials can include food, liquid, or minerals, among others. Generally,
these pieces of equipment deal with the items in loose form, such as conveyor belts or elevators
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designed to move large quantities of material, or in packaged form, through the use of drums and
hoppers.
* Conveyor belts
* Stackers
* Reclaimers
* Bucket elevators
* Grain elevators
* Hoppers
* Silos
A SYSTEMATIC APPROACH TO MATERIAL HANDLING MAINTENANCE
The term material handling equipment refers to conveyors, sorters, spirals, carousels, and
a wide assortment of electrical and mechanical devices. Proper maintenance of this equipment is
essential, because it prevents the loss of business or production caused by mechanical failure.
This article introduces a systematic approach to material handling maintenance based on these
lessons, and includes important tips to prevent common material handling maintenance mistakes.
The first step in material handling equipment maintenance is to list all material handling
equipment. This step is important because it creates a starting point from which a company can
develop ways to improve its physical assets. Within manufacturing operations, the list should
include all physical assets, production equipment and processes as well as facility assets. For
large distribution centers (DCs), this should include all major areas: mobile equipment, conveyor
systems, sorter systems and facility-related assets, as well as bar code scanners, printers and
other devices that keep a DC functioning.
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Because maintenance depends on more than just knowing what equipment is in place, companies
should take into account other factors that could affect how the equipment runs while an
equipment list is being compiled. For example, equipment operating in the desert of Nevada with
blowing sand requires more maintenance than in a mild climate on the East Coast. Other factors
include terrain, border regulations and the implications of schedules, calendars, cycles and peaks.
Some questions to answer are:
* Is my facility's material handling equipment running three shifts and on weekends?
* Is it indoors or outside?
* Is there moisture or harsh operating conditions?
After the list is made, the company should evaluate the current state of maintenance,
determining its strengths and weaknesses, as well as potential results from improvement
opportunities. The next step is to develop and implement a strategic maintenance plan. This plan
must include a customized preventive maintenance (PM) program to ensure that the equipment
runs with high reliability. PM is a continuous process, the objective of which is to minimize
future maintenance problems. A PM program costs extra on the front end, but savings come
quickly. Studies have shown that operations with PM spend less for maintenance than reactive
run-to-failure operations.
The best approach to customizing PM is to let the craftsmen generate the PM program
from the ground up. What do they think? They are the ones who will be inspecting the
equipment, and if they think it should be looked at daily instead of weekly, this should be the
approach to take. It gives them ownership of the equipment and the empowerment to make it
work more reliably. When this comes together, look out. The results are significant gains in
equipment uptime. All equipment should be organized in this fashion according to what type of
material handling equipment it is. Even the racks should be a part of this (the air-operated flow-
through systems will need PM attention.) The air compressor will always be a part of a PM
program.
The final steps in the systematic approach to material handling maintenance are
validating results and return on investment. Companies should also identify priority areas for
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improvement based upon a total benchmark evaluation of the maintenance operation. They
should also take note of the common mistakes made by those charged with maintaining material
handling equipment and make sure that they are eliminated from the process. Of course, the
ultimate success will be determined by whether the customer is satisfied. Avoiding common
mistakes.
The most common and most obvious material handling maintenance mistake is to
operate in a reactive, firefighting mode.
The following are tips for avoiding other maintenance mistakes:
* Do not over lubricate and always be certain to use the correct lubricant.
* Do not assume that your craftsmen can automatically handle the new high-tech equipment.
They may need additional training and refresher courses from time to time. Develop an
individual training plan for each craftsperson based on his or her level of expertise.
* Use manufacturers' recommendations as a starting point for a PM program, but be sure that the
crafts-people themselves drive the program because they are most familiar with the demands on
the equipment and the inspection processes. In some cases, strictly adhering to manufacturers'
recommendations is counter-productive.
* How many spares should I have on hand? Make a concentrated effort to identify critical spares.
By critical, look at downtime cost and long lead times. If 100 people are scheduled to work on an
operation, then it must have spares to keep it in operations. Paint and plumbing supplies that are
10 minutes away are not critical spares.
* Keep manuals, documents, PM procedures and other inventories near the machines where they
can be used. This is valuable information and an effort should be made to organize it at the
machines. Successful organizations do this seriously, calling the process "facilitated assets."
With a systematic material handling maintenance approach that includes PM, a company
obtains a proactive planned maintenance operation. This operation can prevent common
maintenance mistakes and provide greater levels of service.
Best of all, it eliminates the firefighting mode that wastes scarce maintenance resources.
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Periodic Maintenance
Truck Frame & Chassis
Frame Damage
Counter Weight
Overhead Guard
Seat & Seatbelts
Tires
Data Plate
Upright Inspection
Channel & Rollers
Chains & Adjustment
Carriage
Forks
Load Backrest
Hydraulics
Operation
Oil Level
Hydraulic Cap
Filter Inspection
Hoses & Leaks
Linkages (if applicable)
Engine & Transmission
(I.C.E. Trucks Only)
Oil Level & Change
Lube Chassis, Chains
Filter
Air Filter (cleaned or replaced)
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Cap & Rotor
Points (if applicable)
Wires
Plugs & HR inspection
Leak inspection
Belts
Coolant
Radiator & Air Cleaner
Radiator Hoses
Governor & Carb. Linkage
Transmission Fluid Level
Differential
Linkage (if applicable)
Fluid & Filter Inspection
Brakes
Fluid Level
Pedal Height
Pedal Pad
Operation
Parking Brake
Electrical Systems
Gauges
Warning Lights
Battery Connection
Battery Condition
Wiring
Horn
Other Safety Features
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Hour Meter
Electric Trucks ONLY
Drive Motor Brushes
Hydraulic Motor Brushes
Power Steering Brushes
Contactor Tips & operation
Switches & Adjustments
Cables & Wiring Condition
Battery Connectors & Charger
SCR/Drive Control Panel Blowout
EQUIPMENT RECORDS
Equipment records play a critical role in effectiveness and efficiency of the maintenance
organization. Usually, equipment records are grouped under four classifications: maintenance
work performed, maintenance cost, inventory, and files. The maintenance work performed
category contains chronological documentation of al l repairs and preventive maintenance (PM)
performed during the item’s service life to date. The maintenance cost category contains
historical profiles and accumulations of labor and material costs by item. Usually, information on
inventory is provided by the stores or accounting department. The inventory category contains
information such as property number, size and type, procurement cost, date manufactured or
acquired, manufacturer, and location of the equipment/item. The files category includes
operating and service manuals, warranties, drawings, and so on. Equipment records are useful
when procuring new items/equipment to determine operating performance trends,
troubleshooting breakdowns, making replacement or modification decisions, investigating
incidents, identifying areas of concern, performing reliability and maintainability studies, and
conducting life cycle cost and design studies.
WORK ORDER SYSTEM
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A work order authorizes and directs an individual or a group to perform a given task. A
well-defined work order system should cover all the maintenance jobs requested and
accomplished, whether repetitive or one-time jobs. The work order system is useful for
management in controlling costs and evaluating job performance. Although the type and size of
the work order can vary from one maintenance organization to another, a work order should at
least contain information such as requested and planned completion dates, work description and
its reasons, planned start date, labor and material costs, item or items to be affected, work
category (preventive maintenance, repair, installation, etc.), and appropriate approval signatures.
JOB PLANNING
Job planning is an essential element of the effective maintenance management. A number
of tasks may have to be performed prior to commencement of a maintenance job; for example,
procurement of parts, tools, and materials, coordination and delivery of parts, tools, and
materials, identification of methods and sequencing, coordination with other departments, and
securing safety permits. Although the degree of planning required may vary with the craft
involved and methods used, past experience indicates that on average one planner is required for
every twenty craftpersons. Strictly speaking, formal planning should cover 100% of the
maintenance workload but emergency jobs and small, straightforward work assignments are
performed in a less formal environment. Thus, in most maintenance organizations 80 to 85%
planning coverage is attainable. Maintenance scheduling is as important as job planning.
Schedule effectiveness is based on the reliability of the planning function. For large jobs, in
particular those requiring multi-craft coordination, serious consideration must be given to using
methods such as Program Evaluation and Review Technique (PERT) and Critical Path Method
(CPM) to assure effective overall control.
USE OF COMPUTERS IN MAINTENANCE
Computerized maintenance management system (CMMS) is also known as enterprise
asset management and computerized maintenance management information system (CMMIS).
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A CMMS software package maintains a computer database of information about an
organization’s maintenance operations, i.e. CMMIS – computerized maintenance management
information system. This information is intended to help maintenance workers do their jobs more
effectively (for example, determining which machines require maintenance and which
storerooms contain the spare parts they need) and to help management make informed decisions
(for example, calculating the cost of machine breakdown repair versus preventive maintenance
for each machine, possibly leading to better allocation of resources). CMMS data may also be
used to verify regulatory compliance. CMMS packages may be used by any organization that
must perform maintenance on equipment, assets and property. Some CMMS products focus on
particular industry sectors (e.g. the maintenance of vehicle fleets or health care facilities). Other
products aim to be more general. CMMS packages can produce status reports and documents
giving details or summaries of maintenance activities. The more sophisticated the package, the
more analysis facilities are available. Many CMMS packages can be either web-based, meaning
they are hosted by the company selling the product on an outside server, or LAN based, meaning
that the company buying the software hosts the product on their own server.
Key features that a computerized system must have are -
• Hard copies of records must be available to be produced on request;
• The system must be tamper proof (e.g. records can't be changed at a later date);
• It needs to be clear what has been checked (which should be at least the items described
in the Guide to Maintaining Roadworthiness) and by whom;
• There is a clear end to end audit trail showing that identified faults are clearly logged
and once dealt with, signed off by a person who has authority to decide whether a vehicle is fit
for service.
Any planning tool software needs to be drawn up in accordance with the maintenance
regime agreed as part of the operator’s license requirements. If the computerized system does
not meet any of the above points then it will not meet the necessary requirements as identified by
the Guide to Maintaining Roadworthiness.
A first step Freight Best Practice offers a Guide to Preventative Maintenance and a free
simple planning spreadsheet to help you with improved maintenance. The Vehicle Maintenance
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Planner is an easy way to electronically track and store data on your fleet's maintenance helping
you to plan servicing, inspection and MOT schedules effectively and provide a log for any
unplanned maintenance your fleet may incur. This spreadsheet also provides a yearly planner
that can be printed and wall mounted.
1.The Daily Walkaround check
The daily walkaround check can be undertaken using a handheld device and
stored in an electronic format.
Providing a written report
Any defects found during the daily check, while the vehicle is in use or on its
return to base must be the subject of a written report.
The details that need to be recorded are:
• Vehicle registration or identification mark;
• Date;
• Details of the defects or symptoms; and
• The reporter's name.
It is common practice to use a composite form that also includes a list of the items
checked each day. It is advisable that where practicable the system should incorporate 'Nil'
reporting when each driver makes out a report - or confirms by another means that a daily check
has been carried out and no defects found. Electronic records of reported defects must be
available for 15 months along with any other record of repair. Hard copies must be able to be
produced where required.
Regular safety inspections:
Safety inspection information can be collected by the use of a handheld device
and stored electronically. The records MUST show a clear audit trail from inspection to repair
sign off – should one be required.
Safety Inspection Report Forms
Key Information
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A record must be completed for each safety inspection separately for both vehicles and
trailers. If the record of the safety inspection is to be stored electronically then the checklist used
for the inspection need not be retained. You may use an electronic device (e.g. PDA) in place of
a checklist.
Electronic Capture and Storage of Safety Inspection Data Electronic capture and / or
storage on computer of defects found or work done (e.g. bar coding or scanning), is acceptable
providing that a means of interpreting each code is readily available.
Safety inspection records stored electronically, using a computer, must be tamper-proof
and capable of producing hard-copy information for use at public enquiries held by Traffic
Commissioners. Computer records must contain:
• Name of owner / operator;
• Date of inspection;
Regular safety inspections
• Vehicle identity;
• Odometer (mileage recorder) reading (if appropriate);
• A full list of the items inspected (or these can be indicated on a paper report if used for
the inspection);
• An indication of the condition of each item inspected (however, it is sufficient to
provide details of defective items only)
• Details of any defects found;
• Name of inspector;
• Details of any remedial / rectification or repair work and by whom it was done; and
• A statement that any defects have been repaired satisfactorily. Internet-based systems
are becoming more common. These provide significant opportunities for improving the ease with
which operators can plan and monitor the maintenance of their vehicles, thus leading to higher
standards and improved compliance.
Safety Inspection Programme
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Safety inspections must be planned in advance. Vehicles that are subject to a
statutory annual test may have their year's programme planned around the anticipated test date to
avoid duplication of work associated with the test, such as cleaning and major servicing.
Planning a safety inspection programme :
A simple method of drawing up a programme is to use a year round planner or
flow chart. Computer-based systems are equally acceptable and electronic vehicle maintenance
record management and storage systems available will often incorporate an electronic planning
feature. The information, which should be kept in the simplest form possible and displayed
prominently, will serve as a reminder of programmed inspections or of any changes that have
been necessary. All vehicles subject to programmed attention should be included. Ideally
planners or charts should be used to set safety inspection dates at least six months in advance.
Vehicles' annual test dates should be included, as should servicing and other ancillary equipment
testing or calibration dates, e.g. tachograph, lifting equipment, etc. The planner should be
updated regularly by indicating the progress of the programme and recording any extra work
carried out. Vehicles that have been taken off the operator's licence or other vehicles temporarily
off-road should have their period of non-use identified, and a note should be made when vehicles
have been disposed of. The planner or chart may be used to record other items in the vehicle
maintenance programme, such as servicing, unscheduled work and refurbishing. Each activity
should be clearly identified.
Maintenance Engineer
The Maintenance Engineer enhances service quality and equipment reliability by
improving workflows and optimizing maintenance processes using Lean Six Sigma practices and
Reliability Centered Maintenance (RCM) methodology. This position, a key link between the
field maintenance organization and the engineering and manufacturing centers, provides input to
improve equipment design, reliability, and maintainability.
The Maintenance Engineer has a critical role in connecting field operations with the
maintenance organization, helping minimize downtime and failure rates and maximize
equipment productivity. This position reports directly to the Maintenance Supervisor or
Maintenance Manager.
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Essential Responsibilities and Duties
* Identifies and captures opportunities for improvement in equipment maintainability and
reliability.
* Uses Lean Six Sigma and RCM concepts to optimize work processes, adapt maintenance
tools and procedures to improve equipment utilization and reliability, and minimize
service quality incidents.
* Keeps current with latest equipment, technologies, and maintenance methods.
* Promotes importance of data and service quality within maintenance community.
* Assists with development and coaching of junior maintenance staff.
* Supports Maintenance Manager and Location Manager in planning for equipment and
maintenance resources and correcting existing discrepancies.
* Ensures application of asset management and maintenance systems data and accurate,
timely data entry and reporting.
* Participates in technical audits and compliance assessments, and follows up on closure of
remedial action plans.
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