failure prediction by condition monitoring (part 1)

Failure prediction by condition monitoring (Part 1) B. J. Woodley , BSc, ARCS

Michael Neale and Associates Ltd (Consulting Engineers), 43 Downing Street, Farnham, Surrey, England

Condition monitoring is concerned with extracting information from plant, machines and components to indicate their condition, and to enable them to be operated and maintained with safety and economy. This general review o f the sub/ect initially discusses the principles o f condition monitoring and its relation to the maintenance activity. It is noted that a condition monitoring method is an extension o f the age-oM principle o f engineers in charge o f machines using their own senses to obtain some general indication o f machine condition.

A summary is given o f all the available monitoring methods. For histori- cal reasons these are divided into two types: those for machine condition monitoring and those for plant cor- rosion monitoring. Particular emphasis is placed on relatively low-cost techniques o f relevance to materials engineers. The power o f such methods is illustrated by the use o f practical examples.

The practical benefits o f carrying out condition monitoring and the possible economic benefits with the probable cost o f setting up a condition monitoring system are discussed.

2

A Long overhaul period J

Many failures occur -ll

1. Compromise

I i perio~ "

Short ~1 " period " 1 ~

Safe [

I

Time between machine failures

Fig. l. Thechoiceofanappropriate maintenanceinte~al

The principles of condition monitoring The crudest method of operating machines is to run them until they fail and then try to repair them to make them fit for further service, i.e. break- down maintenance. This method of operation can be very expensive in terms of lost output and machine destruction and, in addition, can involve hazards to personnel. It is now well recognized that, particularly in the case of large and expensive plant, it is much more economical and operationally satisfactory to stop machines at regular intervals for preventive maintenance in order to reduce the chance of failure during a time that they are required to be available for use.

A major difficulty in planning regular preventive maintenance is the choice of an appropriate regular maintenance interval as illustrated schematically in Fig. 1. This difficulty arises because machines do not fail at regular intervals and so:

1. Too frequent maintenance wastes production time and increases the incidence of additional problems arising from mal-assembly due to human errors.

2. Maintenance with too long an interval results in an unacceptable number of failures occurring during machine operation.

A compromise regular interval between these two is usually chosen on the basis of past experience, but, unfortunately, by definition some failures will still occur during machine operation.

A more satisfactory compromise in terms of maintenance strategy is to carry out preventive maintenance at what may be irregular intervals, but to determine these intervals by the actual condition of the machine at the time. For such condition-based ma#ztenance to be possible, it is essential to have a knowledge of machine condition and its rate of change with time. The main function of condition monitoring is to provide this knowledge.

The knowledge may be obtained by

selecting a suitable measure of deterio- ration in condition of the machine, some of which are shown in Fig. 2. The measurement can then be inter- preted in two basic ways often called trend monitoring and condition checking. Trend monitoring involves recording the value of the measurement at intervals and then, by assessing the trend of this measurement, a useful lead time in warning of incipient machine failure is obtained (Fig. 3). On the other hand, if there is considerable experience of the machine and its failure characteristics, a single check reading can provide sufficient data for determining a safe period of further running. This approach is called condition checking and is particularly effective when there are a number of similar machines in a plant and so a comparison of the check readings between the machine being monitored and a machine known to be in new or good condition can be made. In addition to the above basic methods the readings taken may also be analysed in some detail to indicate the likely cause of the problem - defect diagnosis. These three methods of trend monitoring, condition checking and defect diagnosis are compared in Table 1. A common complementary alternative is the use of alarms where the alarm indicator may be set to give a warning when the safe running period has fallen to a critical value. However, the alarm level usually has to be set high to avoid false alarms and so the useful lead time is substantially less than by using trend monitoring.

The methods of condition monitoring The methods of condit ion monitor ing are selected to measure the deterio- ration in condition of the plant, machine or component to be monitored. This then requires a knowledge of the device and its failure characteristics and the purpose of this section is to outline all the main monitoring methods and their capabilities as a guide to their general selection. How- ever, whatever technique is chosen the most critical part o f the condition

MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, September 1978 19

I I

DETECTING DEFECTS IN THE MACHINE COMPONENTS

I I STATIONARY MOVING

COMPONENTS* COMPONENTS

I I Boroscopes Vibration and Detecting

Radiography noise analysis changes in Acoustic emission operating

Resonance change character Strain gauges [ Brittle coats Crack detection

Temperature

measurement

Pressure measurement Flow measurement

DETECTING CHANGES IN I

I THE COMPLETE MACHINE

I P e r f o r m a n c e m e a s u r e m e n t

I Overall levels of vibration and noise

I JOINTS AND SURFACES WITH RELATIVE MOVEMENT

i L

Loaded surfaces

I I Detecting Detecting debris changes in lost from component component surfaces

surfaces I I , ,

Surface casts Direct collection Witness Detect- in machine

indents ion in for regular Movement or the examination clearance machine change Special vibration techniques

I

Seals

I JOINTS BETWEEN FIXED COF/~ONE NTS

*Stationary components are those which are either permanently fixed, or monitored only during periods at

rest.

Fig. 2. Some methods of monitoring machines and components

I ;lear debris Rattle and noise Noise Observing stains Leaks (fretting)

(leaks) I

Leak detection by:-

I i Regular samp- Visual inspection ling of lubricant Noise or working fluid Sniffing for analysis of its contents

monitoring activity is the decision whether a reading taken f rom a

machine represents an incipient failure condition. This is o f ten called condi t ion assessment or failure predic t ion - hence the title of this article - and some examples of how this is done in practice are given later in the article.

The object of condi t ion moni tor ing is to obtain an indicat ion of machine cond i t ion which can be at two levels:

1. The indicat ion that a problem exists.

2. The def ini t ion of what that problem is.

The principle is not new in that engineers in charge of machines have always used their own senses to obta in some general indicat ion of machine condi t ion:

Sight: Leaks. Smoke or casing colour change, indicat ing overheat ing.

Max. Allow~t~ Level i /I

prr~ ~.c tod Trend

x x

[ lead

. . . . . . . . . . . . . . . . . J . . . . I [lunninq ttlae

Fig. 3. The general principle of trend monitoring

Smell: Overheating. Leaks. Hearing: Abnormal noise, indicat ing

some mal func t ion . Feel: Abnormal vibrat ion, indicat ing

some mal func t ion . High casing temperatures , indicat ing overheat ing.

General sense: Assessment of machine per formance .

A great improvemen t in assessing the exis tence of a problem can be ob ta ined , however , by the use of the simple ins t ruments , such as pressure gauges, t empera ture indicators and tachometers , which are usually f i t ted to most machines, to give some numerical indicat ion that a problem

exists. These numerical readings el iminate the errors of personal opinion and can be colnpared with data from the machine manufac ture rs for normal opera t ion and with pre- vious readings on the same ins t rument .

The recogni t ion of the value of this approach has resulted in the develop- ment of special i n s t rumen ta t ion which can give a more accurate indicat ion of the exis tence of a p r o N e m in a machine and somet imes def ine its

nature, for example:

Shaft posi t ion indicators giving static and dynamic , axial and radial move- ments ;

Table 1. A comparison of methods of condition monitoring and of failure diagnosis

Trend monitoring Condition checking Defect diagnosis

Timing of Readings taken at measurements regular time intervals

while the machine is running

Qualitative r-SSkill-~d operators can measurements do subjective trend I

I monitoring if they are close enough to the ir machines

Quantitative measurements The taking of regular

measurements and their recording and analysis gives a lead time on machine problems

Readings taken at one time while the machine is running

When the problem has become manifest or after failure has occurred

! Typical activi ty o f an I When machine is stopped,

engineer when I inspection o fcnmpon- checkinga m a c h i n e I ents can indicate the during operation cause o f the problem

Numerate values al low comparison wi th established standards or other similar machines to give knowledge of machine condition

C o n d i t i o n m o n i t o r i n g

Measurements may be analysed in considerable detail to provide guid- a n c e on possible causes of the problem

20 M A T E R I A L S IN ENGINEERING APPLICATIONS, Vol . 1, September 1978

Vibrat ion measuring ins t rumen t s a t t ached to the machine casing;

Devices inser ted in the machine oil s t ream to de tec t and /o r collect wear particles f rom machine com- ponen t s ;

Boroscopes for observing the condi t ion of inaccessible c o m p o n e n t s ;

Thermograph ic cameras to give a thermal map of a machine casing.

Th'e subject has also developed along two dis t inct lines, namely , machine condi t ion moni to r ing which tends to de tec t failures due to fracture and wear and is usually the province of the mechanical engineer: and plant corros ion moni to r ing which measures corrosive a t tack in, say, chemical plants and requires such chemical knowledge tha t it is usually carried out by chemis ts or chemical engineers. The general principles of bo th types of cond i t ion moni to r ing are identical , but because of their t radi t ional separ- a t ion they will be t reated separately in the next two sections. However , at all t imes it is i m p o r t a n t to apprecia te that it is usually essential to have a suitable combina t ion of comple- men ta ry m e t h o d s in typical practical s i tuat ions.

Machine condition monitoring methods In spite of the large number of techniques and ins t rumen ta t ion available, there are only four basic me thods :

1. Visual monitoring. Machine com- ponen t s are visually inspected to de te rmine their condi t ion .

2. Wear debris and contaminant monitoring. The condi t ion of critical

c o m p o n e n t surfaces, subject to loading and relative movemen t , is assessed by collect ing and examining the wear debris which they generate .

3. Vibration monitoring. The condi t ion of machines and c o m p o n e n t s is assessed f rom the amoun t and nature of the vibrat ion which they generate.

4. Performance monitoring. The condi t ion of a machine or c o m p o n e n t is assessed by measuring how well it is per forming its in tended duty.

The general way in which they are used is that the exis tence of a p rob lem is usually de tec ted f rom the general level o f the measurement or its rate of change, while the nature of the problem can generally be de te rmined f rom a more detai led analysis o f the measurements (Table 2). The me thods descr ibed in the table are in effect a mechanism of commun ica t i on between a machine and a moni tor ing

Table 2. The general application of monitoring methods to the detection and definition of machine problems

Detect ion of problem existence by measure-

Monitoring ment of level and its method rate of change

Determination of the nature of the problem by analysis of the measurement

Visual Overall appearance monitoring

Wear debris Amount of debris Size distri- Shape of Chemical monitoring bution of debris composition

debris of debris Vibration Overall vibration level Frequency Signal Signal statistics

monitoring content waveform Performance Rate of output Uniformity of Quality level Uniformity of

monitoring rate of quality output

Colouring Shape Texture

Table 3. Some applications of monitoring methods

Wear debris and Component to be monitored Visual contaminant Vibration Performance

S t a t i o n a r y c o m p o n e n t s

Cas ings • •

M o u n t i n g s a n d f o u n d a t i o n s • •

Tanks and containers • • Pressure vessels • • Pipes • • H e a t e x c h a n g e r s • •

S c r e e n s a n d s e p a r a t o r s • • S t a t o r b lades •

R o t a t i n g c o m p o n e n t s

Shafts • •

Machine rotors • T u r b i n e b lades • • •

I m p e l l e r s a n d p r o p e l l e r s • • •

Wheels • • Gears • • • Chain drives • • F l e x i b l e c o u p l i n g s • • •

P u l l e y s a n d be l t s • •

G o v e r n o r s • •

R e c i p r o c a t i n g c o m p o n e n t s

Pistons •

L i n k a g e s and levers • •

Cams and tappets • • Valves • • •

Cab les a n d cha ins • •

B e l l o w s • •

Diaphragms • •

Springs • • •

G u i d e s a n d s l ides • • •

Sp l i nes • •

F r i c t i o n c o m p o n e n t s

B rakes • • Clutches • • •

V i b r a t i o n d a m p e r s • • •

Bear ings

P la in • • • Rolling • •

Flexure • •

Seals

Lip •

Mechanical • • Packed glands •

Windback • •

L a b y r i n t h • • •

P i s t on r ings • •

Wear res i s tan t su r faces

Hard •

Elastic •

Manufacturing tools Cutting tools • •

Metal working tools • • Casting and moulding dies • •

Working fluids H y d r a u l i c • • •

C o o l i n g and hea t t r a n s f e r • • •

L u b r i c a n t s • • •

M A T E R I A L S IN E N G I N E E R I N G A P P L I C A T I O N S , V o l . 1, S e p t e m b e r 1 9 7 8

4 *

21

engineer. It will be observed that in relation to the others, visual monitoring requires negligible technological back-up. This is because the necessary analytical facilities already exist in the human observer. The lack of these natural analytical facilities in the other three methods of monitoring is the reason why more complex technological facilities and methods - often electronic need to be applied.

As a general guide to the selection and application of these methods, Table 3 lists the possible monitoring methods for common mechanical components. The selection of methods is component based because machines consist of a large number of assembled components and when machines fail they do so because one or more components have failed, rather than because the whole assembly has failed simultaneously. Consequently, the most sensitive methods of machine condition monitoring work by detecting the symptoms of individual component failure, since the greatest degree of deviation from normal conditions will be concentrated in these symptoms.

A measurement which is particularly sensitive to machine condition:

1. Gives the longest lead time on failures;

2. Compensates for the effect of vari- ations between successive readings that is typical of any experimental measurement :

3. Reduces the effect of any external factors which might interfere with the operation of the monitoring method.

Therefore, although it is possible to do condition monitoring at three levels, namely, the complete plant, key machines in the plant or key components in key machines in the plant. the last of these is the most generally applicable.

The available equipment within each of the four basic methods is now summarized.

Visual mon i to r ing Visual inspection is one of tile simplest and most familiar methods of assessing condition and gives an immediate and direct indication without the need for processing the results. Although its application is nominally limited to stationary components and direct eye access, these limitations can be reduced by the various ways described in Table 4.

The interpretation of the results of

Table 4. The ways in which direct visual inspection may be assisted or extended

Extension of direct visual inspection Method Remarks

Lighting of dark Light probes Cheap, simple system internal parts

Improved access Boroscopes

Increased mag- Magnifying glasses nification or low power

microscopes Boroscopes

Ability to inspect Stroboscopes rotating or reciprocating components

Better definition Dye penetrants of small cracks

Extension of visual Infrared range by use of thermography radiation outside X-ray examinat ion visible spectrum

-r-ray examinat ion

Ability to visualize Thermograpbic surface tern- paints peratures

visual monitoring is usually done directly by the engineer on the basis of his general experience and therefore it is essentially a condition checking technique. However, when the sig- nificance of the observation is in doubt, it is useful to record it using one of the methods in Table 5 and repeat the recording at intervals to give a series of trend photographs or images so that qualitative trend monitoring can be employed.

Surface inspection only, but many nominal ly inaccessible parts can be inspected, particularly if small access holes are included in the machine design

Very effective for surface inspection of components to which there is direct access

Most have some optical magnification incor- porated. Some incorporate television transmission of the image which can give a useful magnif icat ion

Direct visual surface inspection of moving parts is possible. By operating the s troboscope at a slightly different frequency to the component movement , any pronounced vibration m~,des, or crack or gap stretching, can be observed

Only suitable for cracks which break the surface

Very effect ive for detecting hot spots and other undesirable temperature gradients

Can give useful information on the condit ion of internal components . Equipment tends to be large and not readily portable. Output is a paper, rather than a direct, image

Very useful for inspection of ho l low machines and components since ray source is small and can often be placed inside them. Output is a paper, rather than a direct, image

The paints need to be selected to match the temperatures to be measured. Some prior knowledge of the probable temperatures is therefore necessary

Wear debris and c o n t a m i n a n t m ol7 itorillg Wear debris monitoring involves the monitoring of machine lubricants for the presence of wear debris. This debris is generated by wear processes at the relatively moving surfaces of load carrying components such as bearings, gears and pistons, and by monitoring the quantity and nature of the wear debris it is possible to obtain an indication of the condition of the various machine components which are in contact with the lubricant.

In some situations which are very sensitive to particulate debris, such as hydraulic systems, the monitoring of contaminant levels can be very useful. These checks for dirt or other contamination often require the use of the same types of equipment as wear debris monitoring and so are considered together in this section.

The three main types of wear debris monitoring method are illustrated in Table 6.

Direc t de t ec t i on me thods . These give continuous surveillance while the machine is running and so can warn of rapid failures and activate alarms. They give a quantitative measurement which can be trend monitored, but are not applicable to non-circulating (self- contained) oil systems or grease lubri- cation. Typical types of equipment are;

Optical t echn iques - instruments which use the principle of light scattering to detect particulate debris and

22 MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, September 1978

Table 5. Methods of recording visually observable features

Visually observable features to be recorded Method Remarks

Surface features visible Photography Familiar established method directly, or through a microscope

Surface features observable through boroscopes

Surface pitting or other damage arising in service

Photography Videotape

recording

Surface prints

Surface casts

Progression o f Witness indents surface wear

Familiar established method Suitable for boroscopes using television, but is

very expensive

Simple method. Surface wiped with coloured ink or marking paste, and pressed against paper to record surface damage

Simple method. Surface area is surrounded by dam of modell ing clay and a setting resin poured in to make a reverse cast record of the surface. Alternatively, a plastic sheet material softened with a solvent may be pressed against the surface

The surface is impacted or locally machined to produce a shal low and measurably wide indent. Photographs or measurements of the indent widths then give a measure of the amount o f material worn from the surface

light attenuation for chemical• calibrating and are applicable to oil, thermal degradation. They measure fuel, hydraulic and process fluids. the wear of all oil-wetted corn- However, they are very expens ive - ponents, are continuously self- several thousand pounds each -

Table 6. The main types of wear debris monitor ing method

and can give problems if a relatively large amount of non-critical debris, e.g. carbon particles, air bubbles or general fluid opacity, is normally generated as this can shroud metal debris readings.

Electrically conducting filters - these are located upstream of any filter and have'a specially designed mesh screen (Fig. 4) which indicates the amount of electrically conducting debris and the output is electronic- ally processed and displayed. The mesh size can be chosen so that a wide range of particle sizes is collected or only large particles, e.g. gear teeth chips, are detected. They cost several hundred pounds each and are commercially available.

Inductive detection methods -severa l designs of these exist but are yet to become commercially available even though extensive developlnent has been carried out. Such methods must include temperature compen- sation - typically by using two inductive gaps: one for temperature

Wear debris monitoring method Type of information obtained

1. Direct Detection Methods

2. Debris Collection Methods

Debris sensitive ~////2 detector ~

I Electronic signal processing and display facilities

Wear debris

Removal

Debris I l Collection

from machine

3. Lubricant Sampling and Analysis

Cleaning, removal

and examination of debris

Oil Detailed laboratory analysis

A direct indication of the amount of wear debris being generated in the machine.

On inspection, an indication of the amount of wear debris being generated.

On examination of the debris, an indication of the source of the debris:

On analysis, a quantitative indication of the amount of wear occurring, and an indication of where the wear is occurring.

MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, September 1978 23

Mounting Collar

Resistor

Banks

Oil Flow Cone-type

bulkhead

Particles

Build Up

Here

Terminals

Resistor Banks

(Alt Location) Sensing Loops

Fig. 4. An electrically conducting filter design

correct ion and one for the wear debris measurement .

Capacitative detect ion methods again, not yet commercia l ly available, but typical designs use changes of a.c. capacitative coupling to detect debris. Near symmet ry of oil flow through the measurement and reference capacitors must be assured to give good tempera ture compen- sation.

Debris collection methods. These give periodic surveillance, so cannot usually activate alarms and can only warn of rapid failures if f requent ly inspected. Such methods usually require machine access, require trained personnel and give quali tat ive outputs . In terpreta t ion of these quali tat ive indications is usually carried out by looking for large amounts and/or sizes of particles collected and then by considering the physical form - size, shape, colour, texture - of the debris.

Typical debris col lect ion methods are:

Existing fil tration system - this is basically an extension of the age- old habit of looking for muck in

Sensing Screen

the oil filters and rubbing the oil between the fingers to feel for large particles when the machine is shut down for overhaul. This can readily be extended to inspect ion on a regular basis at little cost and with surprisingly useful results, particularly with machines using centri- fugal filters which catch even very small wear particles. However , special debris col lect ion methods have been developed because of the fol lowing practical problems with typical industrial filters:

Structural Fins

1. The machine usually has to be s topped for filter removal , i.e. work has to be carried out at weekends, nights, etc.

2. Normal industrial f i l tration systems are not usually designed for ease of removal and reinstalla- t ion - oil draining and replenish- ment may be required.

3. Most existing filters do not per- mit convenient display or easy removal of wear particles.

Special filters these mesh filters (Fig. 5)2 collect all particles down

Oil inlet Pressure of temperature transducer

Oil

Filter mesh

Fig. 5. Some special filters for debris collection

k

'° ,icles 24 MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, September 1978

Fig. 6. A magnetic plug

to their mesh size and the complete filter unit can usually be extracted from its housing without breaking any pipe connections. The machine need not be stopped if a by-pass valve is fitted, but the debris must be removed from the filter and so quick 'pop out and look' checks are not usually possible. They are mainly used for detecting the non- ferrous debris not collected by magnetic plugs and, once fitted, such filters must not be forgotten as machine damage can result from filter blockage on some designs.

Magnetic plugs - the modern magnetic plug or chip detector housings are usually of the self-closing type which prevent oil loss during removal and the plug itself, which is normally held in place by a bayonet (push and turn) quick release con- nection, is highly magnetized and has a smooth, simple geometrical shape to give easy removal of adherent particles (Fig. 6). 3 Although they only detect ferrous debris, the ease with which they can be inspected makes them a relatively cheap, useful maintenance tool. Details on the interpretation of wear debris collected in this way is given in a later section of this article on practical applications.

Lubricant sampling and analysis. This consists of taking a clean sample of lubricant from the machine which is representative of the system as a whole. Typical methods use either a suction device (sometimes called a sampling gun, syringe or thief) or a special sampling valve or a special adaptor on a magnetic plug housing or involve taking a sample from the drain stream during oil changes. Although the choice is usually on the basis of convenience, the importance of obtaining a good sample with a laboratory standard of cleanliness in

.the machine's working environment cannot be over-emphasized.

Two main methods of analysis are used:

1. Analysis of the sample to determine the concentration of chemical elements which it contains, i.e. elementat analysis.

2. Analysis of the sample to determine the amount, size arid shape of contaminant particles contained in it, i.e. wear particle analysis.

Each of these are now described.

1. Elemental (spectrometric) analysis. These are very sophisticated analysis methods which require skilled operators and a willingness to spend over £6000 on an analyser. A typical set of results is shown in Fig. 74 and the usual analysis methods are:"

Atomic absorption analysis - this operates on the principle that every atom absorbs only light of its specific wavelength. The equipment is very useful if relatively small numbers of samples are to be analysed (say, 25 samples for four elements per man day).

Atomic emission spectroscopy - this measures the characteristic wave- lengths of light which are emitted when the various elements in the oil are excited by, say, an electrical discharge. As many elements can be analysed simultaneously, it is a very useful method if relatively large numbers of samples are to be analysed.

X-ray fluorescence analysis - this measures the emission of characteristic X-rays by a medium when the medium is exposed to a source. A new method which is promising for on-line applications.

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F ig . 7 . A n o i l a n a l y s i s r e p o r t f o r a n e w h i g h - s p e e d r e c i p r o c a t i n g c o m p r e s s o r

M A T E R I A L S IN ENGINEERING APPLICATIONS, Vol . 1, September 1978 25

0 . 0 1

10 s d N

0

u 10 4 ' 0 r ,

-,-I

e -

10 3 QJ

0

2 10 4~

",'1

.-4

",'t E

~ 1 0

.,-I

0

QJ

"~ 0 4

i

1 5 10 20 30 40 50 60 70 80 90100 120 140160 180 200

Particle Size, microns

Fig. 8. A particulate contamination chart for hydraulic fluids based on machine tool experience

2. Wear particle analysis. There are two main methods of wear particle analysis:

Particle count ing - this gives data on the size dis t r ibut ion o f particles in a fluid by count ing the particles individually and adding the results. It is part icularly applicable for analysing con tamina t ion of hydraulic fluids when results can be compared with charts such as Fig. 8. t This graph can be used as a very rough initial guide for typical industrial s i tuations but in very critical applicat ions, e.g. aircraft, the re- qu i rements may need to be made much tighter, and in heavy industrial applicat ions, e.g. steel rolling mills, much higher con taminan t levels are of ten acceptable.

Ferrography - this technique separ- ates wear particles f rom any oil sample and arranges them according to size on a purpose designed glass slide using a magnetic field. The me thod is sold as a comple te package and typical equ ipment is shown in Fig. 9. More details on the in terpre ta t ion of the data f rom ferrography is given in a later sect ion of the article on practical applications.

A c k n o w l e d g e m e n t

Much of the in format ion for this article is taken from a practical engineering guide to machine condi t ion moni to r ing which is being published by Her Majesty's S ta t ionery Office later in 1979)

10 6

Fig. 9. The ferrograph analyser

References I. Neale, M. J.,Woodley, B. J. and Wood-

cock, J. S. A Guide to Hze Condition Monitoring o f Machinery, Report TRD 200. Her Majesty's Stationery Office, 19"/8.

2. Botstiber, D. W. Wear monitoring systems. Machine Design, 26 October 1967, 170--176.

3. Wiggins, R. A. Equipment Reliability Through Oil Monitoring. Vactric Con- trol Equipment Ltd, 1968.

4. Drost, J. G. Preventive maintenance through oil analysis. Chemical Engin- eering Progress, 68 (8), August 1972.

26 MATERIALS IN ENGINEERING APPLICATIONS, Vol. 1, September 1978

failure prediction by condition monitoring (part 1)

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