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Basic Electric Motor Basic Electric Motor MaintenanceMaintenance
Ed RobinsonEd Robinson
Regional Service MangerRegional Service Manger
[email protected]@weg.net
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Eli Lilly Study Eli Lilly Study Primary causes of failure:Primary causes of failure:
• Misalignment of sheaves and couplingsMisalignment of sheaves and couplings
• Over/under lubrication of bearingsOver/under lubrication of bearings
• Improper tension in belted systemsImproper tension in belted systems
• Temperature: Readings taken on the drive-end bell housing Temperature: Readings taken on the drive-end bell housing within 1 in. of the drive shaft closely approximate the within 1 in. of the drive shaft closely approximate the internal internal winding and bearing temperatureswinding and bearing temperatures
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Basic Maintenance Basic Maintenance
1.1. Bearing & Lubrication Bearing & Lubrication MaintenanceMaintenance
2.2. Coupling MaintenanceCoupling Maintenance
3.3. Electrical InspectionElectrical Inspection
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Bearing Failures
• Bearing Failures are the #1 cause of motor failures. Over 60% of all motor failures are attributed to the bearings. However, many external factors contribute to the actual failure of the bearings. The bearing itself is not usually the culprit.
• So, let’s start by talking about the original design and what affect that has on the life.
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Bearing Design Shape
Deep Groove Radial Ball Deep Groove Radial Ball Bearings are used for Bearings are used for “general purpose” “general purpose” applications. They work applications. They work with both radial and axial with both radial and axial type loading.type loading.
Ball Bearings Roller Bearings
Cylindrical Roller Cylindrical Roller Bearings are used for Bearings are used for heavy radial loaded heavy radial loaded applications, like a V-applications, like a V-belt drive. There is NO belt drive. There is NO axial loading.axial loading.
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Bearing Design Size
200 Series
Ball Bearing
200 Series
Ball Bearing
300 SeriesBall Bearing
300 SeriesBall Bearing
Same Bore Size
And……Given the same load, the 300 Series will last twice as long as the 200 Series.
The 300 Series has larger diameter balls or rollers and a larger outside diameter which results in the heavier load carrying capacity
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Bearing Design Size It’s very easy to see the difference in the
300 series bearing on the left, and the 200 series on
the right.
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Bearing DesignFactors in the life of a bearingFactors in the life of a bearing
Load and Speed. Those are the two determining factors in designing a bearing for a particular application and the life expected from that bearing in that application.
The size of the bearing will help determine how much load, and for how long the bearing can handle that load. The size and type of the rolling element also plays an important part in the load rating.
Since you can’t change the design characteristics or the size of the bearings in your motors, it is a good idea to include these things in your analysis for selecting a new motor, in the beginning.
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Bearing Design Life
L10 Life
Of 100 bearings Of 100 bearings tested, under tested, under the same load the same load and speed and speed conditions, the conditions, the L10 life is where L10 life is where 10% of the 10% of the bearings will fail bearings will fail (or 90% will (or 90% will survivesurvive).
Of 100 bearings Of 100 bearings tested, under tested, under the same load the same load and speed and speed conditions, the conditions, the L10 life is where L10 life is where 10% of the 10% of the bearings will fail bearings will fail (or 90% will (or 90% will survivesurvive).
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Bearing Design Life
L50 Life
Of 100 bearings tested, under the same load and speed conditions, the L50 life is where 50% of the bearings will fail(and only 50% will survive).
Of 100 bearings tested, under the same load and speed conditions, the L50 life is where 50% of the bearings will fail(and only 50% will survive).
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Bearing Design Life
L10 Life
L50 allows them to downsize the bearing and save money. But who
really pays?
Some Manufacturers use L10 life in engineering their motors.
And some use L50 life in engineering their motors.
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Bearing MaintenanceBasic Inspection
Shaft “Feel”, without the motor running
Turn the shaft by hand … • Before 1st use to distribute grease.• Does it turn easily ?• Feel for flat spots, rubbing, stiffness, etc …
Bearing Noise while running (unloaded)
Listen to the motor bearings … • Listen for abnormal sounds, squealing,
squeaking, grinding, etc …• Unloaded roller bearings will definitely make a
“rattling” noise..
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Bearing MaintenanceHi Tech Inspection
Vibration Analysis
RTD Thermal Analysis
Infrared Thermography
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Bearing Failure Factors
Leading causes of bearing failure
Lack of lubrication
Too much lubrication
Incompatibility of greases
Contamination of lubrication
Internal Winding Temperatures
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Bearing Lubrication
Re-lubrication IntervalsRe-lubrication Intervals
MOTOR BEARING GREASE RELUBRICATION INTERVALS (In Months)
8 hrs / day 8 hrs / day 24 hrs / day 24 hrs / day
RPM HP Range clean dirty clean dirty
0.5 - 7.5 12 6 8 3
3600 10 - 40 9 4 4 2
50 - 150 9 4 4 2
0.5 - 7.5 36 18 18 9
1800 10 - 40 24 9 12 4
50 - 150 18 9 9 4
0.5 - 7.5 48 24 24 12
1200 10 - 40 36 12 18 650 - 150 12 12 12 6
Believe it or not, too much lubrication in a ball bearing can have the same result as not enough lubrication
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Nameplate Information
The nameplate tells you exactly what type, how much, and how often to add lubrication.
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Bearing Lubrication Grease
Did you know you can’t mix just any grease with any other grease? It’s true - doing so can cause catastrophic damage to equipment. And it’s true even if the grease types are the same. Why is this?
Grease consists of two parts - the actual lubricant (normally oil) and a carrier, or base. Each has it’s own function. It’s the carriers that are not compatible. Always know the base of the grease you are adding and the base of the grease you are adding to. Once you know those things, you can use the chart on the next page to determine if you can add one grease to the other.
The grease compatibility chart will help ----
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Bearing LubricationBearing Lubrication
Grease types Li
thiu
m
Com
plex
Lith
ium
Al
Com
plex
Ca
C
ompl
ex
Ba
Na
Ben
toni
te
Si G
el
Poly
urea
Lithium Complex
Yes Yes No Yes No No No Yes Yes
Lithium Yes Yes No Yes No No No Yes Yes
Al Complex
No No Yes No No No No Yes No
Ca Complex
Yes Yes No Yes Yes No No No No
Ba No No No Yes Yes No No Yes No
Na No No No No No Yes No No No
Bentonite No No No No No No Yes Yes No
Si Gel Yes Yes Yes No Yes No Yes Yes No
Polyurea Yes Yes No No No No No No Yes
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Bearing LubricationHow does the grease get into the rolling element?How does the grease get into the rolling element?
PositivePressure
LubricationSystem
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Basic Maintenance
1. Bearing & Lubrication Maintenance
2. Coupling Maintenance
3. Electrical Inspection
1. Bearing & Lubrication Maintenance
2. Coupling Maintenance
3. Electrical Inspection
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Belt Drives
Belt Over-Tension is a Primary Source of Bearing, Belt and Motor Shaft Failure.
Example
D - Large Sheave = 18”d - Small Sheave = 12”C - Ctr Dist = 36”
C2 = 1296”18 - 12 = 6” - - 6/2 =3”
32 = 9”1296 - 9 = 1287”
t 1287”Span = 35.8”
35.8 / 64 = .5”
1/2” Belt Deflection
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Belt DrivesIf you have a V-belt tension gauge and want to tension by force, you can use a chart like this. This is a much more accurate method of tensioning.
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Belt DrivesLife Expectancy
The load has a great deal of effect on the
life of the belt.
A 25% overload on the belt drive, will result in only 50% of the design belt life.
A 25% overload on the belt drive, will result in only 50% of the design belt life.
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Belt DrivesLife Expectancy
The tension also has a great deal of effect on the
life of the belt.
A 18% over-tension on the belt drive, will also result 50% of the design
belt life.
A 18% over-tension on the belt drive, will also result 50% of the design
belt life.
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Belt DrivesLife Expectancy
Heat is also a major factor. Designing good ventilation for the V-Drive can make a big difference.
A 20o temperature increase above ambient, will also result 40% loss in the design life of the belt.
A 20o temperature increase above ambient, will also result 40% loss in the design life of the belt.
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Belt Drives Installation
Most of the belt problems begin with the original installation of the belts. Prying the belts on (as pictured below) can break the cords in the belt, not to mention external groove damage along the sides of the belts where they ride in the groove of the sheave.
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Belt Drives Alignment
Another problem that is often overlooked is the sheave alignment. This type of misalignment can result is belt overheating, shortened life, as well as bearing overloading.
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Belt Drives
Maintenance
Another much looked over issue is the sheave groove wear. When the sheave grooves wear, and dish out, as shown below, you lose belt contact on the sides of those grooves. Then, you have to over tension the belt to try and make it work. That puts excess overhung load on the bearing and shaft.
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Belt DrivesMaintenance
As you can see, a great deal of cost effective maintenance can be done on a simple V-belt drive.
Critical Factors
Original DesignAlignment
TensionClean
Cool
Critical Factors
Original DesignAlignment
TensionClean
Cool
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Direct CouplingShaft Alignment
TABLE.2 RULES FOR OFFSET MISALIGNMENT AND INBOARD BEARING LIFE
Maximum offset (direct measurament and percent of maximum for three expected bearing life)
Coupling Type 90% life expectancy 80% life expectancy 50% life expectancy
Maximum coupling offset
recommended by manufacturer
Link 3 mils (12% max)
5 mils (19% max)
20 mils (77% max) 26 mils
Elastomeric 8 mils (11% max)
21 mils (30 % max)
70 mils (100% max) 70 mils
Grid 1 mil (8% max)
2 mils (17% max)
5 mils (42% max) 12 mils
Gear 5 mils (10% max)
10 mils (20% max)
35 mils (70% max) 50 mils
Using average offset values for various life expectancies, it can then be broadly stated for the couplings used in this study that: 1. If the motor is offset misaligned by 10 percent of the coupling manufacturer's allowable offset, then one can expect a 10 percent reduction in inboard bearing life. 2 . If the motor is offset misaligned by 20 percent of the coupling manufacturer's allowable offset, then one can expect a 20 percent reduction in inboard bearing life. 3. If the motor is offset misaligned by 70 percent of the coupling manufacturer's allowable offset, then one can expect a 50 percent reduction in inboard bearing life.
* University of Tennessee, College of Engineering
Laser Alignment
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Basic Maintenance
1. Bearing & Lubrication Maintenance
2. Coupling Maintenance
3. Electrical Inspection
1. Bearing & Lubrication Maintenance
2. Coupling Maintenance
3. Electrical Inspection
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Electrical Inspection
There are a lot of items included in this area, but it
starts with good wiring practices, including good clean
connections, and good grounding procedures. Many
electrical failures involving motors happen during the
initial connections. If your electrician doesn’t
understand the wiring diagram, get some outside help.
Don’t use the trial and error method. Most electrical
failures that happen because of miss-connection, will
do so quickly. This is an expensive mistake.
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Preventative Electrical Maintenance
Checking the initial
wiring procedures, before starting the motor the first time.
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Motor ProtectionProtecting the motor from Protecting the motor from overloadsoverloads
Thermocouple / Thermostat
Bi-metal construction (dual expansion rate) Alarm (or) Overheat Inexpensive
Thermistor: Solid State Switch Increase in temperature causes additional
resistance in sensor , energizing the external relay.
Alarm (or) Overheat Inexpensive
Thermocouple
Thermistor Relay
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Motor ProtectionProtecting the motor from overloadsProtecting the motor from overloads
RTD’s: Resistance Temperature Detectors
Resistance change fed to external instruments allow for :
Alarm Signal
Temperature Readout
Automatic Shutdown
Continuous trend chart recording
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Motor ProtectionProtecting the motor from overloadsProtecting the motor from overloads
RTD’s: Resistance Temperature Detectors
Change in resistance proportional to temperature Linear signal allows for exact temperature reportMore expensive than Thermistors or Thermostats
RTD - Resistance vs. Temperature
100
150
200
250
300
0
10
20
30
40
50
60
70
80
90
10
0
11
0
12
0
13
0
14
0
15
0
Temperature (ºC)
Re
sis
tan
ce
(o
hm
s)
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Preventative Electrical Maintenance
PROTECTION AS A FUNCTION OF CURRENT
PROTECTION WITH MOTOR THERMAL
SENSORS OVEHEATING CAUSES FUSE FUSE AND
THERMAL PROTECTION
1. Overload with 1.2 x Rated Current
2. Duty Cycle S1 to S8 EB 120
3. Braking, reversals and operation with constant starts
4. Operation with more than 15 starts per hour
5. Locked rotor
6.Phase Failure
7. High voltage oscillation
8. Frequency oscillation
9. High ambient temperature
10. External heating caused by bearings, belts, pulleys, etc.
11. Cooling obstruction
LEGEND: UNPROTECTED PARTIALLY PROTECTED TOTALLY PROTECTED
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Preventative Electrical Maintenance
Effects of voltage variationEffects of voltage variation
Periodic checks of Voltage and Current measurements might save some real problems down the road.
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Preventative Electrical Maintenance
Unbalanced Voltage supply lines are responsible for a lot more failures than are usually thought of
A motor with a good service factor and low temperature rise might cover for the 3% unbalance, but not for 5%.
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Application vs. Design IssuesApplication vs. Design IssuesCustomers Control Customers Control
(Preventable)
Correct Correct Electrical Electrical Connections Connections
Bearing Bearing Lubrication Lubrication
V- Drive V- Drive MaintenanceMaintenance
Shaft Shaft AlignmentAlignment
Thermal Thermal Overload ProtOverload Prot
Connections – Connections – loose bolts, loose bolts, nuts, and nuts, and screwsscrews
Manufacturers ControlManufacturers Control
(Warranty)
Short Circuit Between Turns
Short Circuit in Stator Slot
Phase to Ground Short
Rotor Failures
Rotor Bar Breakage
Excessive Rotor Heating
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Preventative Maintenance
Preventative Maintenance compares to Reactive Preventative Maintenance compares to Reactive
Maintenance as a checkup or physical compares to Maintenance as a checkup or physical compares to
emergency room treatment.emergency room treatment.
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Equipment Life/Cost Curve
Cost of FailureCost of FailureCorrective
MaintenanceCorrective
Maintenance
Predictive Maintenance
Predictive Maintenance
Preventative
Maintenance
Preventative
Maintenance
Cost Cost Time or Life Time or Life
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Predictive Maintenance
Differs from Preventative Maintenance by using scheduled analysis of:
Shaft Misalignment
Relubrication
Balancing
Vibration
And then using that analysis to predict the longest period of life you can get from your equipment, BEFORE it fails.
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Predictive TechnologiesVibration Analysis:
Couplings Bearings Gears
Infrared Thermography:Infrared Thermography:
Electrical Connections
Insulation Deterioration
Mechanical Considerations
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Infrared Thermography
Motor Starter Overload
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Infrared Thermography
Heat pattern caused by an improperly aligned motor
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Conclusions
You can get more for your maintenance dollars with Preventive and Predictive maintenance routines. The old days of “Don’t fix it if it ain’t broke” are going by the way. Downtime and lost production is simply too expensive to let it run ‘til it quits. Identifying the critical applications, and starting a routine to log the failures, their causes and repairs is the first step.
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Conclusions
Also, identifying the right motor in the first place is a great place to start. Some of the most important features to look for are:
Motor Insulation and Temperature Rise Service Factor Bearing Sizes and Load Capacities Construction Materials Efficiency ( Not only does the efficiency of the motor save you operating costs, but it helps to ensure the quality of materials the motor is built with )