ELECTRIC MOTOR BEARING LUBRICATION AND REGREASING
More than any other prime mover, electric motors drive industry. A number of factors influence motor reliability, including design, manufacture, selection, installation, operating environment, & maintenance. In order to operate efficiently and dependably, electric motors must be properly lubricated. EPRI estimates nearly half of all motor failures are bearing related. Many of these failures can be traced to improper lubrication practices such as over lubrication, under lubrication, contamination, incorrect selection, and mixing of incompatible grades.
Motor Bearing Lubrication
Some industrial motors are fitted with journal bearings, but the majority - more than 80% - is fitted with rolling element bearings. The choice of bearing type and lubricant delivery mechanism depends upon application details.
Oil-lubricated bearings can operate at higher speeds, run cooler and last longer than grease lubricated bearings. Oil typically contains fewer contaminants than grease (because of settling and filtration) & oil analysis can be used for monitoring lubricant condition. However oil is more difficult to contain in the housing & oil lubrication typically requires more complex design and more routine maintenance. Also, oil runs off of the bearings easily when the motor is not running, and so risk of dry start is higher. The common oil lubrication methods are Bath Oil Lubrication, Ring Oil lubrication, Forced Oil circulation, and Oil Mist lubrication.
These are discussed in APPLICATIONS section of KNOWLEDGE LIBRARY.
Grease lubrication is by far the most common method of motor lubrication. Grease lubrication offers several advantages, including immediate availability during start up, reduced risk of grease escaping bearing and contaminating product and better exclusion of contaminants past the housing seals. Key disadvantages are lower bearing speed limits, increased heat generation, inability to remove contaminants once ingress has occurred, shorter oxidative life, compatibility concerns, difficult condition monitoring of the lubricant.
It is unusual to find a grease-lubricated motor equipped with journal bearings. Typically, motors fitted with rolling element bearings are grease lubricated. Grease may be applied manually or automatically through single point or multi point lubrication system.
Motor bearing lubricant selection
Open bearings have no seals or shields. They can run cooler than either sealed or shielded bearings, pose no risk of shield collapse into the bearing elements or motor, and are easy to re-grease. However, open bearings pose greatest risk for grease churning, entry of contaminants and hardened thickener into the bearings, and entry of grease into the motor windings, which compromises heat dissipation and weakens the conductor insulation.
Sealed bearings are equipped with elastomeric seals that completely enclose the bearing elements. Sealed bearings are factory-filled with grease and they may not be re-lubricated. These bearings are also called “lubricated for life” bearings or “non-grease able” bearings. However sealed bearings will not last as long as properly maintained re-lubricable bearings, because the oxidative life of the grease becomes the limiting factor of the bearing’s life. While sealed “lubricated for life” bearing are typically limited to small motors of < 7.5 KW. motors as large as 200 KW have also been fitted with sealed bearings
Shielded bearings are fitted with metal shields that are snap fitted into the outer race. At the inner race a gap of approximately 125 to 375 microns allows grease to enter and exit the bearing cavity. Bearings may be fitted with a single shield facing the grease supply, a single shield facing the motor or double shields. Advantages of shielded bearings include reduced churning of grease, regulated flow of grease to the bearings, restricted entry of contaminants and hardened thickener, and reduced risk of grease entry into motor windings. However shields cause bearings to run hotter than open bearings, and for shields that face the grease supply the risk of the shield being pushed into the bearing.
Lubricant selection is based on many factors, including bearing type, operating conditions, lube delivery mechanism, etc. The Bearing Speed Index or Ndm of the rolling element bearing is an important variable in lubricant selection
N = RPM
Dm = Bearing mean diameter, in mm = (d+D)/2
Ndm = Bearing Speed Index
d = Bore diameter, in mm
D = Outer diameter of bearing, in mm
• Ndm values below 50,000 are considered relatively slow speed bearings,
• values between 100,000 and 300,000 are average &
• Values above 400,000 are considered high.
• The upper limit for oil lubrication of ball bearings is approximately 700,000 with values up to 1,500,000 possible with oil mist lubrication.
• Grease lubrication in rolling element bearing is typically limited to
• Ndm values of 350,000 for ball bearings &
• 150,000 for spherical bearings.
• Special greases with low viscosity base oils can be used in ball bearings up to Ndm value of 1,000,000.
HOW GREASE LUBRICATED BEARINGS FUNCTION
Grease is about 85% oil and 15% thickener and additives. Principally the oil lubricates and the thickener keeps the oil in place. The spongy matrix of the thickener holds the oil in suspension and allows the oil to flow during operation. The oil lubricates between the bearing balls/rollers and the inner and outer races and the cage, reducing friction. Grease also protects surfaces from rusting during long idle periods and unfavourable environmental conditions.
When too much grease is added, the grease is compressed between the bearing surfaces, increasing pressure and heat. With too little grease, the surface friction increases causing heat.
Problems can arise when different thickeners are mixed. Mixing incompatible greases may cause the mix to harden or liquefy. The most common greases used on shop floors and maintenance departments are Lithium soap or Lithium complex based. (>70% of grease produced is Li or Li complex based). However, since the 1990’s many motor manufacturers have switched to Polyurea based greases. Polyurea and Lithium are not compatible. Bearing manufacturers use various greases for factory fill depending on application. Hence a replacement bearing from a bearing manufacturer may have different grease than the bearing fitted by the motor manufacturer.
Many bearing manufacturers provide detailed advice for estimating lubricant type, volume and frequency to match machine operating conditions. The idea is to provide, and replace, just the right quantity of grease that is enough to provide a continuous feed of oil to the element races, but not too much as to crowd the elements and cause churning, overheating and degradation of the grease.
Open bearings are preferred in High load and/or High peripheral speed applications, to allow for cooler operating temperature and longer life. If grease inlet and outlet ports are located on the same side, the bearings are referred to as “conventional flow lubricated”. If inlet and outlet ports are located on opposite sides, they are referred as “cross flow lubricated”. Since the bearing cavity is directly open to the housing, such bearings are at greater risk of over greasing & excessive churning of the grease in the reservoir.
Single side Shielded Bearings
A single side shielded bearing with the shield facing the grease supply is considered by many to be the best arrangement. The shield acts as a baffle against agitation. Since the gap between shield & inner race serves as a metering device to control grease flow, overheating from excess grease is much less likely. The rotating ball-and-cage assembly acts like the impeller of a pump with the gap between shield and inner race acting like the eye of the pump. The grease is drawn in through capillary action as the bearing cage assembly rotates. The grease will be discharged by centrifugal force into the bearing outer raceway. Since there is no shield on the backside of this bearing, the excess grease can escape into the inner bearing cap. Furthermore risk of premature bearing failure due to contaminated grease is reduced, as dirt entry is restricted by the close fitting shield.
Double Shielded Bearings
Confusion reigns on the topic of re-greasing of double shielded bearings. Most bearing manufacturers do not recommend re-greasing double shielded bearings. However many equipment manufacturers do provide grease fittings and drain plugs on the equipment. Double shielding is used in bearings principally to protect against ingress of particulate contaminants, which is the principal enemy of bearings. Hence double shielded bearings are often considered as “lubricated for life”, that is for the life of the Bearing/lubricant combination. Not re-lubricating eliminates the possibility of contaminants from the introduced lubricant and avoids another enemy of bearing life, over lubrication. However the downside of not re-lubricating double shielded bearings is shorter bearing life. All else being equal, bearings like regular infusions of fresh, clean grease. The gap between the shield and inner race does allow for bleeding grease into the rolling element space to give a long and satisfactory life. The decision to lubricate or not lubricate a double shielded bearing comes down to an economic choice:
• Small, inexpensive bearings may not be worth the cost and, instead, should be replaced.
• Larger bearings, particularly those that have not been in use long, may be worth the cost to re-lubricate.
In repair shops double-shielded bearings are often re-lubricated. The shields are popped out and bearings are cleaned with solvents, and the fresh grease filled in.
The housings serve as a lubricant reservoir and are filled with grease. By regulating the flow of grease into the bearings, the shields act to prevent excessive amounts from being forced into the bearing. It is not necessary to pack the housing next to the bearing full of grease for proper lubrication. However, packing with grease helps prevent dirt & moisture from entering. Oil from the grease reservoir can, over a long period, enters the bearing to revitalize the grease within the shields. Grease in the housing outside the stationary shields is not agitated or churned by the rotation of the bearing and consequently, is less subject to oxidation. Furthermore, if foreign matter is present, the fact that the grease in the chamber is not churned reduces possibility of the debris contacting the rolling elements.
Some types of double-shielded bearings are designed so that they can be re-lubricated. They have a hole in the outer race, and grease is introduced within the shields. Excess grease and old grease come out through the clearance between the shields and the inner race
“Lubricated for life” bearings incorporate close fitting seals instead of, or in addition t, shields. The close fitting seals can cause high frictional heat that can accelerate grease deterioration. A bearing manufacturer gives a limiting Ndm value of 108,000 for lubricated for life bearings. New bearings are coated with a rust-inhibiting compound before packaging. This coating is not to be removed. Bearings in original packing can be stored for many years in dry, constant temperature conditions. Sealed bearings and double side shielded bearings are factory filled with grease. Grease in such stored bearings will age and become stiff if kept too long. Storage life of such bearings may be considered at 2 to 3 years.
A general procedure for greasing a new rolling element bearing or completely renewing grease of a used bearing of open or one side shield or one side sealed type is:
• For low speed bearings fill the bearing completely and if Ndm < 50,000 also fill the housing
• For normal bearing speeds fill the bearing to between 50-100% full and fill housing to between ⅓-⅔ of its capacity
• For high speed bearings fill the bearing 25% full and the housing to between ⅓-⅔ of its capacity.
It is best to follow the motor manufacturer’s recommended quantity of re-grease. However if no specific information is available then the following generic formula may be used as a starting point to develop guidelines for particular equipment:
G is the replenishing grease quantity in grams
D is the bearing outside diameter in mm
B is the total bearing width in mm
X = 0.005 (Based on NLGI/SKF)
A) Re-lubrication quantity m1
, for weekly to Yearly re-lubrication
=D • B • x [g]
Where x = 0.002 for weekly
x= 0.003 for monthly
x= 0.004 for yearly
B) Quantity m2
for extremely short re-lubrication Intervals
= (0.5...20) • V [kg/h]
C) Re-lubrication quantity m3
prior to restarting after several years of standstill
= D • B • 0.01 [g]
V = free space in the bearing ≈ π/4 • B • (D2 – d2) • 10¯⁹ – M/7800 [m3]
d = bearing bore diameter [mm]
D = bearing outside diameter [mm]
B = bearing width [mm]
M = bearing mass [kg]
For ease of plant implementation, the number of grams of grease should be converted into strokes for each different type grease gun that is used in the plant.
Several general tables and guidelines from bearing manufacturers exist for determining how often to grease electric motor bearings. A calculated interval is more precise. One such equation based upon the bearing’s size and speed is indicated below. It requires the input of the following adjustment factors to produce a net re-greasing interval in operating hours.
Motors that run hot require more frequent re-greasing because the grease tends to get used up faster and is more likely to run out. Hotter running bearings are also more prone to cake up with old, used up thickener.
Motors are often re-greased with greater frequency when risk of dirt contamination is high. The idea is to make sure there is sufficient grease to keep housing seals from drying out.
Moisture causes serious problems for grease. It reduces the lubricants film strength, causes hydrolysis of the base oil, additives & thickener, increase oxidation ratr< and in some cases washes out the grease from the bearing. Increase frequency when water is present.
Vibration aids shearing of the grease thickener and simply shakes the grease out of the bearing. More frequent re-greasing is required to compensate for the vibration.
• Shaft Position:
Gravity causes grease to flow out of the motor bearings and their housings regardless of position. However rate of loss increases as the shaft moves away from horizontal. Non-horizontal shafts require more frequent re-greasing. Vertical shafts require the most frequent.
• Bearing Design:
Tapered and Spherical rolling element bearings use up grease up to 10 times faster than radial ball bearings. So re-greasing rate needs to be adjusted for different bearing designs.
If no specific re-greasing interval information is available, then a simple generic grease replenishment interval formula such as below may be used as a starting point
T = Re-lubrication interval in hours;
N = RPM;
d = bearing bore in mm
K = Product of all correction Factors
Ft x Fc x Fm x Fv x Fp x Fd
: For intermittent duty cycle motors, the greasing intervals should be the same time frame as continuous duty cycle motors measured by their operation.
: Most bearing manufacturers recommended re greasing intervals are based on the use of lithium grease with mineral oil base at 70° C. Re-greasing intervals can be increased if operating temperature is lower or if superior quality grease is used.
: Re greasing intervals must be shortened if the bearing temperature is higher than the rating basis.
: A Re-greasing interval chart suggested by a motor manufacturer (TOSHIBA) and another chart developed by EPRI for clean operating conditions, are available for reference in KNOWLEDGE CENTRE.
Motors in Layup or Standby Mode
Since most degrading influences are present only while the motor is operating, except oil separation, the re-greasing intervals will be longer for motors in layup/standby mode than those given for motors in continuous operation (typically 1.6 to 2 times). However, if the motors are in layup/standby mode for several years, the motor should be disassembled, the old grease removed, and new grease hand packed before the motor is placed into operation. This recommendation is made because periodic motor rotation only mixes the grease in the bearing and not the grease in the bearing grease cavity.
Grease degradation is a gradual process; grease does not abruptly cease to be an effective
Lubricant. Most grease degrading influences are present only while the motor is running.
Lubrication practices must therefore take into account the motor's operating cycle. Grease degradation can occur because of the following:
• Hardening of grease, such that it no longer freely feeds oil to bearing surfaces. This can result from absorption of dirt or moisture, and can occur because of oxidation over a long period of time.
• Chemical breakdown caused by excessive heat. This can occur by overfilling the grease cavity and can be aggravated by high winding temperatures, especially in totally enclosed motors.
• High bearing loads, usually caused by excessive belt tension on side loaded bearings and by misalignment.
• Oil separation from the base material of the grease. This usually occurs when a motor is not rotated for a long period of time and the grease is not mixed in the bearing. Although rotating a motor periodically will mix the grease in the bearing it does not mix the grease in the bearing grease cavity around the bearing.
Best practices for re-greasing of existing, mounted & enclosed rolling element bearings
||The only true way to prevent over greasing is to disassemble the motor, clean the bearing and grease cavity, and hand-pack the bearing and grease cavity as per guidelines for a new bearing.
The following general re-greasing guidelines/best practices may be considered for in-service open or shielded bearings or bearings with seals only on one side, and does not apply to both sides sealed bearing.
• Determine that the bearing is re-greasable. Sealed bearings cannot be re-greased. Grease fittings may be put in place on motors that are not greasable.
• Ensure Grease is compatible with existing grease. Thickeners represent the greatest risk.
• Dedicate a grease gun to a grease type & label/tag it using a colour or shape code.
• While pneumatic or electric powered grease guns are convenient, an experienced lube technician can feel problems in motors, such as excessive back pressure, when manually greasing.
• Ascertain how much grease quantity is delivered per pump stroke of grease gun. This should be written on the gun. A typical low pressure hand grease gun may deliver 1.25 grams per stroke.
• Ensure it is safe to work with motor running. Else shutdown and lock out motor before proceeding .Large Motors fitted with open bearings may require re-greasing to be done in stationary condition.
• Remove the drain (purge) plug from the housing while equipment is operating (assuming it is safe to do so). With stationary motor, remove plug while motor is still warm.
• Ensure that the motor to be re-greased is at a stable operating temperature, if possible. This allows the hot grease in the bearing grease cavity to be purged through the bearing and out of the drain hole more efficiently than it would if the grease was at ambient temperature. Greasing the motors while they are hot will provide better grease distribution of the new grease entering the bearing grease cavity.
• Using a nylon cable wrap, metal rod or similar tool, to ensure that the drain/purge path is clear of any hardened grease. (Not allowing the spent grease to be expelled causes much of the old product to be forced into the bearing).
• Wipe any dirt, debris & paint off of the grease fitting with lint free cloth. Prevent any foreign material from entering grease cavity
• Wipe the nozzle of the grease gun & purge it a little
• Pump the required quantity of grease slowly into the bearing housing, allwing 3 to 5 seconds per shot so the grease will distribute evenly in the bearing and grease reservoir.(for quantity and frequency of re-greasing see below).
• Stop pumping if back pressure is felt. Pushing too hard can damage housing seals. If the bearing is equipped with a shield facing the grease supply, it is possible to push the overhung shield into the bearing if too much pressure is applied and there is no other relief. A grease gun can develop up to 700 bar pressure.
• Stop pumping when desired quantity of grease has been pumped in, or grease exits past the housing seals, or appears from shaft clearances or purged grease looks fresh.
• After greasing is complete, the motor should run until stable operating temperatures have been reached (typically 1 to 2 hours if motor is greased while hot, 2 to 3 hours if the motor is greased while at ambient temperatures) with the drain plug removed to allow further purging of the excess grease in the bearing grease cavity due to thermal expansion.
• After the excess grease ceases to be pushed out of the drain plug, the drain plug area should be cleaned to remove purged grease and the drain plug installed.Do not be concerned if no grease exits from the purge pipe, this is not unusual.
• The grease that is purged from the bearing grease cavity should be visually inspected for signs of water, oil separation, soapiness, or indications of metallic particles. If rust or other abrasives are observed, schedule the motor for overhaul.
• Leave an extra blob of grease on the grease fitting, to prevent contamination from entering fitting until next re-greasing. Avoid painting of grease fitting.
• Tag/paint motor bearing housing requiring special grease that is different from the standard grease being used in the plant.
It is better to grease some motors while running and others while de-energized depending on how the bearing pumps the grease, the bearing cap clearance, grease viscosity, bearing grease cavity design, and other factors. There is no universal method that will work for all motors. Only experience can dictate which is best for each motor.
It is generally recommended that a motor be greased while in operation unless experience or operating conditions dictate otherwise. The reason for this is that bearing balls act as tiny viscosity pumps and will aid in purging the old grease out and pulling the new grease in if the motor is running. This action does not take place when the motor is stopped. However in motors where grease cavity is not well sealed from the motor housing, excess grease that comes into contact with the rotating shaft may be pumped through the opening and sprayed on the stator windings, raising stator temperature and reducing stator winding insulation life.
If grease cannot be added while the motor is operating or while the bearings are near operating temperatures, then the possibility of over greasing the bearing grease cavity should be considered and some consideration should be given to reducing the amount of grease added to possibly the same amount as recommended for the motors in standby.
Instructions to Motor Repair-shop
Eventually most motors will require rewinding or overhauling. Following instructions should be provided to the repair-shop :
• Feedback required: Even if lubrication was not the cause leading to motor repair, repairer should report lubricant related details, including signs of varnish, scorching, caking of thickener, grease spread on windings, etc
• Specify required grease: Grease used by repairer & grease used in plant for later regreasing may be incompatible. Ideally use the same grease.
• Proper fill volume: Define the initial fill volume for the bearings, considering bearing type, speed & motor design and leave supply pipe filled with grease.
Several technologies are now available to provide feedback to the technician undertaking re-greasing. These technologies add precision to the calculation based regrease quantity & interval s and best practices outlined above. Some of these are:
• Grease-gun mounted pressure gauge
• Grease gun mounted flow meter
• Vibration analysis
• Acoustic emission analysis
• Thermo-graphic analysis
• Grease analysis
||Bearings which operate in harsh environments will demand the shortest re-greasing intervals. Conditions such as high temperatures, vibrations, high loads and high levels of contaminants such as dirt and water demand the most frequent greasing.
Single point automatic grease lubricators can be considered for difficult to access bearings or bearings requiring very frequent lubrication. Some single point lubricators can be configured to feed up to 4 bearings.
Plant wide automatic grease lubrication system may be considered when a very large number of bearings need to be lubricated such as conveyor systems, paper mills, etc.
Refer to KNOWLEDGE CENTRE for section on AUTOMATIC GREASE LUBRICATION.
(Readers may also refer to FAG Publication WL 81 115/4 EA for a detailed exposition on bearing lubrication.)