Lubrication, Bearings, and Oil Seals
Lubrication
Of all the important items of maintenance, lubrication ranks as one of the highest. Incorrect oiling or greasing will produce as disastrous results as any other type of motor mistreatment.
Excess oil may get into the windings where it will collect dust and other foreign matter. Too much grease in antifriction bearings causes heat and sometimes failure of bearings and may also coat the windings. Most manufacturers furnish data on correct oiling and greasing, and numerous articles have been written on the subject. The important point is to set up a definite lubrication schedule and follow it. Years of experience have demonstrated that it is as bad to use too much as too little oil and grease.
Of equal importance is the type of oil or grease used. In general, the recommendations of the manufacturer or experienced oil companies should be followed. In some cases, for design reasons, manufacturers insist on the use of particular lubricants that have been adopted after exhaustive test by the manufacturer. It will pay to follow these recommendations.
Sleeve Bearings
Some oil-lubricated machines are shipped without oil and, in the case of large machines, the journals are often packed and treated for protection during shipment. The rotating elements may also be blocked to prevent damage to the bearings and journals during shipment. Where lubrication is required, the bearing must be opened, the packing removed, and the journal cleaned and flushed before filling the housing with oil. All motor and generator bearings should be checked for oil before starting up.
The bearings of all electrical equipment should be carefully inspected at scheduled periodic intervals in order to obtain maximum life. The frequency of inspection, including the addition of oil, changing the oil, and checking the bearing wear, is best determined by a study of the particular operating conditions. If makeup oil is required in excessive amounts, an investigation for oil leaks should be started immediately.
The more modern types of sleeve-bearing housings are relatively dust and oil tight and require very little attention, since the oil does not become contaminated and oil leakage is negligible. Maintenance of the correct oil level is frequently the only upkeep required for years of service with this type of bearing.
Older types of sleeve bearings require more frequent inspection and check- ing for wear, and oil changes should be made more often. Never add oil to bearings when the machine is running.
In most cases, the safe temperature rise for a bearing is considered to be within 40°C above the room ambient.
Small sleeve-bearing motors use either wool packing or fluid wick for transferring the lubricant to sleeve bearings instead of oil-ring lubrication. Some of these small motors have provision for relubrication.
When electrical equipment must operate under extreme differences in air temperatures, the use of a lighter oil may be found desirable during cold weather.
Care should always be exercised in the use of reclaimed lubricating oils. The filtering operation should be positive and should remove all foreign and injurious matter.
A hot bearing is usually due to one of the following causes:
• No oil.
• Poor grade of oil or dirty oil.
• Failure of the oil rings to revolve with the shaft.
• Excessive belt tension.
• Rough bearing surface.
• Incorrect fitting of the bearing.
• Bent shaft.
• Misalignment of shaft and bearing.
• Loose bolts in the bearing cap.
• Excessive end thrust due to incorrect leveling. A bearing may become warm because of excessive pressure exerted by the shroud of the shaft against the end of the bearing.
• Excessive end thrust due to magnetic pull, with the rotating part being sucked into the stator or field because it extends farther beyond the magnetic structure or field poles at one end than at the other end.
• Excessive side pull because the rotating part is out of balance.
If bearing becomes hot, the load should be reduced if possible and lubricants fed freely, loosening the nuts on the bearing cap. If the machine is belt connected, the belt should be slackened. In case relief is not afforded, the load should be removed and the machine kept running slowly, where possible, until the shaft is cool in order that the bearing will not freeze. The oil supply should be renewed before starting the machine again.
A new machine should always be run unloaded or at slow speed for an hour or so to make sure that it operates correctly. The bearings should be carefully watched to observe that the oil rings revolve and carry a plentiful supply of oil to the shaft.
Antifriction Bearings
Ball or roller bearings carry the load by direct contact, as opposed to sleeve bearings, which carry the load on lubricating film. Lubrication is necessary to minimize the friction and generation of heat caused by the balls rubbing on the outer race as they roll over the top or on the retainer of the cage.
Antifriction bearings require considerable care to prevent loss of end clearance, distortion of balls, and marking of races. If too much force is used in pressing the bearing on the shaft, the clearance may be destroyed. It is recommended that antifriction bearings be heated in a hot bath of clean oil rather than by the use of dry heat. When the bearing is pulled off, with all the stress on the outer race, both races may be damaged, with resultant failure when put back in service. The bearing manufacturer’s recommendations should be followed when removing and reapplying this type of bearing.
Bearing manufacturers produce a bearing known as the prelubricated shielded bearing. Several years use of this bearing has demonstrated that, for many applications, no further lubrication is needed. Such bearing construction is usually indicated on the nameplate.
In general, to obtain maximum service, ball-bearing motors should be relubricated at intervals determined by the type, size, and service of the bearing. Many motor manufacturers offer as a guide a table suggesting the intervals between lubrication. These tables show time intervals between greasing that range from 3 months or so for motors operating in very severe service, as in conditions involving dirt or vibrating applications, those where the end of the shaft is hot, or subject to high ambient temperatures, to inter- vals of up to 3 years for easy service, where motors operate for short periods or infrequently.
The bearing housing is usually arranged to introduce new grease and purge the bearing of old grease, allowing it to discharge through a partially restricted escape port or relief hole. This will, in general, allow filling to the desired degree, which is one-third to one-half full, leaving some space in the housing to allow for expansion of the grease.
It is again stressed that overgreasing can be just as harmful as undergreas- ing. Overgreasing causes churning and internal friction that can result in heating, separation of the oil and soap, oxidation of the grease, and possible leakage through the retaining seals.
Installation of Oil Seals
The importance of correctly installing an oil seal cannot be overemphasized. Failure to observe correct installation procedures probably accounts for more cases of the incorrect functioning of oil seals than any other single cause. To secure the ultimate in satisfactory service, it is recommended that the following precautions be observed.
Correct seal
It is essential that the seal be the correct size for the installation. Oil seals are made for a specified shaft size. When they are installed on a shaft of a larger diameter, there will be drag, frictional heat, and excessive wear on the sealing element and shaft. When installed on a shaft having a smaller diameter, immediate leakage can occur.
Fluid contact
The seal should be assembled with the toe or wiping edge of the sealing element pointing toward the fluid to be retained. Exceptions for unusual applications must be by specification in manuals or instructions furnished with the assembly.
Bore
The bore should be checked for adequate chamfer (30° angle to a minimum depth of 1/16 in.). The bore should be inspected for scratches and all sharp edges removed. The seal outside diameter should be correct for the bore in the assembly. When a leak at the outer edge of either metal or rubber-covered seals is caused by abrasion of the oil seal, it may be directly related to incorrect chamfer on the bore of the use of incorrect installation tools.
Shaft
The surface of the shaft should be uniform and free from burrs, nicks, scratches, and grooves. The surface finish should be between 10 and 20 μin.
and, on a repair job, should be buffed to this thickness with crocus cloth.
Lubrication
In all cases, a lubricant should be applied to the shaft or to the sealing element of the oil seal. This aids installation and reduces heat buildup during the first few minutes of run. The application of a lubricant to the outer periphery of a synthetic rubber-covered seal will reduce the possibility of shearing or bruising.
Pressing tools
In pressing the seal into the bore, it is imperative that the correct-sized pressing tool be used to localize the pressure on the face of the seal and in direct line with the side walls of the seal case to prevent damage and distortion to the seal cases during the installation. When a seal must penetrate the bore below the surface, the correct pressing tool should be 1/32 in. smaller than the bore diameter. On installations where the seal is flush with the housing, the correct pressing tool should be at least 1/8 in. larger in diameter, and more if room permits. Care should be taken to avoid hammer blows, uneven pressure on seal surfaces, and cocking of the seal during this operation.
When an oil seal of open channel construction is pressed-fit heel first into the bore, an installation tool will be helpful. The tool is designed to have contact with the inside diameter of the seal case.
Shaft end If the seal is to be installed toe first, the end of the shaft should have a 30° by 3/16 in. taper, or an installation tool must be used. If the seal is to be installed heel first, no special precautions are necessary other than to remove burrs or sharp edges from the end of the shaft.
Shaft with keyways and the like
When an oil seal is installed over the keyway, splines, and the like, an installation thimble should be used with the outside diameter not more than 1/32 in. over the shaft.
Pressure-lubricated bearings
Because of speed and bearing loading, it is necessary to pressure lubricate the bearings on some larger motors and generators. Pressure gauge readings may not show the amount of oil flowing, but machines have a sight oil-flow detector where oil flow may be checked. Orifices in the feed lines may clog, and oil-flow detection devices will protect the bearings.
Bearing insulation
If the bearing is insulated, care must be taken so that the insulated bearing is not grounded by bearing temperature detectors or relays.