SOLVING BEARING OVERHEATING PROBLEMS
Overheating is a major indicator, along with vibration and noise, of an underlying problem affecting a bearing or related components. Because normal operating temperature vary widely from one application to another, no single temperature is a reliable sign of overheating in every situation. By observing an application when it is running smoothly, a technician can establish a benchmark temperature for a particular bearing arrangement. Wide deviations from this accepted norm generally indicate troublesome overheating.
The list of possible causes of overheating ranges from out-of-round housings and oversize shaft diameters to excessive lubrication and bearing preloading. These causes fall into two major categories: improper or faulty lubrication and mechanical problems, such as incorrect fits and tolerances.
A bearing must be lubricated correctly to function properly. Lubrication provides a thin film between a bearing's rolling elements and raceway, protecting metal-to-metal contact, which increases friction and causes heat to build up.
The type of lubricant is also critical. The wrong lubricant causes overheating. Contact the lubricant manufacturer to determine the correct grease or oil for the application at hand.
Lubricant quantity is a factor in overheating. Excessive lubrication can cause a condition called churning, leading to a sharp temperature rise in all but exceptionally slow-speed bearings. In this case, the static oil level should be reduced to the center of the lowest ball or roller. If grease is used, the lower half of the housing should be half full.
The opposite problem, insufficient lubrication, can cause bearing temperatures high enough to produce discoloration and softening of bearing steel. If this occurs, the bearing should be replaced. To prevent this type of damage, fill the bearing housing with grease or oil to recommended levels.
Problems can arise in the lubricant delivery system. If lubricant return holes in the housing's seal area are blocked, the pumping action caused by certain types of seals causes oil to leak. The oil level is reduced, allowing heat to build up. The blockage should be removed, the used oil drained, and the housing refilled with the proper amount of fresh lubricant.
External oil sight gauges can have clogged vents. This blockage causes inaccurate reading with the gauge showing a greater amount of oil than is actually present in the system. Over time, the bearing oil level drops, and damage occurs without the operator's knowledge. Oil gauges should be checked and cleaned regularly.
Overheating is often a symptom of problems within a bearing arrangement's basic mechanical components. If lubrication is correct, troubleshooting should focus on bearing shaft fits, housing fits, internal clearances, and housing shaft seals.
Bearings with inadequate internal clearances are prone to overheating. This is often due to faulty bearing clearance selection. The appropriate clearance can be misjudged, for example, in cases where external heat is conducted through the shaft, expanding the bearing inner ring. To ensure correct internal clearance, the replacement bearing should have internal clearance specifications identical to the manufacturer's specifications.
Insufficient bearing internal clearances can be caused by excessive shaft interference. A cylindrical bore bearing mounted on a shaft with an oversized diameter can result in excessive expansion of the bearing inner ring, reducing clearance in the bearing. To correct this condition, have the shaft diameter ground to obtain the specified fit between bearing and shaft.
Too little clearance in tapered bore bearings can occur when the bearing is forced too far along the tapered shaft. To resolve this, loosen the lock nut, dismount the bearing, and remount it properly, following the bearing manufacturer's installation instructions for retightening. The lock nut should secure the bearing on the sleece, but allow it to rotate freeely.
Another common source of overheating occurs when the arrangement consists of two locating bearings on a single shaft, restricting the axial bearing movement that accommodates shaft expansion. The problem is solved by ensuring that only one bearing is held, and the other has adequate axial clearance in the housing bore to allow for shaft expansion. The problem is solved by ensuring that only one bearing is held, and the other has adequate axial clearance in the housing bore to allow for shalf wxpansion. This approach restores sufficient axial bearing motion to the arrangement. Pedestal-mounted housings should be located with the floating bearing centered in its housing seat, permitting shaft expansion or contraction.
A potential problem can occur when rotating seals rub against a bearing's stationary parts. This results from insufficient clearance in labyrinth seals, or from seal misalignment. Check the running clearance of the troublesome seal and correct it. Realign the seal to eliminate rubbing.
Contact seals may also be excessively tight, leading to a buildup of heat. Tightness occures when composition seals have excessive spring tensions or have died out. Replace tight seals with those of lower spring tension. If the seals are dry, lubricate them.
Heat-producing mechanical problems are not confined to bearing and seals, but can also be caused by the housing. These problems include out-of-round and undersized housings, oversized housings, and housings enlarged by wear.
A housing bore that is out-of-round and undeersized can pinch a bearing and cause it to run hot. The best solution is to replace the housing. In some cases, the housing bore can be remachined to eliminate pinching. Excessive housing distortion is often associated with an uneven housing platform or improper shimming of the platform. Check the housing platform for flatness and for shims that fully cover the entire pillow block base.
An oversized housing bore can cause a bearing to overheat by allowing the bearing outer ring to spin. Replace a housing of this type with a correctly sized one.
Outer ring spinning also occures when nonferrous housings are gradually worn away because of the relative softness of the metal. Rebore the housing and insert a steel bushing into the bore with an interference fit. machine the bushing bore to the correct size.
Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.
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1998 SKF USA Inc.
The above information was supplied by the SKF USA, Inc. The SKF Bearing Maintenance Institute recommends the above bearing maintenance tips to maximize bearing performance. SKF USA, Inc. may be contacted at www.skfusa.com. Solving Bearing Overheating