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Proper maintenance keeps large slewing ring bearings going

Large-diameter bearings are custom-engineered and manufactured almost exclusively for lifting equipment that requires rotation. These devices typically have a complex load spectrum: the slewing ring bearing can simultaneously handle all combinations of thrust, radial, and tilting moment loading. Large bearings in heavy-duty applications such as cranes, excavators, shovels, tunnel boring machines, and bucket trucks are particularly prone to premature failure, especially if proper maintenance is not performed.

Because of their size (generally 8" to 10'), these bearings are very expensive. A regular schedule of raceway lubrication, torque checks on the bolts, and gear lubrication should be followed if the equipment is being used regularly or even if the unit is idle.

Bearing lubrication
The bearing should be lubricated at regular intervals, with a heavy-duty, extreme-pressure grease. Slowly rotating equipment or oscillating applications, such as backhoes, excavators, and cranes, should be relubricated about every 100 hours of operation. More frequent lubrication—every day or every eight hours—may be needed on rapidly moving or continuously rotating equipment such as trenchers and boring machines.

While adding grease, the bearings should be rotated to spread the grease throughout, with enough grease added to purge the old grease out through the seals. This procedure helps to clean out contamination.

Over long periods of time, grease tends to dry out and condensation can form within a bearing. Grease should therefore be introduced at least every six months, whether the equipment is operating or idle.

Gear lubrication
Most large bearings incorporate gear teeth and the gear lubrication requirements are not the same as for the bearing. Because the meshing action of the teeth tends to squeeze out lubricant, the gears should be lubricated every eight hours on slow or intermittently rotating equipment, and more often on rapidly or continuously rotating equipment. Small amounts of grease should be introduced at the point of mesh between the gear and pinion.

A check of bolt torque should be part of any routine maintenance procedure. The frequency of these checks depends on severity of service. Vibration and shock tend to loosen bolts, so periodic torque checks will ensure that the proper preload level is maintained.

Mounting bolts should be torqued to the proper level and checked periodically to ensure proper pre-tension. Improperly pre-tensioned bolts can fail, causing damage to equipment and, more important, injuries to workers.

Additionally, tightened bolts tend to creep or elongate over a period of time, which reduces preload. Elevated temperatures will also increase bolt creep and this factor should be considered in developing a torque-check schedule.

Seals on the bearing should be visually inspected periodically to be sure they are fully intact. Insignificant as these seals may seem, they aid considerably in the prevention of bearing raceway contamination.

Because of the severity of typical large-bearing applications, bearings eventually will become worn. But with proper maintenance, a worn bearing will still be deemed repairable, saving the time and cost associated with purchasing a new replacement bearing.

As all industry sectors struggle to become more cost-efficient, a preventative maintenance program can save valuable time and hard-earned money. Extending the life of costly equipment with proper maintenance and, when needed, bearing refurbishment, is a sure way to help increase cost-efficiency.


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