The classic mode of failure for slewing rings is fatigue, which is of subsurface origin. But today most fail due to lubrication problems, contamination, overloading, improper installation, or poor mounting fits, all modes of failure that tend to be surface-originated. They also occur much earlier than classic fatigue failures and are less predictable.
The main problems are inadequate lubrication, contamination and overloading.
Inadequate lubrication: Slewing rings require a heavy-duty pressure grease, reapplied at least every 100 hours when equipment is slow-rotating or used only intermittently. Otherwise, surface damage develops. It first takes the form of roughening or waviness; later, fine cracks develop, followed by flaking.
Contamination: Dirt and debris that get into the lubrication system act as an abrasive and accelerate wear. Usually they scratch or indent the bearing raceways, again leading to fine cracking and, ultimately, spalling.
Overloading: Turntable bearing life calculations are based on flat, rigid mounting structures. But when a structure distorts significantly under load or gets out-of-flat, loads are less evenly distributed, resulting in deformation. While turntable bearings are designed to handle combination loads efficiently, a load that exceeds the bearing capacity can lead to permanent deformation, and eventually bearing failure.
There are many signs of problems. If metal particles or flakes are apparent in the lubricant, some sort of wear is taking place. If the turning torque of the bearing increases substantially or has tight spots, this usually indicates an uneven load distribution. A grinding noise emitted from the bearing typically means inadequate lubrication and/or excessive wear in the raceway.
Play, or clearance, is an important indicator. A new slewing ring is assembled with enough clearance to compensate for the slight out-of-flatness encountered in a mounting structure. Wear can increase this clearance dramatically and increased clearance in turn accelerates the wear. This should be closely monitored.
Excessive bearing clearance due to accelerated wear results in less overall stiffness in the entire system. In construction equipment, this is magnified at the bucket, causing a potential safety problem for the operator. Poor work distribution (common among digger derricks where most of the work is done on the curbside) is another concern that can lead to uneven wear and excessive play. And a severely worn bearing can in turn cause costly damage to other components, such as the drive pinion and gear box.
- Read a technical feature by Ron Shaw on how to determine whether to rebuild or replace bearings in the September 2010 digital edition of Machinery & Equipment MRO (Rebuild or Replace?, page 18).
Ron Shaw is the bearing remanufacturing manager for Kaydon Corporation, Bearings Div., Avon, OH. For more information, visit www.kaydonbearings.com.