Page 303 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 283

Inadequate Lubrication

Any load-carrying contact between the rollers and the inner and outer
races in a bearing requires the presence of lubricants for reliable opera-
tion. All bearing rollers undergo varying amounts of sliding motion in
addition to the primary rolling motion present as they transmit the load
between the inner and outer races. In addition, the rollers must carry the
bearing cage as the bearing rotates, so they also slide on the bearing cage.
This sliding motion can be very detrimental to a bearing unless the lubri-
cant film is thick enough to prevent contact between the sliding parts.
The viscosity of a lubricant is the most important characteristic of the
oil either as oil itself or as the oil in grease lubrication. An oil with too
low a viscosity allows metal-to-metal contact between the rollers and the
inner and outer races, which results in bearing failure. Also, an insuffi-
cient quantity of lubricant at medium to high speeds generates a tempera-
ture rise which in turn can cause lubricant failure. Lubricant failure gen-
erally causes surface damage in the bearing ranging from frosting to
spalling, discoloration, glazing, or smearing.

Ineffective Sealing
The effects of dirt and other abrasives in bearings can result in changes
in bearing internal geometry. Freedom from abrasive matter is so impor-
tant that some bearings for very high precision equipment are even as-
sembled in air conditioned white rooms. In addition to abrasive matter,
corrosive agents must be excluded from bearings. Water, acid, and other
agents that deteriorate lubricants result in corrosion and premature bear-
ing failure. Acids formed in the lubricant with water present etch the
bearing surfaces and reduce the load-carrying capacity.

Vibration

Rolling dement bearings exposed to vibration while the shafts are not
rotating are subject to a damage referred to as false brinelling. This is
usually indicated by either bright polished depressions at each roller or a
corrosive type stain or fretting. The vibrating load causes minute sliding
in the area of contact between the rolling element and raceways and sets
free small particles of material that are oxidized and cause accelerated
wear. Many bearing failures are probably caused by false brinelling
which is never discovered since the unit is usually operated until the
bearing is destroyed before it is inspected.