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Twelve ways to kill bearings.

Twelve ways to kill bearings

Properly designed, properly installed, and properly cared for, precision ball bearings will enjoy a long and useful life. Because we don't always do all the right things, healthy bearings can have shortened, yet distinctly memorable lives. Because bearing failure can be as painful for you as it is for the bearing, it would help to have clues on what you did wrong.

Here, from a recent booklet published by Barden Corp, Danbury, CT, are distinct graphic identifications of common forms of bearing abuse. It will help you perform "autopsies," learn from your mistakes, and hopefully prevent reoccurrences. For copies of the complete booklet from Barden, circle 374.

A: Excessive load

Severe spalling indicates major premature fatigue failure caused by excessive load (although tight fits, brinelling, and improper preload also so cause fatigue to a lesser extent). Heavy ball wear paths, widespread spalling, and some overheating are additional clues. The remedy is to reduce load or increase bearing capacity.

B: Overheating

Discoloration of rings, balls, and cages (ranging from gold to blue) indicate overheating. Temperature rise degrades or destroys lubricant, and temperatures over 400 F can anneal ring and ball materials, reducing their hardness. Common culprits are heavy electrical heat loads, inadequate heat paths, or insufficient cooling at excessive loads or speeds. Adequate heat paths, supplemental cooling, or thermal-over-load detectors are the cures.

C: False brinelling

Elliptical wear marks, axially at each ball position, indicate excessive external vibration, or false brinelling, when the bearing is stopped and not creating a lubricating film. Remedies include isolating the bearing and switching to bearing greases with antiwear additives.

D: True brinelling

Spaced indentations in the raceway indicate loads have exceeded its elastic limit, usually a static overload or severe impact (such as using hammers to install bearings). The remedy is to apply force only to the ring being pressfit (don't push the outer ring to force the inner ring on a shaft).

E: Normal fatigue failure

This is the normal spalling or flaking of material from any running surface, rings or balls, and not an indication of abnormality. Once initiated, it progresses, causing increased vibration. If the resulting normal bearing life is unacceptable, the remedy is a bearing redesign to upgrade fatigue life.

F: Reverse loading

Angular-contact bearings have an elliptical contact area to absorb thrust loads in one direction only. Installing the bearing in the opposite direction creates grooved wear bands on the side-loaded balls, as they ride over the edge of the raceway. Corrective action is to simply install the bearings correctly next time.

G: Contamination

Irregular dents or material embedded in the raceway or balls with scratches or dents indicate contamination, a leading cause of bearing failure. Causes are contaminations in the lubricant from dirty tools, hands, or work areas. Cures are adding seals and more careful handling during installation and prior storage.

H: Lubricant failure

Discolored (blue/brown) ball tracks and balls are initial symptoms of lubricant failure. Excessive wear follows, with overheating and catastrophic failure. Causes are restricted lubricant flow or excessive temperature. Cure is to seek advice from the bearing supplier on lubricant selection, fits, preloads, and cooling.

I: Corrosion

Red or brown staining of balls, raceways, cages, or bands indicates exposure to corrosive fluids or atmospheres. This causes an increase in vibration, which can be followed by wear, increased radial clearance, loss of preload, and, in extreme cases, fatigue failure. Cure is to divert fluids away from bearing or add bearing seals, either integral or external.

J: Misalignment

A skewed ball-wear path, not parallel to raceway edges, indicates misalignment. More than 0.001 ipi can cause temperature rise and heavy wear. Causes are bent shafts, burrs on shaft or housing, and locking nuts not square to their thread axes. Cure is to inspect for runout.

K: Loose fits

Circumferential wear or any discoloration of the bearing's outer or inner mounting surfaces indicates fretting from a loose fit of the bearing and mating parts. Fretting oxidants are a distinctive brown color and very abrasive. Results are wear, heat, noise, and runout problems. Cure is to correct bearing fit.

L: Tight fits

Heavy ball-wear paths at the bottom of raceways indicate that an interference fit has distorted bearing radial clearance. Bearing temperature rises, as does torque, and failure is quick from wear and fatigue. Cure is to decrease total interference and restore proper radial clearance.
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Sep 1, 1990
Words:731
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