Extending bearing life
Focus on specifications, proper lubrication and diligent on-site management.
Nothing in a plant runs smoothly and efficiently without bearings, yet their important role is not always well understood so they frequently fail earlier than expected. The result? Costly lost production, although the bearings themselves are relatively inexpensive (typically less than 10% of the machine value).
The lifespan of ball or roller bearings (designed for either radial or axial loads) is influenced by many factors, but two of the most important are lubrication and contamination. According to Swedish bearing manufacturer SKF, 36% of premature bearing failures are caused by incorrect specs and improper lubricant application. Some bearing companies and tribologists suggest the percentage is as high as 50%. Inevitably, any bearing deprived of proper lubrication will fail long before its normal service lifespan.
Because bearings are usually the least accessible machine components, wrong or neglected lubrication compounds the problem. Sealed-for-life bearings, automatic lube systems, and using only lubricants, tools and techniques recommended by bearing manufacturers will alleviate most problems.
Sealed-for-life bearings account for only a small portion of all bearings in use; all others are subject to contamination, a bearing’s worst enemy. Bearing manufacturers strive to improve designs that minimize opportunities for contamination that cause fretting and corrosion. Examples are SKF’s LGAF 3E anti-fretting agent and its LHRP 1 anti-corrosion agent. The former is a greasy, smooth paste specially developed to prevent fretting corrosion between metal surfaces in loose-fit arrangements. The latter provides long-term corrosion protection in both ferrous and non-ferrous metals. It leaves a stable rust protection film over metal bearing components.
About 16% of failures are the result of poor fitting. Correct fitting tools and methods for properly mounting and dismounting bearings are available from suppliers. There is no need to use brute force. Simple fatigue accounts for another 34% of premature failure from overloaded machines, abuse, incorrect service or neglect. However, overstressed bearings emit early warning signals that are detected by hand-held instruments, hard-wired systems or continuous monitoring sensors.
Randy Ikonouye, engineering manager for industrial OEM and heavy-duty truck fleets at NTN Bearing Corp. Canada Ltd. in Mississauga, Ont., recently delivered a presentation about bearing failure analysis and lubrication at the Toronto Section of the Society of Tribologists and Lubrication Engineers (STLE). He noted that although lubricant is an integral component of every bearing system, it’s often overlooked.
Bearing lubrication reduces friction, dissipates friction heat, prevents corrosion and extends service life. Adequate (not too much, not too little, uncontaminated and properly applied) lubrication is vitally important. Inadequate lubrication results in spalling or flaking and pitting, and cage failure (due to vibration and moment loading). Heavy and abrasive wear symptoms are excessive bearing heat and discolouration.
Damage is also due to water etching (water, moisture, snow or ice); rust and corrosion resulting from fretting; or contamination caused by improper lubricant storage, mixing incompatible lubricants, and/or not wiping grease fittings before lubrication. When any of these occur, take immediate action by:
- keeping a sample of the lubricant;
- getting the full story; and
- relubricating when possible and at the right time.
Measuring grease noise assesses the quality of lubricated rolling bearings. The latest equipment for this is the Grease Test RigBeQuiet+ developed by the SKF Quality Technology Centre in Steyr, Austria. It assesses cleanliness and damping characteristics.
Bearings are important machine components. Correct choice and fit, the right lubricants and application, plus diligent onsite maintenance will go a long way to preventing premature failure and costly lost production.
Steve Gahbauer, an engineer and Toronto-based freelance writer, is the former engineering editor of PLANT.
This article appears in the January/February edition of PLANT.