11 bearing myths debunked
How to avoid the easily avoidable problems.
The wheels of industry turn on bearings, so why do they vibrate, clatter, squeak, drag and overheat? Bearings fail for lots of reasons. Most failures are related to lubrication and contamination, but myths and misconceptions handed from one generation of maintenance engineers to the next help perpetuate many easily avoidable problems. Here are 11 of the most common false notions and what to do about them.
1. It’s okay to hammer a bearing into position if needed. A direct hammer blow leaves dents in the raceway that cause noise and dramatically reduce bearing life. If installation is difficult, first check the shaft diameter and look for burrs, dirt or corrosion. If needed, use a press to slide the bearing on. Apply pressure equally on the face of the inner ring to avoid damaging the raceways and rolling elements.
2. Off-the-shelf TGP shafting is the best option. It’s more important to know the shaft’s tolerance range to ensure it meets the spec for diameter and roundness. Review the manufacturer’s recommendations and measure/specify the correct diameter.
3. It’s fine to hand-tighten setscrews one at a time. Under tightening allows the bearing to slip on the shaft. Over tightening distorts the raceway or cracks the inner ring. Tighten the first setscrew to half the recommended torque, the second setscrew to the full torque, then go back to the first setscrew and apply full torque.
4. Bearings should not be hot to the touch. Normal bearing operating temperatures range from 27 to 66 degrees C, but some applications run higher or lower. Most bearings are rated for -29 to 121 degrees C, but special grease, seals or heat stabilizing processes allow them to operate at higher temperatures. Bearings normally run hotter at start up or right after re-lubrication because excess grease increases drag and friction. Spikes up to 10 degrees C are normal at start-up, and -1 degree C after re-lubrication. Steady-state temperatures resume as the rolling elements purge excess grease through the seals.
5. Bigger bearings are always better. They may show a higher fatigue life, but if the load does not achieve the minimum requirement, rolling elements skid along the raceway. This causes high temperatures, excessive wear, lubrication breakdown and failure.
6. Sealed/lubed-for-life bearings will last forever. Bearing life depends on the grease, which is affected by the operating conditions (speed and load) and environment (temperature and contamination). Grease life improves with enhanced seals, proper installation and proper selection.
7. Re-lubrication once a year is sufficient. Start by reviewing the manufacturer’s lubrication recommendations, but actual intervals may vary quite a bit, depending on load, speed, temperature or environmental conditions. Applications with higher speeds, temperatures or heavy contamination sometimes require weekly or even daily relubrication. By contrast, a mounted ball bearing in a lightly loaded, low-speed, clean environment may do fine with 12- to 24-month intervals. Certain applications may need to be monitored and lubrication intervals/amounts adjusted accordingly.
Re-lubrication replenishes grease that temperature breaks down or deteriorates. Pumping in new grease also helps flush away contamination.
8. Always add grease until it purges from the seal. Doing so will probably fill the bearing cavity, which increases operating temperature and may create enough pressure to blow out the seal. However, in low-speed or dirty conditions where contamination may easily enter the seals, filling a bearing may help improve performance. Application experience will dictate when the entire bearing cavity should be filled.
9. Add grease if a bearing makes noise. Noise indicates internal damage has likely occurred. Adding grease may provide temporary relief, but a noisy bearing should be closely monitored and replaced at the first opportunity. Be sure to investigate the root of the failure.
10. Any grease will do. Greases do differ. Some may be incompatible because of the different thickeners (soaps) used. For example, many electric motors use a polyurea thickener while some mounted ball bearings use lithium-complex thickeners. These greases are borderline compatible, but depending on the actual make up, may not work together. Grease types will also be incompatible based on the viscosity or type of oil in the grease.
11. Just shoot grease through the fitting. It’s better to always clean grease fittings and the grease gun tip. Put the tip in an oil bath or protect it with a plastic cover. A plant’s uptime and OEE may “turn” on bearing health. If you are falling short of desired operational life, ask the bearing manufacturer to assist you with selection and troubleshooting.
This article was provided by industrial technology provider Emerson Industrial Automation, Power Transmission Solutions, based in Florence, Ky., with Canadian offices in Rexdale and Unionville, Ont. Ian Rubin is director of marketing, mounted bearings, for Sealmaster, System Plast and Browning-branded products. Visit http://powertransmissionsolutions.com.