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Are bearings making you spin your wheels?

Where would automation be without bearings? That's the premise behind this white paper by Ian A. Rubin, Director of Marketing, mounted bearings, for the Sealmaster, System Plast, and Browning brands at Emerson Power Transmission Solutions. Rubin also addresses a number of myths surrounding bearing use.

Pw 60899 Pie Chart2

The wheels of industrial automation turn on bearings, so why do those wheels often vibrate, clatter, squeak, drag and overheat? Bearings can fail for lots of reasons. Most failures (as shown in Figure 1) 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. These myths fall into three general areas of bearing use: installation, misapplication and lubrication.

Installation myth #1: It's okay to hammer a bearing into position if needed - FALSE.
Never strike a direct blow to bearing. The rolling elements and raceway are hardened, but can still be damaged. A hammer blow can leave dents in the raceway that can cause noise and dramatically reduce bearing life. If installation is difficult, first check the shaft diameter, look for burrs, dirt or corrosion on the shaft. 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.

Installation myth #2: Off-the-shelf TGP shafting is the best option - FALSE.
It's much more important to know the shaft's tolerance range to be sure it meets your bearing manufacturer's spec for diameter and roundness. Review the bearing manufacturer's recommendations and measure/specify the correct shaft diameter.

Installation myth #3: It's fine to hand-tighten setscrews one at a time - FALSE. Setscrews should be tightened to the manufacturer's recommend torque. Under tightening can allow the bearing to slip on the shaft. Over tightening can distort the raceway or crack the inner ring. Use the "half-full/full" rule for tightening setscrews - 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.

Application myth #1: Bearings should not be hot to the touch - FALSE.
Normal bearing operating temperatures can range from 80°F to 150°F, but certain applications may run higher or lower. Most bearings are rated for -20 to 220°F, but can be supplied with special grease, seals or heat stabilizing processes that 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 in the bearing. Spikes up to 50°F are normal at start-up, and 30°F after re-lubrication. As the rolling elements purge excess grease through the seals, the bearings return to steady-state temperatures.

Application myth #2: Bigger bearings are always better - FALSE.
Bigger bearings with a higher load capacity may show a higher fatigue life, but if the load does not achieve the minimum requirement, the rolling elements can skid along the raceway instead of rolling. This can cause high temperatures, excessive wear, lubrication breakdown and bearing failure.

Application myth #3: Sealed/lubed-for-life bearings will last forever - FALSE.
Bearing life depends on grease life, which is affected by the operating conditions (speed and load) and environment (temperature and contamination). Grease life can be improved with enhanced seals, proper installation and proper grease selection. Ultimately, the best bearing is the properly lubricated bearing.

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