Electronic gadgets rely on a host of equipment for their creaseless performance. One of them is power generating equipment—spinning generators, motors, and shafts that create and transmit power. In fact rotating machinery is the basis not only of electronic gadgets, but also of modern civilization itself. A wide variety of lubricated and self-lubricating bearings keeps these machines operating smoothly and efficiently. Heat is a major factor affecting bearings, degrading its lubrication and damaging the bearing, ultimately increasing friction and decreasing efficiency.
Normal operations rarely merit concerns over temperature rise of bearings. However, exposure to abnormally high loads and speeds, high ambient temperatures, and hot process fluids can cause problems.
For instance, roller bearings can start to run hot because rolling elements microslip on the races. Another reason is the contact stress generated hysteresis of rolling elements and race materials. Even the sliding between the pilot surfaces or rolling elements and their separator and the sliding between guide flanges and rollers generates heat. Shear and turbulence in the lubricant also generates some heat.
One of the best means of heat control in bearings is by using cooled oil, particularly useful in gas turbines and pumps for hot liquids. Although lubrication needs only a thin film of oil, high flow rates of oil can also help to cool bearings. The common practice is to set the nominal oil level to the center of the bottom ball bearing. This works satisfactorily except for extremely high speeds, when an oil flinger providing a mist of oil becomes necessary. In places where the above cannot be used, heat is removed by convection from the bearing housing to the ambient air.
Thrust bearings are susceptible to the viscosity of the lubricant they use. Viscous friction can cause power loss, which can be substantially reduced by lower lubricant viscosity. However, the best way of keeping thrust bearings cool is increase the volume of oil flowing over each pad segment. Another method often followed by engineers is to directly cool the oil feed or cool the bearing housing from the outside. Using copper plates in place of steel backing also helps to keep bearings cool, as copper conducts heat away better than steel does.
Sleeve or journal bearings have a longer life when running cool. Therefore, it is important to take steps to bring down the operating temperature. Although sleeve bearings also benefit from lower viscosity oil, the cooling effects are somewhat limited. This is because the viscosity of the oil film rises as the overall temperature drops. Nevertheless, low viscous oil helps in cooling sleeve bearings in high-speed machinery.
Engineers strive to maintain an optimal radial bearing clearance as this has a pronounced influence on bearing performance and hence its operating temperature. If normal clearance is small, the sleeve bearing can run hot and the cooler housing can constrain it, leading to a seizure.
Too much clearance can also lead to vibration, unbalance, and other instabilities. To avoid such undue vibration and temperature rise, engineers must follow recommended diametrical clearances.
