Sleeve Bearing Load Limits

Sleeve bearing load limits Electrical Apparatus , Jun 2003 by Nailen, Richard L ShareEmailDiggFacebookTwitterGoogleDeliciousStumbleUponNewsvineLinkedInMy YahooTechnoratiRedditPrintRecommend0How their load is calculated and their limits TRUE OR FALSE: SLEEVE bearings are unsuited to motors driving belted loads. Most handbooks or application guides agree that sleeve bearings won't handle radial load ("side pull"). However, like some other well-known motor "facts," it's not so. Until ball and roller bearings began to be popular during the 1930's, motors applied to belt or chain drives used sleeve bearings just like directcoupled machines. The bearings had to be sized to carry whatever the load was. How is sleeve bearing load calculated, and what limits apply? The relationships are quite different from those for anti-friction bearings and are based on different physical concerns.

More Articles of Interest Sleeve bearings: modern use for an old technology Coping with hot rotor stress Evaluating the motor without a load Motor noise Thermal stress in two-layer stator windings For a ball or roller bearing, radial load causes a deformation (strain) in the components that's associated with fatigue stress. Eventually, material failure occurs in the form of pitting or spalling. In a sleeve bearing, as long as proper lubrication is maintained, an oil film separates the babbitt bearing liner from the steel journal surface on the shaft, so that neither friction wear nor crushing stress acts to bring about failure in the metals involved. That's why we so often see the comment that "sleeve bearings last forever," or (more properly) "no specific load/life relationship exists for a sleeve bearing." However, the sleeve bearing does have one important enemy: heat. The heavier the load and the higher the speed, the greater the heat developed through friction-not metal-to-metal rubbing, but shearing within the oil film. That's why large, high-speed sleeve bearings must be pressurelubricated in some way, with an ample volume of oil flowing through the bearing to carry away that heat. Also, compared to the ball or roller bearing, the sleeve bearing is necessarily much wider axially (parallel to the shaft). Inevitably, the result is often an angular difference between shaft and bearing centerlines. That causes "pressure points" at the top of one end of the bearing and at the bottom of the other end. Concentrated loading there tends to break down the oil film and lead to destructive rubbing.

That condition was of particular concern years ago, when bearing length was relatively greater than in modern designs. The length-to-diameter ratio (L/D) for bearings 50 years or more ago was typically 1.5 to 2.0. Today, it may range from 0.8 to 1.2. What determines allowable radial load on a sleeve bearing, then, is the oil film. For at least the last 70 years, authorities worldwide have agreed that the tin-based babbitt bearing commonly used in electric machinery, with journal diameters from 2" to 20" or more, can safely operate at pressures of 100 to 200 pounds per square inch of projected bearing surface. What does that mean? In a 3" bearing with L/D of 1.0, for example, the bearing diameter and length are each 3". The "projected area" is simply the product of those two dimensions, or 9 square inches. Therefore, the allowable radial load on the bearing may be from 900 to 1,800 pounds, depending upon lubrication and shaft speed. Of course, the true area supporting the load is not exactly 9.0 square inches. This will vary depending upon the direction in which the load is applied, because of oil grooves and relief pockets in the babbitt. Even a vertically downward load, such as the weight of the rotor-shaft assembly, doesn't apply pressure uniformly at right angles to the babbitt surface through the lower half of the bearing. As the shaft rotates, the journal "climbs" somewhat up the side of the bearing, supported by the oil film, so the contact pressure between journal and babbitt is unevenly distributed. All those variables are accounted for in the 100-200 psi limit. The allowable maximum is chosen by the bearing manufacturer; the accompanying graph is typical. Those load limits are far below what is customary for main crankshaft bearings in Diesel or gasoline engines. That may seem surprising, because most electric motor or generator bearings operate at far lower speeds and temperatures. Engine bearings are subjected to severe "pounding" from the crankshaft. However, they are normally bronze, a much harder and stronger material than babbitt (and more heat resistant), as well as more costly to manufacture or replace. Furthermore, engine bearings are lubricated by a continuous flow of high-pressure oil. That oil pressure gauge or "idiot light" in your vehicle is there for a good reason; loss of oil pressure can destroy bearings in minutes. Nevertheless, driving a car 15,000 miles a year may mean only a few hundred hours of operation at 2,000 to 3,000 RPM. A large electric motor may experience 15 or 20 times that many hours of annual operation, with a great many more revolutions, even at a lower speed. Certainly, the radial load permitted for any sleeve bearing in electric machinery is far below that which a ball or roller bearing could carry on the same shaft. And the allowable load applied horizontally to the side of the bearing may have to be less than what can be applied vertically downward. But that does not mean radial load and sleeve bearings are incompatible.