The choice between sleeve or ball bearing for use in small motors
Bearing type and quality is an important factor in determining the reliability and life expectancy of fractional horsepower electric motors and fans. The choice is generally between sleeve bearings and ball bearings. When choosing, design engineers must consider factors such as: application, fan longevity, mounting, angle of operation, noise, ambient temperature, duty, lubrication issues, etc.
When the ambient temperature is relatively low, sleeve bearing motors can last as long as ball bearing motors. However, sleeve bearing motors can have problems starting at very low temperatures due to the oil viscosity and bearing stiction – especially with low power models. Shaded pole motor designs are inherently inefficient and the surplus heat generated can help to free bearings in some cases.
Motor reliability is crucial in achieving long life. Small motors and fans operating in the range from 25 to 60 degrees centigrade using ball bearings will outlast their sleeve bearing equivalents by an average of 50 percent. Ball bearing motors can operate beyond 70 degrees centigrade, with reduced life.
Ball bearing motors can be mounted in any position without affecting life expectancy. Sleeve bearing motors are designed to operate with the shaft in a horizontal position and any variation in angle can reduce life span. Ball bearings are preloaded with a spring or wave washer during manufacturing. A ball bearing motor can be used with heavier fan blade when the shaft angle is vertical up or down.
Bearing End Float
Motors with ball bearings have no axial movement. The relative position between shaft and bearing is fixed. To allow for thermal expansion, sleeve bearing motors are manufactured with a clearance which ranges from 0.1mm to 1.0mm. This prevents the motor from seizing when it gets hot. This “end float” is not usually a problem, though exact fan positioning with ball bearings is preferable.
Ball bearings are manufactured using precision components. Steel balls are precision ground and super finished to provide exact tolerances. Sleeve bearings can deteriorate at high temperatures because they are made from porous powdered metals in a sintering process. Micro hardness makes secondary machining difficult. Sleeve bearing quality depends on shaft clearance and oil supply.