Have been used in almost all sizes of electric motors since motors were invented. Sleeve bearings are still used for large motors where the desired bearing life cannot be achieved with rolling-element bearings. The limiting factors in larger motors are the diameter of the bearing and the speed of rotation. Sleeve bearings are deceptively simple in appearance. Made of tin- or lead-based Babbitt, they are machined slightly larger than the shaft. A film of oil continuously lubricates the bearing and shaft, minimizing surface wear and cooling the parts.