Thrust ball bearings are composed of washer-like bearing rings with raceway grooves for the balls. The ring attached to the shaft is called the inner ring or shaft washer while that attached to the housing is called the outer ring or housing washer. In double direction thrust ball bearings, there are three rings with the central one being fixed to the shaft. Single direction and double direction thrust ball bearings can support thrust loads in one or both directions respectively. There are also thrust ball bearings with an aligning seat washer beneath the outer ring in order to compensate for shaft misalignment or mounting errors. Pressed-steel cages are usually used in the smaller bearings and machined cages are generally used in the larger ones. Series 51100