In a tape recorder the capstan is the rotating shaft that drives the tape past the heads. Usually the tape is squeezed between the capstan and a rubber wheel known as the pinch roller. The capstan is what controls the speed of the tape and is usually attached to a “flywheel” so that a smooth and consistent tape speed is maintained.
Interestingly (or maybe not), the tape speed is not always equal to the surface speed of the capstan as one would expect. A phenomenon known as “belting” can cause tape to be moving faster than the capstan turns, especially with small diameter capstans. Also, thick tape will move slightly faster than thin tape. This means that a very slight, but audible, rise in pitch occurs whenever a spliced (therefore thicker) piece of tape moves over the capstan.