The spindle engine is the central engine of the disk drive. All platters are connected to it. Typically the spindle engine runs at 7200 rpm (rotations per minute). Other common speeds are 5400, 4200 (some 2”5), 10000 or even 15000 RPM. An increased number of RPM leads to better performance of the drive since data passes the read/write head more often.
See also: HDA overview image.