Spindle Motor Definition
A spindle motor is a small, high-precision, high-reliability electric motor that is used to
rotate the shaft, or spindle, on which the platters are mounted in a hard disk drive (HDD).
A platter is a thin, high-precision aluminum or glass disk that is coated on both sides with
a high precision magnetic material and which is used in a HDD to store data. Modern HDDs
typically contain multiple platters, all of which are mounted on a single shaft, in order to
maximize the data storage surface in a given volume of space.
All HDD spindle motors are configured for direct driving of the platters. That is, the
rotating shaft of the motor and shaft on which the platters are mounted is a single, integral unit.
This is in contrast to most electric motors, which use gears or belts to transfer and modify their
output.
Most HDD spindle motors spin the platters at a constant rate ranging from 3,600 to 7,200
RPM. The precision of the rotational speed is maintained through the use of feedback loop
circuitry.
Among the other important characteristics of spindle motors are small size, low power
consumption, high reliability (including the ability to run for thousands of hours and tolerate
thousands of start and stop cycles without failure), minimal wobbling and vibration (due to the
tight tolerances of the platters and magnetic heads), low heat output and minimal noise output.
Minimizing the wobbling and vibration makes it possible to reduce track spacing on the
platters, thereby increasing HDD data storage capacity. A track is any of the concentric circles on
the magnetic coating on a platter over which one magnetic head passes while the head is
stationary but the disk is spinning.
A critical component for reducing the wobbling of the shaft along with vibration and
noise is the bearings. Spindle motors have long used ball bearings, which consist of small metal
balls that are housed in a metal ring structure around the spindle motor shaft. However, in the
past few years there has been a transition to fluid dynamic bearings, in which the metal balls are
replaced by high viscosity oil, thereby eliminating the metal-to-metal contact. The advantages
include a further reduction in noise, longer life expectancy and lower cost.
Other ways in which spindle motor performance has been improved and size and cost
have been reduced in recent years include the use of more powerful permanent magnetic
materials, higher precision machining techniques and improved electronic control circuits.