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 a 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.

Created May 2, 2006.
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