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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Construction and Operation of the Hard Disk | Hard Disk Spindle Motor ]

Spindle Speed

As hard disks become more advanced, virtually every component in them is required to do more and work harder, and the spindle motor is no exception. As discussed in detail here, increasing the speed at which the platters spin improves both positioning and transfer performance: the data can be read off the disk faster during sequential operations, and rotational latency--the time that the heads must wait for the correct sector number to come under the head--is also reduced, improving random operations. For this reason, there has been a push to increase the speed of the spindle motor, and more than at any other time in the past, hard disk spin speeds are changing rapidly.

At one time all PC hard disks spun at 3,600 RPM; in fact, for the first 10 years of the PC's existence, that was all there was. One reason for this is that their designs were based on the old designs of large, pre-PC hard disks that used AC motors, and standard North American AC power is 60 Hz per second: 3,600 RPM. In the early 1990s manufacturers began to realize how much performance could be improved by increasing spindle speeds. The next step up from 3,600 RPM was 4,500 RPM; 5,400 RPM soon followed and became a standard for many years. From there speeds have steadily marched upwards. Usually, faster PC hard disk speeds "debut" on SCSI drives that are used in higher-performance applications, and then filter down to IDE/ATA a few years later. At one time 7,200 RPM spindles were only found on top-of-the-line SCSI drives; they are now being used in consumer IDE/ATA disks sold at retail while SCSI has moved on to loftier heights. This table shows the most common PC spindle speeds, their associated average rotational latency, and their typical applications as of early 2000:

Spindle Speed (RPM)

Average Latency (Half Rotation) (ms)

Typical Current Applications

3,600

8.3

Former standard, now obsolete

4,200

7.1

Laptops

4,500

6.7

IBM Microdrive, laptops

4,900

6.1

Laptops

5,200

5.8

Obsolete

5,400

5.6

Low-end  IDE/ATA, laptops

7,200

4.2

High-end IDE/ATA, Low-end SCSI

10,000

3.0

High-end SCSI

12,000

2.5

High-end SCSI

15,000

2.0

Top-of-the-line SCSI

Note: Hard disks for laptops and specialty applications come in a wide variety of spindle speeds, even beyond the several speeds listed above. I have not exhaustively researched and listed these here.

Increasing spindle motor speed creates many design challenges, particularly aimed at keeping vibration and heat under control. As discussed here, when the motor spins faster these become more of an issue; some high-end drives have very serious heat, vibration and noise problems that require special mounting and cooling work to allow them to run without problems. To some extent, there is a tradeoff between spindle speed, and the heat and noise issue. Engineers generally focus on keeping these matters under control, and usually improve them significantly after the first generation of drives at any given spindle speed. However, in some applications, using a slower and quieter drive can make sense.

Next: Continuous Power vs. Peak Power at Spin-Up


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