The spindle motor also must be precisely controlled for speed. The platters in hard disk drives revolve at speeds ranging from 3600rpm to 15000rpm (60-250 revolutions per second) or more, and the motor has a control circuit with a feedback loop to monitor and control this speed precisely. Because the speed control must be automatic, hard drives do not have a motor-speed adjustment. Some diagostics programs claim to measure hard drive rotation speed, but all these programs do is estimate the rotational speed by the timing at which sectors pass under the heads.
There is actually no way for a program to meause the hard disk drive's rotational speed; this measurement can be made only with sophisticated test equipment. Don't be alarmed if some diagnostics program tells that your drive is spinning at an incorrect speed; most likely, the program is wrong, not the drive. Platter rotation and timeing information is not provided through the hard disk controller interface. In the past, software could give approximated rotational speed estimates by performing multiple sector read requests and timing them, but this was vaild only when all drives had the same number of sectors per track and spun at the same speed. Zoned-bit recording combined with the many various rotational speeds used by modern drives, not to mention built-in buffers and caches-means that these calcualtion estimates can't be performed accurately by software.
On most drives, the spindle motor is on the bottom of the drive, just below the sealed HDA. Many drives today, however have the spindle motor built directly into the platter hub inside the HDA. By using an intermal hub spindle motor, the manufacture can stack more platter in the drive because the spindle motor takes up no vertical space.
Traditionally, spindle motors have used ball bearings in their design, but limitations in their performance have now caused drive manufactures to look for alternatives. The main problem with ball bearings is that they have approximately 0.1 micro inch (millionths of an inch) of runout, which is lateral of modern drives, it has become a problem. This runout allows the platters to move randomly that displus the metal-to-metal contact nature of ball bearings allows an excessive amount of mechanical noise and vibration to be generated, and that is becoming a problem for drives that spin at higher speeds.
The solution is a new type of bearing called a fluld dynamic bearing, which uses a highly viscous lubrication flud between the spindle and sleeve in the motor. Thi flud serves to dampen vibrations and movement, allowing runout to be reduced to 0.01 micro inches or less. Fluid dynamic bearings also allow for better shock resistance, improved speed control, and reduced noise generation. Several of the more advanced drives on the market today already incorporated fluid dynamic bearings, especially those designed for very high spindle speeds, high areal densities, or low noise. Over the next few years, I expect to see fluid dynamic bearings become standard issue in most hard drives.
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