Hardware Reference
In-Depth Information
speeds ranging from 3,600 rpm to 15,000 rpm (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
diagnostics 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 measure the HDD's rotational speed; this measurement can
be made only with sophisticated test equipment. Don't be alarmed if some diagnostics program tells
you that your drive is spinning at an incorrect speed; most likely, the program is wrong, not the drive.
Platter rotation and timing information is not provided through the hard disk controller interface. In
the past, software could give approximate rotational speed estimates by performing multiple sector
read requests and timing them, but this was valid 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
calculation 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 internal hub spindle motor, the manufacturer can stack more platters in the drive because the
spindle motor takes up no vertical space.
Note
Spindle motors, particularly on the larger form-factor drives, can consume a great deal of 12-
volt power. Most drives require two to three times the normal operating power when the motor
first spins the platters. This heavy draw lasts only a few seconds or until the drive platters
reach operating speed. If you have more than one drive, you should try to sequence the start of
the spindle motors so the power supply does not have to provide such a large load to all the
drives at the same time. Most SCSI, SAS, and some ATA drives have a delayed spindle-motor
start feature.
Traditionally, spindle motors have used ball bearings in their design, but limitations in their
performance have now caused drive manufacturers 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 side-to-side play in the bearings. Even though that might seem small, with the ever-
increasing density of modern drives, it has become a problem. This runout allows the platters to move
randomly that distance from side to side, which causes the tracks to wobble under the heads.
Additionally, the runout plus 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
fluid dynamic bearing
, which uses a highly viscous
lubricating fluid between the spindle and sleeve in the motor. This fluid 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. The
first drives on the market to use fluid dynamic bearings were advanced drives designed for high
spindle speeds, high areal densities, or low noise. Over the past few years, fluid dynamic bearings
have become standard issue in most hard drives.
