Hardware Reference
In-Depth Information
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
motorfirstspinstheplatters.Thisheavydrawlastsonlyafewsecondsoruntilthedriveplat-
ters 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 and some ATA drives have a delayed spindle-mo-
tor start feature.
Traditionally, spindle motors have used ball bearings in their design, but limitations in
theirperformancehavenowcauseddrivemanufacturerstolookforalternatives.Themain
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 con-
trol, 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.
Logic Boards
All HDDs have one or more logic boards mounted on them. The logic boards contain the
electronics that control the drive's spindle and head actuator systems and present data to
thecontrollerinsomeagreed-uponform.OnATAdrives,theboardsincludethecontroller
itself, whereas SCSI drives include the controller and the SCSI bus adapter circuit.
Many disk drive failures occur in the logic board, not in the mechanical assembly. (This
statement does not seem logical, but it is true.) Therefore, you sometimes can repair a
 
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