Hard Drive Mechanics, platters, read/write heads, the actuator & spindle.

hard drive mechanics guide

Welcome to our guide to hard drive mechanics.

In this section we will look inside a hard drive and find out exactly what makes it tick. We will be looking at platters, read/write heads, the spindle, the actuator and its arms.

Inside the hard drive

Hard Drive Mechanics
Hard Drive Mechanics
The mechanical technology used inside hard drives hasn't changed that much over the years, in fact the basic mechanics in today's hard drives are much the same as they were 10 or 15 years ago.

In fig 1.1 to the left you can see the internal mechanics of a hard drive:
  • A - Platter/s
  • B - Read/Write Head/s (and slider)
  • C - Actuator Arm/s
  • D - Actuator
  • E - Spindle

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Platters (A in fig 1.1)
Most Platters are made of either an aluminium/alloy or glass/ceramic composite and are like glass to the touch. There is always more than one platter inside modern hard drives and each platter is double sided (with some exceptions).

In modern hard drives the platters spin at 5400RPM and above. The distance (flying height) between the read/write heads and the platter is around 50nm (0.05, a human hair is around 100).

You should never open your hard drive's sealed casing (unless you are in a controlled environment, such as a clean room) as a single speck of dust is bigger than the gap between the platters and read/write heads. If dust/debris does enter the drive, it can have catastrophic results when the heads crash into it.

The platters have a magnetic coating which allows them to store the data magnetically (more on how the data is stored later).

Read/Write Head/s (B in fig 1.1)
These are attached to the end of each actuator arm and as the name suggests they are responsible for reading and writing data to/from the platters. There is usually a set of read/write heads on each side of each platter (it is common to have hard drives with an odd number of heads).
Actuator Arm/s (C in fig 1.1)
Actuator arms move across the platters to position the read/write heads in the right place to read/write the data required. The actuator arms are manufactured so they have a spring action causing them to close (if no platter present). This ensures they do not move away from the platter with time, as the drive winds up to full speed, the air lifts the sliders (and therefore the heads) around 50nm (0.05) above the platters.
Actuator (D in fig 1.1)
This refers to the device that physically moves the actuator arms. Years ago they used to use stepper motors for controlling the actuator arms, but the problem with stepper motors in applications such as this, is that over time with a lot of use, they lose their integrity and can cause data corruption.

Another problem with stepper motors is that when they get hot (which hard drives do) they lose their precision. For example, data that is saved with a cool drive can sometimes be unavailable when the motor gets hot due to this displacement effect.
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Stepper motors also required the read/write heads to be parked (moved to a data-free area of the drive) manually. If the read/write heads were not parked and the drive suffered any shock (such as transportation), the data could be corrupted.

Nowadays most (if not all) hard drives use a voice coil instead of a stepper motor, unlike stepper motors, voice coils are linear and don't suffer from the same integrity problems.

Because voice coils are magnetically driven by electrical currents there is no mechanical wear and tear. Voice coils also negate the need for parking the heads before shutting off the power, as when they lose power the heads return to the parked position automatically.

Spindle (E in fig 1.1)
The spindle revolves the platters, when you see an RPM (Revolutions Per Minute) specification of a hard drive it is referring to the spindle speed.


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