Head Stack Assembly

The image below shows a the head stack assembly and other related parts. Each part is identified and described.

1 Actuator Magnetic Flux
This forms a vital part of the Voice Coil Motor (2 below) and consists of magnet pairs (top and bottom) that encapsulate the Voice Coil Motor (VCM). When electricity is applied to the coil, it functions in a similar way as a standard electric motor and as the coil 'rotates' it moves the entire head stack assembly to position it over the platters.
2 Voice Coil Motor
When the drive needs to read or write data, first of all it needs to position the heads over the correct area of the drive. To do this, power is applied to the coil that is proportional to the position on the platters where the required data is located on the disk. As the voltage is applied to the coil, it 'rotates' the whole assembly on its axis (3) and positions the heads over the correct area on the drive, waits for the correct sectors to pass under the head and then reads the data. Modern drives use a feedback mechanism so that minute changes in voltage can position the heads very precisely.
3 Actuator Axis
This consists of the shaft and the actuator-bearing and is the axis on which the actuator arm 'rotates' when power is applied to the VCM.
4 Actuator Arm
The far end of the arms is where the head suspension arms are attached. The arms are made to as long as practical to make the largest arc for the read write heads making maximum use of the available platter surface.
5 Head Suspension Arm
Made from spring steel or nickel plated beryllium-copper so as to be flexible to allow them to take off and land as the power is applied to the motor. The heads are glued or held in place under the suspension arm that is designed to maintain the heads in the best position for reading and writing data.
6 Parking Ramp
Technically speaking this is not a part of the head stack assembly. This mechanism is designed so the heads are not hovering over the platters when the drive is idling and when the hard disk is powered off. This type of parking technology is becoming more common and very few modern drives have heads that park (or land) on the platters when not in use.
7 HSA Connector
Connects the internal head preamplifier circuit and VCM to the external electronics board.
8 Head Preamplifier Circuit
This can just about be seen on the edge of the assembly. Its function is to amplify the signal from the read heads to remove 'noise' so that the data can be clearly identified. A failure in the Pre-Amp circuit is almost impossible to repair so an entire matching head stack assembly is used to replace the failed one in order to recover data.

When the heads fail on a hard disk, the symptoms usually consist of clicking or ticking noises. More often than not, it is one or more physical read-write heads or the pre-amp circuit that have failed. In either of these cases there is a better than 90% chance of data recovery.

The function of the VCM and Heads

The VCM is the head positioning system and is controlled by components on the main circuit board. As each seek is made to read specific data on the drive, the processor on the circuit board calculates the exact position on the platters to read the data. The processor then sends the necessary commands to the VCM controller to get it to set the heads in position ready to read the data. The mechanics of this operation is;

The coil is mounted on the actuator assembly in close proximity to a strong permanent magnet (1 above). When current is applied to the coil, an electromagnetic field is created that forces the actuator arm, and therefore the heads to move to a position according to the amount of current applied. By precise control of the voltage/current, the heads can be moved to the exact position needed ready to read or write the data.