Hard Disk Drive Head Failure

Hard Disk Drive Mechanical Failure.

This article concerns the mechanical design of high capacity hard disk drives and their propensity to fail.

Hard Drive Read Write Head Sliders.

In a hard disk drive  the actuator arm and read write head slider are used to reach out over the magnetic surface of the hard disk platter surface. This actuator arm carries transducers that transform magnetic energy into electrical signals and vice versa. The assembly is called the read write head gimbal and in operation transfers the signals between the controlling electronics and the disk platter magnetic storage media. The components consisting of the disks, head gimbal, spindle, housing, and the other parts together make up the hard drive assembly (HDA).

Hard Disk Drive Read Write Head Operation.

In operation the hard disk platter is rotated at a set speed, typically 7200 rpm, using  a spindle motor. There are concentric magnetic channels for recording data spaced at intervals across the disk surface and these channels are known as tracks. When a request to read data is received, the hard disk firmware communicates the data and track coordinates to the actuator which then aligns the head/arm, over the specific data/track location.

Hard Disk Drive Data Capacity.

The perpetual requirement for increased data storage has led disk drive designers to attempt to cram ever more volumes of data into the available physical format constraints. Vibration, noise specifications, temperature, power requirements, and other factors however, limit the number of platters that can be economically introduced into a single drive. As a result hard disk drive technology developments have concentrated on increasing the areal density of data stored on each disk surface and the rotational speed of the platters. These imperatives mean that to create a viable signal pick-up from the read write heads, the head slider needs to be positioned more closely to the platter media surface.

Hard Disk Drive Head Design.

The read write heads held by the gimbal are lightly spring loaded perpendicular to the media surface. As the platter rotates air is dragged through channels in the head slider creating an air bearing system (ABS) causing the heads to fly above the platter surface, this separation is called the “fly height”. Maintaining a viable fly height is a complex function of air density, spring preload tension, relative speed and slider design. In manufacture head-disk combinations produce variable fly heights. Mechanical tolerances, spring tensions, air pressure, aerodynamics etc., can move the air bearing surface closer or further from the media surface and produce variables in signal strength. With fly height design in single figure nanometre distances the probability of unintended contact between the head components and the moving media surface is greatly increased

Hard Disk Drive Head Failure.

Today’s hard disk drive read write head assemblies must therefore function reliably at ultra-low nanometre fly heights. To militate against the effects of inevitable contact between components;   hard drive platter surfaces are lubricated and head sliders have specially designed control dams. These control dams are intended to divert excess lubrication away from the pick-up in order to avoid Read Write head failure that may occur as a result of the lubricant contaminating the air bearing or the transducer elements of the head. The presence of lubrication on the surface of the platters and its migration to the head slider can adversely affect the fly height of the read write heads. Controlling lubrication migration and redirecting the lubrication flow into a channel on the slider and away from the actual transducer pick-up can reduce the effects of  head crashes, however when the incidence  of contacts or crashes becomes excessive due to a reduced fly height the number of bad sectors and SMART errors is  greatly increased and will generally result in catastrophic hard disk drive failure.