Computer hard disk drives are storage devices for digital data. The digital data is stored on high-speed rotating magnetic surfaces. They are non-volatile and use the random access technique to access data stored on them. It consists of fast rotating discs which have magnetic surfaces onto which data is stored. They are permanently mounted on the computers and cannot be easily detached like floppy disks. It is known as hard disk drive as both the disk and the drive are contained inside the same unit. The costs of hard disk drives have fallen in the last few years and their physical size has also reduced drastically as the capacity increases. The hard disk drives are the dominant devices used for secondary storage of data. There are several types of hard disc drives. However, they can be broadly classified as either external hard disk drives or internal hard disk drives. They can be further classified depending on the method of attachment onto the computer system. These classifications include: USB hard disk (connection via USB port), SATA hard disk (connection via SATA slots), and IDE hard disks (connections via the IDE slots). The performance of a hard drive depends on the number of revolutions the platters can make in a minute (RPM). When the number of RPM is higher then the speed with which data can be read from the hard drives is also likely to be higher. This means that the performance of a hard drive depends entirely on the number of revolutions that the platters can make in a minute. (Christensen, 1997)
Functions of a hard disk drive
This is a permanent storage location for data in a computer. The operating system and other computer software are installed on the hard disk. The hard disk drives store more than floppy drives and accessing the stored data can be easily and faster. (Hayes, 2002)
2. Components of the hard disk and the functions of each part
A hard disk is divided into various parts which have specific functions to perform. They include:
i. Electric motors
a. Disk motor (Spindle motor) – Used for spinning the disks. It has an external rotor which is attached to the disks. Stator windings are also fixed at this point.
b. Actuator – used to position the read/write head assembly across the rotating disks.
ii. Connectors – A hard disk drive has two types of connectors which are used for powering the drive and the other connector used for exchanging data with the computer (also known as interface). The interfaces which are commonly used are ATA or SATA. ATA is also called PATA (Parallel ATA) and is slowly being replaced with SATA (Serial ATA). However, the use of SCSI is also on the rise but it is commonly found on servers and not the ordinary PCs which are in use nowadays.
iii. Logic board – All circuitry which are charged with the responsibility of controlling the hard drive are found here. Most hard disc drives have about three to four big ICs on the logic board which have different functions.
a. The biggest circuit is also referred to as the controller and is in charge of all the operations of the hard drive like exchanging data, controlling the operations of the motors and issuing write or read instructions.
b. Flash-ROM circuit – This is the location of the hard drive firmware which is a program stored inside a Read Only Memory (ROM) that the controller executes
c. Motor driver – This is meant for driving all the motors on the hard drive. It is located between the controller and the motors
iv. RAM – It’s also called a buffer and is used for controlling the execution of programs in a hard drive. A bigger RAM results into a faster hard drive and vice versa.
v. Read/Write Heads – They form an interface between the magnetic media where storage of data is done and the electronic component of the hard disk. They are used to convert information from bits to magnetic pulses for storage or from magnetic pulses to bits during a read operation.
vi. A hard disk platter – This is a circular disk on which is used to store the magnetic data. A platter is capable of storing information on both sides.
vii. Base Castings – They provide the platform for attaching all the other drive components of a hard disk.
viii. Crash Stops – They are used to restrain the HSA from moving beyond the prescribed travel zone
ix. Filters – They are used to filter out airborne particles during the drive operation.
x. HGA – They provide the point of attachment and suspension for the sliders.
xi. HSA – They perform the complete assembly for mounting sliders to the actuator during a drive operation.
xii. Media – They provide the storage surfaces.
xiii. Spacers – They are used to control the disk to disk spacing of the media track.
xiv. Tape seals – They are used to seal the gap between the top cover and the base casting.
xv. Top clamps – They are used for holding the media and disk spacer rings in place on the motor hub.
xvi. Top cover – They are used for covering the drive in order to eliminate external particles from accessing the drive
xvii. VCM – They are used to provide the space where actuator coil is moved to control the positioning of the slider on the media.
3. Materials for making the components
a) Read/write head
It is made up using thin film techniques with hard materials which can scratch through the protective layers. Nanotechnology is used here to make the smaller particles which are used for making the heads. The use of nanotechnology has helped produce materials which are hard enough and also small enough not to cause any damage to the magnetic films. (Christensen, 1997)
b) Spindle motor
There is need for a high speed spindle motor. The materials used for making the spindle and the spindle motor include; cast iron, iron, stainless steel, brass and copper wires. The materials are light in weight and this ensures that the overall size and weight of the hard disk is not affected by the fixation of the motor. When mounting the spindle motor on to the hard disc, split-clamp mounting is used. This has several advantages over other kinds of mounting techniques. This method of mounting allows for easy rotational adjustments of the spindle motor. It also makes it easy to remove and mount the spindle.
The platter has a magnetic surface which is divided into some small sub-regions which represent a single binary unit. The magnetic grains are used here as opposed to continuous magnetic medium so as to reduce the space needed for a magnetic region. The platters are made using aluminum or glass and earthenware. There is a protective carbon-based overcoat which is deposited on top of these materials to bring about the smooth surface and also offer protection of the inner magnetic surfaces. 
Platter Substrate Materials
Substrate supports the media layer. This material must be stiff, magnetically inert, cheap, readily available, light in weight and easy to work with. Aluminum meets all these criteria and has been widely used to manufacture the substrate material. The platters also need to be very smooth and flat. However, aluminum is not very smooth and alternatives to this have been proposed as the use of aluminum is likely to be ineffective especially when operating at very high spinning speeds. Glass and other composites made with glass are so far the best alternative to aluminum.
The glass platters have several advantages over the aluminum platters. They include:
i. Thinner platters – Glass has more enhanced rigidity thus allowing for the manufacture of thinner platters. This also allows for more platters to be used on a single drive without a corresponding increase in physical size. The thinner platters also have a lighter weight which helps in reducing the spindle motor requirements.
ii. Higher quality – The glass platters are smoother and flatter than aluminum. This helps in improving the reliability of hard disk and resulting into a faster spindle speed without destroying the read/write heads.
iii. Rigidity – Glass is more rigid than aluminum. This helps in reducing noise and vibrations especially when the drive is to operate at high speeds.
iv. Thermal stability – They offer a more thermal stability as they expand lesser than aluminum when heated.
The main disadvantage of glass is fragility especially when it is very thin. This is not supposed to be an issue as proper care has to be taken on the hard drives even when using aluminum platters. Some companies are trying the use of glass-ceramic platters which will reduce the problem of cracking though it has not been in use for long. (Schmid & Achim, 2008)
d) Base Castings – It is made up of pure aluminum or aluminum alloys. Bare aluminum is not preferred as they have a relatively rougher surface which is difficult to clean. An alloy of aluminum having a smoother is preferred.
e) Crash Stops – They are made form injection molded plastic piece with an elastomeric bumper and sometimes a permanent magnetic latch. Their small size makes them not be a major contamination source. A permanent magnetic latch is not preferred as there is a likelihood of magnetic contamination taking place especially during the manufacturing process. The elastomeric bumper has a small size and is not a major source of contamination thus making them the best alternative to be used in the manufacturing of crash stops.
f) Filters – They are made using matted synthetic fibers (from glass or carbon) or granules. The fibers are preferred as the only problem they pose is the difficulty during manufacturing when the fibers shed from the edges but the problem is not very common as opposed to the problems experienced when using the granules.
g) HGA –. They are made up of a combination of ceramic or ferrite sliders, copper or gold wires, Teflon wire or plastic insulation, stainless steel. The combination of at least two of the above materials is good but the use of stainless steel results into the suspension components being loaded with stainless steel which are very difficult to remove thus bringing about the problem of contamination. Care should therefore be taken to avoid the use of stainless steel.
h) Spacers – They are made of aluminum or stainless steel or ceramic. In this case we are going to use the Machined stainless steel which is much cleaner than stamped stainless steel. The machined stainless steel is also preferred to ceramic spacers as the ceramic spacers are also difficult to clean and are brittle (they can easily break)
i) Tape seals – They are made of plastic tape or paper tape which is coated with an adhesive making it easy to stick. Paper tapes are preferred to plastic tapes as the plastic tapes can easily generate high voltages when stripped.
j) Top clamps – They are made of aluminum or stainless steel. Machined aluminum is preferred as it is cleaner than the stainless steel and more cleanable. However, it can gall during installation of screws incase the drive needs to be worked on. This is not a big issue as reworking on the drive is not always a common practice.
k) Top cover – They are made using aluminum, stainless steel or plastic. A combination of the three materials is preferred as there is no any single material which is very easy to work with. A combination of the three gives a good package for providing hard disk covers.
l) VCM – They are made using nickel plated stainless steel or rare earth permanent magnets. Using nickel plated stainless steel is preferable as they are not likely to break and have less chances of experiencing magnetic contamination.
4. Summary and conclusions
The physical size of the hard disk has decreased over the last few years. The decrease in size has also come with an increase in speed and a decrease in weight. The materials used for making the hard disk should be robust enough to with stand high temperatures and high rotation speed. Care should therefore be taken when choosing the materials used for making different components of the hard disk so that an increase in speed does not result into an increase in physical size or the overall weight of the hard disk.
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