| HARD DRIVE
The hard drive is an external storage device consisting of one
or more disks driven by one servo motor. The medium of the hard
is flat, circular, rigid aluminum coated with a magnetic type
of material. For each disk there is at least two or more read/write
heads. The read/write head flies above the surface of each disk
at an air gap of 0.002 inches. The assemble which houses the
head is called a slipper. The slipper is designed with the aerodynamics
of an airplane's wings. The aerodynamic design of the slipper
gives it the ability to float right above the boundary layer
of air created from the rotating motion of the disk. As an object
rotates there exist various degrees of air velocity. Near the
surface of the rotating object the air velocity is its greatest
and least at a greater distance. If the head hit the surface
(called a crash) it can destroy all the data at this point and
damage the disk at that point permanently. When a track is destroyed
permanently, it is labeled bad.
A single disk in a hard drive can store between 200 to 700
megabits-per-square inch and rotates between 2400 to 6000 RPM's.
Each disk has tracks and the alignment of a track to corresponding
tracks of other disks of the hard drive creates what is called
a cylinder. Tracks are divided into sectors and a sector is
divided into blocks of data (see fig). In order for data to
be accessed on each disk there is a controller to direct the
actuator. The actuator is a servomechanism which moves and positions
the read/write head from track to track. The controller specifies
the location (track, sector, block), which the read/write head
is to act upon and the actuator positions the head at that location.
The time it takes to carry out Diagram of hard drivethis operation
is called access time. Access time (9-14ms) is a factor of three
operations: the time it takes the head to move to the specified
track (seek time), the RPM of the disk (the time it takes the
disk to rotate to the specified sector and block under the head),
and the time it takes to read the data. The access time increase
if the head is parked. A parked head is considered safe from
doing damage to the disk or itself. A head is normally parked
before moving the equipment. If during a seek operation the
head strikes the surface of the disk (creating bouncing), this
will add (settling time) to the seek time.
FLOPPY DISK DRIVE
The floppy disk drive was developed by IBM in the 1960's. The
disk is made out of plastic which is then coated with a magnetic
material. Among common floppy disks, the storage spaces are:
360k, 80k, 1.2m, 1.4m, and 2m of data. Their sizes are the 5.25
and the 3.5 inches disk. Floppy disks are a removable storage
medium. They are soft-sectoring. The storage medium and enclosure
of a floppy have an index hole to mark the start of a track.
In a read/write operation the head is positioned over the desired
track and pushed against the disk medium. A cut out slot is
provided in the enclosure of the disk for the read/write head
to access the storage medium. The disk rotates at 360 RPM's
and no greater do to friction heat at greater RPM's.
Soft-sectored disk: a process in which tracks and sectoring
are software performed. Floppy disks are soft-sectored.
Exchangeable disk pack: a removable and replaced disk of hard
disk system.
Hard-sectored disk: a physical method of identifying the start
of a sector. A hole in the storage medium is one mean.
Moving-head disk: a single read/write head scans the surface
of a disk by stepping from track to track (used in most PC's
today).
Fixed-head disk: each track has a separate read/write position
over it. The enclosure is normally sealed and pressurized with
inert gas to keep out contamination. Fixed head disk eliminates
seek time.
GEAR (A)=80 TEETH; GEAR (B)=40 TEETH; WHAT IS THE GEAR RATIO
BETWEEN THEM?
To obtain the gear ratio divide the small value into the larger
value. The result is 80/40=2. The ratio is 2 to 1. Gear (A)
being 2 times larger.
 IF
THE GEARS WERE MESHED, WHAT WOULD BE THE RPM OF GEAR (A) IF
GEAR (B) WERE ROTATING AT 50 RPM.
In a mesh configuration the smaller gear (B) always rotate
at a greater RPM than the larger gear (A). When the RPM is given
for the smaller gear divide it's value by the ratio value (2).
The results are (50/2=25). When the RPM is given for the larger
gear (A) multiply it's value by the ration value (2). The results
are (2*25=50).
IN A SITUATION IN WHICH THE RADIUS, NUMBER OF TEETH, AND RPM
ARE GIVEN FOR ONE GEAR AND THE RATIO OF THE TWO GEARS, THE RADIUS,
NUMBER TEETH, RPM AND CIRCUMFERENCE CAN BE FOUND FOR THE OTHER
GEAR. THE RADIUS OF GEAR (B) IS 10CM, IT ROTATES AT 1000RPM,
IT HAS 500 TEETH AND THE RATIO BETWEEN THE TWO GEARS IS 3 TO
1. WHAT IS THE CIRCUMFERENCE OF GEAR (A)?
The circumference equal C=2*pi*r, the area of a circle is equal
A=pi*r^2 and the volume of sphere is equal to V=1/3*pi*r^3.
The circumference of gear (B) is equal 2*pi*10=2*3.14*10=62.80CM.
Multiplying the ratio (3) time the circumference of gear (B)
will yield the circumference of gear (A). The result is 62.80*3=188.40CM.
Multiplier the number of teeth of gear (B) time the ratio (3)
will yield the number of teeth for gear (A). The result is 500*3=1500
teeth. Divide the RPM of gear (B) by the ratio (3) will yield
the RPM of gear (A). The result is 1000/3=333.333 RPM.
Gear series
When gears teeth are meshed in a series all the odd gears rotate
in the same direction and all the even gears rotate in the same
directions. The gears ratio are determine between each pair
of gears. |
