SCREW AND SCREW JACK

SCREW: -

Screw is one of the most important machines. It is used to hold different parts of machines together. It has waste applications in our daily life plus in industries. It is used in every type of device.

CONSTRUCTION AND WORKING: -

It simply consists of a threaded rod with a head known as “Screw head”. It has a number of threads. The perpendicular distance between two adjacent threads is known as pitch of screw. The thread of screw can be regarded as a continuous inclined plane wrapped round a cylinder of radius d .

MECHANICAL ADVANTAGE: -

If we apply an effort ‘P’ on the head of screw then it turns one revolution and at the same time the screw moves forward in to the wood or wall through a distance equal to its pitch “h”. The effort ‘P’ moves through a distance 2pd. The screw remains in the wood due to frictional forces between the screw and the wood. A large amount of energy changes in to heat energy during the process of screwing.

Let us assume an ideal case when there is no loss of energy then; in this condition

Output = input

P x 2pd = W x h

W/P = 2 p d/h

Hence, the mechanical advantage of the screw will be.

M.A. = 2pd/h

The mechanical advantage of the screw depends upon the following factors.

·         PITCH: -

In order to increase mechanical advantage of screw we must use a screw of small pitch.

·         RADIUS OF SCREW: -

Larger is the radius of screw head, greater is the mechanical advantage.

SCREW JACK: -

A screw jack is a simple machine. It is used to lift cars or heavy automobiles. It consists of a long screw rod which passes through a threaded block B and a handle. The distance between two consecutive threads is known as pitch of screw.

MECHANICAL ADVANTAGE: -

When an effort is applied to the handle, the effort moves in a circle of radius “d” while “d” is the length of the rod and the block “B” moves up equal to the pitch of the screw jack. If the handle is turned through one complete revolution in a circle of radius “d” the effort moves through a distance 2pd and consequently the load is raised through a height h. Hence, the mechanical advantage is given by:

M.A. = 2pd/h

M.A. = Distance through which the effort is moved/height through which the load is raised

We know that the pitch of the screw is very small as compared to the length of the rod, so the mechanical advantage should be very large. Due to frictional force between the different parts of screw jack, the efficiency is less than one. It is due to the reason that lot heat energy is used for overcoming the frictional forces.

It should be remembered that in case of screw jack friction is a necessary part of operation, because in the absence of friction, it would unwind at once when applied force is removed.