For procedures, see your mechanical laboratory manual.

Title: Screw Jack

Aim

1. To obtain the velocity ratio of the machine.
2. To plot graphs on a base of increasing load, illustrating the variation of
(a) effort
(b) effect on friction
(c) efficiency
3. To obtain an equation for the relation between load and effort and hence obtain a value for the limiting efficiency of the machine.

Theory

The screw jack is a device used for lifting heavy loads which are usually centrally loaded by apply smaller effort. It is a mechanical device that can increase the magnitude of an effort force.

A screw jack consists of a heavy-duty vertical screw with a load table mounted on its top, which screws into a threaded hole in a stationary support frame with a wide base resting on the ground. A rotating collar on the head of the screw has holes into which the handle, a metal bar, fits. When the handle is turned clockwise, the screw moves further out of the base, lifting the load resting on the load table. In order to support large load forces, the screw usually has either square threads or buttress threads.

The screw jack consists of two simple machines in series; the long operating handle serves as a lever whose output force turns the screw. So the mechanical advantage is increased by a longer handle as well as a finer screw thread. However, most screw jacks have large amounts of friction which increases the input force necessary, so the actual mechanical advantage is often only 30% to 50% of this figure.

The axial distance moved by the screw when it makes one complete revolution is known as the Lead of the screw.

The distance between two consecutive threads is called a Pitch. For a single-threaded screw,
Lead = Pitch
For a double-threaded screw
Lead = 2 x Pitch

Mechanical Advantage

It is the ratio of the load lifted "W" to the applied effort "P"
Where
M.A = W/P

Velocity Ratio

It is the ratio of the distance moved by the effort (y) to the distance moved by the load (x).
Where
V.R = y/x

In one complete revolution of the lever by effort P:
Distance traveled by effort = 2Ï€R (circumference of effort)
Distance traveled by the load = Pitch of screw jack
V.R = 2Ï€R/Pitch

Efficiency

It is the ratio of the output to the input. In the screw jack, it is the ratio of the M.A to the V.R
Where
Efficiency = M.A/V.R x 100

Pitch

It is the axial distance in one complete revolution
Frictional loss = effort (V.R – load)
Frictional loss = P (V.R – W)

Applications

1. linear motors
2. linear actuators
3. Mechanical lifts

References

1. Jack Screw, wikipedia.org
2. How a screw jack works, www.powerjack.com
3. Screw Jack, www.engineeringtoolbox.com