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Screw Jack

What is a screw jack? How does it work? Which principle of physics does it use? How to calculate effort required and efficiency of a screw jack?

The principle, on which a screw jack works is similar to that of an inclined plane. If one complete turn of a screw thread is imagined to be unwound, from the body of the screw jack and developed, it will form an inclined plane, as shown in Fig. 1.22.

From the geometry of the figure,
Screw Jack
In a screw jack, the effort (P) required at the circumference of screw is same as discussed for inclined plane, i.e.

For raising the load, P = W tan (α + ϕ)
and for lowering the load, P = W tan (α - ϕ)


Notes:
1. When friction is neglected, then ϕ = 0. In that case

2. The efficiency of a screw jack is given by
The efficiency of a screw jack
3. The efficiency of a screw jack is maximum, when helix angle,
Maximum efficiency of a screw jack
and the maximum efficiency is given by
Maximum efficiency of a screw jack

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