Screw Jack

A Screw Jack consist of a cylinder with a spiral ridge runs round it. The spiral is called the thread (T) and the distance between two adjacent threads is called the pitch (p) of the screw.
Consider the following diagram.

Mechanical advantage of the screw jack

                           M.A = Load
                                      Effort

Velocity ratio (V.R)

                           V.R = Circumference of circle of radius (R)
                                                      Pitch of screw

                           V.R = 2πr
                                       P 

Example 1

 A screw jack with a pitch of 0.2cm and a handle of length 50cm is used to lift a car of weight 1.2 x 10⁴N. If the efficiency of the screw is 30%, find:

a)    The velocity ratio and mechanical Advantage of the machine

b)   The effort required to raise the car

Solution

          Date given.

          Pitch (p) = 0.2cm
          Radius (r) = 50cm
          Load (L) = 1.2 x10⁴N
          Efficiency of jack (ɛ) =30%
          Velocity ratio (V.R) =?
          Mechanical advantage (M.A) =?
          Effort (E)=?

           From

a)    Velocity ratio (V.R) =?

The Velocity ratio = 1570

From

The Mechanical advantage = 471

b)    Effort (E) =?

From 

The effort required to raise the car = 25 .5 N