EXPERIMENT NO. 9
Aim: To find the Mechanical Advantage, Velocity Ratio and Efficiency of simple and
compound screw-jack.
Apparatus Required: Simple and compound screw jack, Load to be lifted (W), Weights or
Effort to be applied (P), Vernier caliper, Pan, Weight box.
Theory:
Screw Jack
It is a device used for lifting heavy loads which are usually centrally loaded by applying
smaller effort. It works on the principle of inclined plane. The device consists of a nut and
screw. The load is carried by screw head. The body consisting of a nut is fixed and screw is
rotated by means of a lever.
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 Pitch
of the screw. For single threaded screw Lead = Pitch, and for double threaded screw L = 2p
Mechanical Advantage
It is the ratio of weight lifted to effort applied.
M.A. = W/ P
Velocity Ratio
It is the ratio of distance moved by the effort (y) to the distance moved by the load (x).
V.R. = y/ x
In one complete revolution of the lever by effort P:
Distance traveled by effort = 2 π R
And, distance traveled by the load = p
Therefore, Velocity Ratio = 2 π R / p
Compound Screw Jack
In one complete revolution of the effort wheel, the distance moved by the
effort, y = π ( D + d )
But since it is a compound screw jack,
Therefore for the load to be lifted through a distance p, the no. of revolutions required by the
effort wheel = No. of teeth on the gear, N.
Therefore for p distance moved by the load, the distance moved by the effort =
π ( D + d )N
Therefore, Velocity ratio = π ( D + d )N / p
Mechanical Efficiency = M.A./ V.R.
=W/P
π ( D + d )N / p
= W. p
P π ( D + d )N
Procedure :
Put load on the jack and start applying efforts gradually and record the observations as
the load just moves.
Observations :
Let, Load Lifted = W
Effort Applied = P
Effort Wheel Diameter ( D) = 130 mm
Diameter of the rope (d ) = 5 mm
Pitch of the screw ( p ) = 2.5 mm
No. of teeth on the gear ( N ) = 40
Weight of the Pan ( w ) = 150 gm
S. No. W (kg )
1.
2.
3.
4.
5.
P( kg )
MA = W/P
VR = π(D+d)W/p
15
17
19
21
23
Precautions:
1. Lubricate the jack well before use.
2. Apply effort gently.
3. Note the effort readings as the load just moves.
Fig. 9.1 Simple Screw Jack
MA/VR =
Wp/Pπ(D+d)N