Page 94 - CITS - WCS - Mechanical

WORKSHOP SCIENCE - CITS

Efficiency and Their Relations: The efficiency output of a machine is simply the output work divided by the
input work, and is usually multiplied by 100 so that it is expressed as a percent. A machine cannot be 100 per
cent efficient because the output of a machine is always less than the input. A certain amount of work done on a
machine is lost to overcome friction and to lift some moving parts of the machine. If a machine is supplied with 100
joules of energy and it manages to convert 80 joules into useful work, then the efficiency of the machine would
be (80/100)x 100%= 80% . The remaining 20% of the energy is wasted, often as heat due to friction or sound.

Hence, the relationship between mechanical advantage, velocity ratio, and efficiency isղ=MA VR × 100 %.
Mechanical advantage is the ratio of load to effort of machine or we can say it is the ratio of output of machine to
the input of the machine.

Velocity ratio is also defined as velocity of effort to the velocity of load.
Let a machine overcome a load ‘L’ by the application of an effort E. In time ‘t’. Let the displacement of effort of dE
and displacement of load be d1

Thus, the mechanical advantage of the machine is equal to the product of its efficiency and Velocity ratio.
Note:
Mechanical Advantage (M.A.), efficiency (η)and velocity ratio are interconnected with each other. Basically
mechanical Advantage of machine is equal to the product of its efficiency and velocity Ratio. Therefore,
M.A.=V.R.×η.

Work input = Effort x displacement of effort = E x dE
Work output = Load x displacement of load = L x dL
Work output
Efficiency (η) =
Work input
L x dL
= ------------ 1
ExdE
Load (L)
Also, Mechanical Advantage, (M.A) = -----------2
Effort (E)
dE
And velocity Ratio (V.R)= ----------3
-dL
By equation 1,2 and 3
M.A.
Efficiency (η) =
V.R.
M.A. = V.R. x η

Working Principles of Inclined Plane:
Inclined plane, simple machine consisting of a sloping surface, used for raising heavy bodies. The force required
to move an object up the incline is less than the weight being raised, discounting friction. The steeper the slope,
or incline, the more nearly the required force approaches the actual weight.

81

CITS : WCS - Mechanical - Exercise 8