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Theory of machines
Prepared by :
Urvesh Dungarani 140050119506
Guided by :
Sometimes the brakes are not only used to retard or
stop the motion but may also be used to measure
power absorbed during braking. Such brakes which
have a provision to measure absorbed power are called
Thus dynamometer is a device which is used to
measure the frictional resistance. By knowing
frictional resistance we can determine torque
transmitted and hence the power of engine.
Dynamometer can be used either to measure
force, torque or power.
In this type of dynamometer, the work done by
engine is absorbed by frictional resistance of
brake is converted into heat during process of
These dynamometers can be used for
measurement of moderate powers only.
Various types of dynamometer are discussed
It is simplest form of an
consists of two wooden
blocks clamped together
on a revolving brake drum whose power is to be
The bolts and nuts are used to clamp the blocks.
In order to adjust the pressure on pulley to control its
speed, helical spring is provided between nut and
Friction between blocks and pulley tends to
rotate the blocks in the direction of rotation of
drum. Suspended weight W at the end of lever
prevents this tendency. Applied weight is so
adjusted that the lever remains in horizontal
position by tightening the nuts to get the
constant engine speed.
With the help of above equation, we can
calculate the brake power.
Brake Power P = 2π NT
where, T = Weight applied (W) × distance (l)
N = speed of shaft
It consists of one, two or more ropes wrapped over
rim of a pulley keyed to shaft of engine. In order to
prevent slipping of rope over pulley, U shaped
wooden blocks are placed at certain intervals on
circumference of pulley.
The upper end of rope is attached to a spring
balance while the lower end carries a dead weight
If high power is produced, then heat is generated
due to friction between rope and pulley or drum.
To prevent this, water is provided to cool the
W = dead weight or suspended
S = spring balance reading
D = diameter of pulley or drum
N = speed of engine
d = diameter of rope
(W-S) = net brake load
If diameter of rope is neglected,
Brake power, P = (W-S)πDN/60 Watt
consists of two elements
called rotating disc and
The rotating disc is
attached to engine shaft
whose power is to be
measured and revolves inside the casing.
The casing and rotating disk have semi
elliptical recesses forming a chamber through
which water is flowing.
When water flow through chamber with high
pressure the vertices are set up in the chamber
which tends to rotate the casing also.
This tendency of rotation of casing is restricted
by braking system that measure the torque and
the power of engine.
It consists of endless or continuous belt run over the driving
pulley A, driven pulley B and intermediate pulleys C and D.
The driven pulley A is rigidly fixed to shaft of an engine
whose power is to be measured. The driven pulley B is fixed
to another shaft to which power is to be transmitted.
The intermediate pulleys C and D rotate on a pin fixed to a
lever having fulcrum at midpoint of two pulleys.
A balancing weight is provided in lever to initially keep it in
equilibrium. Two stops S1 and S2 are used to limit the
motion of lever.
The weight of suspended mass at one end of lever
the difference in tensions of tight and slack sides of
Power of the engine :
P = (T1-T2)×πDN/60
The working of belt transmission
dynamometer is shown in the video…
Epicyclic train dynamometer measures power
while it is being transmitted from driving to driven
It consists of simple gear train .
The lever is pivoted about common axis of driving
and driven shaft.
The power is transmitted from one gear to another
through pinion. A balancing weight is adjusted to
one end of lever to keep it in equilibrium at rest.
Two stops are used to limit the movement of lever.
Power transmitted = WLR×π/(60×a)
where, W = dead weight
L = lever arm
N = speed of engine
When power is transmitted along a shaft, the driving end
shifts through a small angle relative to the driven end.
Torque transmitted is directly proportional to the angle of
twist. Therefore, a torsion dynamometer works on principle
of angle of twist.
Torsion dynamometer can measure large powers as in case
of power transmitted along the propeller shaft of a turbine
or a motor vessel.
Torsion equation is :
where θ = Angle of twist in radians, and
J = Polar moment of inertia of the shaft.
It consists of two discs A and B
fixed on a shaft at convenient
distance. Behind disc A powerful
electric lamp is fixed on bearing
To the right of disc B an eye piece
is fitted on shaft bearing.
When torque or power is
transmitted through shaft, it
twists so that disc B lags behind disc A.
In this case, all four do not remain in same line,
hence the light of lamp is not visible through
the eye piece as shown in fig.
The eye piece is moved circumferentially so
that all four are again in same line and light of
lamp is visible through the eyepiece as shown
The angle of twist can be measured up to
1/100th of a degree.