Learn to force Measure ment

1. Measurement of force
Force measurement can be classified into two basic categories:
1. Direct
2. Indirect

2. Direct Method
• Direct methods involves comparison of
gravitational force on the standard mass.
an unknown force with a known
• A force is exerted on a body of mass m due to the earth’s gravitational field,
represented by the following equation:
W = mg
m = standard mass
g = acceleration due to gravity
W = weight of the body

3. Analytical Balance / Equal arm balance
• An unknown force is directly compared with a known gravitationalforce.
• Comparison of masses is done by beam balance.
• It is sufficient to find only the magnitude, as the unknown force and the
gravitational force act in direction parallel to eachother.
• The working principle is illustrated schematically.

4. Analytical Balance / Equal arm balance
• The fig. shows the balance in an
unbalanced condition.
• dG = Distance between the fulcrum and the
centre of gravity pointCG.
• WB = Weight of the balance arms and
pointer.
• W1 & W2 = Two weights acting on either
side of the balance.
• When the two weights W1 and W2 are
equal, angle θ = 0.

5. Analytical Balance / Equal arm balance
• Deflection per unit unbalance gives a
measure of the sensitivity of the balance.
• The difference between the two weights, i.e.
W1 − W2, gives theunbalance.
Disadvantage
• Set of weights should be equal to the
maximum weight to be measured.

6. Platform Balance
• For measurement of large weights, a platform balance or multiple-leversystem is
preferred.
• In a platform balance, two smaller weights are used;
Wx = poise weight & Wy, = pan weight for measurement of W (Large weight)
• The initial zero balance is set by an adjustablecounterpoise.

7. Platform Balance

8. The multiplication ratio gives an indication of the weight that should be applied to
the pan in order to balance the weight on the platform.
Example
• If the multiplication factor is 100, then a weight of 1 kg applied on the pancan
balance a weight of 100 kg placed on the platform.

9. Load Cells
• In load cells, elastic members act as primary transducers and straingauges as
secondary transducers.
• A load cell is an indirect method of force measurement, i.e. Force or weight is
converted into an electrical signal.

10. Load Cells
• A load cell comprises four strain gauges; mounted at
90° to each other.
• Two are used for measuring the longitudinal strain &
the other two for measuring the transversestrain.
• Two gauges experience tensile stresses, the other two
are subjected to compressivestresses.
• At the no-load condition, resistance in all the four
gauges will be same & potential across the terminalsB
and D are same.
• The Wheatstone bridge is balanced and outputvoltage
is zero.

11. Load Cells
When the specimen is stressed due to the
applied force, the strain induced is
measured by the gauges.
• Gauges R1 and R4 measure the
longitudinal (compressive) strain and
gauges R2 and R3 measure the
transverse (tensile) strain.

12. Measurement of Torque
Measurement of torque is important for the following reasons;
• Toobtain load information for the analysis of stress.
• Toevaluate the performance characteristics of machines.
• Todetermine the mechanical power.
• T = Fr (in N m)
• P = 2πNT
• Dynamometers are used to measure torque.

13. Measurement of Torque
Dynamometers Classification
Absorption Dynamometers
 Prony Brake dynamometer
 Rope Brake dynamometer
Transmission Dynamometers
• Belt transmission dynamometer
• Epicyclic dynamometer
• Torsion dynamometer

14. Measurement of Torque
Absorption Dynamometers
 Prony Brake dynamometer

15. Measurement of Torque
Absorption Dynamometers
 Prony Brake dynamometer

16. Rope brake dynamometer:
It is another form of Absorption type Dynamometer which is most
commonly used for measuring the Brake Power of the Engine . It
consists of one , two or more ropes wound around the flywheel or rim
of a pulley fixed rigidly to the shaft of an engine . The upper end of
the ropes is attached to a spring balance while the lower end of the
ropes is kept in position by applying a dead weight .

17. Transmission Dynamometers
Power-measuring dynamometers may be
transmission dynamometers or absorption
dynamometers. The former utilize devices that
measure torque, in terms of the elastic twist of
the shaft or of a special torquemeter inserted
between sections of the shaft. The torque is
produced by the useful load that the prime
mover, motor, or machine is carrying.

18. Belt Transmission Dynamometer

19. Hydraulic dynamometer
• It works on the principle of dissipating the power in
fluid friction rather than in dryfriction.
• It consists of an inner rotating member or impeller
coupled to the output shaft of engine, this impeller
rotates in a casing filled withfluid.
• The heat developed due to dissipation of power is
carried away by a continuous supply of working fluid,
usually water.
• The output can be controlled by regulating the sluice
gates which can be moved in and out to partial or
wholly obstructive flow of water between impeller and
the casing.