Torsion Test

Torsion is the stress associated with twisting (torque). The torsion testing device has two sockets, one fixed and the other can rotate. The fixed socket is attached to an instrument which senses torsional moment and displays this value on a graduated dial or digital torquemeter.

The torsion specimen is made of a hexagonal stock to minimize the slippage in the sockets of the testing machine and is turned to round in the center portion. Torque is most commonly encountered in members which are circular in cross-section. Shafts and threaded fasteners are the usual torque resisting and torque transmitting machine elements.

A device for measuring twist angle, which is called torsiometer, is mounted on the specimen before it is inserted into the sockets.The parts of the torsion testing machine are shown below: 

A

Dial gauge

G

Input handwheel

B

Leveling handwheel

H

Hexagonal sockets

C

Deflection arm

I

Torquemeter output socket

D

Adjustable feet

J

Torque shaft

E

Gearbox carriage locking screws

K

Input shaft

F

Base

 

 

Torquemeter and Calibration Procedure:

The following controls are located in the back of the torquemeter:

1. ZERO adjustment knob - this is used for zeroing the meter prior to applying the load

2. CAL screw - this can be used to adjust the calibration of the meter in the SI units

3. SI/IMP adjust screw - this adjusts the ratio between the SI and British units as selected by the SI/IMP switch in front of the meter.

4. Decimal points - this can be used to determine the number of decimal points displayed. 

Calibration:

1. Fit the calibration arm onto the square end of the torque shaft, then set the deflection arm (C) approximately level by adjusting the handwheel (B). Set the dial gauge (A) to zero by rotating outer bezel.

2. Select SI units and set the torquemeter to zero by adjusting the ZERO knob at the rear end of the instrument.

3. Add a load of 5 kg. To the calibration arm (L)and return the reading on the dial gauge (A) to zero by rotating the handwheel. Check that the reading on the torquemeter is 24.5 0.5 Nm. If the error is greater than 0.5 Nm, the claibration should be adjusted using CAL screw to set the reading to 24.5 Nm when the load is 5 Kg.

4. Remove the load and check that the meter returns to zero.

 Torsiometer:

The torsiometer is used to measure the angle of twist in radians both in elastic and plastic regions.

Operation:

Place the torsiometer on the specimen. This is done in three stages:

1. Push one end of the specimen firmly into the socket mounted on the tailstock of the torsion machine. Separate the torsiometer into its three main components; two end clamps (A) and (B) and the cylindrical center spacer (C). Slide the end clamp (B) onto the specimen and frimly tighten the cap screw (D) at the appropriate distance from the specimen and using the allen key set the rod (H) so that its radial support is at approximately 45 o from the vertical (towards the handwheel side) and tighten the knurled nut (F) to lock it in position.

2. Slide the cylindricial spaver (C) over the specimen and onto the spigot on end clamp (B).

3. Place the remaining end clamp (A) onto the specimen taking care to locate the spigot on this end clamp as far as possible into the open end of the spacer (C). Turn the end clamp until the dial gauge plunger contacts the flat on the end of the rod (H). Firmly hold the three components together and tighten the cap screw (D) in the end clamp (A). The spacer should be free to rotate without end play. The whole assembly is now firmly fixed to the test specimen and the tail stock can be slid along the bed until the free hexagonal end of the specimen is inside the head stock socket. Load the straining head in position. The torsiometer is now ready for use. During the experiment, if an adjustment is necessary, slacken the knurled nut (F) and reset the position of the rod (H).

A = End clamp

E = End caps

B = End clamp

F = Knurled nut

C = Spacer

G = Dial gauge

D = Cap screw

H = Rod

 

Experimental Procedure:

1 . Measure the diameter of the test specimen.

2. Note that gage length is 2.00 inches.

3. Check the calibration of torquemeter, adjust if necessary.

4. Mount the torsiometer on the specimen.

5. Increase the angle of twist at an incremental value of 0.005 radians and record the corresponding value of torque. Repeat this for 10 incremental readings.

6. Reduce the load to zero and remove the specimen from the machine.

Warning: Do not attempt to remove the specimen when under load.

Report:

1. Prepare a report, describing the purpose of the experiment, the equipment and setup used and the results obtained.

2. Using the following equations, calculate the shear strain and shear stress for each increment. Tabulate the stress and strain values.

Experimental values

   

Calculated Values

Torque

T

Lb-in

Angle of Twist

q

Radians

Shear Stress

Ss

Psi

Shear Strain

e s

Radians

       
       
       
       
       
       
       

 

3. Using the vertical axis for shear stress and horizontal axis for shear strain, plot stress -strain diagram.

4. Determine the shear modulus from the slope of the straight line.

5. Compare your result with the published value of the shear modulus. Calculate the % error. List the possible sources of error.

Shear Strain = e s = r q / L

Shear Stress = Ss = T r /J

Angle of Twist = q = TL/(JG)

 

e s = Shear strain (radians)

Ss = Shear Stress (psi)

q = Angle of twist (radians)

L = Gage length (in)

r = Radius (in)

G = Shear Modulus (psi)

J = Polar Moment of Inertia (in4)

T = Torque (lb-in)

 

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Last Update: March 24, 2000

Prepared by : Serdar Z. Elgun