Torque Measurement.docx

Mechanical Torsion Meter Mechanical torsion meter is used to measure torque developed by a machine. Basic principle of mechanical torsion meter When a shaft is connected between a driving engine and driven load, a twist (angular displacement) occurs on the shaft between its ends. This angle of twist is measured and calibrated in terms of torque. Construction of mechanical torsion meter The main parts of the mechanical torsion meter are as follows: A shaft which has two drums and two flanges mounted on its ends as shown in the diagram. 1. One drum carries a pointer and other drum has a torque calibrated scale. 2. A stroboscope is used to take readings on a rotating shaft.

Operation of mechanical torsion meter: One end of the shaft of the torsion meter is connected to the driving engine and its other end to the driven load. 1. An angle of twist is experienced by the shaft along its length between the two flanges which is proportional to the torque applied to the shaft. 2. A measure of this angle of twist becomes a measure of torque when calibrated. 3. The angular twist caused is observed on the torque calibrated scale corresponding to the position of the pointer. As the scale on the drum is rotating, reading cannot be taken directly. Hence a stroboscope is used. The stroboscope’s flashing light is made to fall on the scale and the flashing frequency is adjusted till a stationary image is obtained. Then the scale reading is noted. Application of mechanical torsion meter  Simple and inexpensive method  Power of shaft can be calculated (flashing frequency gives information about speed).  Limitation of mechanical torsion meter Poor accuracy due to small displacement of the pointer. Sensitivity is reduced even due to small variation in speed. It can be used only on shafts rotating at a constant speed Electrical Torsion Meter (Torque Measurement using slotted Discs) Basic principle

Due to the applied torque, there is a relative displacement between the two slotted discs. Due to this relative displacement of the slotted discs, a phase shift exists between the pulse generated by the transducers. When these pulses are connected to an electronic unit, it will show a time lapse between the two pulses. This time lapse between the two pulses is proportional to the twist of the shaft and the torque of the shaft. Description of Electrical Torsion Meter 1. The main parts of an electrical torsion meter are as follows: 2. A shaft connected between a driving engine and a driven load. 3. Two slotted discs attached on either side of the shaft. 4. Transducer (magnetic or photo electric ) to count pulses from the slotted disc.

Operation of Electrical Torsion Meter 1. The teeth produce voltage pulses in the transducers. 2. When torque is not applied on the shaft, the teeth of the bth the discs perfectly align with each other and hence he voltage pulses produced in the transducers will have zero time difference. 3. But when torque is applied on the shaft, there is a relative displacement of the slotted discs due to twist experienced by the shaft and hence the teeth of both the discs will not align with each other and hence the voltage pulses produced in the transducer will have a time difference (that is , time lapse). 4. This time lapse between the pulses of the two discs is proportional to the twist of the shaft and hence the torque of the shaft. 5. A measure of this time lapse becames of torque when calibrated. Application of Electrical Torsion Meter 1.

Used to measure torque on rotation shafts.

Advantages of Electrical Torsion Meter 1. 2.

There are no signal leakage problems. There is no noise creation.

Optical Torsion meter Basic Principle Due to torque, an angular twist (angular displacement) occurs on the shaft between its ends. This angle of twist is measured by using optical means where in angular deflection of light rays is proportional to twist and hence the torque. Construction of optical torsion meter

The main parts of an optical torsion meter as follows: A shaft is used on which two casting M and N are connected at a known distance. A tension strip linking the two castings. Two mirrors which are fitted and aligned on the castings. A light beam falling on the mirrors, an optical system and a torque calibrated scale. Operation of optical torsion meter hen the shaft is transmitting torque, a relative movement occurs between castings M and N, and due to this, the mirrors will change position ( partial inclination between the two mirrors) since they are attached to the castings. As the mirrors are constantly made to reflect a light beam on the torque calibrated scale, due to the changed position of the mirrors, there will be an angular deflection of the light rays which is measured from the calibrated scale. This angular deflection of the light rays is proportional to the twist on the shaft (relative movement of casting M and N) and hence the torque of the shaft. Applications of optical torsion meter It is used in steam turbines and I.C engines

strain gauge torsionmeter With this device four strain gauges are mounted onto the shaft, as shown in Figure below. The twisting of the shaft as a result of an applied torque results in a change in resistance of the strain gauge system or bridge. Brushes and sliprings are used to take off the electrical connections and complete the circuit, as shown. More recently use has been made of the resistance change converted to a frequency change.

A frequency converter attached to the shaft is used for this purpose: this frequency signal is then transmitted without contact to a digital frequency receiver. When a torque is applied to the shaft, readings of strain and hence torque can be made.

Fig: Strain gauge torsionmeter