Figure a shows the construction of load cell used to measure torque using strain gauges connected to the rotating shaft.. Figure b represents the bridge arrangement to measure torque. .The strain gauges are fixed at 450 with the shaft axis. Two strain gauges are subjected to tensile stresses while the other two experience compressive stress. Slip rings are used for connectivity with the bridge. When torque is applied to the shaft, the strain gauges change their properties and the strain is measured by the bridge circuit. Output of the bridge network will be proportional to the torque .
Figure above represents a strain gauge bridge circuit used for torque measurement. Four bonded wire strain gauges are mounted on a 450 helix with the axis of rotation. They are placed in pairs diametrically opposite. When the gauges are accurately placed and have matched characteristics, the system is temperature compensated and insensitive to bending and thrust.
Working: When the shaft is under torsion, gauges 1 and 4 will elongate as a result of the tensile component of a pure shear stress on one diagonal axis while gauges 2 and 3 will contract due to the compressive component on the other diagonal axis. The Wheatstone bridge output is proportional to torsion and hence the torque. .