How is Shaft Torque Measured?
1) Electric power meter
The meter calculates the input power by measuring the current and voltage in the line driving the motor. By knowing the shaft speed and the motor efficiency. The input electric power is converted to an estimated torque. This is considered an indirect measurement of torque. Because it is not the true mechanical torque. But the relationship between the electric power measurement and the theoretical torque value.
The accuracy of this method is poor. Because the efficiency of the motor can vary greatly. Vary depends on a variety of factors such as speed, input voltage, output load and temperature. This method is only applicable to transmission systems with electric motors. It cannot accurately respond to rapidly changing torque conditions. Therefore, they are only suitable for situations where a “total” torque measurement is required.
2) Clamp-on torque sensors
Clamp-on torque sensors are physically clamped to the shaft. These systems include a pre-mounted sensor (usually a strain gauge or optical sensor). The sensor measures the strain on the shaft. It converts into a torque measurement through a set of calculations. It depends on the geometry and characteristics of the shaft.
The main advantage of these systems is that the installation does not require the use of a sensor. The sensor is located away from the surface shaft, which can cause high uncertainty in the measurement. The stacking of mechanical components between the shaft and the sensor increases the uncertainty of the measurement. And it exacerbates the effects of environmental changes (such as temperature) on the system. Clamp-on systems are also quite bulky. They require a large amount of available shaft area to be properly installed.
3) Surface mount torque telemetry
Surface mount rotary torque measurement systems rely on a strain gauge sensor mounted directly to the surface of the shaft. This sensor works in the same way as a clamp-on torque sensor (by measuring the “twist” on the shaft). The data is transmitted off the shaft using telemetry. This is a wireless, contactless method of data transmission.
How to Calculate Shaft Torque?
The torque of a shaft can be calculated by the following basic formula: 𝑇=𝐹×𝑟×sin(𝜃)
Where:
𝑇 is the torque in Newton meters (𝑁⋅𝑚)
𝐹 is the force acting on the shaft in Newtons (𝑁)
𝑟 is the distance from the axis of rotation to the point of force application (lever arm) in meters (𝑚)
𝜃 is the angle between the force vector and the lever arm. This formula reflects that torque is a measure of the rotational effect of a force on a shaft. When the force is perpendicular to the lever arm (i.e. 𝜃=90∘), sin(𝜃)=1, and the torque reaches its maximum value.
