What Is The Accuracy of TorqueTrak Systems?
Using the TorqueTrak telemetry systems, you can achieve accuracy better than ±1.0% Full Scale. To understand the accuracy of our systems, it is helpful to break up into the two primary sources of error: Instrumentation Error and Calibration Uncertainty.
All TorqueTrak systems are inherently precise, and competitive to any other instrumentation that measures torque. Instrumentation accuracy is dependent on items such as temperature and linearity of the system. By the time all of the instrumentation errors are added up, they account for a small portion of the overall error (typically ~.1% or less).
The calibration of TorqueTrak telemetry systems usually dominates the amount of error in most applications. How much error exists due to calibration is dependent on which method of calibration is used.
Method #1 – Full Scale Calculator
The most common method of calibration is using the properties of the shaft and strain gage to calculate the theoretical full scale torque. Due to uncertainties in the inputs to the calculation (such as exact shaft diameter, gage factor, poisons ratio, and modulus), we say a 3%-5% error is possible.
Binsfeld provides calculators that help determine the full scale torque for a given application. Shaft Diameter has a major influence in the Sensitivity calculation – a precise value should be used. Gage Factor is supplied with the package of strain gages. Modulus of Elasticity and Poisson Ratio were probably recorded by the shaft manufacturer, but it may be difficult to recover those values. Using nominal values for Modulus of Elasticity and Poisson Ratio introduces an uncertainty of perhaps ±3% Full Scale. This is not to say that using nominal values introduces error, because the true shaft characteristics may be exactly equal to the nominal values used.
Method #2 – Deadweight
This is the most precise way of calibrating the output of a TorqueTrak telemetry system. In a deadweight calibration, a known torque (a known force/weight on a known moment-arm) is applied to the shaft and the corresponding output signal is recorded. In this case, the error is dominated by whatever error is inherent in the set-up of the dead-weight test.
The known moment arm should be affixed to the shaft using a clamp or perhaps by bolting onto a flange. The length of the moment arm should be measured accurately from the axis of the shaft to the point of applied force/weight. The known force/weight could be calibrated weights free-hanging from the arm, in which case the moment arm would be the horizontal dimension. Or the known force could be applied using a calibrated torque wrench. On large shafts the known force could be applied using a crane or a jack and measured with a calibrated load cell. Be Careful! The applied force and the moment arm must be perpendicular to each other. Be careful with your measurements, especially if the moment arm drops when free hanging weight is applied. In this case, the length of the moment arm becomes shorter in the dimension perpendicular to the weight/force.