Torque is an important mechanical quantity in the area of machine building although it is not confined to that alone. This article aims to cover the applications of torque transducers and offers a reference for resolution of issues impacting applications.
The key methods of torque measurement with reference to the electrical and mechanical configuration of torque transducers based on the strain gage principle are also discussed. The main fields covered include selection criteria, the environment within which applications operate, installation, startup, vibration analysis, calibration, and the metrological principles applicable to measuring with torque transducers (figure 1).
Figure 1. Different generations of torque transducers.
Torque Measurement Methods
Calculation from Electrical Power
One can calculate torque from the speed of rotation and electrical power. The rotation speed and electrical power of electrical machinery can be determined using modern measuring equipment.
The key application areas, however, are more commonly found in process monitoring, such as mechanical agitators, rabblers and the like as it is required to monitor additional electrical parameters such as reactive power or efficiency.
A significant advantage of determining torque by this method is that there is no need for any kind of mechanical intervention in the power train.
Measuring Reaction Torque
The HBM range includes several forms of force transducers and load cells that can measure reaction force by means of a lever arm (figure 2). The main selection criteria are:
- Direction of force: tensile and/or compressive force
- Required accuracy
Figure 2. Using a U2B force transducer to determine the reaction force acting on a lever arm.
Reaction Torque Transducers
Reaction torque transducers integrate into a single device - functionalities which the bearing and the force transducer have, in the case of the lever arm-based torque measurement. Their key application is non-rotating torque acquisition. Typical examples are process monitoring in agitators, rabblers and similar types of mixing equipment.
Measuring In-line Torque
In this method, the torque is obtained in a rotating train of shafts and is commonly known as in-line torque measurement. Torque transducers are divided into:
- Torque shafts
- Torque hubs
- Torque flanges.
Figure 3. The inline torque measurement method.
SGs in Torque Measurement
SG torque transducers include spring elements combined with strain gages (SGs) and compensation elements as well as adaptation accessories for the torque connections to input and output sides. The key features of the strain gauge principle as being important to torque measurement are:
- Strain gages used in the SG measuring bridge together with their means of compensating for the effects of interference variables have superior characteristics with regard to linearity, hysteresis and reproducibility.
- As SGs have very little mass, the frequencies involved in processes being studied can be very high (>50 kHz).
- It is possible to obtain static and dynamic moments.
- SGs exhibit high strength in the presence of vibration, making them highly stable under alternating loads.
- Torque transducers with SGs show excellent long-term stability when suitably configured for the application concerned.
- It is possible to measure torque in positive and negative directions regardless of whether the shaft train is rotating.
Selection Criteria and Application Environment for Torque Transducers
The main criteria to be considered while choosing a torque transducer and configuring an application are:
- Measuring the speed of rotation and maintenance requirements
- The size of the torque to be measured, from the point of view of quasi-static processes
- Dynamic torque from rotary acceleration and retardation
- Mass and mass moment of inertia of a transducer
- Torsional stiffness and stiffness with respect to other parasitic loads
- Maximum speed of rotation, oscillating torque
- Dynamic torque peaks from electrical machinery
- Parasitic loads and required accuracy in the light of various aspects
- Environmental effects: dust, foreign bodies, fluids, chemicals, temperature, atmospheric humidity, EMC conditions
- Dynamic torque from torsional vibration and parasitic loads from bending and axial vibrations.
Dimensions and Basic Mechanical Properties
The parameters that need to be considered include the following:
- Mechanical installation - The mechanical connection method is also significant for instance in choosing between a flanged connection or a shaft stub connection.
- Mass, mass moments of inertia – The mass of a torque transducer effects the possible sag and the bending vibration. The higher the mass, the more the bending and the lesser the natural bending frequencies. The mass moment of inertia plays an identical role in torsional vibration as mass plays in bending vibration. Hence the more the mass moment of inertia, the lower the natural frequencies for the same degree of stiffness.
- Stiffness – Irrespective of whether bending, torsional or axial vibrations are considered, it is always the case that higher stiffness results in higher natural frequencies in the corresponding vibration; whereas, less stiffness leads to lower natural frequencies.
Types of stiffness includes:
- Bending stiffness
- Radial stiffness
- Axial stiffness.
Operating Conditions and Equipment Features
The operating conditions and equipment features that need to be considered are:
- Maximum operating speed - The torque transducer and coupling must survive undamaged at all operating speeds.
- Measuring the speed and angle of rotation – Several HBM torque transducers are fitted with a speed measuring system either as standard or as an alternative and some are also fitted with a measuring system for angle of rotation.
- Maintenance requirements – Transducers with slip rings need regular maintenance, especially since there is a limited service life for slip ring brushes. It is required to periodically change a lubricant in tooth couplings. Torque transducers with contactless measurement signal transmission and bearings have a much lower maintenance requirement due to the very high service life of their bearings, but cannot be said to be maintenance-free. Torque transducers built without bearings and have contactless measurement signal transmission are maintenance-free and wear-free, as are couplings such as the bellows or multi-disk type.
Measuring Range and Maximum Torque
Parameters considered under measuring range and maximum torque include:
- First rough estimate of the torque in an application - In almost all cases, a rough idea of the torque expected to be present in an application will be available at the design stage. This will be based on the nominal torque of the machines that drive the configuration or the nominal torque of the driven application.
- Dynamic torque - The expected dynamic torque must be known accurately when selecting a torque transducer, as the actual maximum torque is known from the dynamic torque peaks. Nominal and maximum torque can now be used in combination with the appropriate safety factors to select a particular torque transducer or a particular nominal torque.
- Parasitic loads - The term parasitic loads refers to all forces and moments which can act on a transducer in addition to the intended measured quantity. In the case of a torque transducer these are chiefly lateral and longitudinal forces, and bending moments.
- Accuracy - Quantitative determination of these aspects uses a large number of quantities that are to be found in a torque transducer’s specifications. These include:
- Accuracy class
- Classification under the calibration regulations
- Measurement uncertainty
- Repeatability and reproducibility
Environmental influences on torque transducers include the following:
- Dust and foreign bodies
- Thermal conditions
- Electromagnetic compatibility (EMC)
- Torsional vibration
- Bending vibration
- Axial vibration.
This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.
For more information on this source, please visit HBM, Inc.