Torque sensors

How does a torque measuring shaft work?

1. Torsion shaft and wire strain gauges (DMS)

When a torque is generated, the torsion shaft twists at an angle of 45° to the longitudinal axis.

A wire strain gauge (DMS) is flatly bonded with the shaft following the deformation of the torsion shaft. The wire strain gauge consists of two unfolded wires arranged in a way that depending on the influence of torque one of the two wires lengthens, thus changing its electrical resistance.

2. Data transmission

Since the measuring shaft is suitable for rotating applications, the data need to be transmitted off the shaft. That is why there is an electronics assembly next to the wire strain gauge preparing data and transmitting them via a transmitter.

A coil pair built up coaxially serves for power supply of the rotating electronics assembly and wire strain gauge. A fixed coil induces a voltage into the rotating coil powering the rotating electronics assembly.

Thus there is contactless and maintenance-free power and data transmission.

3. Output

The upper fixed part of the measuring shaft includes a receiver that receives the data, verifies them and displays them via the connector. 

In this way about 10,000 measurements per second are transmitted and displayed.

The accuracy of torque measurement is about one part per thousand of the measuring range which corresponds to 0.1%.

4. Speed measurement

In addition to torque measurement each measuring shaft includes integrated speed measurement. An encoder scans a code disk generating two speed signals with each 720 pulses per revolution.

Since two signals out of phase are generated, the torsional direction can be identified as well.

Apart from the square wave signals of the code disk the information on speed is displayed as DC voltage. This allows to display the speed via simple tools, e. g. a multimeter.

5. Dual-range torque sensor

Users often want to measure torques on test specimens that differ widely in their performance. In order to achieve sufficient measuring accuracy, the measuring shaft usually needs to be replaced by a smaller type - in many cases this is time-consuming and expensive.

Throughout the entire DATAFLEX series, the measuring range can be switched in a fixed ratio of 1:5. This quintuples the output voltage range so that the full voltage on the output (+-10 V) is already achieved with one fifth of the maximum torque.
A bigger ratio of the measuring range is not feasible, as components of any size cannot be mechanically connected. 

The development of the measuring range extension considered maintaining the high resolution of the subsequent digitisation in the small measuring range. Here the DATAFLEX achieves a relative inaccuracy of 0.2 %, in the large measuring range 0.1 %. 

Especially with drives that generate high torque peaks, the usual overload limits of the sensors of 150 % of the rated torque are often too low. This is different with KTR's dual-range sensors: When choosing the small measuring range, the measuring shaft can be selected considerably bigger without having to forego accurate measured values.

With dynamic tests it has to be evaluated whether the mass moment of inertia of the torque sensor may have an impact on the measurement results.

Connection housing DF2

KTR supplies torque sensors with matching connection cable and connection housing. This makes sure that the measuring shaft operates properly and the accuracy can be ensured.

How is a torque measuring shaft mounted?

1. Direct assembly

Depending on the application there are several ways to integrate a torque measuring shaft in the drive. Basically it is mounted between drive and load so that torque that is generated does not result in a torsion of the shaft. The aim is to operate the sensor free from shear force as far as possible.

The most popular and easiest structure is direct assembly. Here the measuring shaft is mounted directly between two couplings operating as a spacer of a double-cardanic structure. With this structure motor and load must provide for a bearing that is able to center the components. The large axial distance allows for big radial and angular displacements as long as the radial motions do not generate too high vibrations.

The structure can be arranged at low cost, but must be rearranged with each change of a component.

2. Structure with additional bearing

Specifically with frequent change of load or specimen we would recommend to use an additional floating/fixed bearing uncoupling the measuring shaft from load.

Replacing the specimen does not require any support and realignment of the sensor and can quickly be realized by means of a plug-in coupling.

3. Structure with safety coupling

In some drives peak torques are generated which cannot be prevented and which may endanger the overall drive train. In this case it is necessary to separate the load mechanically from the drive. A solution is to use idle-rotating safety couplings the slipping torque of which can be set and the release of which can be sensed by a limit switch allowing the control to switch off the drive.

4. Fixed assembly

With a fixed assembly the measuring shaft is fixed on the work surface by means of a stand. Since drive and load each have radial and angular displacements, the measuring shaft has to be equipped with double-cardanic couplings on both sides. Otherwise shear forces may arise on the bearings of the measuring shaft that may falsify the measurements. The short distances to compensate for radial displacements only allow for small displacements with this structure.

5. Vertical assembly

With vertical assembly you have to make sure to use couplings having a support device or those which are able to bear the extra weight. To center the components KTR manufactures centre housings the measuring shaft is integrated in.

6. Selection of couplings

Depending on the structure single- or double-cardanic couplings have to be used. In every case the coupling should be backlash-free and suitable for the torque of the sensor. Depending on the application we would recommend additional safety considering torque peaks and critical operating conditions.

As a standard KTR supplies rigid steel lamina couplings. Plug-in ROTEX GS couplings are also frequently used. The latter have a higher damping effect with torque peaks being sensed differently by the torque sensor than with the use of rigid couplings.

7. Calibration

During calibration the parameters of a torque sensor are evaluated which is done via references that are verified by an accredited test laboratory at regular intervals.

KTR performs a factory calibration following VDI/VDE 2646.

Several load cycles are run to determine the main properties of the sensor (relative and absolute deviation, range, zero drift, reversal error, etc.)

With calibration each sensor is set such that the output voltage corresponds to the scaling that is specified in the data sheets (e. g. 10 Nm/V).

We would recommend inspection of calibration following a specified time interval the period of which depends on the frequency of use and load.

What sizes and types are available?

Typ Messprinzip Messbereich Ungenauigkeit Kupplungen Zubehör
DATAFLEX® 16/10 DMS -10 ... 10 Nm 0,1 % RADEX® NC 20 DF2, cable 2, 5, 10m
DATAFLEX® 16/30 DMS -30 ... 30 Nm 0,1 % RADEX® NC 25 DF2, cable 2, 5, 10m
DATAFLEX® 16/50 DMS -50 ... 50 Nm 0,1 % RADEX® NC 25 DF2, cable 2, 5, 10m
DATAFLEX® 32/100 DMS -100 ... 100 Nm 0,1 % RADEX® N 42 DF2, cable 2, 5, 10m
DATAFLEX® 32/300 DMS -300 ... 300 Nm 0,1 % RADEX® N 60 DF2, cable 2, 5, 10m
DATAFLEX® 32/500 DMS -500 ... 500 Nm 0,1 % RADEX® N 60 DF2, cable 2, 5, 10m
DATAFLEX® 42/1000 DMS -1 ... 1 kNm 0,1 % RADEX® N 80 DF2, cable 2, 5, 10m
DATAFLEX® 70/3000 DMS -3 ... 3 kNm 0,1 % RADEX® N 90 DF2, cable 2, 5, 10m
DATAFLEX® 70/5000 DMS -5 ... 5 kNm 0,1 % RADEX® N 115 DF2, cable 2, 5, 10m
DATAFLEX® 110/10000 DMS -10 ... 10 kNm 0,1 %   DF2, cable 2, 5, 10m
DATAFLEX® 110/20000 DMS -20 ... 20 kNm 0,1 %   DF2, cable 2, 5, 10m
DATAFLEX® xxx/xxx DMS -xx … xx kNm 0,1 % on request DF2, cable 2, 5, 10m


Special types for torques >20 kNm

In addition to the standard series KTR manufactures customized torque sensors the equipment and couplings of which can be freely specified. KTR‘s test field is equipped with load testing machines up to  500 kNm.

Where are torques measured?

The use of torque sensors is not limited to individual industries. Normally they are used on test benches. Anyway, during the past few years they have been used more and more in quality assurance and for machine control. Many years of experience allow KTR to support you with the selection and combination of the optimum components.

Measuring technology

The question concerning the suitable output equipment follows the application. Generally all data collection systems having a voltage input come into question. These can be simple data loggers or displays, or you can decide for a software with data acquisition card. The demand on the software/hardware  depends on the function of the drive. 

In many cases a low-cost universal software is sufficient. However, as soon as an analysis of data is required or a large number of data need to be saved, we would recommend to use a specific software. KTR will be pleased to assist you here as well.

Your technical engineering support

Technical Support
Jürgen Kösters
T +49 5971 798-467

T +49 5971 798-0