Tension and compression forces often act along the same mechanical axis in test benches, presses, clamping systems, linear drives and special-purpose machinery. A suitable tension/compression force transducer can measure both force directions and convert them into an electrical signal.
However, selecting a sensor solely on the basis of the maximum expected force is not sufficient. Even a highly accurate force transducer will produce unreliable values if the load is applied eccentrically, transverse forces occur or the surrounding structure deforms under load. The decisive factor is therefore how the sensor is integrated into the actual load path.
Installation height, threads, rod ends, mounting adapters and freedom of movement also affect the measurement. Particularly with alternating tension and compression loads, the connection must be free from play, centred and designed for both force directions.
Table of contents
- What is the difference between tension and compression force?
- Why is the load path so important?
- Which designs are available?
- How does the available installation space affect the selection?
- Threads, rod ends and mounting adapters
- Avoiding transverse forces and bending moments
- Selecting the measuring range and overload capacity correctly
- Static and dynamic force measurement
- Output signals and measuring amplifiers
- Calibration and installation conditions
- Practical example on a clamping system
- Important selection criteria
- Which measuring instruments / products are suitable?
- Conclusion
- Frequently asked questions
What is the difference between tension and compression force?
A tension force pulls two components apart. Typical examples include tie rods, cables, clamping devices and suspended loads. A compression force acts in the opposite direction and presses components together, for example in presses, clamping fixtures or between a hydraulic cylinder and a tool.
Tension/compression force transducers can measure both force directions. Depending on the electrical configuration, one direction is output as a positive signal and the other as a negative signal. This is particularly useful in applications where a drive alternately pulls and pushes.
However, the ability to measure both directions does not mean that every mounting arrangement is automatically suitable for tension and compression. A loosely positioned compression plate, for example, can transmit compression force but not tension force. Bidirectional measurement requires a positive and sufficiently rigid mechanical connection.
Why is the load path so important?
The load path describes the route taken by a mechanical load through a structure. If the entire force is to be measured, it must pass completely through the elastic measuring body of the force transducer.
If an additional load-bearing path exists parallel to the sensor, only part of the actual force will be measured. Such force shunts can be caused, for example, by guides, preloaded housing components, cables, stops or components installed in parallel.
A typical example is a force transducer installed between a cylinder and a tool. If the tool is additionally supported by a tightly adjusted guide, part of the cylinder force may be transmitted through this guide. The sensor will then indicate less force than is actually acting within the system.
For reliable measurement, the following points must therefore be clarified before selecting the sensor:
- Where is the force to be measured generated?
- Through which components is it transmitted?
- Can the entire force be routed through the sensor?
- Are there any parallel load paths or mechanical stops?
- Does the load path change between tension and compression loading?
Which designs are available?
Tension/compression force transducers are available in different geometries. The design should be selected not only according to the measuring range and accuracy, but also according to the intended load introduction.
| Design | Typical load introduction | Suitable applications |
|---|---|---|
| Cylindrical force transducer with female threads | Threaded rods, adapters or rod ends on both sides | Tensile testing, linear drives, clamping systems and small test benches |
| Force transducer with threaded studs | Direct screw connection into existing components | Linkages, cable and tensile force measurement, and compact machinery |
| Low-profile load cell with central female thread | Central screw or tie rod through the sensor axis | Material testing machines, presses and applications with limited installation height |
| Force transducer with rod ends | Pinned connection with limited angular compensation | Cylinders, lever mechanisms and moving structures |
| Miniature force transducer | Small threads or customised adapters | Restricted installation spaces, laboratory equipment and low nominal forces |
| Strain transducer | Measurement of the strain in an existing load-bearing component | Retrofitting to machine frames, presses or beams |
A low installation height is often advantageous, but may require additional mounting plates or special adapters. A longer cylindrical sensor, by contrast, can often be integrated more easily into a tension and compression linkage using rod ends.
How does the available installation space affect the selection?
The available installation space determines more than just the external dimensions of the sensor. Space must also be provided for adapters, the cable connection, installation tools and possible movement of the structure.
On a linear drive, an excessively long force transducer can reduce the usable stroke. A very low-profile load cell, by contrast, requires sufficiently wide and flat contact surfaces. Rod ends compensate for small angular deviations, but increase the overall installation length.
The sensor cable also requires a protected outlet. Under alternating loads, it must not be kinked, crushed or used as a mechanical support. Moving axes require an adequate cable loop or a suitable energy chain.
If a conventional force transducer cannot be integrated into the load path, a strain transducer may be an alternative. It is mounted on an existing load-bearing component and measures its strain or compression. The measured deformation must then be correlated with the actual force acting on the structure.
Threads, rod ends and mounting adapters
The load introduction connects the sensor to the machine. Even minor design errors at this point can influence the measurement deviation more strongly than the specified accuracy of the force transducer.
Threaded connections
Female or male threads enable a compact and rigid connection. The engagement depth must be sufficient to transmit the maximum force, including possible overloads, safely. Insufficient thread engagement can damage the thread or introduce additional play.
The connected threaded rods should be aligned. If they are forced against one another during installation, a mechanical preload or bending moment is generated even before the working load is applied.
Rod ends
Rod ends can compensate for small angular deviations and help keep bending moments away from the sensor. They are particularly useful on cylinders, levers and moving test arrangements.
However, a rod end cannot eliminate an offset load introduction. If the two pivot points are not located on the same line of action, a moment will still occur. The rod end, pin and retaining device must also be suitable for the maximum tension and compression force.
Mounting adapters
Adapters should be short, rigid and rotationally symmetrical. Long or slender adapters can bend under load and introduce the force into the sensor at an angle. Play-free connections are particularly important under alternating loads, as existing play can cause impacts and jumps in the measured value when the load direction changes.
Avoiding transverse forces and bending moments
Force transducers are generally designed for a defined primary measuring axis. Forces acting transversely to this axis, as well as bending and torsional moments, can cause additional deformation of the measuring body.
Possible consequences include:
- deviating or non-reproducible measured values,
- different results under tension and compression,
- zero-point shifts after loading,
- mechanical overload even though the measured force is below the nominal range,
- reduced service life under dynamic loading.
Transverse forces are often caused by misaligned threads, uneven mounting surfaces, jammed guides or lateral movement of a cylinder. For compression measurements, the contact surface must be flat and sufficiently rigid. For tension measurements, the connection points should be articulated where possible, but without unnecessary play.
Guides must absorb lateral movement without carrying a relevant portion of the axial force past the sensor. This conflict should be considered during the design of the test arrangement.
Selecting the measuring range and overload capacity correctly
The nominal force range should match the normal working range as closely as possible. A sensor with a nominal force of 100 kN can measure a force of 5 kN, but this uses only 5 percent of its measuring range. Resolution, signal-to-noise ratio and the relative uncertainty of the complete measuring chain may therefore be less favourable.
At the same time, the sensor must not be selected so tightly that normal load peaks already cause an overload. The following factors must be considered:
- maximum regular tension and compression force,
- start-up and shutdown peaks,
- blocking and end-position forces,
- vibrations and load cycles,
- installation preload,
- possible operator error or process malfunction.
The permissible operating force, limit force and breaking force must not be confused with one another. A high breaking force is not a usable reserve for normal operation. Repeated overloads can shift the zero point or permanently deform the measuring body, even if the sensor appears undamaged externally.
Where high, unpredictable forces may occur, an additional mechanical overload protection device should be provided. Suitable solutions include adjustable stops, force shunts, spring systems or electronic shutdowns. Such protection must be designed so that it does not affect measurement within the normal operating range.
Static and dynamic force measurement
In static measurements, the force changes only slowly. Examples include maintaining a clamping force or determining a stationary load. Accuracy, zero-point stability and creep are the primary considerations.
In dynamic measurements, the force changes rapidly or periodically. Examples include pressing operations, alternating cylinder forces, material testing and force profiles during an assembly process.
For dynamic applications, the sensor, measuring amplifier and data acquisition system must be considered together. A high sampling rate alone is not sufficient if the measuring amplifier applies strong signal filtering or the mechanical structure oscillates.
The mass of adapters and tools also affects the result. During rapid movement, acceleration forces occur and are measured by the sensor in addition to the actual process force. Depending on the measuring task, these forces must be considered, reduced through a suitable design or compensated mathematically.
Output signals and measuring amplifiers
Many force transducers use strain gauges and provide an unamplified mV/V signal. The output voltage depends on the excitation voltage and the applied force. A suitable strain gauge measuring amplifier or corresponding measuring input is required for evaluation.
Force transducers with an integrated measuring amplifier are also available for industrial machinery. Common output signals include 0–10 V and 4–20 mA. Depending on the series, digital interfaces or redundant signals may also be available.
| Output signal | Characteristics | Typical application |
|---|---|---|
| mV/V | Unamplified strain gauge signal offering a high degree of flexibility when selecting the measuring amplifier | Test benches, laboratory measurement systems and high-resolution data acquisition |
| 0–10 V | Easy to integrate into machine control systems, but more susceptible to interference over long cable runs | Short cable runs and existing voltage inputs |
| 4–20 mA | Interference-resistant transmission and effective fault detection in industrial environments | PLCs, plant control systems and longer cable runs |
| Digital signal | Direct transmission of measured values and, in some cases, diagnostic information | Networked machinery and automated systems |
With a 4–20 mA output, the scaling of the downstream PLC or indicator input can be checked using a Druck UPS4E loop calibrator. The sensor signal is simulated electrically during this test. This does not test the mechanical function or calibration of the force transducer.
Calibration and installation conditions
During calibration, the force transducer is loaded with defined reference forces. Tension, compression or both force directions can be tested. The calibration direction must match the subsequent application.
A sensor may exhibit slightly different characteristics under tension and compression. If it will later be used in both directions, the calibration should cover both loading directions and, where appropriate, several loading cycles.
Factory calibration assesses the sensor under controlled conditions. In the actual installation, however, adapters, rod ends, mounting surfaces and the machine structure are also involved. These can influence the measurement result.
For particularly demanding measuring tasks, calibration of the complete measuring chain or a comparative measurement in the installed condition may therefore be appropriate. The force transducer, amplifier, indicator and mechanical load introduction should be considered together.
The installation orientation may also be relevant if heavy adapters or tools are suspended from the sensor. Their dead weight creates a preload that must be tared before measurement or taken into account in the evaluation.
Practical example: Measuring tension and compression force on a clamping system
A pneumatically operated clamping system is intended to secure components under compression and actively retract them after machining. The normal compression force is 8 kN and the retraction force is 3 kN. During rapid closing, short-term peaks of up to 11 kN occur.
A force transducer with a nominal force of 10 kN would be too tightly specified despite the normal working value of 8 kN, because the recurring peaks exceed its nominal range. A tension/compression force transducer with a nominal force of 15 kN is therefore selected.
The sensor is mounted axially between the cylinder and the clamping mechanism. Rod ends are used on both sides to compensate for small angular deviations. The guide of the clamping mechanism absorbs transverse movement without creating a parallel axial load path.
During commissioning, the unloaded sensor already indicates a force of 0.4 kN. The cause is a preloaded threaded rod whose connection points are not precisely aligned. After realignment and reinstallation, the zero value falls to almost zero.
During operation, the compression and tension directions are evaluated separately. A shutdown threshold is additionally set at 12 kN so that a blocked component does not overload the sensor or the clamping mechanism.
This example shows that the measuring range and sensor accuracy alone do not determine the result. Reproducible measurement is only possible with a suitable design and centred, stress-free load introduction.
Important selection criteria
The following information is particularly important when selecting a tension/compression force transducer:
- minimum, normal and maximum tension force,
- minimum, normal and maximum compression force,
- possible load and overload peaks,
- static or dynamic loading,
- load cycles and expected number of cycles,
- available axial and radial installation space,
- type of load introduction and existing threads,
- possible transverse forces, bending moments and angular movement,
- required output signal,
- required accuracy and resolution,
- ambient temperature, humidity and degree of protection,
- required calibration in tension, compression or both directions.
For safety-related shutdowns, it must also be verified whether the sensor, evaluation electronics and control system provide the required safety function and architecture. A standard measuring signal is not automatically a safety-related signal.
Which measuring instruments / products are suitable?
WIKA tension/compression force transducers
The tension/compression force transducers category includes different designs for test benches, mechanical engineering, automation, lifting technology and special-purpose machinery.
Depending on the series, the load is introduced through female threads, threaded studs, rod ends or central bores. Compact sensors for low forces are available, as are robust versions for high nominal forces and industrial environmental conditions.
WIKA F2812 for low nominal forces
The WIKA F2812 is suitable for lower tension and compression forces up to 1,000 N. The load is introduced through two female threads. This allows the compact sensor to be integrated into small test benches, assembly fixtures and industrial measuring systems.
WIKA F2822 for low installation heights
The WIKA F2822 has a low installation height and a central through-bore with female thread. The design is particularly suitable for material testing, presses and industrial measuring points with limited axial installation space.
WIKA F2303 with rod end
The WIKA F2303 is available with a load introduction suitable for moving force connections. Rod ends can compensate for small angular deviations and facilitate integration into linear drives, lever mechanisms and special-purpose machinery.
Special force and strain transducers
The special force transducers / strain transducers category includes solutions for special geometries, restricted installation spaces and existing structures.
A strain transducer such as the WIKA F9302 is mounted directly on a load-bearing component and measures its strain or compression. This solution is particularly suitable for retrofits where the load path cannot be routed through a conventional force transducer without major design modifications.
Conclusion: The mechanical design determines the quality of force measurement
A tension/compression force transducer can measure both force directions reliably if the load is transmitted completely, centrally and without relevant transverse forces through the sensor. The load path is therefore just as important as the measuring range, accuracy and output signal.
Threads, rod ends and mounting adapters must be suitable for the application and must not introduce additional stress or bending. Under alternating tension and compression loads, play-free connections and sufficiently rigid, aligned connection components are essential.
If there is insufficient space for a conventional force transducer, low-profile designs, miniature sensors or strain transducers may be suitable alternatives. Selection should therefore always take account of the specific installation conditions and the complete load path.
Frequently asked questions about selecting and installing tension/compression force transducers
Can every tension/compression force transducer measure both directions with the same accuracy?
The sensor is generally designed for both directions, but may exhibit slightly different characteristics under tension and compression. For bidirectional applications, the calibration should cover both force directions.
Why does the sensor already indicate a force when it is unloaded?
Possible causes include mechanical preload, stressed threads, the dead weight of attached components, a temperature change or an electrical zero-point shift. The mechanical cause should be checked before applying an electronic tare.
How can transverse forces be avoided?
The connection points must be aligned and the load should be introduced along the sensor axis. Rod ends can compensate for small angular deviations. Guides must absorb transverse movement without creating a parallel axial load path.
Should the force transducer be rated above the maximum operating force?
An adequate reserve for actual load peaks is necessary. However, a measuring range that is significantly too large reduces the usable resolution. Nominal force, peak load, permissible operating force and overload capacity must be considered together.
Can a strain transducer replace a force transducer?
A strain transducer initially measures the deformation of an existing component. This deformation can be assigned to a force through calibration. The solution is particularly suitable for retrofits, but does not automatically achieve the same accuracy as a force transducer installed directly in the load path.
Why are rod ends useful for tension/compression measurements?
They can accommodate small angular movements and thereby reduce bending moments. However, they cannot fully compensate for lateral offsets or major alignment errors.
Does the complete measuring chain have to be calibrated?
Sensor calibration may be sufficient for standard applications. For high accuracy requirements, the force transducer, measuring amplifier, indicator and, where appropriate, the mechanical adapters should be tested together or compared in the installed condition.
Which information is required for selecting the instrument?
The required information includes the tension and compression range, maximum load peaks, loading dynamics, installation space, load introduction, threads, possible transverse forces, required output signal, environmental conditions, accuracy requirement and required calibration direction.