Force Washers for Production, Testing and Monitoring

High forces, exacting requirements for the robustness of the measuring equipment – not to mention demands for a low height of construction and easy integration on a limited budget – there are many reasons for using annular force sensors. Their diverse uses range from monitoring forces in virtually countless industrial applications, such as crimping, joining, pressing and riveting, to the long-term monitoring of threaded connections in wind turbines or on rails.

Therefore, the requirements facing the force washers are as varied as the measurement tasks:

A low discrimination threshold, to ensure excellent sensitivity and enable even the tiniest forces to be measured
Rugged version with compact dimensions
Sizes suitable for popular screw, bolt and pin diameters
Drift-free quality when used in monitoring tasks, such as screw and bolt monitoring, for example
Very low repeatability error after installation and calibration

Even with the latest technology, it is impossible to satisfy all requirements with one physical measurement principle. For this reason, solutions based both on strain gauges and piezoelectric force washers have become widespread.

Physical principles

Strain gauge-based force washers

strain gauges in force washer — Fig. 1: Principle of the force washer based on strain gauge technology. Under the effect of a force (F), the annular measuring body (2) deforms. Strain gauges (3) are glued onto it. The housing (1) hermetically protects the measuring equipment.

Strain gauge-based force washers consist of an annular spring element to which strain gauges are fastened with adhesive. As in common in strain gauge sensors, the acting force deforms the spring element. This causes a strain, which the strain gauges convert into a change in resistance. The strain gauges form a Wheatstone bridge circuit, so that – if electric voltage is applied to the measuring washer – this gives rise to a measurable electric voltage that is proportional to the applied force.

When selecting the strain gauges, the sensor designer focuses on achieving the greatest possible angular coverage, so that the washer has uniform sensitivity over its entire surface. Newer series, such as the KMR+ from HBM, also feature welded hermetic seals. These make the sensors suitable for long-term use even in harsh conditions, such as use on outdoor structures, on railroad beds or in wind turbines. The key advantage of strain gauge technology is that the sensors are practically drift-free. This in turn delivers an invaluable advantage in countless monitoring tasks (e.g. the monitoring of threaded connections or cable tension), as the sensors continue to measure accurately over the long term with no need for interim zeroizing or resetting.

Piezoelectric force washers

Piezoelectric force washers consist of two crystalline plates of piezoelectric material, which is frequently quartz (turquoise). An electrode (red) is mounted between these crystal plates. The other side of each crystal plate is connected to the housing (yellow and green) of the force washer.

When a force is applied, these sensors produce a charge (piezoelectric effect), which a special coaxial charge cable feeds to a charge amplifier, where it is transformed into a measurable voltage signal. If we make the surface of the crystals larger or smaller, the sensitivity remains unchanged – in great contrast to the sensors, which are based on strain gauges where sensitivity is dependent on the nominal (rated) force.

Piezoelectric force sensor — Fig. 2: Design of a piezoelectric sensor. The crystal disks (green) convert applied force into a charge that is picked up by the electrode between the sensors

Thus, the sensitivity of piezoelectric sensors is not dependent on the size of the transducer and therefore not on the nominal (rated) force either. Consequently, any sensor can be selected for measuring even the tiniest forces. This results in greater freedom for other parameters, such as high overload stability or geometrical requirements, for example. There are further advantages, too: the discrimination threshold is smaller, enabling a very broad measuring range.

Moreover, the sensor has identical sensitivity over the entire load application surface. On the other hand, the electrical connection is subject to stringent requirements, as very high insulation resistance is needed. Even if measurements are possible in the Newton range and HBM components have exceptionally low drift, all piezoelectric sensors have drift, so that long-term monitoring tasks cannot be performed using this technology.

Charges can short-circuit, and a resulting reset of the piezoelectric sensor’s electrical output can set the sensor to zero. The advantage is that very small forces can be reliably recorded even if the existing acting force is very large. 10 N can be measured without problem by a piezoelectric sensor with a initial load of several kN, if a reset previously took place.

Which Technology Should I Use When?

Strain gauge technology and piezoelectric sensors complement one another perfectly. Below are a few measuring tasks and the preferred technology in each case:

INTENDED USE / REQUIREMENTS	RECOMMENDED SOLUTION
Measuring range over several powers of ten required	CFW or CLP piezoelectric sensors
Monitoring tasks over long periods	KMR+
Process control of joining processes, presses, and similar	Both measurement principles can be used
Use in extreme conditions, high humidity	KMR+
Use without pre-stress	KMR+
Extremely high overload stability required	CFW or CLP piezoelectric sensors (select a larger sensor)
Measurements of the tiniest forces under a high initial load	CFW or CLP piezoelectric sensors
Rapid force measurements	Both measurement principles can be used

Tips and Know-How for Use

Small displacement, prerequisite for rapid force measurements

All force washers feature minimal displacement. This is an advantage, as the rigidity of a force sensor can also be calculated by dividing the nominal (rated) force by the displacement: Hence, a small displacement equates to high rigidity, which in turn means that the structure has a high natural frequency. Since the sensor’s maximum measurement frequency depends on the natural frequency of the overall system, the small displacement means that the sensors are also suitable for very rapid measurements.

Distributing forces uniformly over the force washer: Suitable contact surfaces, load application plates

Likewise, the small displacement means that the sensor must be subjected to uniform load over its entire surface, to prevent overload in individual areas. This applies to both types of sensor. Therefore, the surfaces in contact with the sensor should be ground to ensure they are even. A hardness of 40 HRC is also required.

The KMR+ covers a very large angular domain with the strain gauge. It features load application plates for distributing the force uniformly onto the force washer and so improving sensor behavior. This dramatically reduces the repeatability error and bending moment sensitivity, while also imposing less strict requirements on the contact points of connecting parts.

Piezoelectric force washers: Pre-stressing is absolutely essential

In piezoelectric force washers, pre-stressing is absolutely vital. A screw is generally used to do this. Property class 10.9 or 12.9 is required. The pre-stress is important for pressing the components of the piezoelectric force washer, i.e. the crystals, electrode and housing, onto one another:

To ensure correct dissipation of the charge, when the crystals are in firm contact with the housing and electrode.
To guarantee contact between the components even when bending moments occur. A bending moment that completely relieves the tension on one side of the piezoelectric force washer must never occur.

max bending moment force washers — Fig. 3: Maximum bending moment for force washers (strain gauge-based and piezoelectric). If a bending moment (MB) acts on the measuring ring, the force applied increases on one side (3), thus relieving the other side (2). Visible: the hermetic encapsulation of the measuring ring (1) on a KMR+ force washer

As you can see from the diagram, the permissible bending moment of piezoelectric force washers depends on the load: the maximum bending moment can be applied if the sum of the pre-stressing force and the force to be measured equals exactly 50 % of the nominal (rated) force. Example: You are using a CFW/330KN, the process force you wish to measure is roughly 95 kN. The optimum pre-stressing force is therefore 70 kN, as the sum of 70 kN and 95 KN is 165 kN, i.e. precisely half the nominal (rated) force of the force washer.

Pre-stress, change in sensitivity

If the force washers are installed pre-stressed with a screw or pin, the pre-stressing force of the screw acts on the force washer as well as the force to be measured, F, as shown in the diagram below. Consequently, the force washer is working in a force shunt.

If a force is applied to the construction, a very small deformation results. This causes the screw to be relieved of strain to a slight extent, and the pre-stress drops. This makes the measuring point less sensitive than in the washer without pre-stress. If you require quantitative measured values, the force washer must be calibrated. Of course, qualitative (comparative) measurements can also be performed without calibration.

Fig. 5: Pre-stressing of a measuring ring (2) using two nuts (1, 1).

Calibration procedure

Calibration means comparison with a known quantity. Force washers can be calibrated while installed, by positioning a load cell with fitted force washer in series, e.g. by replacing the tool with a load cell. There is of course always the option of sending machine elements containing the force washer to the factory for calibration. In this case, we need to clarify beforehand whether the construction can be installed in a calibration machine. As a rule, calibrating the installed sensor using a calibrated load cell is the easiest option.

If a sensor with DAkkS (national accreditation body for the Federal Republic of Germany) calibration certificate is used, force washers can also be connected in the field with metrological traceability, as required by numerous quality standards.

Special designs

A force washer with a different diameter is frequently required, or a different nominal (rated) force for the diameter in question. Special designs are generally also possible with smaller batch sizes, and often offer a sensible and space-saving alternative to the conventional load cell.