Force Application with Compressive Force Sensors

A force transducer’s technical features and service life are not only determined by the force transducer itself but also by its design and the design of its force application parts. Structural elements to which force sensors are connected must meet specific technical requirements. Detailed information on this topic is provided in the operating manuals.

The load application parts should meet the following conditions to enable users to get the most out of the sensors’ accuracy and service life:

A major requirement is that the force application parts must have a sufficient hardness (40 HRC) to minimize wear.
The force application parts should have a polished surface (Ra=0.8 µm).
The load application parts must not be severely deformed by the effect of the load.
The load application parts are to ensure that the force is centrally and uniformly applied to the sensor.

Thomas Kleckers, HBM — Thomas Kleckers, Product Manager Force Sensors at HBM

Convex Load Application Knob

Many compressive force sensors have a domed load application knob on their upper side, which ensures automatic centering, i.e. central force application. To achieve this, the load application part’s highest point is positioned exactly in the sensor’s center.

C10 force transducer with a convex load application part

Very high mechanical stress (Hertzian stress) is created at this point, especially with force transducers for very large forces; the sensor designer can influence these by choosing a force application part radius that favors a high accuracy.

It is essential to bear in mind these stresses when dimensioning the counterpart since there is an actual risk of the counterpart being deformed. Therefore, so-called thrust pieces are offered for all the compressive force transducers as well as for some transducers for both tensile and compressive force; these meet the above requirements and prevent damage to the connected structural element.

C10 force transducer with a thrust piece

Special features of force sensors with an internal bore and compression bars

Force sensors with an internal bore and compression bars such as the C6B from HBM offer numerous advantages:

A very high stiffness and a high fundamental frequency – the ideal choice for dynamic measurement tasks
A robust design that ensures immunity to corrosive influences as well as vibration or shock
A cost-effective solution
Availability for extremely high capacities (HBM products up to 10 MN)

Since these compressive force transducer types do not have convex load application parts, mechanical connection them is easy.

Uniform force application

The high stiffness and the absence of a central force application part require reasonable care to be taken to ensure proper sensor mounting. The strain gauges that convert the applied force into a measurable change in resistance are installed at four different points over the circumference of the transducer. The force must be applied uniformly to the circumference to attain good results and to be converted into strain as uniformly as possible. Unevenness and tilting are not compensated for by those sensors because they have very small displacements.

Compact versus long design

An easy way to ensure a uniform mechanical stress distribution is to build sensors that are as long as possible, which ensures that the strain distribution over the circumference is becoming increasingly more favorable. However, it goes against frequently expressed customer requests for a compact design and low weight. This approach is acceptable with regard to high-precision reference force transducers, and these sensors are designed accordingly.

The new C6B force transducer with an internal bore that enables the use of a fold-back arm. Transducers with capacities of 5 MN and higher do not have this internal bore.

HBM offers loading fittings for the C6B compressive force transducer that meet the above-mentioned requirement, i.e. they ensure a uniform mechanical stress distribution. In general, compact force sensors with an internal bore and compression bars are, however, slightly more likely to be affected by the mounting conditions. Therefore, load application parts are provided that can be ordered alongside the C6B to address this issue. The special feature: The sensor and the load application parts that will be used in the application will be calibrated together. The parts are marked accordingly at HBM to ensure that it is always only the combination calibrated by HBM that is used for taking a measurement. Both spherical caps (see fig. below) and load buttons can be used, which, in turn, ensure a convex load application; matching thrust pieces are available.