Ensuring fasteners are done up tightly

Strain gauges inserted in fasteners make certain the correct clamping force is applied

For proper operation many products rely on bolts or threaded studs. Where a gasket is involved in a joint the correct tightness is essential: too little clamping force and the joint leaks, too much and there is a danger of stripped threads.

Torque wrenches are the traditional way of ensuring that studs and bolts are tightened and doing their job properly, unfortunately it does not necessarily mean that the correct clamping force has been applied. This method is inaccurate because the torque figure will depend far more on the design of the fastener and the prevailing frictional conditions than it will on the clamping action.

Only a small proportion of the applied torque is actually used to extend the bolt so even assuming a properly trained operator and a perfectly calibrated torque wrench, errors of the order of 50% can and very often do occur.

Figures have been produced claiming that loose fasteners cause 23% of all service problems in the automobile industry, and that up to 12% of all new cars have problems as a result of incorrectly tightened fasteners.

The ideal way to measure the clamping force in a fastener, says Variohm Components, is to use a system that has been around for a while and measure it directly by means of strain gauges; and the company is able to offer this service on bolts down to 5mm diameter.

In order to provide the best accuracy as well as the best environmental protection the gauges are placed in holes drilled down the longitudinal axis of the bolt or stud. These holes can be as small as 4mm diameter. Performance of bolts drilled in this manner is normally unaffected as the hole only protrudes into the plain part of the body, which, even with the hole in it, is still far stronger than the thread. It is the thread which is limiting the force that the bolt can apply.

On a stud, traditionally threaded at both ends, some de-rating does occur, but this can be offset by selecting an alternative material.

Gauges installed by this method sit on the bolt’s neutral axis where errors due to torsion or bending are at a minimum whereas the tensile force is at a maximum, so accuracy is enhanced and linearity in the order of a fraction of a percent is quite normal. Being on the centreline of the fastener the gauges are not as subject to radiant heat as they would be if mounted on the surface. Conduction means that the heating effect will be slower and more even, so the straingauge bridge will not suddenly go out of balance when the temperature changes.

Variohm Components Tel: 01327 351004