TechArchive

Original article date: April 2000

By measuring the clamping force of threaded fasteners, says ROY MOFFATT, you can be more confident of bolts or studs performing to expectations than you can by relying on a torque wrench alone

How many products rely on threaded studs or bolts for their proper operation? Consider, for instance the importance of correctly tightening the bolts in a joint which incorporates a gasket. Too little clamping force and the gasket might as well not be there. Too much force, on the other hand, and we have stripped the thread.

The conventional way to try to guarantee that the bolts or studs are doing their job properly would be to tighten them to a specified value using a torque wrench. But the problem with this method is that it will not necessecarily produce the correct clamping force. 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 under ideal conditions with a perfectly calibrated torque wrench errors of the order of 50% can and do occur.

According to recently published figures, loose fasteners cause 23% of all service problems in the automobile industry, and as many as 12% of new cars have problems with incorrectly tightened fasteners.

The ideal way to measure the clamping force in a fastener is to measure it directly by means of strain gauges. Variohm is now 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.

Fastening performance

Drilling bolts in this manner doesn’t normally affect their fastening performance since the hold 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, therefore, which is limiting the force the bolt can apply. On a stud, which is normally threaded both ends, some de-rating does occur, but this can normally be offset through the selection of a different 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 linearity in the order of a fraction of a percent is quite normal.

Being placed at the centre of the bolt or stud, 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 that the strain gauge bridge doesn’t suddenly go out of balance whenever the temperature changes.

If the figures quoted for the automotive industry are correct, then this technology could offer far reaching benefits. Consider, too, what it could offer to aerospace research and testing. Areas such as nuclear engineering would be another typical application for this type of technology.

• Variohm Components

April 2000