Emerald Group Publishing Limited
Copyright © 2000, MCB UP Limited
Keywords: Fasteners, Accuracy
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 a stripped 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; the problem with this method, however, is that it will not necessarily 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 assuming a properly trained operator and a perfectly calibrated torque wrench, errors of the order of 50 percent can and do occur.
Figures quoted in the July/August issue of the Engineering Designer magazine claimed that loose fasteners cause 23 percent of all service problems in the automobile industry and that up to 12 percent 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 Components are 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. Drilling bolts in this manner does not normally affect their performance as a fastener 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 therefore which is limiting the force that 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 and linearity in the order of a fraction of 1 percent is quite normal. Being 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, does not so the straingauge bridge doesn't suddenly go out of balance when the temperature changes.
Typical applications for this type of technology are Aerospace research and testing, Automotive development and Nuclear Engineering.
For further information contact Roy Moffatt, Managing Director, Variohm Components Limited, Williams' Barns, Tiffield Road, Towcester, Northants NN12 6HP. Tel: +44 (0)1327 351004; Fax: +44 (0)1327 353564.