TechArchive

This article was originally written in the period 1995-2000

How much torque should be put into a joint? This value is often not defined, so a design engineer often has to base the value on experience and values which the fastener manufacturer may recommend. It is also possible to establish the approximate optimum torque by a trial and error method:

• 1 Take a small number (say, 10-20) of new screws and make sure they are free of any lubricant. Using the screwdriver, tighten the fasteners to within 10% of the estimated torque.
• 2 The assemblies should then be subject to normal service loadings, including vibrations, temperature changes, etc. Note any loosening or relaxation in the joint. Adjust the screwdriver torque level and repeat the test with new screws and components until total joint integrity in service is achieved.
• 3 The establish the ultimate over-torque level by applying a low friction lubricant to the screw threads and hand tightening them to specified torque +10%. Observe the degrees of rotation in the screw in case they suddenly increase, indicating “necking” or fatigue in the screw. Then continue to apply torque until either the thread strips or the screw head breaks off.
• 4 The established safe torque should then be inserted on the production drawings.

Fastener torque values

Clamping force is what actually keeps a joint together and applying torque to a threaded fastener achieves this clamping force by using the thread pitch to pull the screw head down onto the surface of the work place. The table shows recommended torque values for metric fasteners of different tensile strengths or grades. The values shown are for lightly oiled, plain steel screws, having a coefficient of 0.125. The most common grade is 8.8, but 10.9 and 12.9 are also used. It is not recommended that grades below 8.8 are used for machine screws which will be assembled by power tools.

Screwdrivers and Screwdriving is a handy, 24 page, A5 guide to component jointing using threaded fasteners from Desoutter. Its five main sections cover: Threaded Fasteners (illustrating the different types of self-tapping screws and screwheads); Types of Joint; Torque, Screwdriving Tools; and Problem Solving. Single copies are available free of charge by circling the enquiry number immediately below. Also, to assist manufacturers in building up training programmes, the company tells us that the material is available in 8.5 x 11in format for copying onto overhead transparencies.

• Desoutter
• Tel: 0181 205 7050
• Fax: 0181 205 5167