During the past decade it has become extremely important to optimise all aspects of product design. Competition is tougher than ever and steel costs have skyrocketed. For economic reasons each component must be utilised to its maximum capacity. Bolted joints became the centre of attention after recent findings that they are often used to as little as 30 per cent of their capacity.
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Moreover, a failed critical bolted joint could lead to expensive warranty claims or maintenance costs. Control of the clamp load in a bolted joint is vital. However, when faced with a problem joint, it is not surprising that the design engineer will not have an answer if asked about the clamp load. Torque calculations must always be based on the existing conditions that often are very vague. Unless all parameters are correct, the calculation will be unreliable. Examples of parameters are:
- Thread condition of the fasteners
- Hardness of contact surface
- Material (steel, aluminium, copper, etc.)
- Extra friction from a locking fastener
- Extra friction from an adhesive
- Lubricant on the thread
- Type of bolt head (flanged, regular or serrated)
- Surface coating of the bolt
- New or reused fastener
Major automotive companies test incoming fastener batches on the actual material of any specific bolted joint to obtain the torque/load relationship and its deviation. Their engineers therefore know the clamp loads in the joints. However, smaller and
