Design bolt preload

DESIGNING WITH PLASTICS AND COMPOSITES A HANDBOOK Bolt Preload... 

Bauhof, M. Maintaining Bolt Preload in Plastic Assemblies. Du Pont Engineering Design, pp. 14-15, summer 1987. 

Many designs incorporate the phenomenon of stress-relaxation. For example, in many products, when plastics are assembled they are placed into a permanently deflected condition, as for instance press fits, bolted assemblies, and some plastic springs. In time, with the strain kept constant the stress level will decrease, from the same internal molecular movement that produces creep. This gradual decay in stress at a constant strain (stress-relaxation) becomes important in applications such as preloaded bolts and springs where there is concern for retaining the load. The amount of relaxation can be measured by applying a fixed strain to a sample and then measuring the load with time. 

Flat gasket seals must be preloaded with a force equivalent to the force that will be exerted by the pressure within the vessel. A flanged bolt closure or split ring bolt closure design can be used to achieve this preload. Closure bolts should be evenly tightened to a specified torque that will provide the proper closing force. 

Note Hydraulic and mechanical tensioners have to translate hydraulic pressure or torque on the small jack screws, respectively, into preload. If more accuracy for measuring the tension in the bolt is required, and hydraulic or mechanical jacking has been specified, a device such as the RotaBolt or the equivalent can be incorvorated into the anchor design. 

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