Machine Design

Direct Tensioning

Usually, tension is induced in a bolt or stud by torque applied to the nut. The torque applied to the nut becomes force applied to stretch the bolt, which works much like a spring. However, tension may be applied directly to the bolt or stud, with the nut used to maintain that tension.

One method of direct tensioning uses hydraulic stud-tensioning equipment to stretch the bolt in a straight line, producing the proper axial tension. The nut is then reseated, because it has been lifted off its bearing surface during the stretching operation. Rather than doing the work of stretching, the nut merely maintains the fastener in the stretched position.

Advantages of direct tensioning include the elimination of torsional stress caused by torquing the nut. It also eliminates overstressing the bolt. Because the relationship of applied torque to axial stress is variable, assemblers must often overcompensate. Manufacturers claim time, safety, equipment life, noise reduction, and repeatability advantages over standard methods.

Direct tensioning is suitable:

  • When even loading is required, as with a gasketed flange.
  • When fasteners are designed to operate near yield, because the additional stress produced by torquing could cause failure.
  • When the process of loading the fastener is dangerous, as in high-pressure, high-temperature reactor-tube crimping operations. Tensioning permits loading the fastener with the crew operating at a safe distance.
  • When replacement of fasteners is expensive, galling and nut damage can be eliminated. Tensioners require no mechanical force to turn the nut.
TAGS: Fasteners
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