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Consider the following statements regarding the ends of the pressure vessels flanged by pre-tensioned bolts: 1. Pre-tensioning helps to seal the pressure vessel. 2. Pre-tensioning reduces the maximum tensile stress in the bolts. 3. Pre-tensioning countermands the fatigue life of the bolts. 4. Pre-tensioning helps to reduce the deleterious effect of pressure pulsations in the pressure vessel. Which of the above statements are correct?

Consider the following statements regarding the ends of the pressure vessels flanged by pre-tensioned bolts: 1. Pre-tensioning helps to seal the pressure vessel. 2. Pre-tensioning reduces the maximum tensile stress in the bolts. 3. Pre-tensioning countermands the fatigue life of the bolts. 4. Pre-tensioning helps to reduce the deleterious effect of pressure pulsations in the pressure vessel. Which of the above statements are correct? Correct Answer 1, 3 and 4 only

Concept:

  • If proper preload and clamp force is not developed or maintained, then problems such as fatigue failure, joint separation, and self-loosening from vibration can developed in the bolted joint leading to joint failure.
  • The bolt is an elastic member. It has a spring constant k, and will develop a load based on the stretch of the bolt. In just tighten a bolt it only overcome the friction of the bolt and nut or threaded hole, there would essentially be no load on the joint, and any vibration-induced loosening would immediately render the joint.
  • Pre-tensioning does not reduce the maximum tensile stress in the bolts instead it helps to reduce the harmful effect of pressure pulse in the pressure vessel by increasing stiffness of the bolted joint.
  • External tensile load applied to the unpreloaded joint transmits entirely to the bolt. The stiffness of a bolted joint without a preload equals the bolt stiffness i.e. Ke = Kb.
  • For a preloaded bolted joint, joint stiffness is the sum of bolt & clamped member stiffness
  • That is ke = kb + kc
  • Hence preloading stiffness bolted joints will also increase the resonant frequency of an assembly.

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