From what i remember...(long) - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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From what i remember...(long)
Posted by Jeff S. (more from Jeff S.) on Wed, 28 Mar 2001 10:14:25
In Reply to: Re: Headbolts and re-torquing...., Dean, Tue, 27 Mar 2001 13:31:09
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From what I remember in my machine design class (which isn't as much as I would like) "proper" torque is such that the initial tension is nearly equal to the proof load ( maximum load that does not produce a measurable permanent deflection). Translation: the bolt is tightened to a near yield condition.
Why:
1. Maximum protection against overloads. Bolt load cannot be increased very much unless the members seperate (bad for an engine). Likewise, the higher the initial tension the less likely the members are to seperate.
2. Max. protection against thread loosening. Tightening a bolt imparts torsional stresses as well as tensile stress. When an external load is first applied the bolt unwinds and the tensile stresses are removed thus lowering the bolt tension. Also, greater initial tension creates larger friction forces that keep the bolt from loosening.
3. Dynamic load on the bolt is reduced because the effective area of the clamped members is larger. Take a bolt tightened to proof load, now apply an external fluctuatuing load (i.e. combustion gasses). Upon initial operation, the bolt yields locally and clamping force is decreased. When the external force is released the bolt shrinks elastically (moves down the elasticity curve but with a permanent deflection). Likewise, the clamp force and bolt force have been reduced slightly. This shrinking is reversible, meaning that the load can be applied without the bolt yielding further.
Please feel free to add/disagree,etc.
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