Best practice %preload for bolted joint
Best practice %preload for bolted joint
(OP)
I am designing a bolted joint to take a high static load (over 5 tons). I have done the analysis and sized the bolt accordingly. With this high load, the bolt was found to be about an inch. It will need to be prelaod to 75% proof load (number can be slightly different from different source, not important at this point). The problem is the preload together with the static load is quite close to the yield point of the clamp material. The question is whether there is any industrial standard to allow reducing the preload. I did the calculation to determine the jaw separation force and the static load is nowhere close to that number( factor of safety of 4). If that is the case, does it mean the preload can be reduced (say 60% instead of 75%)? To make sure the bolt will not get loose due to reduced preload, I am planning to have lock wire in place.






RE: Best practice %preload for bolted joint
Dik
RE: Best practice %preload for bolted joint
But to answer your direct question; you can use snug tight for bearing type bolts under static load. It sounds like you meet this criteria and thus the pretension only needs to be "sufficient to bring the plies into firm contact". Full force of a worker with a spud wrench or a few hits with an impact wrench is all that's required. Fully pretensioned bolts are not required but any pretension you apply between "snug tight" and "full" can't hurt.
Lock wire is great if done correctly but completely useless if done incorrectly. Lock washers are garbage, don't even try. Nylon nuts or double nuts help but not much. I'd go either locktit (if nuts rarely/never need to be removed) or a castellated nut and pin it with a cotter pin (if it needs to be removed regularly).
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
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RE: Best practice %preload for bolted joint
For ASTM A325 & 490 bolts, AISC's standard pre-tension is 70% of the tensile strength. I've specified less before and had no calls from the field. (In fact, for concrete anchors, I've gone as low as 15%.)
So I wouldn't think its an issue as long as you specify what you want.
RE: Best practice %preload for bolted joint
RE: Best practice %preload for bolted joint
Link
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RE: Best practice %preload for bolted joint
RE: Best practice %preload for bolted joint
Typical preload would be between 50% and 70% for bolt strength. With 70% preload, and 30% load increase, so that Pb = Pp+0.3*Pa ... Pb = bolt load, Pp = bolt preload, Pa = external applied load; then the bolt gaps and fails at the same load.
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RE: Best practice %preload for bolted joint
Usually reload is applied to reduce the deformations (rotation) of a connection
For the design we usually do not consider the preload...the connection must be able to carry the loads without preload also
best regards
Klaus
RE: Best practice %preload for bolted joint