Best practice %preload for bolted joint

[engtiuser2]

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.

 

[dik]

The load in the bolt stays the same as the pre-load up to the point the pre-load is reached... then the load in the bolt increases.

Dik

 

[TehMightyEngineer]

What dik said, you don't add the preload and the static load.

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

https://www.facebook.com/AmericanConcrete/

 

[WARose]

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%)?

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.

 

[engtiuser2]

The reason I add the preload to the static load (Fi + C * P, where C is joint constant) is to understand what is the maximum bolt load is. I used C=0.3 as worst case. I hesitate to go less than 50% as I am using torque wrench and too low pretension can be more likely to prone for error (due to variability in friction.). Sounds like as long as I positively lock the bolt from loosening and clamp load maintained, the preload can be reduced.

 

[Shotzie]

These blog posts are pretty interesting reads as an aside, credit to JGard1985 and his colleague:
Link
Link

 

[Shu Jiang PEng SE]

Two points: 1, the point should be pre-load to 70% minimum tensile strength of the bolts if slip critical bolts are required in the structures; 2, the tension load in bolt does keep constant until the the bolt tension due to the external load is larger than the pretesion.

 

[rb1957]

constant load in the bolt is a simplification. The load in the bolt increases slightly (as engtiuser2 notes above) until the joint gaps when the bolt load is equal to the applied load. This load increase is due to the external load relaxing the joint clamp up.

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.

another day in paradise, or is paradise one day closer ?

 

[klaus.]

You do not add preload to the external loads

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

 

[engtiuser2]

The fraction of external load which transfers into the bolt is called joint constant (C). It varies between 0.1 to 0.3 (typically around 0.2). That means, 100 pound load will be 20 pounds go into the bolt in worst case. Typically we preload the joint to 75% proof load of the bolt and that is typically much larger than the portion of external load went into the bolt. That mean, we need to consider both preload and external load when sizing the bolt. Having said that, with the bolt preloads at 75%, there will be 25% left to handle the external load. If you size your bolt purely based on the load, that means the load will be equal to the proof load of the selected bolt (or better). This load will imposed 20% of its weight to the bolt which is available from the 25% left from the preload. In other word, you can size the bolt by ignoring the preload. This is for static case and for dynamic load, it is different story.