Air Compressor Foundation Keeps Breaking Bolts

Try putting a resilient mounting under the faces that the bolts are pulling together, and tightening the bolts so that there is not so much tension in them.

Too much tension?

Well, perhaps. However, before getting to that point, one can't discount that there isn't enough tension in the bolts. Installing resilient mountings is not going the solve the problem. In fact, this will surely make matters worse

There must be sufficient preload in the bolts in excess of the dynamic loads that they see when the compressor is running. If there isn't, the bolts are likely breaking due to fatigue failure. "Too much tension" could be the problem only if an over-zealous mechanic is tightening the bolts beyond yield.

Although the former is often the cause in situations such as this, either scenario is plausible.

How are the bolts being tightened? Is there any control to the process or, do you simply torque the bolts (with or without a calibrated torque wrench)? Do you periodically check to see that they are still "tight enough"?.

The first step to resolve the issue is to incorporate some sort of control, even if it is simply using a torque wrench. However, this isn't enough since torque has a limited and shady relationship to clamp load. Consider this: You use a calibrated torque wrench to apply the proper "torque" to all of the foundation bolts. If the desired torque is x-ft-lbs, the wrench will register this once sufficient force is applied. Hence, the bolts will be "tight enough", right? No, not necessarily: What if a couple of those bolt holes had crud in the bottom? The bolts in the afflicted holes could have bottomed-out before the head (or nut, if it's a stud application) had begun to clamp the foundation. Thus, even though proper torque was applied, the bolts would still be too loose - and likely break due to fatigue failure .

One must verify the effects of torque before assuming that the process has resulted in the correct bolt load. Otherwise, it's a big guessing game.

There are a number of ways to do this ranging from elongation measurement to load-indicating fasteners. The latter are quite interesting since they change colour depending on the degree of load - and revert if they become loose. Hence, they provide an easy way to quickly determine whether or not the fastener is still tight enough.

Good luck!

I just realized that there seems to be a misunderstanding about whether the foundation itself or the bolts are breaking. The subject field suggests one thing yet the body of the post says another.

If the foundation is breaking, the probability of Mr Slacks's contention that the bolts are too tight and thereby damaging it, is higher. However, if the bolts are too loose, the foundation can move and hence break.

In either case, to resolve this issue, the proper preload must be ensured;the bolts shouldn't be too loose nor should they be too tight.

"Too much tension" could be the problem only if an over-zealous mechanic is tightening the bolts beyond yield.

As you very well know in some applications bolts are tightened beyond the elastic limit especially for a better fatigue behaviour since the force variation on the bolt is a lot less.

If bolts break you are right to stress that most probably pre-load is too small.

Now since we do not know what really happens it should be also considered as possibility that bolts are too short in foundation concrete and destroy it via too important shear strains.

A better more detailed information would help.