Bolting

1Q. What is the one rule that should be followed in bolting?

1A. Keep them tight! Some feel it's a good idea to try to keep the preload at least twice the externally applied load if possible. Under many situations this will eliminate fatigue failures due to cycling loads. The preload has to be large enough so the bolts won't relax and loosen.

2Q. We tightened the bolts carefully to produce a nominal stress of 50,000 lb/in2 but it still failed in fatigue, what happened?

2A. Not knowing anymore about the problem there is a good chance that the joint relaxed. On short bolts or studs with gaskets there is not enough "stretch" in the bolt length. In cases where the bolt stretch is less than 0.002 inch, a gasketed joint can easily relax this much due to the gasket, threads and surfaces. The more surfaces clamped together the greater the problem. Once the bolt stretch is gone and there is no rebound in the gasket or joint the bolt will become loose. With loose bolts and cyclic loading a fatigue failure is usually not far away.

3Q. We are changing from carbon steel bolts to stainless steel bolts to prevent rusting. Is this a good solution?

3A. You need to be careful. Stainless bolts may have a lower strength and be more susceptible to corrosion mechanisms such as stress corrosion cracking. Temperature, and environment are important factors to consider before changing. Check with your engineering department first.

4Q. I've heard a lot about Superbolts. What are they anyway?

4A. There a patented method and that is a trademark for a bolt that uses cap screw bolts to put a lot of stretch in a stud with only hand tool. I've used them for compressor rods, split line bolting on large compressors and other places high accurate loads with limited space are needed. Make sure you follow the manufacturers instructions carefully especially on reciprocating compressor rod bolts. Because of the high loads you can "kink" the rod.