Bellows couplings are an excellent choice when selected for the proper application. We asked Bobby Watkins of Ruland Manufacturing to give us some insight on proper coupling selection. The following is an excerpt from the bellows coupling section of his Webinar, Master Coupling Selection: A Guided Journey by Bobby Watkins.
Bellows couplings are for precise movement applications. They’re torsionally stiff, have a high RPM capability-10,000 RPM, and precise movement. However, they are not easy to align. They do require precision alignment and they are going to give you a little bit of forgiveness. When you have a little bit of misalignment to deal with, you want accuracy. You want high torsional stiffness. This is the coupling.
Some benefits. Suitable for high torque misalignment application requiring minimal positioning error. A low inertia coupling, the bellows portion in the middle is thin wall stainless steel. The hubs are aluminum to keep the weight down. You’ve got a low weight coupling? Low inertia. No maintenance required. There’s nothing to do on this coupling as far as regular maintenance is concerned. Again, these are available in step bore combinations, inch to metric, with keys, without keys.
Some drawbacks. No provision for impact loads. We talked about those high impact load or shock load applications with aggressive move profiles that the jaw coupling does so well. The bellows coupling does not. High shock loads, very aggressive move profiles where there’s some torque involved? This coupling does not like it. It tends to fatigue the bellows and crack it, and eventually tear it. If you need that precise movement, you’re probably going to have to forgive a little bit or be a little more forgiving with your move profile, maybe not quite as aggressive a move profile as you could with a jaw coupling.
Limited misalignment capabilities, we talked about that. It needs precision alignment. Not suitable for vacuum applications. Our bellows coupling in particular, is not suitable for vacuum. It’s epoxied together, that bellows portion in the middle is actually epoxied to the hubs and the epoxy will out gas in a vacuum application. A couple of interesting things about bellows couplings that are good things. When a bellows coupling is in failure mode, that wall accordion portion in the middle has cracked and it’s only a matter of time before the coupling is going to fail. It will make noise. It will make a snap, crackle, pop noise as the coupling is rotating. In this situation, order a replacement right away because that’s an indication that it’s going to fail.
The mode of failure with the bellows coupling has a good side to it and also maybe a drawback. The good side of a failure on a bellows couplings is that bellows will tear and you’re now disconnected. You’ve got to tear through it. It’s in two pieces at this point, and you’re not going to keep going and making that product because you’re not driving anymore so that’s a good mode of failure because you know when you are no longer in zero backlash with the bellows couplings, you’re also disconnected. On a vertical application, like we discussed before, where you want a positive drive, you don’t want to drop that load in a vertical application. A bellows coupling is going to tear and you’re going to disconnect and you’re going to drop, possibly drop that load. Again, the mode of failure can be important as to which type of coupling that you choose.