Elastomer bellows seal tolerance

[zunny]

Does everybody know where to find the elastomer bellows seal tolerance standard? I've found the John Crane's type 21, but I suspect that it can be used for the standard. How about ANSI or ISO standard? I heard from a colleague that the usual one is +0mm/-0.5mm, but if the shaft tolerance is +-0.005mm, is it too tight to install the seal? Thanks for your input.

 

[flexibox]

I am not sure that I fully understand your question but if you have a shaft allow +0/-0.05mm if you are looking for axial setting tolerancing for assembly then +/- 0.5mm is fine on a single spring even +/- 1.0mm for axial. When you fit the seal push the sliding face against the spring and ENSURE that the spring can slide it back without effort. If the seal does not slide then it is too tight. the seal needs to be free to move axially when in operation. I doubt whether you will measure the diameters.

trust this helps

 

[zunny]

Thanks, flexibox. I made a typo in my last post, the shaft tolerance should +/-0.05mm rather than +/-0.005mm. It makes perfect sense that the spring should slide it back, but is the seal itself stantionary? Otherwise, how can it seal the fluid? My understanding is the ID of the Type 21 mechnical seal must be with the minus tolerance because the rubber bellows need to hold the shaft tightly and this "holding" ensures the rotary union to rotate with the shaft. In terms of the seal/shaft tolerance, is there any industrial standard which I can look up?

 

[bk19702]

In a rubber bellows seal like the Type 21, only the front portion of the bellows moves axially. The portion of the bellows in contact with the shaft seals statically to the shaft and does not move axially. This is why rubber bellows seals fall into the non-pusher seal category. The tolerance is tight on the rubber bellows for this reason as once you get the bellows installed on the shaft at it's set dimension, there it stays. The front of the bellows, where the spring contacts, is still free to move in the axial direction.