When do I need a vacuum bellows, and how do I size one?

Fit a bellows when two rigid parts of the system must move relative to each other — thermal growth, pump vibration, or an alignment mismatch you cannot design out. A bellows is not a way to fix sloppy pipework, and using it that way is what breaks them.

The three jobs are worth separating because they call for different products. Vibration isolation between a rotary or Roots pump and the chamber needs a short, soft bellows close to the pump — its purpose is to stop mechanical vibration reaching the chamber and the gauges. Thermal compensation needs travel in one axis, sized from the expected temperature swing and the run length. Alignment compensation is the one people misuse: a bellows will absorb a small offset, but it is not a universal joint and a permanent lateral offset will fatigue it.

Size on the movement, not on the pipe. The specification that matters is permissible axial compression and extension, lateral offset and angular deflection, each of which reduces the others — a bellows given its full rated lateral offset has little axial travel left. Give us the actual movement in each axis and the cycle count, and the selection follows.

Construction changes what it will survive. Formed (hydroformed) bellows are the economical choice for moderate movement and long life on clean duty. Edge-welded bellows give much more travel per unit length and are used where space is short or movement is large, at higher cost and with more sensitivity to particulate. Braided metal hose is a different product again: it absorbs vibration and routing offsets but has very little axial travel, and squeezing axial movement out of a braided hose destroys it.

Two failure modes to design against: torsion and over-compression. Bellows tolerate almost no twist, so if one end rotates during assembly you have already used up life; fit them so the last connection made is a rotating flange, not the bellows itself. And a bellows compressed beyond its rating on install has no travel left for the thermal cycle it was bought for.

For the flange end, match the standard the rest of the run uses — KF or ISO for elastomer-sealed systems, CF where the run is baked. A bellows is also where a standard boundary often lands naturally, so a KF-one-end/CF-other-end version can save an adapter and one more joint to leak.