Do bolts become tighter or looser at cryogenic temperature?

Question

In general, will screws tightened at room temperature become tighter or looser at very low temperature (5 K)?

On the one hand, the screw should loosen because it will contract radially, while the surrounding material will contract slightly less (or even expand, in the limit of a very large screw radius and a very thin ring of surrounding material).

On the other hand, the screw should tighten because the screw should contract linearly along its axis, making the threads tighter, while the surrounding material should contract slightly less because it has other boundary conditions.

Is there a general answer, or does it depend on material, screw size, and/or the exact geometry?

Answer 1

The bolt tension could increase, decrease, or stay the same.

It depends on the thermal stress distribution in the whole structure, which is likely to be eventually supported by something that remains at (approximately) room temperature. This is essentially the same situation as any bolted joint that will carry a load. The "pre-stress" put into the bolts by tightening them when the structure is assembled must be sufficient to accommodate the "working stress" in the joint, whatever causes it.

Of course there are ways to join structures that allow for thermal expansion without creating thermal stresses. They would involve sliding joints, and/or connecting links that can pivot to accommodate the expansion/contraction, etc. A simple example is the tail-pipe of a car, which hangs "freely" from suspension points on links that can pivot as it expands, rather than being rigidly bolted to the car at both ends.

If the bolts are a different material from the parts they are connecting, that is an additional complication. What happens depends on the relative sizes of the coefficients of thermal expansion for the materials.