Threading material

[ScottS]

We are having an unusually high failure rate in one of our products. The components are both made of 303 S.S. and are held together by a 2"-20 unc-3A thread. The environment in which they are used is very harsh and we have tried to keep them as clean as possible but the threads are still binding. What are the rules of thumb when it comes to choosing material for threading parts together? For example, is 303 S.S. more likely to have galling problems than 316 S.S.? Any help would be greatly appreciated.

Thanks,
Scott

 

[Ron]

Are the threads actually galling or is there corrosion or product buildup in the threads? Have you considered other thread profiles(Acme, UNF, NPT, etc.) or would loading conditions preclude their use?

 

[ScottS]

The threads are not really galling but they are binding over time. There is some debris getting by our seals and we have plans to use different seals but I'm not convinced that this is the sole cause of the problem. What might be occuring is they might be drying out, making them very susceptible to debris or it could be a material problem or maybe I have choosen the wrong type of thread as you have suggested. I haven't experimented with other types of threads but they are under a high axial load and need to have min. backlash. One thought is to use dissimilar metals so one will re-cut the thread in the other when debris gets in the way. I'm open to any suggestions.

Thanks again.

 

There will always be a binding problem if the mating parts are of the same material. I have got over the problem by making one item from austenitic and the other from martensitic. There is also an old standard for modifing the threads. This increases the clearences, removes the crests and modifies the root radius.

 

[Waterjetz]

Jail is right on target. My company deals with this kind of problem occasionally, and it becomes more acute as your thread size (diameter) increases. I would suggest leaving the male threaded part as 303, and change the female threaded part to 17-4, heat treated to H1025 or H900. It's a little more difficult to machine than 303, especially if you heat treat it prior to machining. But with sharp cutters, you shouldn't have a problem. If you decide to heat treat it after machining, be sure to stand it on end so your threads can't 'egg' out of shape, especially if it's a thin walled part. Good luck.