Thread Strength in Delrin

[GL]

5/16 wall for a 1/4” bolt/set screw is on the edge. Delvin is pretty tough, but that’s pushing it. If I’m thinking about this right, there’s an inner sleeve you are making. The sleeve fits between the inner tube and an outer tube. The outer tube has a hole in it for the set screw to tighten down on the inner tube - set screw is threaded into the sleeve? Ideas maybe - what would happen if you put two set screws in opposite of each other so you don’t have to tighten them up as much. What about making an outer sleeve, over the top of the tube, with a heavier wall to get more thread engagement, set screw going through the side of the tube and the sleeve so everything stays in place? Or make the sleeve longer with a step that goes inside the tube and a bigger OD for a thicker wall that hangs down a little?

As we are frequently reminded, a picture would be nice. Good for you on trying to fix it instead of just chunking it in the dumpster on the way to the umbrella store.

 

[rwm]

5/16 wall for a 1/4” bolt/set screw is on the edge. Delvin is pretty tough, but that’s pushing it. If I’m thinking about this right, there’s an inner sleeve you are making. The sleeve fits between the inner tube and an outer tube. The outer tube has a hole in it for the set screw to tighten down on the inner tube - set screw is threaded into the sleeve? Ideas maybe - what would happen if you put two set screws in opposite of each other so you don’t have to tighten them up as much. What about making an outer sleeve, over the top of the tube, with a heavier wall to get more thread engagement, set screw going through the side of the tube and the sleeve so everything stays in place? Or make the sleeve longer with a step that goes inside the tube and a bigger OD for a thicker wall that hangs down a little?

As we are frequently reminded, a picture would be nice. Good for you on trying to fix it instead of just chunking it in the dumpster on the way to the umbrella store.

I have thought about an outer sleeve with threads instead of threading the inner sleeve. It could really just be a short collar over the OD. That will probably be my bail out solution if things strip out. It would add some weight but not much. This is for my brother so I don't have the item in hand to take a pic.

 

[piper184]

20 TPI x .325 = 6.5 threads.
I doubt that you can put enough force on a hand tightened screw to strip the threads.
In steel you can put more torque on a fine thread but in castings you generally want to use a coarse thread. I don't know the preferred setup for Delrin but my guess is a course thread is better.

It has always been my experience with 'plastic' bushings that running them dry on metal is the best results. The friction for Delrin should be even less than most plastics so you can get more clamping force with finger tight.

I think you are on the right track. Just try to make the threads as close fitting as possible. Shoot for a 95% thread fit instead of the usual 75% and that will add strength to the threads as well. Might even add a self locking (ny-loc) feature to the fit. Use a stainless steel thumbscrew and it should never corrode or jam.

 

[WobblyHand]

3d print with a threaded insert? You can make the threaded area longer with a boss sticking out so you can get more threat. I realize it's not machined, but it's mostly plastic and light weight, and has metal for threads.

 

[rwm]

3d print with a threaded insert? You can make the threaded area longer with a boss sticking out so you can get more threat. I realize it's not machined, but it's mostly plastic and light weight, and has metal for threads.

I thought about 3D printing, even doing it solid. Delrin is cheap and def gonna be stronger.