Optimum thread size for set screw

[Wizard353]

I need to make some quick and dirty, light duty cylindrical couplers for connecting two 0.25" OD round shafts end-to-end. I planned to use 5/8" OD 6061 aluminum rod with a drilled 1/4" coaxial center hole, secured to the two shafts by a pair of set screws. I will have 3/16" of aluminum to work with for each set screw.

What size set screw would give the best holding power -- 4-40, 6-32, 0r 8-32? A smaller thread size gives you more threads, but a larger screw bites deeper into the metal.

Any suggestions on selecting the best screw size?

 

[T Bredehoft]

I'd use a piece of 1045 or so steel. In any case, the finer thread (for a given diameter) the stronger the threads. I'd go with the 8-32.

 

[David S]

Welcome to the forum. Can you give us a bit more info on the application. Will it be in tension? How much torque if any? etc.

David

 

[Wizard353]

I'd use a piece of 1045 or so steel. In any case, the finer thread (for a given diameter) the stronger the threads. I'd go with the 8-32.

I thought a 4-40 thread would be considerably finer than an 8-32.

 

[Chuck K]

LOL...you're really going out on a limb there. 40 is usually finer than 32. You are correctamundo!

 

[RJSakowski]

I thought a 4-40 thread would be considerably finer than an 8-32.

But the larger diameter will give you considerably more strength.

 

[Wizard353]

Welcome to the forum. Can you give us a bit more info on the application. Will it be in tension? How much torque if any? etc.

David

David,

I am connecting two inexpensive axle and roller assemblies that I just picked up from The Electronic Goldmine (no connection to seller, but a frequent customer).

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16155A

They are an ~1" diameter plastic wheel that spins pretty freely on a 1/4" shaft roughly 1.25" long. I haven't taken one apart yet, but I doubt that there are any ball or sleeve bearings inside, so it's just the steel rod rubbing on the slippery plastic.

This project is for a temporary application in which they will see some light radial and axial forces for about 5 minutes without a lot of torque.

I could probably get away with using cardboard and duct tape for this particular application, but for future reference, I was wondering about choosing the optimum set screw thread size.

Thanks,
(Another) David

 

[David S]

David, I think I might use #6 and put flats on the shafts where the set screws will land to provide better rotation security and also won't make a burr if you have to take them apart. Alternately a slight dimple in each of the shafts for better retention and reduces the burr problem as well.

David

 

[Wizard353]

David, I think I might use #6 and put flats on the shafts where the set screws will land to provide better rotation security and also won't make a burr if you have to take them apart. Alternately a slight dimple in each of the shafts for better retention and reduces the burr problem as well.

David

Thanks, David,

That's just what I guessed, and I was going to flatten the shafts as well.

(Another) David S

 

[T Bredehoft]

I thought a 4-40 thread would be considerably finer than an 8-32.

The ultimate strength is a shear cylinder, the diameter of which is the pitch diameter of the screw. Steel in aluminum will ultimately shear at the od of the screw.