Torquing set screws

[Captain Spaulding]

I have a need to torque some set screws with a 1/4" hex key to 40ft-lbs.

I used a 1/2" to 3/8" drive adapter to connect a 3/8" drive 1/4" hex key to my torque wrench. Worked fine for two of the screws, but the other two are recessed about 2" in their holes requiring a longer key to reach. I bought a 6" long 3/8" drive key, but at any significant torque, the shaft twists considerably making me worry that it will break. I cut the key down to the length required to reach the screws to minimize the issue, but I still don't like it.

This is on a trailer hitch, so I might need to service it on the road, and relying on a tool that seems to require use near its breaking point doesn't give me that warm, fuzzy feeling. I know anything might break at any time, but hoping the torque wrench clicks before the key breaks is a little unnerving. The thought of a broken piece of hex key in a set screw at the bottom of a 2" hole while on the road somewhere is more unnerving.

I looked at drilling the hole out to allow the key shaft to be shorter, but the most I could do might let me use a 1/4" drive 1/4" socket with a short hex key section in it. That would end up with a 3/8" to 1/4" adapter experiencing 40ft-lbs though, and I probably like that even less.

I think this is just a bad design. Am I worrying needlessly about the 1/4" hex key shaft at 40ft-lbs? Any better suggestions?

 

[vjquan]

40 ft-lbs on set screws using 1/4 hex keys seems a bit much. Are you sure it isn't 40 in-lbs? Set screws are just used to hold something in place, not for it's main tightening.

 

[Dick in Wisconsin]

I agree. 40ft-lbs sounds like a way too much. No wonder the hex key is twisting. Double check the specs. If they really say 40ft-lbs, contact the manufacturer and have then double, triple check with engineering. I suspect its inch-pounds too!

 

[larry_g]

http://www.safetysocket.com/sites/www.safetysocket.com/files/parts/bd/Products/setscrewtorque.htm

This is a 1/2" fastener that according to the above chart torques to ~ 50 ft/lb. Not if the OP is is referring to a 1/4" dia fastener then it would be less but he is referring to a 1/4" bit size.

For key limits http://www.bondhus.com/features/protanium/body-4.htm

lg
no neat sig line

 

[American Locomotive]

You're really worry about nothing.

Cutting the key shorter does nothing in terms of reducing the chance of breakage. At a given torque, the hex key will twist the same amount per linear unit length of key. For example: A 1" long hex key may twist 10° across its length when 40 lb-ft of torque is applied. If you make that same hex key 4" long, it's going to twist 40° across its length, however it's still only twisting 5° per inch.

You could make the key 36" long, and that key will twist around in a full circle at 40 lb-ft of torque, but it still will not break. If a 1/2" long key reaches proper torque without breaking, the same style key, made from the same material from the same manufacturer will achieve same torque without breaking - regardless if its 1/2" long, or 1/2 mile long.

In fact, I suspect by cutting the key, you've likely weakened it near the end. Especially if you used an abrasive cutting method.

 

[Captain Spaulding]

Definitely 40ft-lbs for the spec. One of the biggest issues with the longer key is that it takes a significant portion of the swing of the wrench to actually apply much torque to the fastener since the key twists substantially. That makes it hard to use proper technique and hold the wrench by the handle.

I'm not sure whether the key was shortened with a shear or saw. I didn't want to cut it with my chop saw so I just took it to a maintenance machinist buddy and had him cut it at work.

 

[American Locomotive]

Is there any particular reason why you can't just start the wrench handle at different part of the swing, so when it's near proper torque, the wrench is in the right position?

You could also get a long 1/4" box-end wrench and slip it onto the hex key to pre-load it, if it makes it easier to get the torque wrench into position.

 

[Captain Spaulding]

Is there any particular reason why you can't just start the wrench handle at different part of the swing, so when it's near proper torque, the wrench is in the right position?

You could also get a long 1/4" box-end wrench and slip it onto the hex key to pre-load it, if it makes it easier to get the torque wrench into position.

You end up against the trailer on one end, and against the hitch on the other, so your arc is limited and your standing position can be a bit restricted.

Box end wrench is a good idea. It would "hold the twist" while I reset the torque wrench.