Tightening M20 Nut to 200 N.m with Worm Gear

[ROGER WALL]

Hello all,
Named this WORK because I don't know if a am talking about Force or Torque !
I am trying to do up a M20 nut on a shaft - would like to tighten to say 200 N.m which I can do with my scawny arms and a 400mm long spanner.
BUT the situation doesn't allow for me to actually place the spanner on the nut. I have to somehow tighten it when perpendicular to the line of the shaft - approx 75mm away.
I plan to machine the nuts so they look like a worm gear (these are mass produced via CNC and so should be of reasonable quality). I will then
use a worm sitting on the nut with a shaft out to where I can place a spanner on it.
(The ability to keep the worm still and worm/worm gear hard against each other, etc is possible).
The worm will be a reuseable tool and can be made to a good quality.
The worm (tool) will have a small handle to quickly turn the worm to quickly take up slack.
And then I can add a bar or spanner to the shaft to give leverage to tighten off to my 200 N.m.
My question is: How much of my 200 N.m will be lost through all the worm stuff?
Will the teeth survive (mild steel)?
Will I be able to tighten up my nut? Thanks. ROger.

 

[DickL]

I don't think I understand the situation. From the description you've given, I would reach for a socket wrench, suitable extension and the appropriate breaker-bar or torque wrench. What is keeping a socket wrench to be used, and still allow your worm drive mechanism to get to the nut. If there isn't room to swing a wrench above the shaft, there are right angle gear drive and pneumatic drive units that can connect to the extension and bring the 'wrenching action' out to the side or eliminate the need to swing a wrench.

 

[ROGER WALL]

Dick,
Pretent we are doing key-hole surgery on a leg. The shaft is the femur with a nut on it.
I can't put a socket on the nut. I can't use an open ring spanner or an Alden wrench as this
would require a large vertical slot in the patient to go up and down (depth to shaft is 75mm which would need a 100mm slot in leg).
I have a small round hole thru the skin down to the bone which is where I will insert the worm.
I don;t need do know how to resolve the problem I just need to know how much of
the 200N.m I use on the worm (tool) will be lost. I assume this will depend on teeth size, etc. BUt in general I would like to know what the efficiency or lack of the 'a' worm drive would be.
I am thinking of making the worm have 4 starts as well to speed up the rotating of the nut.
Thanks,
Regards,
Roger.

 

[Unrest]

It should be quite simple to use the friction coefficient of the worm/gear materials to find the torque lost, similar to how you'd calculate the torque lost tightening the nut.

Force = friction coefficient * normal force

You know the normal force from the torque on the nut (200Nm) / radius of worm gear teeth at the contact point.

So that equation gives you the additional force that the worm has to apply on top of what's needed to tighten the nut. Convert it back to torque on the input shaft by multiplying by the radius of the worm at the contact point.

The teeth being strong enough sounds questionable. For a first check, find the shear stress in a tooth due to the force.