Square Bar in Torsion
Square Bar in Torsion
(OP)
I have a motor that is 3 HP and 1730 RPM. It has a 102:1 gear reduction set on it. I'm having a problem with it twisting the transmition shaft, which is 1 3/8 inch solid steel. I calculate the full speed Torque to be ~930 lbft...Correct? I calculate the Maximum shear stress to be ~20600 psi...correct? Usually Shear yield is .58 of Tensile yield. So, are the bars we've been using just really weak or am I missing something.
Yes, I know it is unusual to have a transmission shaft be square
Yes, I know it is unusual to have a transmission shaft be square





RE: Square Bar in Torsion
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RE: Square Bar in Torsion
Your calculations are right. What is the steel spec? The material yield strength should be ~42,000psi with no margin of safety.
For the 8000 psi limit your shaft size should be 1 7/8 to 2 inches square for the material you are using.
Ted
RE: Square Bar in Torsion
Hydtools: what is the ISBN for Machinery's Handbook or where can I pick that up?...Always interested in new reference material. Under what conditions does the book give for 8000 psi? The spec is a little weak, as in non-existent. I was using rule of thumb for standard steel yield strength to get an idea of what was going on. I'm relatively surprised it would have sizing for square shafts.
I've kind of shifted my focus somewhat. The problem I was brought is that they twisted the shaft. I started with the thought that I needed to redesign and to check my calculations with others, I'm one of only two ME's that work at my location and the other one is too busy on something much bigger to check this.
Now, I'm to the point of clarification: Isn't this what I want to happen? This setup has a motor rigid coupled to a gearbox, which is rigid mounted to the shaft. I certainly don't want the gearbox to fry or the motor to fry. The failure point that I want is the shaft as opposed to the other two. If I make the material stronger then I risk a much worse failure, either costly or things actually shattering and shooting out shrapnel. I think the way to go is a shear pin of some kind. The problem is I have no way of knowing for all applications how much torque I actually need.
In the end the operator failed to use the machine properly.
Thank you for your help. I'm going to have to figure out what I can do.
RE: Square Bar in Torsion
I have dealt with Falk couplings in the past.
Don
RE: Square Bar in Torsion
As electricpete said, inrush current can give 250% more torque on start up, and if load is variable this can be an issue, plus deceleration forces can come into play on conveyors etc.
Not good practice to use the shaft as the safety valve!
Suggest you do a proper evaluation of the load, both start up and running.
VSD's have electronic shear pins plus also control starting/stopping torque.
Just checking, I estimate a 2" round shaft is the minimum required to handle 3 HP at 17 rpm @8000 psi, whereas the real startup requirement could be much higher.
Do some current checks on start up and running-compare to the motor nameplate specification and this will give some indication where you are at.
RE: Square Bar in Torsion
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Ted
RE: Square Bar in Torsion
I was trying to find a controlled torque coupling and I'll check Falk out. It still gets back to the problem that we can't predict how much load this will be under in worst conditions.
I don't want the shaft to twist but it is the cheapest member. I thought about a shear pin. I haven't calculated the needed size but I'd have to re-design the shaft because we don't have a spot where we have a round connection, they're all square. I'd also have to put some sort of a bushing in around the shear pin to protect the metal. I'm leaning this way.
I really ant to hear any suggestions anyone has, this one has me somewhat stuck since there are so many variables.
RE: Square Bar in Torsion
Once you get the coupling and install it, you can adjust the setting (by trial and error) on the coupling to the necessary torque. If you are running your motor close to 200% for most of the operations then you may want to increase the size of the motor and/or reducer. Do you know what the reducer is rated for (HP and torque rating)?
RE: Square Bar in Torsion
Ron Volmershausen
Brunkerville Engineering
RE: Square Bar in Torsion
RE: Square Bar in Torsion
I was wondering how you solved the problem in the end. Perhaps you could share the results with us.
Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
RE: Square Bar in Torsion