Help with Material Selection Process

[chessguy103]

Hi all,

I'm working on designing a device, and I'm having trouble with material selection for a shaft, modeled below.

I have found the maximum stress due to bending on the shaft in question by using

σ_max = (M_max*y)/I​

where σ is stress, M is bending moment, y is distance from the neutral axis (aka radius of shaft), and I is polar moment of interia.

I got σ = 31.2 MPa. This is where I'm stuck. Now that I have this stress, how do I choose a material? Do I just look at yield strength?

Any help is appreciated. Thanks :)

 

[FEAnalyst]

You should also check the deflection. But generally yes, compare the maximum stress with yield strength.

 

[chessguy103]

Thank you.

And once I find the deflection, what do I do with that value?

 

[Baluncore]

And once I find the deflection, what do I do with that value?

Does the shaft rotate? At what RPM?
Is the shaft subject to axial forces?

 

[chessguy103]

In the scenario I'm looking at, no it doesn't rotate. I'm trying to model a hand winch with a force applied through the rope, but with the brake activated so that no parts are moving.

 

[Baluncore]

I'm trying to model a hand winch with a force applied through the rope, but with the brake activated so that no parts are moving.

Warning. There is a hidden trap here.
To prevent damage to the (wire?) rope, the shaft = drum = spool must have a large radius. It will therefore be fabricated from a hollow tube. How thick must the tube wall be? The tension in the rope, multiplied by the maximum number of turns will provide an external hoop compressive force to the drum. That will cause the drum to fail through external compression, long before the shaft deflects.

 

[chessguy103]

I’m modeling my winch based on a winch that I bought and disassembled. It had a hollow drum, but inside the hollow drum was another hollow tube to allow for spinning. Inside the second hollow tube there was a bolt that went through the tube (and therefore the drum), and held everything together against the shell.

The compression force makes sense, and i should add that to the model. But would failure through external compression be that much of an issue in this case?

 

[Baluncore]

But would failure through external compression be that much of an issue in this case?

It is n times more significant than the deflection; where n is the number of turns posible on the drum.

 

[chessguy103]

Also, let’s say tension on the rope is 1000 N. If the total number of turns on the drum is 20, then the compressive force is 20000 N? I assume that would be applied as a uniform distributed load across the surface area of the drum, in addition to the 1000 point load tension?

 

[Baluncore]

20 turns at 1000 N tension will be 20 kN as a hoop stress around the outer surface of the drum. The drum section is under that hoop compression, as if an external pressure was being applied to the drum surface.

FYI: https://www.irjet.net/archives/V6/i4/IRJET-V6I4746.pdf