overhung load shaft stress/failure?

[mechanicalboonie]

I'm designing a wire spool feeder (de-reeler?) for one of our processes.

We take wire off the spool, run it through a straightener and wrap it onto a drum. The drum is powered and pulls the wire, the spool is free spinning.

Currently we mount the spool onto a rickety stand with a significantly undersized shaft. No bearings, bushings, or anything of that sort. As you might imagine, the spool rattles around a bit and the stands seem to be wearing out quickly. A 2x4 weighted down with some scrap steel is wedged under the spool to provide a braking/tension action.


The objective in redesigning the wire spool holder is to eliminate slack during winding and create a better, more controllable feed. (slack causes loose wires on the drum, causing trouble later on). Additionally, we change spools frequently, so it needs to be quick and easy to change spools.

(it would also be really nice to have a quieter, smoother running spool feeder)
I was looking at commercial spool de-reelers by Accra Inc, and they do not fit into our budget.



My original idea was to mount the spool to a cantilevered shaft.
Two bearings on the left, spool hangs on the right.
Some kind of locking mechanism to hold the spool on the shaft.
A braking device somewhere in the center or on the left and/or something to create tension in the line.


This would leave the right side open for access; the operator could drive his pallet jack/spool right onto the shaft, then lock it in place.


I really like how well this configuration would fit our fast changeover requirement, except I'm not sure if it would actually work safely. I'm aware of lathe crashes involving a long unsupported rod hanging out the headstock, so I'm looking at it from that angle.


I can do the math and find the forces on the bearings, and their expected life.
I can calculate the stress and deflection in the shaft while stationary.

But how do I find the force and stress in the shaft generated by rotating it with a large overhung load?
I was initially approaching this like an eccentric mass on a shaft. Initial deflection coming from the weight of the spool, with the centripetal force feeding back into the deflection.

After thinking about it more, I'm not sure that this is a valid simplification.

I've attached a quick sketch of the shaft, bearings and spool.

I'd appreciate any tips, pointers, suggestions or help you have.
Thanks
Brian


Spool weight is ~900lbs max, ~30"diameter x 13", through hole diameter ~1.625".
We wind the wire off the spool for maybe a minute at a time, accelerating from a stop to a max 300rpm (at the spool), then down to a stop again. Low duty cycle, maybe 1 minute (or less) on to around 10-20 minutes off.

 

[Cockroach]

Overhanged shafts are handled as dynamic loads where centrifugal force increases further away from the support. In essence, the shaft starts to take a parabolic shape as rotation increases.

Failure of long shafts subjected to rotation is typically by fatigue due to cyclic loading. In one rotation, a zone or band of material is subject to compression/tension bending load along every point of circumference. As RPM increases, an element of wall experiences increasing cycles of compression/tension. So as the loose end of the shaft whips up and down, the zone immediately outside the fixed boundry becomes fatigued and breaks.

This is a classical analysis and can be found in most textbooks dealing with rotational balancing.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[BillBirch]

Why not have a static (non-rotating) shaft and fit a hub to the spool? That is, fit the hub to the spool, then mount the whole assembly to the shaft. This hub need only to be a plain bearing for the light duty that it is required. You could have a generous lead in on the hub to allow location by the fork-lift.

As for brakes - why don't you look in a fishing tackle store to look at their devices for filling fishing reel spools?
Regards,
Bill

 

[mechanicalboonie]

Kenneth,
Thanks for the pointer. I've got a list of books to get and I might as well add one that covers rotational balancing. I probably shouldn't have sold my old dynamics textbook either...

Bill,
Thanks for the suggestion. I hadn't thought of looking at fishing reel brakes.

The spools are usually beat up and the through hole isn't always that round, I had been thinking of a cone shaped "wedge" on the shaft, with a matching cone that threads onto the free end of the shaft to grab and lock up the spool (or something like that).

Mounting a hub to the spool hadn't occurred to me. I'll look into it.



Thanks
Brian

 

[hydtools]

Brian, if you are thinking of some sort of cones to mount the spool, look at those used to mount wheels to tire mounting/dismounting spindles. A fixed cone on one end and a threaded cone on the other end of a threaded shaft.

Ted

 

[rob768]

Make sure you don't hit a natural frequency for whirling..

 

[mechanicalboonie]

Kenneth, I haven't had a chance to pick up a book on the subject yet, could you recommend one?

While I'm looking for that, can I approximate this kind of fatigue failure by treating it as a cantilevered beam with a cyclic/alternating load on the end?

It seems too simple to be correct, but I guess I'm asking how to make a quick first pass approximation right now. I haven't found anything dealing with this in my references yet.


Ted,
I looked for some photos and that is pretty much what I had been thinking.

Rob,
I've only looked at it briefly, but it looks like the first natural frequency for this will be above 1000RPM.



Thanks for the help again
Brian