Shaft Failure
Shaft Failure
(OP)
See photos of the setup.
http://i.imgur.com/WHNwACl.png
Hi Folks,
We have some equipment that used a stepped shaft to fit an off the shelf gear with a smaller bore. The shafts are failing right where one would expect. I'm no expert at metallurgy but thought someone might recognize the patterns. I assume that it's a cyclical bending fatigue failure onset by the stress concentration at the shoulder compounded by the shaft's diameter size reduction.
My proposed solution is to machine the entire unit from a single bar of 4340 steel and include the face relief as shown at the bottom rendering. The extremely tight space limits any dimensional expansion outside of the current envelope.
Thoughts?
http://i.imgur.com/WHNwACl.png
Hi Folks,
We have some equipment that used a stepped shaft to fit an off the shelf gear with a smaller bore. The shafts are failing right where one would expect. I'm no expert at metallurgy but thought someone might recognize the patterns. I assume that it's a cyclical bending fatigue failure onset by the stress concentration at the shoulder compounded by the shaft's diameter size reduction.
My proposed solution is to machine the entire unit from a single bar of 4340 steel and include the face relief as shown at the bottom rendering. The extremely tight space limits any dimensional expansion outside of the current envelope.
Thoughts?
-Kevin





RE: Shaft Failure
maybe shotpeen the relief cut ?
currently there are two pieces ... the gear and the shaft, yes?
how about shrink fitting them together ?
maybe put the relief cut in the shaft ??
Quando Omni Flunkus Moritati
RE: Shaft Failure
Was the junction a very small radius or even a corner?
If the gear had a larger diameter square and flat face and could be clamped hard against a similar feature on the shaft the fillet would not be (would not have been) subjected to bending. I'd still incorporate a proper radius
RE: Shaft Failure
RE: Shaft Failure
This is interesting, I never investigated this treatment. Is it costly?
Two parts, yes, a gear and a stepped shaft.
The gear comes keyed from the factory and fully hardened. We'd avoided machining it.
Original shaft material was probably 4140 or 4340 but it would have been machined from the normalized stock and not received any heat treatment. Would a heat treatment have been appropriate for this?
Junction was a fairly sharp corner, with a radius no more than that of the cutting tool.
I think I understand what you're describing, it was a very tight space which didn't allow any extra vertical clearance.
Yes, it's adjusted with threaded screws and locked down with a firm clamp.
Admittedly, not that great.
Yes.
4,000 lb load, 180deg back and forth in 10 seconds, every 90 seconds or so.
It's sharp from the cutting tool. Small chamfer already on the ID of spur gear, we were avoiding any machining on the spur gear so that it would be a drop-in replacement for wear but that would certainly have helped our fatigue.
Back and forth motion or budget may have negated the use of a rotary indexer.
As best as possible.
A protective kickplate. Personnel frequent in the area.
-Kevin
RE: Shaft Failure
Is it possible to fit a radial bearing in the kickplate to support the distal end of the shaft?
Mike Halloran
Pembroke Pines, FL, USA
RE: Shaft Failure
I hate buying fancy material to improve reliability when the fatigue strength could easily be boosted 10X or more with improved geometry.
Page 82 here has some data about general relationship of hardness (heat treatment) and fatigue life improvements.
http://www.timken.com/en-us/Knowledge/engineers/ha...
Compare the possible improvement to hit by geometric stress concentrations that can increase local stress 3X, and surface conditions that can lower fatigue strength 30%.
The improvement shotpeening can make on fatigue strength can be remarkable.
http://www.metalimprovement.co.uk/documents/englis...
RE: Shaft Failure
kgwhipp said "I think I understand what you're describing, it was a very tight space which didn't allow any extra vertical clearance."
In your first image it appears the shaft extended a little beyond the face of the gear. If the shaft was shortened about 4X the protrusion a thick stepped or T shaped washer could be made that would be no taller than the shaft protrusion.
http://www.amainhobbies.com/images/large/yei/yei-n...
it would have a counterbore for the head of a SHCS bolt. The end of the shaft could be tapped and counterbored to receive a bolt with 4 diameters grip length to permit some healthy elongation when torqued to provide 1000s of pounds of clamping force to hold the gear against the shaft shoulder, and the washer would bear in the gear face and not touch the shaft end.
http://www.beaudaniels-illustration.com/technical-...
RE: Shaft Failure
The smaller section of the shaft went into the pinion. The larger section is in the gearbox.
I'm not familiar with the separation force you mention- where does this come into play? I definitely want to include this in future calculations.
There's only 1/8" of clearance above the pinion before we encroach into the area of the spinning surface above, so definitely not fitting anything else in there.
Tmoose:
Nice find on the Timkin reference, I would have somehow imagined the opposite was true, but I agree that geometry improvements and shotpeening are the correct route to move forward with. I'll probably keep the fancy alloy for a consistent heat treat on the hobbed part, some of the heat treat folks by me don't like 1045, which would be my go-to less exotic alternative. That said... I'm probably remembering incorrectly and might have been talking to a flame hardening technician.
-Kevin
RE: Shaft Failure
http://en.wikipedia.org/wiki/Design_and_manufactur...
Mike Halloran
Pembroke Pines, FL, USA
RE: Shaft Failure
Mike Halloran
Pembroke Pines, FL, USA
RE: Shaft Failure
A cantilevered pinion is almost always a bad idea. Your shaft likely failed due to a combination of bending stress concentration, torsional shear stress, full reverse cyclic fatigue bending in the shaft at 1P frequency, and surface fretting damage at the gear/shaft interface.
If you're going to redesign the pinion shaft, I would suggest adding an outboard bearing to support the pinion at both ends. The approach of using a stronger material that you are proposing is just a band-aid fix.
Good luck to you.
Terry
RE: Shaft Failure
http://www.sankyoamerica.com/cam-indexers_dial-ind...
RE: Shaft Failure
RE: Shaft Failure
Looking at page 14 on this site and studying your pics it looks like a rotational bending failure to me.
http://www.rexnord.com/sites/process/services/rene...