Shear shafting?

[slip knot]

As we thaw out from the big freeze in south Texas, I'm starting to go through the damaged items and set up repairs. One of our screw conveyors had a sheared shaft at the gear reducer. The shaft almost looks like cast in the sheared area. The shaft goes through a lower bearing and seal and connects the gear reducer to the flights tube with two through bolts.

The maintenance supervisor suggested taking the shaft to a local machine shop and have them duplicate it but one of the new maintenance guys said it looked like a shear shaft to him and we should order a new one from the manufacturer to protect the machine.

This kind of peaked my curiosity. Why would a manufacturer use a shaft for a shear point when there are two 1/2 in bolts in use that would make a lot easier repair after a shear event. But the fact that it does look like some kind of soft material has me questioning if it may actually be designed to shear. The new guy has had a few good ideas but he's also been one of those who doesn't know how to say he doesn't know when asked a question.

I dont do much work in the field anymore so I'm a bit out of touch. I figured this group would set me straight. is this shear shafting a reality or is the new guy blowing smoke?

[digr]

Never heard of such a thing

[toprecycler]

The two bolts probably should have been the weak points.

But, if the shaft had vibration in its life, it may have started cracking if it had any nick or other damage in the shaft. Does the break look like it’s completely fresh, or does it have some spots where it looks old or rusty?



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[LKeithR]

Originally Posted by toprecycler

...The two bolts probably should have been the weak points...

Yeah, doesn't make sense to me to have an expensive--and possibly hard to replace--shaft engineered to fail when a simple shear bolt will accomplish the same thing.

I've seen a lot of sheared, broken or shattered shafts over the years and I think it's almost impossible to tell what caused the failure if all you have to go on is just a visual inspection...

[slip knot]

Break looks fresh, no rust noticed in the break. The screw conveyor was completely iced up and frozen so I'm sure it was a significant load on it. It seemed to me that the bolts or the key way in the gear reducer would have been the usual shear points. This unit has been in service for 4 years and this is the first problems we've had so I cant complain too much.

I had a picture of it on my phone but cant seem to find it now.

A lot of broken piping will need repair, We dont see these temps very often and the normal way to keep pipes from freezing is heat tape or heat lamps, neither work very well when the power goes out. The non potable water system froze completely. The operations guys tried firing it up this afternoon and reported 14 leaks thru-out the facility. Lots of little leaks is a good thing, the distribution systems still sound.

[Samcord]

I’d probably start with a call to the manufacturer for price availability and specs.

[toprecycler]

Another thought, if the bolts should have been the shear point, are they the correct type? Maybe they sheared before, and someone replaced them with stronger ones to avoid that happening again?

Definitely get a price on a new one. Helps decide wether it is worth trying to fix old one. And have to factor in the time needed to fix old one, vs ordering a new one, and who is doing the work. One offs for a job shop can be more expensive, unless they have a very low shop rate, and is a very simple shaft, depending on the machines and machinists they have.


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[Scotts]

Ahh the joys of extreme weather in unexpected places.

Do you have the manual for the equipment? If so it should show shear bolts in the repair parts or recommended spares to have on hand. this should give some indication of what was there to start with.

Also what drives the motor? Is it across the line or have a soft start or VFD?

I would go with someone put bolts in that are not correct for the application.

Scott

[slip knot]

Job shop costs are pretty reasonable right now. The collapse of the oilfield has these guys cutting prices and cutting each other throats. I can probably get this shaft made cheaper than I can get an OEM overnighted.

I've liked this unit so well that I'm in contact with the company to get another similiar product built. I will certainly give the design engineer a ribbing over this one. I believe these guys sent down a crew for install and start up so I'm assuming the bolts are correct. I dont recall any issues with this conveyor in the past. But who knows.

[digger doug]

I'm intrigued now....what does this machine doo ?