What Causes a Shaft to Walk? (Totally OT)

Hi all

A bit of a vague one this but here goes. If you have a shaft mounted in 2 spherical roller bearings and drive it via a typical chain/sprocket arrangement, what factors will tend to make the shaft move along its own axis (as well as rotating)? The drive sprocket is at one end of the shaft outside the bearings. OK, I can see that if the shaft isn't perpendicular to the drive chain then this would be a likely cause. Also, it is accepted practice to fit locking rings to prevent this wander. But I am trying to find factors which cause this movement rather than remedies at this stage - any other suggestions?

I am talking about large equipment and slow rotation speeds if that makes a difference. The purpose of the shaft is to drive a chain conveyor via a pair of large sprockets within the machine and between the 2 bearings.

TIA

Phil

Reply to
TheScullster
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You will probably find that the shaft is not square to the conveyor axis,putting more tension on one sprocket than the other. The side it moves to needs more tension or alternatively the side it`s moving away from needs less tension. I`m talking here about the conveyor sprockets,the ones between the bearings.

Reply to
mark

well tapered roller bearings of course will make it wander which is why double tapered are normally used.if you are using sphericals, the answer is that not much is really trying to make it go sideways at all, to any great degree, which is why quite flimsy retainers work well.

If the axle is not horizontal weight will cause a side thrust.

If the axle bends or twists under load, that too can cause side thrust.

And of course, with a chain, there is unevenness in the chain itself. It may 'want' to find a position under load where the sprockets are somewhat offset, if one side is shorter than the other.

I suspect its actually this latter that causes most wander. I don't do chains, but I have done a bit with synchro belts and you can see the belt moving across the pulley in a semi random sort of way at speed. Possibly associated with various belt flapping modes that twist the belt.

I think in your case the likely thing is that having shafts sliding - even by a small degree - in bearing inners, is a Bad Thing for wear on the shaft (rather than the bearings) and that's why one tends to lock the shaft in place. And the chain itself will not be uniform, and will cause a little side to side movement: IF that is restrained, there is more likelihood that the chain in time will stretch itself to a more uniform shape :-)

Reply to
The Natural Philosopher

Thanks NP and Mike. Looks like it would be worth examining the tension of the internal chains as a starting point.

Phil

Reply to
TheScullster

I'd speculate that the weight/pull of the chain drive will try to drag the shaft outward (thinking of the shaft as a beam, with the bearing closest the sprocket as a pivot point) as far as slack in the chain/links allows, but then the uneven nature of the chain itself will tend to try and 'bump' the shaft sprocket back in line with the drive sprocket - net result being the shaft wandering back and forth a little.

It might well do - if I'm at all correct on the above then I'd expect the effect to be a lot less noticable at higher speeds or with lighter components.

cheers

Jules

Reply to
Jules

I am by not the greatest stretch of imagination a mechical eng - but just wondering why those flat belts don't wander off the (flangless & flat) pullies on such machinery as eg steam engines used to drive say a log cutting machine - or thresher?

These things have incredibly long drive belts - which rarely seem to slide off. These belts are sometimes crossed over in a figure 8 - maybe something to do with the stability?

Reply to
dave

The pulley faces aren't flat, but slightly convex. The belts twist slightly so to ride up onto the highest point, where they remain reasonably stable.

Reply to
<me9

I believe there's some logic in only making one of the pair convex, though

- I thought I had the explanation bookmarked, but can't find it now. Maybe it was just that the driving and driven pulleys might be slightly out of alignment, and making both convex would stress the belt too much and cause it to fail early.

I always assumed the figure-8 thing was purely to change the driving direction without needing to run the engine in reverse, but I suppose it also means the belt is running around slightly more of the pulleys, and so gives slightly more grip than a imple loop.

More speculation, though :-)

cheers

Jules

Reply to
Jules

Yes, and you have a bit more belt that way, so more "stretch" available for getting it on and off, and more "self weight" to get it to start gripping.

Reply to
newshound

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