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

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.

TheScullster wrote:
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.


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 :-)






TIA

Phil

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

Phil

On Wed, 20 May 2009 10:48:30 +0100, TheScullster wrote:

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.

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.

I am talking about large equipment and slow rotation speeds if that
makes a difference.

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

On Wed, 20 May 2009 17:30:02 +0100, me9 wrote:

On 20 May,
dave wrote:

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?

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.

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

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.


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.

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "TheScullster"
saying something like:

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)?

Hohum... take your pick.
Wear - likeliest in old machinery
Unusual loads
Bad machining of the axial bearing alignment
Bad design, resulting in off-axial loads
etc, etc.

Such creeping movements aren't usually severe and that's why a couple of
clips and washers will probably do the job of restraint. If things have
reached the stage where normal operation wants to send the shaft
shooting out of its bearings, then something's ****ed.