Why tumbler barrels creep, travel, move down the rollers?

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Looking at this photo and trying to figure if the 3/4" shafts may be sagging with approximately 35 pound load in center.
Looks like a 40 inch span.
I would say yes, especially if the shafts are far enough apart to create a wedge from the barrel.
The wedge is most likely happening, the deeper the barrel sits in the saddle the more the wedge force.
The wedge force splitting the shafts will darn sure push the barrel away from the end.

I have seen home made tumblers with a good bit of shaft sag and wedge out. And they require no end rollers. Because the barrel(s) seek the center hammock style.
A clever way to avoid end rollers.

I would let it run for a while barrel starting in center, tumbler sitting flat.

Span is 38".  In an earlier post (the build thread) you said the 3/4" shafts probably wouldn't sag under 150 pound load.  From eyeballing it I think your earlier statement is closer to being correct and that they aren't sagging.   The shafts are not spaced very far apart...5-5/8" center-to-center.  They seem pretty solid at this span.

I can see how the "wedge force" and gravity would encourage the barrel to travel to the wider space.  I think the shaft spacing is ok though naturally I'm sure they're not perfectly parallel and thus contribute to a wedge force.   What amazes me is the quickness of travel that the barrel exhibits....in less than two minutes the barrel will travel an inch.

I first started out with the tumbler level and the barrel in the middle...it quickly traveled to one end and the subsequent wear showing on the rollers is what prompted me to start this thread.

Interestingly, it doesn't matter which direction the barrel is pointed in...it travels consistently to the same (opposite from motor) side. 

For a moment I entertained the thought that the pvc fittings might be a bit "off" in matching dimensions and were maybe contributing to a wedge force, but if that was the case then the barrel would reverse travel direction when it was reversed on the tumbler.  So, I'm scratching off the barrel contributing to the creep...there must be something inherit in the tumbler itself.

The shaft spacing is pretty dog-gone close but I may widen the spacing excessively on the motor end and see if I can get the barrel to move in that direction.  If that doesn't make a difference I'll have to say it must be sprites and fairies doing it.

jackangeline, I got to thinking and the tumbler is aimed east and west...I wonder if I should turn it so it's situated north and south.  Then the question of the direction of rotation of the barrel matching (or not matching) the rotation of the earth...lots to think about!    captbob 

Barrel creep really doesn't matter in the functioning of the tumbler being as I'm going to have a roller stand-off in place regardless.  Just something that really makes me wonder what's going on with a barrel climbing uphill.   

Thanks for the feedback, Chuck. I've about convinced myself of this. I'll be intentionally widening the spacing on the non-frequented end of the roller pair soon to hopefully see for myself this happening. Apparently the spacing has to be more or less perfect from end-to-end of the roller pair for the barrel not to creep to one end, or at least to creep at a very slow speed. Stand-off rollers are a must!!

I have used rollers from rollerblades, very tough compared to dryer rollers. I agree the spacing of the shafts are probably the culprit. Not sure if it would go to the wider spacing depending on rotation of the drive. Aligning belts on conveyor with a tapered tail pulley is easy, but sometimes with straight or flat tail pulleys it is backward from common sense which makes no sense.

As you are aware. If the side rollers go bad it can slow the barrel and the shafts can grind a notch in the barrels.
I tilted mine a bit, it walked a bit when level, now the barrel barely touches the side roller.

It is a puzzle that your barrel is walking like it is. if shafts are parallel and the barrel walks when reversed it is an enigma.

I wondered if the reason for the walking when everything was level and lined up was the rocks impacting the internal shape of the different shaped barrel ends.
Rounded cap end and wedged reducer end. 46 times a minute.
Next time you do a clean out you could try the barrel with no rocks, or full of water so there will be weight w/no impact forces.
Maybe the rocks are pounding it to one direction by striking the inside of the cap or bushing.
I would be suspicious of the cap.

About 2 inches of tilt over it's 4 foot width kept mine from walking. That would be a double shaft, all 4 barrels with caps pointing west.

The barrel walking is actually somewhat moot. Good end rollers solves that !! As toil0 mentioned, roller blade rollers or in my opinion something similar w/ball bearings.
My best end roller that lasts forever is mounted high and at an angle and rolls flush on the end of the cap. Maybe there is a pic

Yes, the one on the left is just a castor. But it is not in shear the way it is angled.
The one on the right must get replaced every few months because of it's crappy contact point.

Best thing I did was moved the end roller up high and out of the way of the pillar blocks. It is the left one that is 3 years old and has lots of pressure on it.

I'm going to keep my eyes open for something with ball bearings in it. Preferably something that has multiple wheels (stock for the future ). I know the roller-blade wheels are highly durable...seems there might be a pair somewhere at the shop that belonged to my nephew...I'll plunder around a bit when I get back to work. Otherwise, I'll keep my eyes peeled for "whatever".

Yeah, I've got plenty of room on the wood to mount a standoff. I can see how the angled roller works a lot better...the sheer force is what's working on my dryer rollers. I think the dryer rollers would probably last for years and years if aligned properly to the barrels. The ones I'm using had been in the dryer for 4-5 years with lots of use. I had to re-felt the drum so I replaced the rollers while I had the dryer torn apart...the old ones looked about like the new ones but they had no sheer force working against them in there former/intended application.

I was looking at the Covington 40 pound tumbler at $770, then shipping.
Noticed it uses a 1/3 HP motor. Seems over powered to me.
Comparing budgets you should be way cheaper than $770 and have two 20 # barrels which I find more flexible.
And space for additional set of shafts.