Abrupt change in tracking

[kelLOGg]

A week before this tracking problem I sawed 20" live edge RO with no problems. I left the band on the mill and did no sawing for about a week (and made no adjustments) and then put SYP on the mill to edge boards and after tensioning the band and engaging the drive wheel the band almost flew off the wheels. The band was riding on the corner of the roller guides at about a 30° angle resulting in a scored line on the band about 1/4" from the back where it was riding on the guide wheel.  See pic. (The contact angle is difficult to see with the blue background from the horse feed bag in the background but it is easier to see the half the band overhanging the drive wheel.) I thought it was the band about to break so I changed bands and observed the same problem. I changed the tracking to bring the blade into proper alignment with the wheels and it sawed perfectly. I am baffled as to why this happened. The band wheels have no wobble or play. Has anyone observed this?



 

[ladylake]

 If your bearings  are good check  your head mounting bolts and band wheel mounts for loose bolts or maybe something broke.   Steve

[firefighter ontheside]

I had a similar problem and thought it was tracking.  It turned out I needed new belts on my wheels.  I changed belts and everything was good again.  @MartyParsons helped me with this.  May not be your issue, but worth looking at.

[WV Sawmiller]

   Did you leave your band on the mill when you finished your last job or did you remove it? If you left the band on did you leave it under tension or back off on it? I know chainsaws warn not to tighten the chain while the saw and leave it under tension when finished because when it cools it contracts and can damage the bearings and such. I assume that would be true on the rollers on a mill too.

[kelLOGg]

The drive belt is beginning to get cracks. Hard to understand how that is the cause but it is a cheap fix should I need it.

I did leave the band on the mill during that idle week but it was not under tension. (I never leave it tensioned.)

I didn't check for loose or broken bolts but I will after lunch.

[barbender]

I have had a couple if times I put a band in that came from a different supplier and they tracked off a bit, but when something moves as much as you have there it's always been something mechanical.

[Joe Hillmann]

The drive belt is beginning to get cracks. Hard to understand how that is the cause but it is a cheap fix should I need it.

I did leave the band on the mill during that idle week but it was not under tension. (I never leave it tensioned.)

I didn't check for loose or broken bolts but I will after lunch.

I don't think he meant the drive belt.  He meant the belts on your wheels that the blade rides on.
It could have been caused by a build up of sap.  Or frozen sawdust, or a big piece of bark that got between the blade and wheels.
On my mill if I flood my blade with too much diesel I can't keep the blade on.  I have to wipe the blade and tires down with handfuls of sawdust before the blade will stay on once that happens.
I would also look for play in the shafts or loose bolts.

[ladylake]

 
 I think he's using a Cooks with steel wheels.   Steve

[kelLOGg]

Steve's right - crowned steel wheels. I usually do get small bits of diesel/sawdust mix on the wheels but they have never caused a problem and they scrape off easily with a finger nail. Perhaps when I blew the dust off the mill after the last use I blew debris on the blade wreaking havoc with tracking. Hard to see how that would a tracking re-adjustment to correct. :
I inspected welds and bolts associated with wheel mounting and all looked AOK to me.

Here's a magnified pic of the band showing the score mark cause by the band riding on the corner of the roller guide for 4 or 5 seconds before I noticed it. Black MagicMarker dot marks the line. It is prominent enough to feel with a fingernail.

[terrifictimbersllc]

Just a thought.  While it seems like nothing significant was done between when the blade was tracking successfully and then when it did not, what was done was to untension and then retension the blade. 

Is it possible that something happened in your tensioning mechanism?  Have you noticed previously that blade tracking position on the wheels is sensitive to amount of tension? If so perhaps it is not being tensioned the same now. 

Or, did the last retensioning finally move one of the band wheels horizontally for some reason?

[ladylake]

 Are you sure your guide rollers don't have a bearing going out locking up sometimes  to make marks  like that on the blade..  Steve

[Gere Flewelling]

I noticed in the picture that the band is only half onto the idle band wheel.  I have never seen it happen on my MP-32, but could the band have been that far forward on the wheel when tensioned?  I am wondering if the band was far enough off the wheel that the back of the band could be on the forward side of the wheel crown.  If that was the case, I think it would explain the angle that the band was hitting the guides.  I think normally if part of the band is hitting the high point of the wheel crown, it would just work its way back to center of the band when turning the wheel backwards by hand.  If it was too far forward of the crown it might want to either stay forward of the crown or try to come off the front of the wheel.
I had a band that wanted to ride back on the wheels once and hit the back of the guides.  I called Cook's looking for advise.  They suggested tensioning the wheel higher that the alignment marks on the tensioner.  When I did that, it brought the band forward and off the back of the guides like it is supposed to be.  The next band I installed didn't require higher tension.  It is only that one band.  That being said, I think it would be likely that if the blade was under tensioned that it might ride forward off the wheels.  

[kelLOGg]

Several weeks ago I realized I was under tensioned so I have corrected that so I have increased the tension and adjusted the tracking to keep the band properly on the wheel. All that was well before the present problem. (Sawed 20" RO w/ new tension setting and all was OK.) The pic shows the indicator for tensioning on my mill. The edge of the silver tape
must align as shown. This gives me 20,000 psi on my 0.045" thick 1.25" band.



 

Tensioning system looks fine - no thread wear on the Acme screw tensioner. I spun the roller guides by hand and they felt smooth, no wobble or play. One is new and I saw no reason to replace the older one.

Gere - that's a compelling observation. I don't really know if the band was mis-aligned or not before I tensioned it. If it was that could be the problem except for the fact that I initially suspected the band so I changed it and got the same problem with the 2nd band. Then I looked at tracking and that "solved" the problem. 

I'm not sawing right now but I plan to start the mill every in hope the gremlin doesn't return.