As an engineer with some knowledge of metal fatigue, I wonder if anyone on this forum would comment on the trade off in reduced blade life when setting guides to push the blade down cf to running at blade tangent position. My calculations for a 3" dia roller indicates the blade exceeds yield every passage past the roller. The rollers would consume most of the blade life cf to the wheels. Does the better control compensate?

Can you post a drawing, as this does not make sense to me as described.

I I think that system will shorten the blades life but I don't think it is so much.
Here are my experience:
My blade guide rollers are pressing down the blade 5 mm from stright untuched position. The blade will not follow the radius of the roller fully. The fixed roller is closest to the left wheel and it is the "worst case". High blade tension makes it worse. I have checked, with a ruler, that the blade passes that roller with a bigger radius than the roller has. (In my log band saw I use Munkfors blades 37 mm x 1.1 mm thickness.) The blade is to stiff to bend fully over the roller as the angle of flexion is so small.
The all length of the blade (4.5 meter) passes the rollers six times/ sek.
Though all this I have an average blade life as I am very satisfied for, more than 100 logs, documented on the card for one blade.

Attached is a jpeg file of the situation. If the guide roller pushes the blade down .25" and is 10" from wheel CL, then the band bends about 1.4 deg. Using conventional beam bending equations indicates a max fiber stress exceeding yield for a .035 thick blade on a 3.0" dia guide.

View higher quality, full size image (5100 X 7014)

!/4 " down pressure with roller guides is no problem as I get up to 20 sharpening's on some of my blades with 3" rollers, 2" rollers might not do as good. The 2 biggest blade breakers are having the back flange or rollers too close to the back of the blade and worn v belts. Steve

It looks like the guides are doing more than guiding. They are also tensioning. Is there a roller guide on both sides of the blade?

Dr. Wengert: Th guide wheels are flanged with no bottom support for the blade. The blade is placed on the mill with the guides fixed in a plane .25" below the tangent to the "drive" wheels. Then tension is applied by a hydraulic cylinder to induce about 20,000 psi tension in the band. My understanding is that this "out of plane" location of the guides provides the up force to hold the blade steady while cutting. Per-Olovs' reply perhaps provides the insight to why small guide rollers don't destroy a blade in short order. A deflection calculation indicates that the blade would not conform to the radius of the guide roller and the stress range would not be nearly as great as going around the drive wheels. All of the information posted here, and further analysis, would make me believe that I should not be concerned of shortening band life by offsetting the guide rollers.

Thank you Per=Olov and Steve N for your insights.

The purpose of a guide is to assure that the blade does not deviate significantly from its desired path. This is done in two ways.

First, if the blade has excess energy (shows up as movement) that develops while cutting, the guide will absorb some of this energy. Overall, this means that the blade will take the shortest path between the two guides, as it would take energy to do otherwise. Sometimes, especially when the guides are in perfect alignment with the natural path of the saw blade (your first figure), there will be a guide on each side of the blade, rather than having offset and then using only a guide on one side.

The second is that the guides shorten the distance between the two main wheels and this results in the blade, where the cutting occurs, having a higher resonance frequency, which means the blade is much less likely to vibrate, as the resonant frequency from the spinning of the blade around the wheels and the frequency between the guides are far apart. If they were close, you can get a standing wave, which means the blade "vibrates" and does not cut straight. Incidentally, this is also why the guides are moved, during sawing, so that they are close to the log surface, again shortening the span and raising the frequency.

Because the guides oftentimes are touching the blade, they are designed to be frictionless to avoid heating. Heating would cause unwanted blade movement and potential failure. So, spinning, roller guides do not frequent inspection to assure they are spinning without restriction.

These concepts apply to band and circular of all sizes.

Hope this helps.

Correction

...guides do need frequent....