Question
I have read that a crown on a band wheel will improve band tracking, but I don't understand why. What is it about a crown that improves tracking, or what is the theory behind crowning?
Forum Responses
From contributor O:
With a crown, the blade wants to ride up to the top of the crown, meaning if the crown is in the center of the wheel, the blade will seek that center.
Where I once worked, our machinists in their wisdom decided the shop bandsaw had worn such that the band wheel had a crown, so they machined it flat. What a nightmare keeping the blade on track, until we found out what they had done.
Let's start with a band centered on a wheel with a crown that peaks at the center of the wheel face. Let's also put ourselves at that point on the wheel where the band first makes contact as it approaches the wheel. At the center of the wheel, the tension is greatest (as the crown stretches the band here more than at the sides).
Although this is beside the point, I suspect this uneven distribution of tension, coupled with crown shape, will cause the band to cup upward toward the wheel, which probably helps with keeping the band on the wheel.
However, I think what truly makes the band seek the center is that point of peak tension, and that it is most likely having a steering effect on the incoming band.
Imagine for a moment that you're pulling a long piece of paper. If you pull from the center, the paper will tend to follow straight behind. However, if you move off center, the trailing edge of the paper will tend to shift in the direction of the offset in your pulling position. In a sense, the paper is trying to steer itself back to being centered with your pull.
Getting back to the band, if a band creeps a little to one side, the point of peak tension will move in the opposite direction, i.e., off center as we did with the paper. That will tend to pull the incoming band back to center, assuming the band will move much like the paper moved.
I have myself just about talked into that theory. Hopefully I've explained it well enough. Interested in your feedback.
As to the shape of the crown, what contributor D suggests makes good sense. We know bands don't like too much curl (e.g., with small wheels) and it's easy to understand that the crown could have the same effect. Also, lots of contact seems quite appropriate. I would assume then that a circular crown would be best, i.e., one with a radius that gives the optimum peak. I haven't done the math, but I suspect a radius of 12 inches (24 inch wheel) would give me some idea of maximum crown height. I suppose however that what we want is the minimum amount of crown that gives the proper (steering?) effect while being least stressful to the band.
Belts may be made to center themselves on their pulleys by the use of crowned pulleys. The usual figure for the amount of crowning is 1/8 inch per foot of pulley width. Thus, the difference in maximum and minimum radii of a 6 inch pulley would be 1/16 inch. ------ Too much crown is undesirable because of the tendency to “break the belts back.”
The above extract applies to flat belts, not necessarily band saw blades.
You don't really need a crown but a radial power in the middle of the wheel. F. ex. on my bandmill there is a V-belt-wheel (SPBØ500) with the shortest SPB V-belt I ever can get on. Band tension is about 5000 N and it pulls the band perfect into the wheels. Radial power is there in the middle and makes a kind of crown, just where the band hits the wheel. The wheels have to be in line and parallel to each other, at least one of them. You need to be able to adjust because nothing is perfect from the beginning. And not later.
You are right that a too high crown isn't good for the band, but you don't need a lot. This is what I've gotten out of successful production of belt conveyors a lot of years. And I'm sorry for my language - I'm a Swede.
Saw doctors have charts for the proper amount of tensioning that should be put on blades, depending on the width of blade and type/condition of lumber to be sawn (softwood, hardwood, frozen logs, etc.). Part of this tensioning implies setting a curve across the blade to fit the crown of the wheels. The curve results in the blade being shorter on both edges and longer in the middle.
These wide bands do not have any backing bearings (no rubber covering either) that prevent them from being thrown off the wheels when the blade enters the end of a log. It's the mating of the blade/wheels curves + tensioning of both wheels that keeps the band on track.
Part of the tensioning on the wheels is also made by one upper and one lower lubricated "scrapers" that match the inside curve of the blade and exert a push on it (deflect the blade) towards the front, where the log carriage will pass.
In general, say for a 12" wide blade, the elevation of the curve is around 1/8 of an inch.
To get a saw to run well, we cool the teeth. Some mills actually drip water (or another magic solution) on the band to cool it. It is much more effective if they cool just the teeth rather than the entire blade. But even if the teeth are cool, we still need tension in the blade due to the centripetal forces (the same forces that, if you remember the old toy, would cause a small ball on an elastic string to fly off when you spin the ball by the string and then release the string, trying to hit your baby brother or sister).
It would have been clearer if I had used centrifugal and not centripetal in the above... sorry.
However, the curve depends primarily on the speed of the band. The width is not a factor, other than that the curve extends for a longer distance; the wheel crown is not a factor, as when the band is running at full speed, it is flat... the curvature of the blade is gone. We do not change the tension based on the species being sawn; we change the tooth style perhaps and feed rate.
Incidentally, the tension (or curvature) put on a wide band is the curve of a 40 foot diameter circle. This is called a "40 gauge" or "40 tension gauge".
"Nearly all mill builders and operators agree that a perfectly flat wheel is best, regardless of the diameter or width of face of wheel, but where crown is used on say a 9-foot wheel with 12-inch face, it should rarely exceed 1/32-inch, in the diameter of wheel. A general rule of thumb for crowned wheels is that the crown on the wheel face should not exceed .001" per foot of wheel diameter.
"The more crown on the wheels, the more tension [that is, the more curvature when hammered] is required in the saw, and the strain on the saw is greater. General practice demonstrates that band saws last longer, have a better “set” on the wheels, and will stand a heavier feed if the wheels are perfectly flat face than if they have even a little crown. It may be accepted as something of a defect in any band mill if it is necessary to resort to much crown in the wheels to keep the saw on them."
There is a seminar on sawblades offered in California by the Wood Machining Institute that has the details on a spinning blade well described.
Gene, please suggest literature, websites, etc. that I (we) could use to research this issue. I'd like to attend the training you mentioned, but it's not an option at this time.
"Strain" is the correct term for the force applied to the blade by weights or springs to enable it to cut in a straight line. Referring to strain as tension can be misleading.
Comment from contributor A:
The reasoning behind the crown on the bandwheel is to help the saw track better on the wheel, and also to keep its position much more stable. A crowned wheel is not always necessary, many sawmills run flat wheels. However, a crowned wheel gives a more instant reaction when you "track" the saw forward or back using the tilt mechanism. A flat wheel doesn't always move instantly and it may continue creeping after you have finished tilting the wheel.
Saw doctors determine the amount of crown depending on many factors such as the size of the wheels, the amount of tension in the saw (tension is different to strain).
The strain itself is what stops the saw from coming off the wheel when using just flat wheels. It keeps the bandsaw taught and well, under strain and helps to maintain a straight cutting line. However, as with everything concerning saws it is just one of many items to be taken into consideration to gain the optimal performance from your saws.
I actually pulled (deliberately) my bandsaw blades with a hook to see how far they'd go to remain on the wheels. Unbelievably the harder you try to pull them off, the harder they resist. They will literally dig in and cut the rubber. That's why the radius of our crowns is carefully machined to equal the radius of the wheel itself.
