Crowns on Band Wheels

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Why do crowns exist, and do they improve band tracking? January 29, 2004

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.

From contributor D:
A belt or blade will climb to the largest diameter of a pulley. If the largest diameter is on the edge of the wheel, that is were the blade will track. On the large sawmill bandsaws with blades a foot wide, the wheels are ground flat.

From the original questioner:
Your responses give me another clue - the band seeks the largest diameter on the pulley. I'm assuming that means the band will tend to straddle that high point, or more specifically, the body of the band (back to base of gullet) will straddle the high point. Now I must figure out why it does that. What are the forces involved? Once I know that, then if it's not obvious, I'll want to understand just what makes a good crown. How high? Triangular, circular, elliptical, parabolic?

From contributor D:
By tilting the wheel, you can make the plate run where you want it on the crown. You should stay away from the triangular crown; you want as much contact as possible with the blade and the sharp corner increases stress in the blade.

From the original questioner:
Based on the tip offered above, I have a theory about what is happening. Let me know if this makes sense or not.

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.

From contributor B:
The Machinery’s Handbook (Revised 21st Edition) page 1030, subheading Pulleys, states:

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.

From contributor S:
The band (belt o w e) wants to hit the wheel in a right angel, 90 (if the sides are parallel, all the width has the same elongation, etc.), but you can't get this effect without tension! If the band comes beside the crown it even goes out of the 90 degree angle and wants to go back.

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.

From contributor A:
Many wide band wheels are crowned not in the middle of wheel but at 1/3 of its width, on the teeth side of the band. This is to accommodate different band widths and make sure that the teeth are always out of the wheel rim. Of course, the band itself is not flat either on its width, but has a curve in it to match the crown on the wheel. This is a process called tensioning of the band. Double-cut bands 8" to 18" wide are crowned (wheels+band) in the middle of width.

From contributor D:
It is my understanding that the tensioning is done so that the band edges where the teeth are can be made taut. I don't think that a crown on the wheels would allow this.

From Gene Wengert, forum technical advisor:
A band is not flat but is slightly curved based on the speed and not the crown of the wheel (except the amount of crown is related to speed). As a band spins faster and faster, it wants to get longer (centripetal force). But of course, the band is restricted from getting longer by all the metal. So, an option to getting longer is to get wavy. We do not want that, however, unless we like wavy lumber. Another option is to curve the band when it is stationary. Then when it speeds up, the curve will flatten and take up the tendency to want to get longer. At full speed, if properly tensioned (or curved or hammered), the band will actually be flat and be without waves of resonance. To reduce resonance or waviness, we also use guides to absorb energy. (This is a brief discussion of a complex subject. Hope it is clear enough.)

From contributor O:
And here I thought tensioning was to offset the heat (and thus expansion) buildup at the teeth when they were cutting wood, not due to "centripetal" force caused when the band goes faster and faster. But I am not the Dr. and have a lot to learn.

From contributor A:
Contributor D, the tensioning that I'm referring to here is the one done on the blade by the saw doctor on a tensioning bench, not the tensioning applied to the blade when set on the wheels.

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.

From Gene Wengert, forum technical advisor:
Actually, we want the heat to be in the center. If the teeth heat, the resulting expansion takes out the curve that was hammered in (takes the tension out), so the blade will not run well at all. (Sometimes, one might put in a little extra tension to account for the heating of the teeth, but then the blade will not run well when it is cool - too much tension.)

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).

From Gene Wengert, forum technical advisor:
Centrifugal force is a force that is directed outward away from the axis when spinning. Centripetal force is the opposite - a force directed inward toward the axis.
Centripetal = -centrifugal

It would have been clearer if I had used centrifugal and not centripetal in the above... sorry.

From Gene Wengert, forum technical advisor:
Contributor A, I also am referring to the same tension hammered into a blade.

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".

From Gene Wengert, forum technical advisor:
I have seen many mills that use flat band wheels, or that use a bottom one flat and a slight crown on the top wheel. When this wheel spins, it will develop a little crown due to centrifugal forces. Here is an excerpt from the Simonds Saw Web Site:

"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."

From contributor D:
Contributor A, I have seen the equipment used to sharpen and tension the wide blades at the Wood Tech shows. They have a machine that has a heavy roller placed on both sides of a wide band saw blade. These rollers stretch the center of the blade, forming the curve you talk about. As explained to me, when the blade is on the saw, the flat ground wheel pulls the edges of the band taunt when tension (force) is applied to the wheels. (The length of the blade is shorter on the edges than in the middle). This stiffens the teeth so the band will cut straight. This is what the people selling the equipment explained to me, if I understood them correctly.

From Gene Wengert, forum technical advisor:
Contributor D, the sales people need to learn a little more about blade dynamics. The explanation that they gave you is not correct.

There is a seminar on sawblades offered in California by the Wood Machining Institute that has the details on a spinning blade well described.

From contributor A:
Gene, I'm just wondering what prevents the band from being thrown off the wheels while sawing a log on those flat-wheeled band saws? Especially for double-cut bands when teeth become clogged and cut less aggressively?

From the original questioner:
With all due respect, I'm not sure I'm buying all of this. For one, why does the centrifugal force only affect the edges? I'd think the entire width would see the effect and thus no flattening. In addition, any flattening would tend to weaken a crowned wheel's grip on the band. And again, if a band goes flat and is riding on top of a crown, what keeps it from walking off the wheel? And none of this tensioning and hammering stuff would apply to those leather or cloth belts I used to see run on farm machinery (with crowned pulleys). I'll asked Lenox. They've been the most technically astute vendor I've talked with so far. Maybe they have a clue.

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.

From Gene Wengert, forum technical advisor:
You could contact:
R. Szymani, Director
Wood Machining Institute
P.O. Box 476 - Berkeley, California 94701
Telephone: (925) 943-5240
FAX: (925) 945-0947

From contributor H:
Just one small point: The contributors referring to tension as being applied by the saw doctor using steel rolls are quite correct.

"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.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

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.

Comment from contributor B:
Why blades center on a crowned wheel: If you wrap tape around a cylinder, you can probably do a fairly decent job of getting each wrap on top of the previous one, but it takes effort. Try wrapping tape around a cone. Unless the tape is very stretchy, it’s impossible to wrap the tape on top of the previous wrap. It wants to spiral towards a larger diameter. Now, if that cone were attached to another one at the large diameter ends (sort of the opposite of a v-belt sheave), the tape would wrap over the crown and spiral toward the other small end but the spirals would tighten up and reverse direction, heading back to the crown. Smooth out the bases of the cones and you have a crowned wheel. I'm sure you've experienced a similar situation wrapping electrical tape around a bulge in connected wires. When you wrap the tape on one side of the bulge wants to wrap toward the thick part of the bulge.

Comment from contributor C:
Band saw blades climb to the top of their wheels' crown for the same reason snow skis turn when you tip them onto their edge. Imagine instead of wheels, bandsaw blades were transported on spheres. The only thing keeping the blade on a flat plane would be the general alignment of the shafts. If the wheels were of equal diameter, the blade would find the plane most perpendicular to their combined orientation. Because the spheres are rotating on a common axis, the tendency to climb that radius prevents the blade from deviating from its plane of rotation most tenaciously. Flat blades on round surfaces being pretty much like the inverse of bent (or curved) skis on a (relatively) flat snow field.

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.