Last month, I opened my article by mentioning the last sale of the season, and guess what? The collection increased by one more! This one is a vertical Crossley VOE, a hot bulb, single-cylinder engine which looks to be about a 6HP, but is in fact, 11HP, achieved by a pleasingly complicated air scavenging system. Originally someone else's restoration project, this one came in do-it-yourself assembly form, with one large chunk of cast iron and several large containers of assorted parts. Thanks to the internet, we had no trouble finding a complete manual to download and print out and this proved to be invaluable in putting the jigsaw back together.

One thing, though, that the internet seems to consistently fail to find is belt dressing. I know I've covered the subject of flat belts before in these articles, but it is the one thing I get most "please help" emails about! It's easy to lose track of how many times I've passed on the discussions from the ATIS (Antique Tractor Internet Service) Stationary Engine Mailing List to the readers of GEM, so I looked up the last time I mentioned belts. Unbelievably, it was 32 articles back, so I don't think I've overdone the subject yet. Last time, it was mainly about the various products which could be used as belt dressing, but the one thing everyone wants is the solid sticks of rosin-based dressing, so please, if anyone knows of a source of this product, could you contact GEM so the information can be passed on to the engine collecting world?

* On more than one occasion, a person has asked at a show, "What keeps the belt from slipping off the side?" - or - "What keeps the belt on."
Can anyone give a simple explanation ?

* The simple answer is that the crown (center) section of the pulley is traveling at a higher surface speed that the outer edges. The belt gravitates to the portion of the pulley that runs the fastest. That is what keeps a flat belt on.
If you were to make a tapered pulley and tried to run a belt on it, the belt would walk right up the taper and off the high side of the pulley.

* And for all these years I've been telling everyone that it was magic!
I think most spectators at shows would be more likely to believe that it's magic than they would that the center of the pulley is going faster than the outside!

* I agree with what you're saying, but still don't understand why it will gravitate toward the larger diameter, higher surface speed. It seems logical to me that the smaller diameter will be the path of least resistance and the belt would go that direction.
In working with conveyor belts for some time, I know that a crowned pulley that isn't perpendicular with the belt's edge will gravitate down hill, or toward the loose side.

* There is a very simple way to understand why a belt will gravitate toward the larger diameter of a crowned pulley. It helps to make a sketch.

1) Make a sketch of the outline of a crowned pulley.
2) Viewed from above, lightly sketch a belt that is about half the width of the pulley; and, it is off to one side of it so that the edge of the belt is at the edge of the pulley.
3) You know that the other pulley at the other end of the belt is going to be directly in line with the first pulley. So, as you sketch the belt, show it centered on the second pulley.
4) Belts are flexible. Taking this into consideration, now finish drawing the belt. The crown of the "half off" pulley will put a curve in the belt, as viewed from above. Right?
5) Now, back at the "straight" portion of the belt before it begins its curve, draw a line right down through the center. Draw the centerline, in other words.
6) Where does that centerline point? It points toward the center of the "half off" pulley.
7) As the straight portion of the belt travels toward the "half off" pulley, it is headed toward the center of it.
8) Thus, you can see that the curvature of the belt caused by the crown causes the approaching belt to aim toward the center of the pulley.

If you're not convinced, draw a reverse crowned pulley and do the same exercise. If the belt is slightly upset off the center of the pulley, it will run off the pulley.

The explanation is clear as mud until one draws the sketch, step by step.

* Thanks to all who responded.
I was looking for a more simple explanation. I can see I went to the wrong place for a "simple" answer.

When I'm asked, "How does the belt stay on", I usually get too complicated in trying to explain how the belt stays on.

Many times you can look down the length of a belt and see that the belt curves. If the belt isn't straight - Why doesn't it fly off the pulley ?
I tell them there is a crown, or ridge, in the center of the pulley. As the belt tries to travel off to the right, the left side of the belt gets tighter than the right side and this makes the belt travel to the left. Now the belt will try to travel off to the left side of the pulley and the right side of the belt gets tighter than the left side and that makes the belt travel to the right.
It's sorta like balancing a bicycle. The belt is constantly "correcting" and stays on.
In the case of the 2 pulleys being flat surfaced, I simply feel the pulleys have to be
lined up pretty good in order not to throw a belt.

* A simple explanation for why a belt self-centers onto a crowned pulley? If there is some belted equipment on hand, the answer can be simple, indeed, provided the crown is sufficiently noticeable.
Just sight down the length of the belt. Where is its centerline aimed? It's aimed toward the center of the pulley. So, that's where the belt is headed, toward the middle of the pulley. That's where it will go and that's where it will stay. End of explanation.

* The belt is tighter in the crown area; the extra grip of the crown just pulls the belt right into the center. I understand someone experimented with running the belt in a trough, meaning the center of the pulley was low and the edges were high. The belt walked right off every time.
If the belt slips, it will go the other way and throws itself off the pulleys. Happens all the time when I am cutting firewood; I get an odd shaped piece that binds a bit and I throw the belt.

* In a perfect set-up, the pulleys are flat, everything is aligned nicely, and the belt goes just where you want it to. But this is the real world and even when we do align things perfectly, they'll eventually vibrate out of position and the belt will fall to the ground.
The behavior of the belt on a crowned pulley could be explained by complex math and big words, but a simple demonstration is much clearer. It's an experiment you can do right now while sitting at your desk (if your desk is as cluttered as mine).
Take off your belt and lay it out flat on your desk. Now take a cylindrical soda can, lay it perpendicular across the belt, and then fold your belt over the can. Note how the top side of the belt lines up with the bottom side. That's great, but if the belt is near the edge of the can, there is nothing to make it want to go back to the center.

Now get something similar in size to the soda can, but tapered or rounded. A water glass will work nicely. Start the same as before, with the belt flat on the desk and the tapered water glass perpendicular to the belt. Fold the belt over the glass. WHOA! The top part of the belt now points off in another direction. It points toward the big end of the glass. This is the same effect that keeps the belt tracking toward the fat side of a pulley.

Now consider the behavior of a belt that is slipping. If the pulley is perfectly flat and the whole system is perfectly aligned, the belt will just slip and stay in position. If the
pulley is crowned, the belt will be pushed off much more quickly.

It takes more of the belt to make it around the larger circumference and there's extra belt on the small circumference.

There's always one, isn't there?

*My pants fell off and I spilt Coke all over my desk. The belts stay on the engine because they do, that's all those folks need to know.

Or maybe two.

* You are not implying that I'm fat and that my pants would have fallen off because of torque around my circumference and the slippage that follows are you?

Now that the surface closest to your reading area is covered with sketches on the back of old envelopes, belts and a selection of household items, we'll move on to a slightly different belting question.

* How tight does a flat belt have to be? I've never used this type belt before. I'm going to run a buzz saw with an old Briggs 14 for the first time.

* I just looked up the specs on my tractor. The belt speed is 3,200 feet per minute at full throttle. I'd guess I'm running about 2,000 feet per minute. With the saw pulley thus running 2,000 fpm, and saw pulley being 8" and the saw 30" diameter, that makes the saw run at 7,500 fpm.
With that info you can figure out ahead of time if your engine setup is in the ballpark.
Both tractor and saw pulleys are nicely crowned, that really helps keep the belt where it belongs.

* I've run several things with flat belts.
One thing about them is when they drop off they tend to just lay down and not cause any damage.
They don't have to be tight at all. In fact they can be very lose and still do what you need.
Many times mine actually flop pretty good and you wonder how they stay on.
When I used my Witte to run a Buzz Saw, I would have the belt taut but still lose enough to be able to slip the belt on and off the saw pulley when not running.
My belts are not too long, maybe 16'-20' total length, 8'-10' belted up.
You'll need to experiment - but it's important to use a belt dressing. Many times you'll have trouble keeping a belt on the pulley simply because it can't get any grip.

I see speed was mentioned earlier. I think the pulley on my Witte was about 6" and the pulley on the saw was about 10". With the Witte sped up to about it's safest fast speed, I was a little slow on the saw, but it worked. On a buzz saw, you'll want to hear the "singing" of the teeth with a nice "whine" when you're not sawing.

* As I recall a 100' belt is self-tightening to about the right tension, with the weight of the belt in the 50 feet between the pulleys.
Shorter belts you have to tension yourself, you'll probably want an idler to adjust it or make the engine movable on your rig.
I'd think the Briggs to be too fast for a buzz saw unless you have a speed reducer. You'll need a pretty small pulley on the engine, and that may be hard to keep a belt on. I run my saw on a JD tractor, running about half throttle or less. A lot of people use hit and miss engines running at maybe 300-500 RPM with a little larger pulley, maybe 12 - 18''.

* I got it going. At first, since the pulleys were kind of rusty it grabbed real good. Then it started to slip, so I used the belt dressing and was having a great time. After a while the Briggs started to really labor and when I shut it off I noticed metal shavings under the gear unit. Closer inspection reveals that they were coming from under the blower housing - this doesn't look good! On the bright side, I have two spare engines in the shed!

There. I always KNEW there was a good reason for getting another engine. And another. And another. They're "spares"!!!
Don't forget to contact GEM if you know of a supply of solid belt dressing. It seems that everyone prefers it to the modern alternatives .
And finally, do take care with your engines, belts and saw rigs when you're cutting firewood for the winter. Happy Christmas, everyone.

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