A great question from Angus

This was the question received a couple of days ago from Angus in America:

Message:  We were wondering: why doesn't the drive belt fall off the flywheel when Atom is driving the sawmill? The belt is fairly slack, and there is no flange on the flywheel. The traction engine flywheels we saw at Pickering 2010 were convex in profile, but still the belt doesn't fall off. How does this work?
Thanks
Angus (6), Angus's Dad (44) and Angus's Grandpa (75)

It is a great question and very observant of them to notice how odd it is.

This is how I tried to explain the puzzling pulley in my reply to them:

Hi Angus, Dad and Grandpa,

Thanks for your email and great question. In fact I think it is one of the best questions I have ever been asked from the books and very observant of you to notice that something odd was happening.

In fact nearly all flat belts run on pulleys without a flange and to make them stay on, the pulley has to be the shape you observed: Domed, convex or barrel shaped.

If the pulleys are parallel or cylinder shape, then the belt will just wander off the side. An if the pulley is more like a convex shape or the opposite of barrel-shaped, the belt will certainly fly off very quickly.

Flat pulleys are not used so much these days, but they were popular until a few years ago. They were used as in the book for traction engines to drive sawmills or threshing machines. More recently, tractors sometimes had a pulley for driving machinery but these pulleys were smaller than a flywheel, about 30 cm diameter, but again distinctly barrel shaped. Also all the machinery in old-fashioned factories was driven by flat belts from “line shafts” up in the factory roof.

Your question though is “Why does the belt stay on?” It is quite difficult to explain in words without being able to gesticulate with my hands! But I will try my best....

It is all to do with what happens if the belt is not perfectly aligned with the pulley and is running to one side. And this is the case if the belt starts to wander off, or the two pulleys are not perfectly lined up.

It is also important to think of the belt as a part of it is approaching a pulley because this is the bit which matters. (The part which is just coming off the pulley,going back in the other direction does not have any real effect.)

Now this is the tricky bit to explain: If the belt is running on one side of the pulley, the bit of belt nearest to the centre of the pulley is going round a slightly larger diameter than the bit nearer to the edge. The belt is flexible and gets a little bent sideways on the pulley (not the normal way it bends to go round the pulley). The slight bend in the belt (which is caused by the barrel shape of the pulley) actually steers the belt back towards the centre of the pulley. If it goes too far and ends up on the other side, then the process happens the other way and the belt steers itself back to the centre again. It is “Self Centring.”

Does that explain it?????

If not, tell me and I will have another go!!!

Best wishes
Chris.