Dear all,
so far I have never written in this forum. I am operating a steam shovel here in Northern Germany, which definitely is something different to the majority of traction-talk threads.
Please let me give you an engineer´s comment on Boadicea´s belt-issue. For 20 years now I am working as an application engineer for the Forbo Siegling Company. We are manufacturing flat belts in Germany since 100 years. On our English website you can also find contact data:
https://www.forbo.com/movement/en-uk/
Forbo Siegling (UK) Ltd.
Unit 4, Fifth Avenue
Tameside Park
Dukinfield
GB-Cheshire SK16 4PP
Until 1943 we made flat belts from chrome leather, after that date only with a tension member made of Polyamide, covered with leather on both sides for friction. These belts are available in a big range, the effective beltpull can be between 45 lbs/inch and 450 lbs per inch belt width (I calculated from metric dimensions). One of my jobs is to calculate and understand belt drives, mostly the heavy ones, which today can reach up to 1500 kW (2000 hp) in waterpowerstations, steel mills or wood-chipping machines. The belt width can reach more than 3 ft. One of the problems we have to avoid are vibrations of the belt strands. Every free tensioned belt part ist like a tensioned string on the guitar, having a resonance frequency, which can be altered by the tension (like you tune the guitar).
In the video you see the upper belt strand, which is transmitting the effective beltpull, running stable and calm. But the bottom strand shakes heavily. This means, that the bottom belt part is in resonance with an external excitation, possibly coming from the uneven torque of the steam engine (compare the belt movement with the engine chuffs!).
You need the pretension force in the bottom strand to build up the effective beltpull when the belt is going around the drive pulley. When you have a big arc of contact, you can transmit more beltpull than a smaller arc of contact with the same pretension.
But when the belt is shaking, the pretension oscillates, which can lead to short-time but frequent complete slippage of the belt on either pulley. When the belts slips in longitudinal direction, there is also no force to hold it in transverse direction, not even on crowned pulleys. So it can easily be pulled off sideways by the belt forces, if the pulleys are not aligned perfectly. I assume it is not possible to align them perfectly in reality. Maybe one of the machines moved on the ground due to vibrations or unsufficient anchors.
There is only one solution to shaking belts: increase tension (=increase the distance between the pulleys), reduce effective belt load or change the speed. They should have stopped the test or reduce the power after the first heavy shaking of the belt. You can see, that it was not possible to come beyond the resonance point, and this is dangerous.
When you watch a belt drive speeding up on a saw bench or similar, in most cases you will see that each belt strand has its shaky seconds when going through resonance. But as long as resonance does not persist in normal operation, this is ok.
We should be glad that no person was injured, there is a lot of energy stored in a tensioned belt. Even with smaller belts that might have a wire-hook-splice there is a risk – do not stand in line with the pulleys, because belt ends with disintegrating wire hooks can produce some nasty scratches.
English is not my native language, so please apologize if some expressions are not 100% correct. But I hope that I could explain some of the theory behind. We should leave resonance to string instruments, and listen to the smokebox rhythm! Have a save steaming!!
Peter
You may also contact me by my company-email:
peter.meyer@forbo.com