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Broken Drive Belt

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David Noble30/10/2020 12:08:28
218 forum posts
9 photos

Here's a bit of an odd problem, for me anyway! I had just finished a facing cut on my lathe and pressed the stop button. There was a loud bang and this is what I found. Now, there was no load on the motor at the time and it snapped just when I pressed the stop button. Any ideas gratefully received before I replace it.


not done it yet30/10/2020 12:25:50
5141 forum posts
20 photos

Are you sure there was no load - like the cross slide just reaching the end of its travel? I have to be careful, if parting off from the rear, on my lathe.🙂

Be relieved that it was a lathe drive and not the cam belt on your car engine.🙂

David Noble30/10/2020 14:45:59
218 forum posts
9 photos

Thanks NDIY, all I can think, is that I must have inadvertantly nudged the reverse switch which is next to the stop button. I did check and it's still on 'Forward' but I suppose I could have given it a nudge. (Shrugs shoulders) I'll put it down to the gremlins.


not done it yet30/10/2020 16:34:43
5141 forum posts
20 photos

Surely your lathe motor does not go from speed in forwards to full power in reverse? If I switch directions at full speed - the motor simply winds down and then winds back up in the other direction - although I don’t even have time to change directions and restart if I use the stop button. Soft start is a definite plus, for me, at all times.

While I don’t do it, if I left the chuck key in, I doubt it would do any damage as that first half turn is at very low speed/power - I know the motor power trips if I have the spindle locked with the back gear engaged ... and inadvertently start the lathe🙂 .

Expensive gremlins if they strike too often. Positive-drive belts are not cheap, compared to V belts.

larry phelan 130/10/2020 16:39:46
868 forum posts
17 photos

Interesting ! I part off from the rear all the time, so might have to watch that !surprise

Tim Hammond30/10/2020 17:05:45
47 forum posts

I suspect strongly that this is a characteristic of cogged belts. The reason I say this is experience with Harley-Davidson motorcycles, which for some time have used this system to transmit power to the rear wheel. There are many reports among owners of belts failing for no apparent reason, usually under light-or no load conditions. It happened to me one afternoon some years ago, at the time I owned an H-D Sportster model, it had just 30,000 miles on the clock, I started it up after visiting a friend, selected first gear, let the clutch out to move off and found I had a motorbike with a gearbox full of neutrals. It had to return home on the back of a lorry. On examination the belt looked exactly like the one in the photograph. Prior to this I'd inspected the belt as per the instructions in the handbook and it appeared absolutely fine. There was considerable discussion on the various owners' fora that belts tightened to the manufacturer's recommended figure would fail prematurely, and to run them a lot slacker - mine was tensioned to the correct figure using the tool sold by H-D for this. Some owners racked up impressive mileages (100,000 miles+) before changing a still-intact belt, other belts failed almost after leaving the dealer's showroom. And who remembers the death rattle of a Ford Cortina engine (?Pinto ) when the camshaft drive belt failed?

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