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Lathe belt broken

Gates 710 belt for a 9x20 lathe

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BOB BLACKSHAW19/11/2019 17:17:18
299 forum posts
64 photos

To answer Bill`s question, when I got the 9x20 lathe the belt was tight on 3 off the pullies so when changing over to another pully it was a thumb push to get the belt off. The tensioner in my opinion gave even more strain on the belt and I didn't use it until the belt came looser or changing to the other three pullies which didn't have the tension and the tensioner was needed. Now its on a inverter only three pullies are used and they are tight on all three so the tensioner is not used.

If I remember a Myford lathe the tensioner was used to slacken the belt to change pullies, the 9x20 as I've said the belt is tight even when the tensioner is off.

The Gates belts are suppose to be the best and are used with cam shaft drives on cars, so changing over on other pullies must be damaging the belt. Is this a design fault? are there any other 9x20 users having this problem.


Ron Laden19/11/2019 17:21:12
1921 forum posts
368 photos

Bob why are you not using the belt tensioner as Bill says that's what it is there for. I have the Warco 918 and the belt I use is 5M but the 730 not the 710 so is a little longer. Belt changing to a different pulley size is a doddle, push the tension lever back, belt becomes slack change belt position release the tension lever and that's it quick, easy with no stress to the belt.

Dave Halford19/11/2019 17:31:56
743 forum posts
6 photos

Hi bob,

There should be about 1/2" of slack if you press on the longest length with your thumb.

Sounds like the belt could be over tensioned

BOB BLACKSHAW19/11/2019 17:43:23
299 forum posts
64 photos

Actually I have just been thinking this, Ive always used a 710 belt as that was the belt supplied with the machine, so I will try a 730.

Thanks Bob.

Howard Lewis19/11/2019 20:55:39
3267 forum posts
2 photos

If it is Z section belt, it should be a Vee belt. In my experience Vee belts turn over if subjected to excessive tension (In my case torsionals ).

As said, it should be possible to press the middle of the belt run inwards about 1/2". If you can't, the belt is overtensioned, which would explain the repeated failures. Suggesting that the 710 is too short so that720 or 730 would be less likely to be overtensioned, and last longer.

On Myfords, and many other lathes, the weight of the motor, on its pivoting bracket provides enough tension to drive satisfactorily. Vee belts tend to tension themseves, on the one side, as they run.

All belts need a difference in tension, between one side and the other to provide a drive.


Mike Woods 121/11/2019 01:16:31
28 forum posts
1 photos

Hello Bob

I had similar problems with a used Chester 920 that I bought some years ago. The drive belt on the machine was a bit sad and tattered, so I bought a replacement. That fitted fine on all but one speed arrangement, where like yours it far too tight, so I bought another but one size larger for use on that one speed. One thing I did find that was the axis of the belt tensioner roller was not aligned with the pulley shafts and main spindle, which caused the belt to run bowed to one side (see picture). This allowed the belt to:

A - ride up the side of the flange on the end of the roller, flipping the belt so the it ran inverted in the pulley vee

B - rub on the edge of the pulley as it exited the vee and also rub on the edge of the larger pulley

The misalignment was the result of slop in the main pivot on the tensioner crank, tensioner roller spindle pressed into the crank at an angle before being welded in place during manufacture and the tensioner crank being anything but straight and untwisted. All relatively easy to correct though.

If you are going to try the larger belts and use the tensioner, it is worth checking alignment. Hopefully yours will be okay.

belt tensioner alignment.jpg

Michael Gilligan21/11/2019 08:37:06
15712 forum posts
687 photos

That’s a great illustration of a likely problem, Mike


Mike Poole21/11/2019 09:05:06
2575 forum posts
60 photos


page 77 of the Gates manual has some interesting recommendations for idler pulleys, it may be interesting to compare your setup with their recommendations.


Mike Woods 121/11/2019 09:33:09
28 forum posts
1 photos

Thanks Michael, since retiring this old dog has been trying to learn new tricks, namely CAD. However, having looked at this the morning after, I have just spotted my schoolboy drafting error. Note the absence of the pulley profile obscuring the bottom run of the bowed belt. It has nothing to do with learning CAD, this is my mistake and mine alone. Back in my days in the drawing office, this would have earned me a day in the print room running off and hand folding diazo prints. More CAD practise tonight, methinks!

Michael Gilligan21/11/2019 10:33:23
15712 forum posts
687 photos
Posted by Mike Poole on 21/11/2019 09:05:06:


page 77 of the Gates manual has some interesting recommendations for idler pulleys, it may be interesting to compare your setup with their recommendations.



I think you probably have found one of the culprits, Mike ...

NOTE: the use of outside idlers is not recommended for Polyflex® JBTM belt drives.

I see that mick H references Polyflex belts on page 1 of this thread.

... is that what BOB is using ?


Edited By Michael Gilligan on 21/11/2019 10:36:08

Ron Laden21/11/2019 14:30:35
1921 forum posts
368 photos

Yes Bob will be using a Gates 5M JB Polyflex belt and the spring loaded tension roller on the 920 lathes does run on the outside of the belt. Despite Gates recommendation not to have outside idlers one can only assume that the original design of the lathe ignored that recommendation. Having said that and based on my Warco 918 (920 lathe) I can only think that the lathe designers got it right, there are thousands of 920 lathes around the world and I have not heard of belt problems being an issue.

Unlike what Mike W1 found on his machine the tension roller, the sprung lever and its mount on mine are all square, aligned, well mounted and the belt runs true between pulleys. I have bought a spare belt for mine but thats only to cover the need of a replacement which with sods law is bound to come on a Friday evening. I dont know how old the belt that came with my lathe is but I know its been fitted for quite some time and still looks in very good shape.

I dont think there is need to start delving into mods to the tension roller, there is no need it works just fine. Providing it is all good on Bobs machine he just needs a replacement belt of the correct length. Bob couldnt use the tension roller because the belt he had was too short and was difficult to fit on and off the pulleys which is what caused the belt damage.

My two penneth anyway.


Edited By Ron Laden on 21/11/2019 14:34:20

BOB BLACKSHAW21/11/2019 16:04:08
299 forum posts
64 photos

Thanks for the replies, my tensioner is misaligned, before I converted to the inverter some of the pullies needed the tensioner and the belt did twist over when on those pullies, so I now know the reason why I have this problem.

The belt that has just split lasted around 10 months with a fair bit of use, so I can still use my spare as its set up. My new belt 730 will need the tensioner sorted, as I forgot that it did twist over.

Surprising the difference in cost for Gates belts, one company £26.00 with postage, another £8.35 each with £10.00 postage but free postage if £33.00 spent, so four belts for £8.35. Trouble is four belts if the set up is ok could be years before the last is used and could perish.

We 9x20 owners should get together and buy a few at £8.50 and send the out to each other for a pound postage.


Dullnote18/02/2020 15:08:19
70 forum posts
18 photos

Hi Bob, can you tell me where you purchased the belts mine snapped last night, operator error, can't balme anything else.

If four belt are too much for you PM and I could do a deal with you for Thanks


BOB BLACKSHAW18/02/2020 16:01:46
299 forum posts
64 photos

Jim I didn't buy the four belts as I cant find the web site, but I brought one on e-bay from bearingshop uk at just under £14 with postage each. If I find the web site for the four I will send a PM, but I got the belt within two days on e-bay, if you cant find it PM me.


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