Powertwist Belt on Myford Super 7

In a search for increased smoothness at high speed, I recently splashed out on a Fenner Powertwist V belt for the motor to
countershaft drive on my Super 7. Unfortunately my experience so far is not good.
The old v belt is z section, nominally 10mm wide at the top of the wedge. My pulleys have v grooves which are about 9.4 mm wide at the top, so the belt outer surface sits a little above the pulley rim. The Powertwist belt, supplied as z section, is 11.2 mm wide at the top, and barely enters the v of the pulleys. It stands so high above the outer rim of the pulley that it rubs on the belt cover. (N.B. Before anyone says I've been sent an A section belt by mistake, I also have an A section Powertwist belt, and it measures 13 mm wide at the top.)
I am in discussion with the supplier about this problem, but in the meantime I wondered if anyone else in the group had encountered similar problems with Powertwist belts.
Mike
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wrote:

It ain't nominally 10mm. It's exactly 3/8", that's why its X=Z section :-)
The T-link and power-twist belts tend to ride very high on the pulleys until they are bedded in and they take a lot of tension to get them bedded in.
Having experimented with them, I came to the conclusion that they are no replacement for a new, correctly tensioned V-belt.
Mark Rand RTFM
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I've got a Z section powertwist belt on my ML7 and have no problems at all with it; tension is important and you must follow the instructions on the sleeve regarding shortening the belt by, IIRC, 2 links per foot. Martin
--
martin<dot here>whybrow<at here>ntlworld<dot here>com



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wrote:

Mar/Martin
Thanks for the replies. I have a feeling I will be returning to a standard v-belt.........
Mike
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Mike, that is exactly what I did. I ran a Powertwist for a month or so as I didn't have time to strip the lathe down at the time. While it gave no real problem in the short term and had one major advantage on my S7B several things concerned me so I went back to the standard belt. The advantage was that with a standard belt and a single phase motor the locking pin in the bull gear used to "hammer"; the powerr twist stopped that completely. The issues that concerned me were that the tension needed to be very high and I was worried about the headstock/layshaft bearings (un-necessarily I'm sure). I have also heard that the belt can wear the profile on the pulley over a long period. I had decided to go to a ploly-vee set up and actually roughed out the required pulleys but have yet to cut the vees and fit them. Changing the motor to a three phase and VFD also stopped the hammering so I guess that issue has more to do with the single phase pulsing.
On my Boxford AUD it has a much deeper profile linked belt that looks as if it should do a better job. I haven't even checked the pulley profile on that so have no idea if it is suitable - it just looks a better belt.
Anyway after all that, IMHO the best drive for smooth high speed would be Poly-vee. It just takes a couple of lumps of metal and hours of making swarf. I haven't checked the Myford site to see if they offer the system for the older lathes (at a price) but I'm surprised that none of "our suppliers" have produced a machined kit (John please note). Although I think that Hemmingway produce (or did produce) drawings and pulley blank castings. If you watch the latest Myfords running with the system they are much smoother/quieter at all speeds.
Best regards
Keith
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Thanks for that Keith. Your experience reinforces my decision to go back to the standard belt - at least in the short term. Poly-vee is interesting, but I wonder how one handles the motor to countershaft drive (which is the belt I'm playing with at the moment)?
Mike
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Mike Hi, the latest Myford super7 connoisseur lathes have Poly-V belts and pulleys on both the motor to countershaft (primary) and the countershaft to headstock drives. Having used Poly-V drives in a number of high speed installations at work that ran for thousands of hours with little trouble; they are in my opinion much under-rated in our world. My only surprise is not how good they are but how long it took for small machine tool manufacturers to catch on. They are more critical of consistent tension and pulley centre distance but both those issues are well controlled in the Myford installation. I have not heard of any problems with them on either part of the drive train. My experience of the standard V belts is that they are inconsistent and prone to vibration when new. After a fair bit of use they seem to become more flexible particularly at the joint which seems on some belts to be stiff when new. I'm sure I will get round to fitting a set of Poly-Vs to my own S7B but it is just a little question of WHEN.
Best regards
Keith
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