Poly V belts

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frogesque

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Problem:

My old Myford ML10 lathe would originally have been fitted with flat belt cone pulleys for changing speed (as well as the usual back gearing) giving 6 speeds in total.

Someone (not me) has changed the flat belt set up for standard 1/2 inch cone pulleys on the lathe spindle and the lay shaft. As well as not matching diameters, meaning the tension needs adjusted for every change, the belt is too thick to get past the middle stud nut on the the headstock. A total pain!

Anyone any experience of using poly V belts (the thin flat ones with mini grooves) and either obtaining or making cone pulleys to suit?

To date, the headstock bearings have been replaced (split tophat oilite bronze bushes), backplate made for a top quality 4 jaw Pratt chuck and I've some work still to do on the tailstock that appears to belong originally an ML7. I like it because it has a cam lever for locking to the bed and the normal offset for taper turning. Unfortunately the centre height is off to the headstock and I've shimmed it meantime but the toolmaker in me will probably make a new shoe for it. I've also sourced a reasonable set of change wheels for screw cutting but the thread dial indicator is missing - another wee job that needs doing!
 
I think you are saying they pulleys suit a vee belt. I guess you are aware that vee belts come in different section sizes (A, Z, etc.) Would a smaller section belt solve the clearance issue?

Fit a VFD and three phase motor and you will only move between pulleys twice a year.

Poly vee belts are said to work well on flat pulleys with the belt vees inwards. Just make sure the crown on the pulley is there for centreing.

Standard J-section poly vee are 40 degree angle. You can look up the vee spacing, but it is very close to 11tpi.

An 11tpi tap or BSPP thread chaser die can be used as a form tool if you want to make a pulley.

The Model Engineer forum would give you more comprehensive answers to your questions.
 
Thanks for the reply. 11tpi chaser sounds interesting, in the distant past I made one by pitching the compound slide using a single V point tool.

Current pulleys are standard 1/2 inch wide groove to suit a v belt. No room to get the belt onto the next pulley without a lot of arm wrestling. I'm still debating the best (easiest) way round the problem.

I take the point about 3ph and a variable speed control but (as always) I'm already way over budget!
 
I would agree, less tension not more. I converted my Kity planer. Used a power steering pulley off a Jeep on the motor, same size and just turned down the original pulley to act as a boss for it. Made the drum pulley using a cut off tool ground down to match the profile of the grooves in the existing pulley. Like the chasing tool idea, hadnt occurred to me. You can find the spacing and other dimensions etc on the Gates site for one. I have a dro on the lathe so setting the spacing was easy.
 
those poly belts work almost as well inside out.....like orig flat belts......

Lots of engines now with serpentine poly belts work on both sides....
IE, the grooved side as well as the flat side.....
what have u got to loose if u try it....
 
The problem with a polyvee belt is that they cannot survive much slippage, you end up with many single bits of belt as I learnt from the past when involved with a FEAD belt auto tensioner and a larger output alternator which put to much load on the belt which just shredded. A normal V belt providing it has not bottomed out drives on the sides which due to the angle wedge into the pulley.
 
Years ago I stripped an oldwashing machine down to nuts, bolts, drum, electrical harness and lumps of concrete. I actually wanted the sheet metal!

Huge poly V pulley on the drum and a drive direct from the motor with something like a 1/2 - 5/8 dia. Spindle and grooves cut directly onto it. Years of cycling high speed, low speed forward, reverse and the belt was absolutely sound. Belt would have been about 1/2 inch wide (4 or 5 grooves?)

I'm currently using a fractional HP motor (home made single V plywood driving pulley to the lay shaft), while testing and restoring the lathe and my feeling is, for a cheapo tempory fix it's the way to go. May well go for a more heavy duty solution later.
 
The problem with a polyvee belt is that they cannot survive much slippage... ... I learnt from the past when involved with a FEAD belt auto tensioner and a larger output alternator which put too much load on the belt which just shredded.

So you are saying that if you overload the belt (attempt to use it in excess of its design), it will fail.

Could you clarify how that failure mode is peculiar to a poly-vee belt as opposed to every other flexible drive transmission mechanism.
 
Huge poly V pulley on the drum and a drive direct from the motor with something like a 1/2 - 5/8 dia.

You make a good point. Domestic washing machines have a hard life and poly-vee works extremely well in them.

All the ones I've had my greasy paws in over the last ten years have had a plain (smooth) pulley on the drum and only the relatively tiny diameter pulley on the motor is grooved. The belt wrap on the drum must be enough to transmit the power without the extra cost of grooving it.

Your example is also good as it reminds us that to get the best power transmission, the area of contact of the belt with the pulley needs to be maximised. Poly-vee has the big advantage that you can increase the wrap by using idlers and putting a reverse curve into the belt, something that is verboten wiht a standard v-belt.
 
The problem with a polyvee belt is that they cannot survive much slippage, you end up with many single bits of belt as I learnt from the past when involved with a FEAD belt auto tensioner and a larger output alternator which put to much load on the belt which just shredded. A normal V belt providing it has not bottomed out drives on the sides which due to the angle wedge into the pulley.
If you have to retain a v belt then its worth getting the notched type as they don't require as much tension to grip well.
 
If you have to retain a v belt then its worth getting the notched type as they don't require as much tension to grip well.
I do have a notched belt for the final drive. To fit the pulleys without bottoming there is insufficient room between the headstock casting to get the belt over the top of the pulley. Looking at pictures of old lathes they were originally have had flat belts and not a problem changing speed.
 
Could you clarify how that failure mode is peculiar to a poly-vee belt as opposed to every other flexible drive transmission mechanism.
Simply down to the way the belt transmits power, if you have ever worked in powertrain then this is basic knowledge. An auto tensioner applies a pre determined tension to the belt to meet the drive needs so if you replace a 90 amp alternator with a 150 amp alternator then it requires more power to drive which the belt has to deliver, not enough tension and the belt rides over the alternator pulley and in doing so shreds. For interest lookup "angle of lap" as it will give another clue as to potential issues. The solution is to use a tensioner that can apply more tension providing it is within the capability of the belt.

The reason for failure is that if you look at the cross section of a polyvee belt they are relatively thin compared to V belts, look at the belt profile and that of the pulley, a sharp crest unlike the V belt that sits in the pulley and drives on the side faces so acts like a blade once the belt slides over it.
 
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