Myford Super 7 Mk1 Motor drive belt size plz

[Piero FranchiParticipant @pierofranchi37209]

Hi Guys

Just a simple question

what is the size of the motor drive belt for the super 7

IF you are giving me the size, plz state if its inside length, out side length or chord length please

Kind regards

[Piero FranchiParticipant @pierofranchi37209]

[HopperParticipant @hopper]

Wrap a tape measure around the outsides of the two pulleys, following the route the belt takes, and measure the distance around. Deduct one inch and order a belt that size. Works for me.

[HughEParticipant @hughe]

Pierio,

The manual specifies the inside length as 33.5 inches with a z profile.

Hope this helps

Hugh

[John HaineParticipant @johnhaine32865]

But I suggest a link belt – adjustable and smoother running.

[KWILParticipant @kwil]

But what is the sizing tolerance on link belts? Every link must be slightly different width to its neighbours, so there must be a vibration at or around link pitch frequency.

[RobboParticipant @robbo]

Re Hugh's post above

The marking on my belt is Z850/Z33 ie Z section 850mm or 33 inch. The measurement is the inside length, pitch length 20-25mm more.

[davebParticipant @daveb17630]

Some V belts are bigger than the marked size, how much bigger depends on the section but can be 2" or more. There appears to be no relationship between the marked size and the actual size (although I am assured there is). I recently ran into the problem when ordering replacement B section belts for my Taiwan made lathe. Just to add to the confusion, how the belt is marked depends on where it it made, some countries mark them actual size so the marked size could too big if UK belts are ordered but could also be too small depending on the suppliers source.

[HughEParticipant @hughe]

Hi Piero,

Robbo's reply made me think again, so I went and looked at my S7. I have fitted a 33 inch belt as this is all I could get at the time. I noticed that the motor is at it highest position for tensioning and that the boss on the pulley is very close to the casing. The spare belt I got from Myford years ago was the 33.5 inch (851mm), this would allow the motor shaft to sit centrally in the casing. So both 33 and 33.5 inch belts would work it just means you will need to adjust the position of the motor to suit. The Myford belt is marked "Z872 Ld / 10×850 Li Z33 1/2".

Re John's suggestion to use a linked belt. I have a linked belt between the countershaft and spindle as it saves dismantling the spindle and shaft to fit a replacement. They are more expensive. When tried the same on the motor drive it hummed which drove me nuts as it was low pitched so I got rid of it. As you do not need to dismantle anything to replace this belt its not worth extra cost.

Simplybearings are a good source of info regarding belts etc

Hugh

[Simon Williams 3Participant @simonwilliams3]

Hello Piero,

My Mk 1 S7 has a motor drive belt marked Optibelt Z870. My impression is that this is a tad long, and if I needed to replace it I would try the next size down. I'm guessing that might be an 850.

But the Z870 is working fine, though I only use it on the low range pulley. The high range is there, but I tried the experiment of fitting a two pole three phase motor to the lathe, and driving it from a VSD. Great improvement, I would recommend it. The belt will drive OK on the high range pulleys, I just don't use them, as I will explain.

For normal turning I use the VSD driving the motor set to about 30 – 40 Hz, giving a working speed a bit higher than the manufacturer's original intentions. With modern high speed steel tool bits and relatively small work pieces this gives a good result. I found using the lathe all the time on the high range pulleys was too much, I wanted something intermediate.

It really wins if I'm threading something, as I can turn the VSD way, way down to say 6 Hz and thread in lowest non back gear. The motor – even at this frequency – gives plenty of torque, though the clutch is inclined to slip if it's not adjusted a bit tight. I have been experimenting with cutting metric threads with the little beast, so I can't disengage the half nuts. Having a three phase motor lets me reverse the chuck easily, and return to the start of the cut at a higher speed than I can use in normal forwards rotation cutting to a shoulder. The VSD allows me to run at a different speed in reverse to that going forwards. Also the soft start/soft stop feature of the VSD means that there is little or no possibility of unscrewing the chuck off the nose of the mandrel. I note that just as you can (in theory) unscrew the chuck by crash starting in reverse, you could also perhaps unscrew it by dynamic braking of the spindle, running forwards.

I just like three phase motors run off VSD's, they are so much more civilised than single phase motors. It was an expensive experiment to change the single phase motor and starter for a three phase one with a VSD, but it was worth it.

Hope this helps, best rgds Simon

[Piero FranchiParticipant @pierofranchi37209]

Thanks for all the help and advice guys

I found my two original belts that came with my Myford when I bought it a couple of years ago.

Funny enough, they where right in front of my eyes LOL (see photos on my profile, the belts are on the wall in front of me, silly me)

The belts I have had 34 written on them both.

Now I do know from ordering belts on other machines that in the older days, these measurments were not read from the same place as they are now.

If memory serves me, the older measurments were from the core line (an imaginery center line)

anyways I now have two motor drive belts and I am VERY pleased to say my myford fired up on its own power for the first time yesterday in over 7 years (ive not had it that long)

It run like a dream, VERY VERY quiet, although I have given the old girl a Full rebuild (and not just a simple paint job)

as to link belts, why do I keep hearing about this rubbish????

the reason you have a V belt over a flat belt is to give a bigger footprint at point of contact with the pulley,

with a link belt, the footprint is reduced at the point of contact, meaning your not going to get the same grip.

Its that simple!!!!!!!!!!! nothing no one can say is going to change this VERY SIMPLE FACT

If any one is interested, I have uploaded a few more photos of my myford on my profile

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Piero Franchi on 25/07/2017 07:17:51:

as to link belts, why do I keep hearing about this rubbish????

the reason you have a V belt over a flat belt is to give a bigger footprint at point of contact with the pulley,

with a link belt, the footprint is reduced at the point of contact, meaning your not going to get the same grip.

Its that simple!!!!!!!!!!! nothing no one can say is going to change this VERY SIMPLE FACT

Gosh, Piero, that's a bit of a rant . If only it were that simple. Firstly, flat belts were considerably wider than v belts for similar power applications. It is the wedging action of the v belts that gives it the enhanced grip but this has a disadvantage. The v belts is stiff because the deep section and has to be compressed as it goes around the pulley and then expands as it goes back to flat. This causes energy loss ( hysteresis). A problem that is exaggerated by the relatively small pulleys we use on our small lathes. Often the the tension in the belt has to be excessive to ensure that it wraps properly around the smaller pulley. Link belts ( and cogged belts ) help to minimise this problem by being more flexible and having gaps that close up while the belt goes around the pulley. So, the convenience of a link belt may not have quite as much compromise on its performance as you might think.

Cheers,

Rod

[SillyOldDufferModerator @sillyoldduffer]

Posted by Piero Franchi on 25/07/2017 07:17:51:

…

as to link belts, why do I keep hearing about this rubbish????

the reason you have a V belt over a flat belt is to give a bigger footprint at point of contact with the pulley,

with a link belt, the footprint is reduced at the point of contact, meaning your not going to get the same grip.

Its that simple!!!!!!!!!!! nothing no one can say is going to change this VERY SIMPLE FACT

…

Let me start by saying that I've no practical experience of link belts! Nor have I seen any figures comparing the grip of V-belts compared to link types. So I admit Piero may well be correct.

Or is he? An alternative analysis is that the reduced area presented by a link belt increases its contact pressure on the pulley. Increased contact pressure could compensate for reduced contact area. Also, the construction of a link belt might allow it to be made of materials with a higher coefficient of friction than a flexible belt.

A dubious analogy I know, but Piero's logic suggests a drum brake (large area, low pressure) should be more effective than a disc brake (small area, high pressure). I think the disc brake does a better job, apart from parking.

My main point though is that – in the absence of evidence – both Piero and I are naughty boys. We have both posted opinions unsupported by any evidence.

For what it's worth, I don't recall users of link belts complaining they lack grip. Rather, they report good results from a type of belt that's more expensive but easy to fit. Does anyone know if link belts are really more likely to slip than an equivalent v-belt, or not?

Dave

[John Stevenson 1Participant @johnstevenson1]

Perhaps the performance is linked relative to the price. No one wants to admit they have bought a pup.
Logically it’s down to surface area and depth of groove hence powerful machines going on to multiple matched belts in stead odds going from C section to D section, to E etc.

This is where poly vee score, large surface area with mimimum thickness,allowing a better grip on smaller pulleys

[Roderick JenkinsParticipant @roderickjenkins93242]

Some people claim a smoother performance with link belts but I think it is the convenience that wins me over. Not worth the extra cost for the motor belt on a Myford because that is easy to change having an open pulley at both ends but for the spindle drive I think the convenience of using a link belt outweighs dismantling a nicely set up spindle. Poly vee definitely makes it easier to use compact pulleys, my Coronet wood turning lathe uses them and I considered changing my mill to poly vee but a trial with a cogged belt gave such an improvement that I abandoned this. I will add that I have tried the cheap moulded plastic link belts and found them wanting. Loads of stretch and not good grip.

Rod

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