Pierio,

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

Hope this helps

Hugh

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.

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.

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

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

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