Lathe motor belts proper tension

conant · Aug 6, 2016

Just wondering if there is a good rule of thumb for setting multiple belt motor drives, on a lathe for example.
Obviously not so extremely tight as to put unnecessary load on the motor bearing but not so loose to allow slippage.

HuFlungDung · Aug 7, 2016

Perhaps about 1/4" deflection per foot of separation of pulley centers, with a good thumb pressing firmly on the back of the belt. If you can safely adjust the tension on the fly, typically, tighten until the belts stop flapping on the slack side. Don't overtighten, I've observed that they may start to flap again if you go too tight.

Limy Sami · Aug 7, 2016

HuFlungDung said:
Perhaps about 1/4" deflection per foot of separation of pulley centers, with a good thumb pressing firmly on the back of the belt. If you can safely adjust the tension on the fly, typically, tighten until the belts stop flapping on the slack side. Don't overtighten, I've observed that they may start to flap again if you go too tight.

Roger that, it's all too easy to over tighten belts, and on lathes that is a very bad thing.

conant · Aug 7, 2016

Thanks guys. I have a set of 4 B81s on my lathe I just got running and they are a mix match. One looking like new belt and the rest used for sure.
Tightened up to the point of a good feel on the 4 and 3 are flopping and one is fine. Buying a new set soon.
Are individual belts sold close in tolerance when manufactured, or should I look into a matched set?
Gates belts are readily available.
Any suggestions on brand?

HuFlungDung · Aug 7, 2016

Last time I asked for a matched set, I was told they generally are manufactured so precisely nowadays (from a given manufacturer) that there is no need to specify matching. Definitely run with all new belts, not a mix of old and new.

conant · Aug 8, 2016

Thanks. I assume Gates are a good brand?

Tyrone Shoelaces · Aug 8, 2016

When you've got them all on and reasonably tight you should be able to push down on any one belt in the middle of the belt. That belt should deflect so that the top of the belt you're pressing on is in line with the bottom of the other belts.

If you can't press the single belt that far the belt is too tight.
If you can press the single belt further than that it's too slack.

That's the best way I can find to explain it.

Regards Tyrone.

Clive603 · Aug 8, 2016

Good brand belts should be well enough made to work as a matched set but I'd ask for a special order supply expecting all the belts to come from the same batch. I've known the "two in stock, two ordered in" supply process not to work out very well. I've no way of knowing what the supplier turnover on that size was like in that particular case but would have been unsurprised to find that the in stock ones had been around for months, maybe years. But this was back in the days when minimum order quantities were a pretty much an absolute rule so odds and ends tended to pile up.

No name, dealer brand or rebadged imports are not safe here. I've seen belts carrying the same nominal size and length annotations differing in length by more than the nominal interval between sizes. Other horrors included dammn great lumps at the join, made to order specials in this case and silly expensive, and sections which matched no official size. Sort of thing that makes me wish folk would ask before buying instead of me finding out on a fix something else visit.

Clive

msimard · Aug 8, 2016

Gates has great quality and their smaller poly-v and micro belts are better than you expect belts to be.

Single vee belts are scary lately, I got a NAPA Auto parts branded one to replace a Chinese belt that had very visible wobble.
The made in Mexico NAPA belt had the exact same wobble!

Jaxian · Aug 9, 2016

I had a similar question about belts about two years ago. Called Gates and got an engineer on the phone when I got too technical for the phone monkey. I was complaining that I can't find matched belt sets, (my No.4 horizontal takes 8), he said "no longer an issue, use the Hi-Power II belts".

I said I had just bought a set, that based on the markings were from different batches and they didn't tension the same when I put them on. He said that is not how you set up the 'new' style belt sets.

You put them on until the loosest belt is at the correct tension then run them for a full 24 hour cycle and they would all normalize and take a 'set' at the same size. I told him it was for a lathe I wasn't going to run 24 hours straight and he said to just run it as you normally would and after enough cycles they would get that same size set. They would just squeak for a while until they do.

This was the explanation from a Gates engineer on why there are no more 'matched' sets. If bedded in properly any Hi-Power II belts will become matched.

conant · Aug 9, 2016

So if I understand this correctly, I put on 4 of the High Power II Belts, tighten up till the looses belt is correct regardless of how much I over tighten the tightest belt?
What about the motor bearing with the strains of overnighted belts?

EPAIII · Aug 9, 2016

That's not their problem.

conant said:
So if I understand this correctly, I put on 4 of the High Power II Belts, tighten up till the looses belt is correct regardless of how much I over tighten the tightest belt?
What about the motor bearing with the strains of overnighted belts?

Jaxian · Aug 9, 2016

That was my feeling too. But he said the over tightened belts will heat up quickly because of the extra friction loosen up and when the friction heat drops off because they are no longer super tight they will take a 'set' at the correct size.

I mean it sounds reasonable enough if you used the right material. But I figure Gates has been making belts forever and this guy had zero reasons to lie to me so unless he was just being an asshat for giggles why not.

Also the belts are working perfectly two years later and now seem to be the same size when I press on them. Weirdly though I lined up the labels when I put them on and after a week or so they weren't anywhere near each other. Since then they seemed to have stayed the same in relation to each other. Not sure how that happened but obviously something was moving around at the start. Also they don't sell matched sets anymore so that would seem to kind of bear out his point.

Hell though, don't take my word for it. Call Gates and say you need to talk to an engineer about a industrial application. Wasn't that hard to get a guy on the phone.

conant · Aug 9, 2016

I see Gates sells the power bands also.

IrritableBadger · Oct 20, 2017

Matched belt sets, for the most part, stopped being made as matched sets quite some time ago. To the best of my knowledge those that are still manufactured that way are MIL spec parts to fulfill contractual obligations, not for better performance.

Even before matching sets got dropped from catalogs the definition of matching had evolved to be nearly meaningless (made on the same day, made in the same plant, made in the same hemisphere, etc...). Orders were filled with the first parts on the rack/bin then banded or bagged as matched sets. It was stupid and really expensive. It was really disingenuous too. If you've purchased matched sets of belts in this century they were either old, or conformed to some nonsense definition of matched.

Gates, among other top tier belt manufacturers all reached the conclusion it was better to invest in some R&D and be done with the hassle and charade of matched sets. But if you absolutely have to have a matching set I can send you a prelabeled poly bag full of belts that'll say whatever you like

rons · Oct 20, 2017

conant said:
Just wondering if there is a good rule of thumb for setting multiple belt motor drives, on a lathe for example.
Obviously not so extremely tight as to put unnecessary load on the motor bearing but not so loose to allow slippage.

I have woodworking and metalworking machines with belts. After looking in instruction manuals it always comes down to about 1" of deflection in the center of the pulley span.

AlexO · Oct 20, 2017

The manufacturer of the belts should spec the ideal tension the belt should be ran at. One tensions under the free part of the belt is at the correct pitch for that length - like a guitar string. Thumb deflection tends to over tighten belts. The thumb of the research engineer is not the thumb of a seasoned machinist.

Here in SA we get all the belt brands under the Sun. I find that single V's have gotten much worth over time and poly-V's are a nightmare to get right. But when right they work well.

tnmgcarbide · Oct 20, 2017

i've recently replaced nearly every belt in the shop, and was surprised to find that nearly all the wrapped
_B_ and 5_ section belts were stiff and lumpy . that's fine on a lawn tractor .

on my machine tools
i've been using cogged bx belts if theres a pulley under 4 inches involved . BX belts have a precision ground raw edge with
no lumpy seam, and run smooth as glass . they don't take the -set- that wrapped belts get from being stuffed into a paper sleeve for retail stores' wall hanging on a pegboard .

AlexO · Oct 20, 2017

tnmgcarbide said:
i've recently replaced nearly every belt in the shop, and was surprised to find that nearly all the wrapped
_B_ and 5_ section belts were stiff and lumpy . that's fine on a lawn tractor .

on my machine tools
i've been using cogged bx belts if theres a pulley under 4 inches involved . BX belts have a precision ground raw edge with
no lumpy seam, and run smooth as glass . they don't take the -set- that wrapped belts get from being stuffed into a paper sleeve for retail stores' wall hanging on a pegboard .

Could you please send a link ? I use cogged auto belts whenever I can and they're fine. But they take a set in the packaging.

Illinoyance · Oct 20, 2017

Check this out.
http://rubberproducts.jasonindustrial.com/Asset/Belt-Tensioning-Instructions.pdf

Lathe motor belts proper tension

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