Thread: Pulley sizes vs RPM?????

This is on WOW

Anyway, here is what Mr. Jones posted

http://www.csgnetwork.com/pulleybeltcalc.html

Joash

Al,

Driving Pulley Diameter times (X) Motor Speed in RPM.

take the product of this and then

divide by Driven pulley Diameter will give you what you are looking for.


Regards Mike

Drive pulley diameter times motor RPM divided by desired RPM equals driven pully diameter.

Joash, thanks, but Mr Jones requires belt length to calculate, and belt length doesn't have anything to do with RPM... that I know of, anyway. Yep, it's on WoW, and another forum also. But I still don't have exactly what I'm looking for, but I'm gettin' there.

Thanks, Mike, but the required size of the "driven" pulley is actually what I'm looking for. I guess there are too many configurations possible to have the perfect formula for what I'm trying to do. I can't install a four foot pulley, so I guess I'll have to have a jackshaft in between the motor and the arbor. Having the formula for figuring that is what I need.

Gil, thanks again, Now, how do I re-construct this formula to work a jackshaft into the mess? Do I just have to take a wag as to the jackshaft "driven" pulley, and then figure another time with the same formula?

Al,

The two formulas are the same thing one gives the answer in pulley size and the other give the speed in rpm. It looks as if you will need to use two lots of pulleys and shafts to get down to 35rpm with the motor you mention. You could use a reduction gearbox which are readily available in all sorts of reduction ratio.

An other alternative is to get a brush motor and vary its speed by a speed controller which operates by varying the voltage provided to the motor.

Good luck with it, sorry I cannnot be of more help. There are a lot of approaches you could apply to your dilemma to bring about same result.

Regards Mike

OGYT,
I’ll have a crack at this because, whilst I know bugger-all about wood turning, your problem seems to be one of gearing.
Now, if I read you right you have a fixed diameter for both the DRIVEN pulley and the DRIVE pulley of 2 inches.
You have a motor capable of either 1750 or 1150 RPM and you want the DRIVEN pulley to turn at 35-40 RPM.
So if you take the case where the motor is turning as slow as it can (1150 rpm) and the DRIVEN pulley is spinning as fast as you want it to (40 rpm) then you must achieve a reduction of 28.75:1
So in a one-step process this means that the DRIVE pulley must turn a DRIVEN pulley with a diameter of 2x28.75 or 57.5 inches . Any of the previous respondents said essentially the same thing, and that web site that Joash pointed you to makes things that bit easier. It does calculate belt size if you want but it’s not a necessary input or output if you aren’t interested. In any case, you’re looking at a 5 foot pulley if you want to do it in one step or, more practically, a multiple set of pulleys to step it down gradually, and you’re right, the combinations are theoretically infinite but I’m sure that available sizes will soon restrict that.
I have no idea what pulley sizes are available but let’s say you could get your hands on a bunch of 6 inch and 2 inch pulleys then you can get pretty close in three stages as per the sketch. Any set of reduction values which, when multiplied together, give you approximately 28.75 will do the trick

Hope I haven't lead you astray but I think my figures are correct

EDIT - that second figure in the diagram should be 383 not 328

Ian

Ian, looks like we had about the same idea.

Gil,
Ah well, you know what they say about great minds and fools

Ian

Ian,
Yup, I do.
Well, now that the world is a safer place, I reckon I will go to bed (1:45am here).

One problem with the proposed ideas, is that for 3 pulley sets (ie. 3 belts) you need 4 shafts, and tensioning the belts can become a nightmare. Adjusting one throws others out, etc. unless you start at the "fixed" end (usually the chuck) and work your way through. It also means that as you work towards the further shafts, they need larger and larger adjustment slots to take up the slack.

One work around is to modify it to use only two shafts :eek: by running some of the pulley clusters on bearings, so they rotate independantly of the shafts. (In pic, these are the yellow clusters.) So long as the two pulley diameters & the belt sizes are kept constant throughout then all the belts can be equally tensioned by adjusting the one shaft.

'Tis an old concept, BTW. I first saw it in an old steam-driven sawmill out Emerald (Vic) way, where they used it as a reduction mechanism on their leather flat-belt system... from vague memory they were running 5 or 7 belts and tensioning was done by adding logs as weights!

A worm drive reduction gearbox could be cheaper, and certainly simpler

At this rate it will probably be cheaper to buy a hamster and make a treadmill.

Skew, that is an outstanding idea!!! Thanks
Using 7" and 2" pulleys throughout (and 1150 motor rpm), the final speed would be 26.8 rpm.
Gil

Dunno, might work... motorbike gear box!
Harley's have a separate Gbox, but even 2nd(or 10th)hand still prob cost a bit.
All these pulleys/shafts and belts would soon add up in cost's too, with a bike's gear box you might be able to incorporate the clutch... just the thing for starting heavy turnings!