Advice on Pulley Ratio/Sizes for variable speeds

[Neil Butler]

I am very new to turning, but very keen.

I received an old lathe from a friend of mine (I would provide the name but I can't find one anywhere-old cast iron).

Currently it has 5" of clearance (10" swing?), but I hope to add some riser plates to increase this. topic for another post.

I've built a very solid stand for it, and purchased a dual voltage Wags 1 HP motor at 1750 RPM.

I will have to do the manual speed control using stepped pulleys.

As I will be interested in doing bowl work, I need to see how low an RPM I can get. I'm anticipating replacing the current pulleys on the spindle (I think that is the word for the lathe portion) and the pulley on the late (currently approx. 3.5" diameter). I also know I want to use a small pulley on the motor and a larger one on the lathe, but I’m not sure what I should be using as guidelines for size vs. RPM.

Can anyone provide advice on what size pulleys I should get and what speeds/rpm I might expect to get from them?

It seems like there is a lot of experienced people on this forum, so I'm very interested to see what people have to say. Any help is much appreciated.

Neil.

[Neil Butler]

I should add.

I've done some math, but I'm not sure I'm right.

I've calculated the circomfrance and ratios using a 1" diameter motor pully.

Assuming a single 1" diameter motor pully.

Dia. of pulley__Cir. of Pulley__Ratio to motor pully_____Estimated RPM
on lathe

1"______________3.14"___________1__________________1750
1.5"_____________4.71"__________0.67______________1167
2.0"____________6.28"___________0.75_____________875
2.5"____________7.85"___________0.8______________700
3"_____________9.42"___________0.83______________583
3.5"___________10.99"___________0.86______________500
4"____________12.56"___________0.88______________438
4.5"___________14.13"___________0.89______________389
5"_____________15.7"___________0.90______________350



My Questions:

1) are these number correct?
2) if these numbers are correct, can I get lower RPMs? Do I need to?
2) what do people recomend the best options/sizes for a step pully situation?

Again- thanks in advance for your help.

[Dennis Ford]

I did not check all of your numbers but the math is correct on the ones I looked at. However the 1" motor pulley size is not realistic. 1-3/4" is about the smallest that will work for v-belts and that will not transmitt power efficiently because the belt has to bend around such a small pulley. If you use the multi-rib belts, they will bend around a smaller radius but step pulleys are difficult to find for them. 350 rpm is plenty slow enough for turning pretty large bowls if (this is a big if) the lathe is strong and heavy enough to spin the blank without self-destructing or walking around the shop.

[Frank Scott]

I have used this site to calculate pulleys, seems to work ok.http://www.gizmology.net/pulleysbelts.htm

Scott

[Paul Engle]

Neil I was going to suggest also a web site like Frank listed , there are others out there and it may be worth checking around , there are mfgrs that make step/ed pullies ( multiple shiv diameters) as well as variable speed pullies . I found this one when I was doing the pully thing on my DC VS conversion. I like it very much as is not too cluttered like the one Frank listed.
Happy trails!
http://www.csgnetwork.com/pulleybeltcalc.html

[Richard Madison]

Neil,
The speed ratio between two pulleys is simply the ratio of their diameters. Thus a 2" motor pulley driving a 10" lathe pulley (just as an example) would have a five-to-one speed ratio. Using a 1750 rpm motor the lathe speed would be 350 rpm. I think this is the last one on your list.

[Neil Butler]

thanks for all the info so quick. I've used a few of the calcs and re-measured a few things in the shop.

If I understand correctly, I'll need at least a 1-3/4" pulley on the motor, and that won't work very good. Will a 2" be OK?

assuming 2" pulley on the motor is fine, I checked my lathe and the max pulley I can put on the spindle is 7". Based on the calculations sites I can expect to get minimum of 500 rpm. Isn't that still too fast for doing rough work? If so, how do I get it lower? would a 2 step pulley system help. If so, any link on 2 step systems?

Neil

[Leo Van Der Loo]

What size pulleys do you have now ??.
If you have any at all, a jackshaft would probably be the way to go, getting the 6 step pulleys in the size you want might be very hard to find. HTH

[Marc Martindale]

Maybe you could put the step pully on the motor side?

[robert hainstock]

My first lathe was a Sears dual tube bed. It had a four pulley, step drive and no motor. To this, I added a used 1/3 hp 1725rpm with a five step pulley on it. I never checked, but I think that gave me twenty different speeds. I suspect that many of those speeds were similar, But it turned a lot of wood from lumpy to round. Good luck with your choices.:)
Bob

[Wilbur Pan]

Your calculations are correct, but if you have a four speed lathe, you're going to be limited by the availability of the sizes of four speed pulleys that are generally available. To get the slowest speeds, you want to have a small pulley on the motor shaft and a large pulley on the headstock shaft. Checking out the Grainger website, the smallest and largest four step pulleys are a 4 - 3-3/8 - 2-5/8 - 2" pulley and a 6 - 5 - 4 - 3" pulley. By having the 2" pulley on the motor and the 6" pulley on the headstock, with a 1750 RPM motor that will give you 583 RPM at the lowest setting.

You could put together a true variable speed system by using a DC motor and controller, or a 3 phase AC motor with a VFD, which will give you lower speeds.

Alternatively, you could install a jackshaft, as Leo suggested, which could lower the speeds further. Here's a picture of one.

[Neil Butler]

OK so I think a Jack Shaft is the only way I'll really be able to get both low and high RPMs. I don't really want to buy a new motor.

Anyone have any resources on building a Jack Shaft or where to get the materials/pulleys?

[Leo Van Der Loo]

Neil ENCO has 4 step sheaves also pillow block bearings and other parts and material.
the 4 step pulley I pulled up is #308-0516, pillow block bearings #505-3236.
Except for the stepped pulleys the other pieces are quite common parts you might find locally and less costly.
Oh 1 heads-up, make sure you use the same size stepped pulley for the sets, or you will have to change the distance between the sets every time you change speeds.
Have fun ;-))