2 step Motor pulley size

[Gard]

I have been working away at restoring a SB 10L lathe, SN 146767 with marks W.E.F. and L.Q.R. There is also a lightly stamped circle with some sort of symbol in it. I removed the under cabinet motor mount assembly by unbolting it and lifting the cabinet off so that I can clean and paint the cabinet. No obvious problems with the motor mount other than the thick oily crud. Spent a couple of hours or so with various brushes and gasoline to get the worst of it off. Everything works smoothly but I may disassemble it to lubricate the various pivot points and shafts because there are no holes or oil cups to lubricate the shafts



The motor is clearly not original, my question is how about the motor pulley? I suspect this was a 12 speed lathe so a 2 step pulley? The parts list I have says contact south bend with the serial number to get the correct pulley. Anyone have any idea what it should be?

I guess I could do some measurements of pulley sizes and RPM to make an educated guess but I suspect there will be some trial and error to get the correct belt tension on both sizes. I am not sure what the maximum RPM should be for the old segmented iron headstock bearings.

Was there ever a table that showed which belt and back gear locations provide what RPM?

 

[Bart.Howard]

 

[johnoder]

So - looking at post #2, we see the following ratios for A to B
1.835, 1.829 and 1.91

The average of those is 1.858

That is the difference between the two steps on that pulley
If - say - the big step was 3, then the little step would be 1.615

 

[Bart.Howard]

these are the measurements that are on mine 1940 vintage, there are some on ebay, but they as always are proud of them. but if you dont have the means to make your own....

 

[johnoder]

Do note the same (within reason?) as I got. The outside of the smaller divided into the outside of the larger is about 1.62

 

[Gard]

So - looking at post #2, we see the following ratios for A to B
1.835, 1.829 and 1.91

The average of those is 1.858

That is the difference between the two steps on that pulley
If - say - the big step was 3, then the little step would be 1.615

The above would be true if you were using the same size pulley on the other shaft, I had to do a little algebra, the 1.858 needs to be multiplied by the ratio of the pulley sizes on the countershaft. To complicate things a little further, the speed ratio is based on the pitch diameter of the pulley which falls someplace between the belt ID and belt OD.

Thanks for those dimensions, I may just buy or make a pulley with that and hope for the best.
Can you tell me the belt size used (width)?
Based on your dimensions you have SB part # PT2131NR1, it says groove width is 0.635". There are a total of 13 of the 2 step pulleys listed with a range of sizes in my SB parts list form 921F
Do you need to adjust belt tension when you change from A to B?

 

[johnoder]

The above would be true if you were using the same size pulley on the other shaft, I had to do a little algebra, the 1.858 needs to be multiplied by the ratio of the pulley sizes on the countershaft. To complicate things a little further, the speed ratio is based on the pitch diameter of the pulley which falls someplace between the belt ID and belt OD.

Thanks for those dimensions, I may just buy or make a pulley with that and hope for the best.
Can you tell me the belt size used (width)?
Based on your dimensions you have SB part # PT2131NR1, it says groove width is 0.635". There are a total of 13 of the 2 step pulleys listed with a range of sizes in my SB parts list form 921F
Do you need to adjust belt tension when you change from A to B?

I enjoyed the math problem - the SB experts will need to address the questions

 

[jmm03]

on my 10L, approximately 1967-68 vintage, large diameter= 3.475", small diameter= 2.230", uses a 5L420 belt. Mine is large outboard, small inboard on the driven. I run it that way and use my VFD for speed changes. It is the way I got the lathe originally,(I believe) from a local high school shop. Jim