Sheldon Lathe "Under-Drive" Speed Selector Explanation

There isn't much information online about Sheldon lathes, I've found out. Atlas/Craftsman, South Bend, Clausing/Rockwell? They have quite a presence online (OK, Clausing not so much, but better than Sheldon). When I picked up my Sheldon 11-inch lathe a few months back, I immediately started scouring the net for information. Sites like VintageMachinery.org and Lathes.co.uk are great sites and have lots of free and valuable information. Yahoo groups also are a good place to find that elusive manual. But enough rambling. This post is about Sheldon's very cool four-speed "Under-Drive" or "U-Drive" system and how it works. Here's a photo of the U-Drive in my lathe (the two handles are removed to get the pedestal door open - they fit onto the two shafts poking out).

And this next drawing is from a patent Sheldon applied for in 1940, submitted by Leo Zupan, which got approved in 1942.

Below is a depiction of the pulleys and their approximate diameters. I made this in Excel and color coded the belts to give a better idea of what's going on (I know the spindle pulleys need to be between the back gears but I drew it this way because I'd added the back gears later on and didn't want to change things - it still conveys the idea).

If we first concentrate on the motor pulley assembly, both my lathe and the Zupan patent drawing have the larger pulley on the left (blue belt), corresponding with a smaller pulley on the lower intermediate shaft, while the smaller motor pulley is on the right (orange belt), corresponding with a larger pulley on the lower intermediate shaft. The left handle shifts between the orange and blue belts/pulleys and has a low and high speed. Move this handle left and it selects the orange set of pulleys (low). Move this handle right and it selects the blue set of pulleys (high).

Then if we look at the right end of the lower intermediate shaft, a smaller pulley is on the left (green belt) and larger one on the right (yellow belt), corresponding with larger/smaller pulleys on the upper intermediate shaft. The right handle shifts between the green and yellow belts/pulleys and has a low and high speed also. Move this handle right and it selects the green set of pulleys (low). Move this handle left and it selects the yellow set of pulleys (high).

This next graphic shows the four lathe speeds on my lathe based on handle positions. I made this in Powerpoint. Someday I might try to etch this graphic into a brass plaque to affix to the lathe.

Left handle moved to the left and right handle to the right (L-low/R-low) gives the lowest spindle speed. Left handle right and right handle right gives 2nd spindle speed (L-high/R-low). Left handle left and right handle left gives 3rd spindle speed (L-low/R-high). Finally, left handle right and right handle left gives highest spindle speed (L-high/R-high).

An interesting observation was that the literature I found online (mainly lathe parts lists) shows the left set of pulleys reversed from what my pulleys are. That would make the left shift handle behave opposite to what my handle does. I think Sheldon made this change from the original patent (and my lathe) layout just prior to or soon after the patent was approved in 1942, as my lathe was made about 1940, about the time the patent was applied for.

The graphic below, which came from a 1944 Sheldon catalog I ran across on OzarkToolManuals.com, shows the lever positions for what I consider the "newer" layout.

Maybe this post will assist other Sheldon U-Drive owners in their quest to unravel the mysteries of this unique drive system.