Increasing the speed on Southbend 10-16” Models. Late 1930s-late 1940s.


These lathes were designed to run on plain bearings for 8-16 hours a day, with daily oilings. My 10R from 1939 has two countershaft speeds of 300 and 579 from a 2 groove pulley on its 1725 RPM motor. Speed changes via  V belt from the  2” and  3.375”  pulley with a slightly smaller corresponding  groove in large countershaft pulley. This was an option I believe. Bit of a pain as you have to reduce belt tension adjuster to move v belt. 

For the standard 2” pulley the Countershaft runs at 300 RPM and the 3 spindle speeds are 700, 434 and 277 PRM. (Back gear drive neglected ). Shifting V belt  to the larger drive pulley,  the spindle goes to 1357, 837 and 535 RPM. Southbend HTRAL has warnings about bearing adjustments and lubrication at this speed, so this is clearly a maximum.

It is  easy to increase the size of the motor drive pulley in order to create a countershaft speed that will yield a top spindle RPM of 1000, but at less than the 579 countershaft speed that is the maximum for this series of lathes. WARNING. I have no operational measurements for bearing temperatures at anything above 700 which will tell you if you are pushing your lathe too far, and if you do this, I would recommend taking your bearing temperatures before and after. Bearings heat up over time, 20-30 minutes before my lathe flattened  out at a 30 deg rise at a spindle of 700 rpm.They will get hotter at 1000 RPM! . How hot is “too hot” is not clear. The decision for you is how many hours you will run at this speed and how good are your bearings and oil passages.

Doing some  math, you can get your countershaft pulley to 429 RPM, and then your 3 spindle speeds will be 999,  643, 395 RPM by increasing the motor pulley from 2” to 2.75”. I would be careful using backgears with this countershaft speed and not for long periods if you do. Use your ears and make sure backgears are well lubricated, with teflon grease on the shaft and heavy gear oil on the bull gear.  Use only cast iron pulleys, not die cast, and can order any size  from McMaster Carr. You will likely need a gear puller and the set screw may be tough to remove. You may want to make sure you can remove your pulley before ordering new. You will likely have enough belt adjustment range to use the same belt but you can order a size longer from McMaster. Without the 2 groove pulley, this is a semi-permanent change and its use depends upon what speeds you normally run. All the VFD users will come out now and point out how it’s easier and better to go that route but my simple phase converter only goes from 60Hz down and my motor is pre-1992 which makes its use with a VFD problematic. This only costs $30.  


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