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I have a 1975 SB10K with the original countershaft system and guard. It has a two step v-groove motor pulley, flat countershaft pulley and cone pulleys.
I think part of the reason for the flat countershaft is ease of changing v-belt positions because it is a royal pain in the behind to loosen the motor mount to release belt tension.
But with the flat countershaft, the belt can be eased off the side of the countershaft and moved without releasing the tension.
The manufacture's decision to have a v belt run (on one side) on a large flat pulley is counter intuitive. Several other manufactures also used this set up. When i noticed that a new to me SB had this set up. I assumed it was a previous owners set up/ mistake. Perhaps a large v pulley was expansion or increases friction from the side of the belt.
On Thursday, December 10, 2020, 11:34:37 AM EST, E A <b-arch@...
Now if I could only find one of those flat countershaft pulleys, I'd be in like Flynn.
Here are the South Bend Forms for the belts and pulleys, and the horizontal and underdrive for 9" and 10K lathes. Also the speed chard from my HTRAL.
Note that while the motor pulleys are V-pulleys, the countershaft is flat. The V-Belt runs on it perfectly.