SteveMaryland wrote: ↑Wed Jul 27, 2022 8:24 pm
Did a graph showing Shopsmith speed curves for 7 belts of lengths from 30 to 25.5 inches. Curious to know what they looked like plotted together. I believe the geometry. If anything else is going on that we are missing, please advise.
All numbers are based purely on geometry, with no consideration of belt stretch, width, slippage, or wear. 15mm (.59") was the belt width in the model. A smaller belt section would widen speed range but might cause problems.
In normal operation, both sheaves move concurrently during a speed adjustment. I cannot think of a standard situation where only one sheave moves but the other remains stationary. As long as there is belt tension, both sheaves will move until traction forces are balanced.
When motor sheave PD = control sheave PD, the sheave ratio is 1:1. At that point, the only reduction is the 1.6 between idler and output shafts. This happens on the graph where all the curves intersect. Regardless of belt length, all the curves intersect at 2156 because 2156 is 3450/1.6.
The 30, 29, and 28" belts are of no practicality but I plotted them to show what happens when a longer belt is used.
A longer belt reduces the speed range on both low and high ends. A shorter belt increases the range. This is what was meant when we previously said "Shorter belt = lower min speed, higher max speed. Longer belt = higher min speed, lower max speed". The graph confirms this.
I think one of the useful results of all this is that the best belt length to use is the shortest one that will not result in a PD going below 1.88" when the drive is at the extremes of range. Going below 1.88" risks the belt jamming inbetween the sheaves.
One point not investigated - what is the belt manufacturer's minimum recommended belt PD? Is it more than 1.88"? A more flexible belt would be good.
If 3450 motor RPM is a real number, then clearly 700 RPM at the quill is an elusive goal on a Shopsmith. To those who claim they get to 700 using the standard motor and the Official Shopsmith belt, or any belt for that matter, let us in on it.