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I think so!
If I understand the listed data for control sheave pitch diameter, WHY are they different for different sized belts? That variance implies the control sheave is NOT at the high speed stop position. ? ? ? ?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.
I don't think it should be used as the slow speed stop. Hopefully the quadrant will limit the spring compression low speed and leave some room for the spring to compress so it can absorb shocks during operation. Without that the belts and the metal of the idler shaft assembly have to absorb the shock from a sudden change in speed like when a drill bit jams. That can burn the drive belt and V-belts or even cause the V-Belt to jam. I think it would be a great way to wreck your button bearing as the V-Belt tries to force the upper sheaves apart.
In reality "full compression" is the absolute "Low Stop simply because the dial can not be turned any further. I would consider that a HARD STOP.edma194 wrote: ↑Thu Jul 28, 2022 6:11 pmI don't think it should be used as the slow speed stop. Hopefully the quadrant will limit the spring compression low speed and leave some room for the spring to compress so it can absorb shocks during operation. Without that the belts and the metal of the idler shaft assembly have to absorb the shock from a sudden change in speed like when a drill bit jams. That can burn the drive belt and V-belts or even cause the V-Belt to jam. I think it would be a great way to wreck your button bearing as the V-Belt tries to force the upper sheaves apart.
If the low speed stop can't maintain some room for compression on the motor spring then operating at the LOW setting should be avoided.
At the high speed stop the positions are, of course, opposite but also a little over 1/8" less extreme on both sets of sheaves. Depends entirely on how you set the high speed stop.
For comparison, what is the dimension of your fully compressed spring. I measure mine to be 1 21/32".DLB wrote: ↑Thu Jul 28, 2022 3:34 pm Here are some pictures of my Mini, taken with the speed set all the way to the slow stop:
Mini Mark.JPG
The belt, located at the maximum diameter of the Idler sheaves:
Mini Idler.JPG
Belt location and spring compression showing it is at the minimum diameter for the motor sheaves:
Mini Motor.JPG
To me, this suggests that the factory belt is very close to the optimum length. From optimum length, I think slowest speed increases with either a longer or shorter belt. High speed operation is different, especially in that we adjust the stop for a specified belt position on the motor sheave. Which therefore is not going to close all the way to give max diameter.
- David