Mark V Speed Control

[SteveMaryland]

"I am curious though about what you have stated as the length of the official SS belt being 26.57" Where is that number stated?"

The Shopsmith belts I have seen say "15 X 675" on them. This is metric for a belt .59" wide and 26.57 long. So I call that the Official Shopsmith Belt.

Others on this Forum could tell us if Shopsmith has used other widths and lengths in the past.

In my materials above, there is no attempt to capture belt stretch, initial belt tension, belt/sheave friction, high or low speed stops, etc. It's all geometry nothing more.

You have 4 factors - belt length, two pitch diameters, and center distance. Pick three, and the fourth one can be solved for.

[DLB]

I'm happy to report that my machine, measurements provided previously, evidently exceeds the performance predicted by this model as well as Dusty's model. Subject machine has a SS belt. First I increased speed from slowest achievable to get the 0.031" between belt and pulley as depicted. I noted that both the control sheave and the floating sheave moved in response to the speed increase. Then I calculated the outside of belt to motor sheave parameter for the 2.48 PD associated in the model with the SS Belt. That came to 1.321". I measured that parameter on my machine at 1.625", a fairly significant difference. So while this isn't the slowest speed achievable on my machine, it is significantly slower than predicted in the CAD. (Note that the 2.48 is a diameter, but what I'm measuring is a radius associated with it. So a ~.3" difference in measured value is a ~.6" difference when converted to diameter or PD, so in the 20%+ range.)

I didn't convert this to speed for a couple of reasons. One is that the PD used in the CAD is 0.189" from the OD. Most information I've seen on-line seems to suggest a much lower number. The second reason is that I have adjusted the Mark to a faster speed than the slowest achievable in an effort to validate the model. The third reason, I still don't have a tach that would either prove or disprove much of this conjecture.

- David

[JPG]

I am having difficulty with a shorter belt allowing slower slow AND higher fast.

A shorter belt will decrease the effective range from smallest to largest pd. That will result is a restricted rpm range.

I think a longer belt will increase the effective range. It is also the likely solution to the original issue.

Now what is this about detents????? What causes them???


P.S. My conception of official SS belt dimensions (since greenies were first conceived) is 25.5" x 17/32 width across widest.(0.531)

Belt length WILL affect ratio between smallest and largest pd on opposing belts. Difficult to predict because of non predictable path of the belt wrap and belt path different between tension and slack paths. We can make drawings til the cows come home, but they will not reflect real world results.

[dusty]

I am having difficulty with a shorter belt allowing slower slow AND higher fast.

A shorter belt will decrease the effective range from smallest to largest pd. That will result is a restricted rpm range.

I think a longer belt will increase the effective range. It is also the likely solution to the original issue.

Now what is this about detents????? What causes them???


P.S. My conception of official SS belt dimensions (since greenies were first conceived) is 25.5" x 17/32 width across widest.(0.531)

Belt length WILL affect ratio between smallest and largest pd on opposing belts. Difficult to predict because of non predictable path of the belt wrap and belt path different between tension and slack paths. We can make drawings til the cows come home, but they will not reflect real world results.

If drawings don't do the trick (in the real world) then please shed some light on the issue. Tell us all who are uninformed just how this monster works.

Pictures please. Drawings not required!

[DLB]

...
Now what is this about detents????? What causes them???
...

I was referring to the features machined into the back of the speed indicator dial. Corresponding approximately to "SLOW" and "S." Remember, I was 'reaching' for something that correlates actual slow speed with high speed adjustments.

- David

[SteveMaryland]

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.

[JPG]

I am having difficulty with a shorter belt allowing slower slow AND higher fast.

A shorter belt will decrease the effective range from smallest to largest pd. That will result is a restricted rpm range.

I think a longer belt will increase the effective range. It is also the likely solution to the original issue.

Now what is this about detents????? What causes them???


P.S. My conception of official SS belt dimensions (since greenies were first conceived) is 25.5" x 17/32 width across widest.(0.531)

Belt length WILL affect ratio between smallest and largest pd on opposing belts. Difficult to predict because of non predictable path of the belt wrap and belt path different between tension and slack paths. We can make drawings til the cows come home, but they will not reflect real world results.

If drawings don't do the trick (in the real world) then please shed some light on the issue. Tell us all who are uninformed just how this monster works.

Pictures please. Drawings not required!

My comment re 'real world' vs drawings was implying one must be very careful when assuming details that make up those drawings. Tis far too easy to assume one's self into irrelevant observations/conclusions.

No pix, but when one sets the high speed stop, the control sheave is at the minimal pulley diameter. A short belt will be positioned further in on the motor pulley thus decresing the range and shifts the low speed downward. The reverse is true with a longer belt. The control sheave sets the reference at the high speed stop. A longer belt will ride further out on the motor pulley thus attaining faster rpm at both the slow and fast settings. The limit is the long belt slipping off the motor pulley at fast.

The high speed adjustment limits the fast positioning of the control sheave and provides a hard stop at fast.

A longer belt will cause faster speeds at the fast setting(due to it's positioning further out on the motor pulley.(up the a point of slipping off) It will provide grater range of travel from that outer position to the inner most position. When the motor belt moves inward, the belt also moves outward on the control sheave that will establish the slow rpm attained.

[dusty]

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.

Thank you Steve. Your graphic pulls all the data onto one page thus allowing a much better analysis of the datum.

It appears to me as though all discussion on this subject can be terminated. I just hope that the OPs questions have been answered. Mine certainly has.

[DLB]

...
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'm using your Capture3 for reference because I think it represents your understanding of the machine with the SS belt. Mathematically, my machine produces low speed under 700 RPM at 3450 motor RPM with the SS Belt.

Capture3.JPG (38.56 KiB) Viewed 812 times

From the dimensions you give near the upper pulley, it appears that you are estimating the pitch diameter (PD) to be 0.189" from the outer edge of the belt. Assuming I've done the math correctly, this means that the drawing reflects the belt position in the motor sheaves is 1.321" in from the rim. This is the most critical dimension to me, as you indicated you are using three 'known' values to calculate the 'unknown' fourth. This is the fourth value. And on my machine it measures 1.875", meaning an actual PD much smaller than the calculated PD shown. And therefore, significantly lower RPM in the real world.

What are the reasons? Some I know, some I don't because they are not shown in the drawing. One that I know, for example, is that I'm running the machine at the stop and do not have the 0.031" margin shown in the drawing between the rim of the Idler and my actual belt position. Offsetting that a bit, is the fact that my sheaves measure 5.475", 25 thou smaller than the drawing. But all in, sample of one real world belt diameter is slightly larger at the Idler. Two effects, one is slightly higher PD regardless of whether I use your estimate of PD location or my own, the other is that more of the belt's total length is consumed on the upper pulley.

More importantly might be how we are measuring belt size. From the drawing, it appears to me that you are using a belt circumference measured at the location where you have estimated PD. Belt circumference is normally measured at the outer circumference. My belt measures approximately 26-5/8" at the outer circumference. If I convert it to how I interpret the drawing, it is more like 26-1/4". The measured width of my belt is 0.543".

I can't say what effect these considerations produce in your CAD/Drawing. IIUC belt width is a factor in how you are modeling it but not really shown in this output. My measurements are valid for a sample of one machine at the slow stop.

- David

[SteveMaryland]

"My belt measures approximately 26-5/8" at the outer circumference. If I convert it to how I interpret the drawing, it is more like 26-1/4".

See attached. If belt outside length is 26-5/8", then pitch length is (about) 25-1/2".