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Mark V Speed Control

SteveMaryland

See attached jpg. Based on the geometry shown, low and high output speeds will be:

low speed = 3450 rpm x (1.63/5.06)/1.6 = 695 rpm

high speed = 3450 rpm x (5.06/1.63)/1.6 = 6694 rpm

I haven't verified the 1.6 ratio but assume it is correct.

Effective PD (pitch diameter) is a fuzzy number because it is driven by the cross-sectional shape of the belt, but where I have it is close enough.

Real output rpms should be expected to be +/- 2 percent of these numbers due to deviations from the given geometry, motor rpm, belt wear, belt width and length.

I would set both the high and low speed limits conservatively to avoid the belt getting stuck between sheaves. That is why I have the top of the belt 1/32" under flush with the sheave OD. We would also never want the belt to extend beyond flush.

Why the OP and Dusty cannot get down to 700 rpm (as measured by a meter?) is not clear to me.

DLB

dusty wrote Maybe I can't do this and still be correct but I did (do) not use pitch diameter. I have done all my calculations and sketches based on the outer surface of the belt.

I'm curious. Which numbers do you find to be in error and what do you have.

When at low end what rpm do you have or calculate. At 700 rpm (or the very lowest you can achieve) where does the v-belt ride on the idler pulley.

I found both of the numbers you used for radius to be pretty high.

I measured the pulley diameter at 5.475". On the Idler my belt runs slightly proud, so outer belt diameter was slightly higher, but I used 5.475/2 or 2.738 for Idler diameter. On the motor pulley I measured 1-7/8" from the lip of the pulley to the belt and subtracted that from pulley radius, so 0.863". I subtracted 1/16" from both as a guesstimate correction for PD, so 2.675" and 0.8". Ratio of 0.299 and RPM of 1032 at Idler. So 645 RPM estimated at upper spindles.

Critique: Measuring small pulley radius is a little crude due to access. Some error is introduced there. And PD correction is estimated, so a bit more error. But my numbers are in the range that Steve's model predicts, with differences that make sense.

Note: Actual belt dimensions and even Idler pulley position in the eccentric are factors that come into play for getting minimum and maximum PDs at the same time, as opposed to hitting one of the two stops. So there is going to be some machine-to-machine variation in actual values.

- David

HopefulSSer

DLB wrote as opposed to hitting one of the two stops.

- David

What is the slow stop? I'm aware of only the high stop.

dusty

HopefulSSer wrote
DLB wrote as opposed to hitting one of the two stops.

- David
What is the slow stop? I'm aware of only the high stop.

Slow stop is imposed by the spring on the motor sheave, As you move toward the low end, the motor sheave gets smaller and smaller. This is imposed by the spring moving the two together. Once the spring is fully compressed - you stop. Thus low end stop.

High speed stop is all together different. There is an adjustable mechanical stop for that.

dusty

DLB wrote
dusty wrote Maybe I can't do this and still be correct but I did (do) not use pitch diameter. I have done all my calculations and sketches based on the outer surface of the belt.

I'm curious. Which numbers do you find to be in error and what do you have.

When at low end what rpm do you have or calculate. At 700 rpm (or the very lowest you can achieve) where does the v-belt ride on the idler pulley.
I found both of the numbers you used for radius to be pretty high.

I measured the pulley diameter at 5.475". On the Idler my belt runs slightly proud, so outer belt diameter was slightly higher, but I used 5.475/2 or 2.738 for Idler diameter. On the motor pulley I measured 1-7/8" from the lip of the pulley to the belt and subtracted that from pulley radius, so 0.863". I subtracted 1/16" from both as a guesstimate correction for PD, so 2.675" and 0.8". Ratio of 0.299 and RPM of 1032 at Idler. So 645 RPM estimated at upper spindles.

Critique: Measuring small pulley radius is a little crude due to access. Some error is introduced there. And PD correction is estimated, so a bit more error. But my numbers are in the range that Steve's model predicts, with differences that make sense.

Note: Actual belt dimensions and even Idler pulley position in the eccentric are factors that come into play for getting minimum and maximum PDs at the same time, as opposed to hitting one of the two stops. So there is going to be some machine-to-machine variation in actual values.

- David

I am not certain about which numbers you refer to BUT in this thread I called out some numbers that depict the required diameter (radius) of the sheave to achieve a low end rpm near 700.

All of my sheaves measure very very close to 5.45" in diameter.

I use that number to calculate the diameter of the other sheave. at either the high or low end.

Remember - sheave sheave axis are separated by a distance of 7.25" thus 14.5" of the 27" belt is always in transition from one sheave to the other.. The remaining 12.5" of the motor belt is always "on the pulley (upper and lower half circumferences).

It is nearly impossible to measure the diameter (or radius) except at the extremes. Therefore I calculate rather than attempt to measure.

dusty

SteveMaryland wrote
See attached jpg. Based on the geometry shown, low and high output speeds will be:

low speed = 3450 rpm x (1.63/5.06)/1.6 = 695 rpm

high speed = 3450 rpm x (5.06/1.63)/1.6 = 6694 rpm

I haven't verified the 1.6 ratio but assume it is correct.

Effective PD (pitch diameter) is a fuzzy number because it is driven by the cross-sectional shape of the belt, but where I have it is close enough.

Real output rpms should be expected to be +/- 2 percent of these numbers due to deviations from the given geometry, motor rpm, belt wear, belt width and length.

I would set both the high and low speed limits conservatively to avoid the belt getting stuck between sheaves. That is why I have the top of the belt 1/32" under flush with the sheave OD. We would also never want the belt to extend beyond flush.

Why the OP and Dusty cannot get down to 700 rpm (as measured by a meter?) is not clear to me.

I just went to the shop to measure. I can not get to 700 rpm. In fact, I don't get close.

With the information you provide in this post I am unable to come to an understanding either. I understand you pulley diameter ratio time motor rpm and I agree that that ratio gets you there. But:

The numbers do not all add up.

With reference to this sketch I am unable to account for about 4" of the motor belt and the only place the difference can be introduced is in the circumference of the idler pulley.

The belt is 27" long. The pullet axis are separated by 7.25" thus accounting for 14.5" of the 27".

The idler pulley is at maximum diameter thus accounting for 15.895" while the motor pulley (by calculation) accounts for 5.1208", Total belt length accounted for is 23.087"

Where is the rest of that belt (4").

I contend that it resides on the idler pulley (by necessity) making the stated sheave ratio incorrect. Represented by the red ring.

Please correct where ever I am wrong.

700 rpm a no-go.png

DLB

HopefulSSer wrote
DLB wrote as opposed to hitting one of the two stops.

- David
What is the slow stop? I'm aware of only the high stop.

As Dusty said, full spring compression on the motor shaft is one. The other is the Idler, there is a hard limit as to how close together the two sheaves will come, roughly where the belt is even with the outside rim +/-. Neither stop is adjustable.

- David

HopefulSSer

dusty wrote
HopefulSSer wrote
DLB wrote as opposed to hitting one of the two stops.

- David
What is the slow stop? I'm aware of only the high stop.
Slow stop is imposed by the spring on the motor sheave, As you move toward the low end, the motor sheave gets smaller and smaller. This is imposed by the spring moving the two together. Once the spring is fully compressed - you stop. Thus low end stop.

High speed stop is all together different. There is an adjustable mechanical stop for that.

OK so there's no slow stop per se. That's what I thought. Thanks!

SteveMaryland

See attached jpg.

Previously I did not consider belt length - but I should have because it matters here.

The "official" Shopsmith drive belt length says 675 mm which is 26.57" - which is not 25.42". But not 27" either.

BUT if the previous 1.63" PD were changed to 2.48", belt length would change to 26.57 = 675mm. And thus with the "official" Shopsmith belt:

low speed = 3450 x (2.48/5.06) / 1.6 = 1057

high speed= 3450 x (5.06/2.48) / 1.6 = 4399

So now I also don't see how 700 can be obtained if the "official" belt length is used. A shorter belt would do it.

Shorter belt = lower min speed, higher max speed.

Longer belt = higher min speed, lower max speed.

DLB

dusty wrote ...

The belt is 27" long. The pullet axis are separated by 7.25" thus accounting for 14.5" of the 27".

...

I think this may be one error in your model. The belt length needed to travel between the two axes is not constant. The belt is vertical, or perpendicular to the axes, at only one speed. Therefore the 14.5" is only correct at one speed. At all other speeds it is at an angle to vertical and the length will be increased.

The belt also does not maintain 180 degrees of contact on the sheaves. A small error will be introduced if you are assuming it does so.

- David

dusty

When I use 2.53" as the max radius (5.03" Max PD) and do the calculations I find the min radius (Min PD) to be 1.449" and the idler speed to 6024 rpm (1234 rpm quill).

DLB

dusty wrote ...

The numbers do not all add up.

With reference to this sketch I am unable to account for about 4" of the motor belt and the only place the difference can be introduced is in the circumference of the idler pulley.

The belt is 27" long. The pullet axis are separated by 7.25" thus accounting for 14.5" of the 27".

The idler pulley is at maximum diameter thus accounting for 15.895" while the motor pulley (by calculation) accounts for 5.1208", Total belt length accounted for is 23.087"

Where is the rest of that belt (4").

...

I'm still having a hard time understanding where you are coming from on this. When I read the words I think you have accounted for 14.5" + 15.895" + 5.1208" or a bit over 35.5" of belt, but you said the total accounted for is 23.087". By my measurements/math, the only number that is close to mine is the 14.5", and I'm not understanding where the rest of them are coming from.

I'm applying my measured values to what I think you are trying to do here. I am using only the outside diameter of the belt. I'm simplifying the belt-path model as two semi-circles with a diagonal line connecting them. The Idler measured R=2.75" (belt was slightly proud), so accounts for 8.639". The Motor measured 0.8625, accounts for 2.71". The straight line distance between the points on the pulleys is calculated. It is the hypotenuse of an artificial triangle with "a" equal to the 7.25" difference in centers and "b" equal to the difference in radii, comes to 7.492". (Per Pythagorus.) Times two. All in, comes to 26.33" of what I believe is usually called a 26.5" (OD, not PD) belt. Pretty close.

My measured values, therefore, seem very close to correct. My spring appears to be fully compressed and my belt rides slightly proud on the Idler, so the pulleys are maxed/min'd out. (Both at, or very near, their mechanical limits aka stops. ;) ) If I had a tach I would expect it to measure about 4% higher than I calculated or I'd be wondering why. (4% because of virtually unloaded motor speed being slightly higher than 3450.)

- David

dusty

DLB wrote
dusty wrote ...

The numbers do not all add up.

With reference to this sketch I am unable to account for about 4" of the motor belt and the only place the difference can be introduced is in the circumference of the idler pulley.

The belt is 27" long. The pullet axis are separated by 7.25" thus accounting for 14.5" of the 27".

The idler pulley is at maximum diameter thus accounting for 15.895" while the motor pulley (by calculation) accounts for 5.1208", Total belt length accounted for is 23.087"

Where is the rest of that belt (4").

...
I'm still having a hard time understanding where you are coming from on this. When I read the words I think you have accounted for 14.5" + 15.895" + 5.1208" or a bit over 35.5" of belt, but you said the total accounted for is 23.087". By my measurements/math, the only number that is close to mine is the 14.5", and I'm not understanding where the rest of them are coming from.

I'm applying my measured values to what I think you are trying to do here. I am using only the outside diameter of the belt. I'm simplifying the belt-path model as two semi-circles with a diagonal line connecting them. The Idler measured R=2.75" (belt was slightly proud), so accounts for 8.639". The Motor measured 0.8625, accounts for 2.71". The straight line distance between the points on the pulleys is calculated. It is the hypotenuse of an artificial triangle with "a" equal to the 7.25" difference in centers and "b" equal to the difference in radii, comes to 7.492". (Per Pythagorus.) Times two. All in, comes to 26.33" of what I believe is usually called a 26.5" (OD, not PD) belt. Pretty close.

My measured values, therefore, seem very close to correct. My spring appears to be fully compressed and my belt rides slightly proud on the Idler, so the pulleys are maxed/min'd out. (Both at, or very near, their mechanical limits aka stops. ;) ) If I had a tach I would expect it to measure about 4% higher than I calculated or I'd be wondering why. (4% because of virtually unloaded motor speed being slightly higher than 3450.)

- David

14.5" is the sum of the 1/2 circumference measurements and can ve calculated by knowing the radius or diameter of the sheaves. The 12.5 " is the distance in space between the axis of the two shafts (idler and motor). 12.5 + 14.5 = 27" The belt length which obviously is a constant.

The diameter of the idler pulley is also a constant when at low speed. The variable sneaks in when PD is taken into consideration. With the idler pulley at max calculate the circumference, Subtract that and the 14.5" from the 27" and the result is the 1/2 circumference of the motor pulley.

The 27" (all of it) must be acvcounted for,

JPG

The 'achieved' slow rpm is indeed affected by the high speed adjustment.

Yes the floating sheave can come to a mechanical stop sorta, but since we are considering the open condition, any stop would also diminish belt tension or the belt dropping down into the vanes of the motor pulley sheaves. The closure of the control sheave can limit the attained slow speed since it cannot close more to increase the effective diameter.

The high speed adjustment starts with positioning the belt near the outer rim of the idler pulley. The hs adjustment is made to limit the positioning of the control sheave. When the adjustment is made, the floating sheave will be responding to changes made in the positioning of the control sheave(affected by the hs stop).

Many things can affect the end result including belt width and lengthdue to wear etc.), sheave wear, as well as the hs stop itself.

We need to realize there are many things that WILL affect the final result and that the HS adjustment is an approximation at best.

Using a tach to quantify things will lead thee on a merry chase( similar to chasing perfect rip fence alignment with a dial indicator).

DLB

I don't 'see' the correlation between high speed adjustment and low speed achieved. I do see the correlations between belt length/width and any speed. The high speed adjust, by setting the dial, does set the relationship between sheave position and where the slow detent will occur, but I'm going past the detent if needed in arriving at the slowest achievable speed. Just as I go past the high speed detent, usually without even knowing it is there.

Note - IIRC the slow speed detent occurs at 'SLOW' on the dial, and is based entirely on dial position. It may or may not be felt at slowest achievable speed, which is based entirely on sheave positions. If you want it to occur at slowest achievable speed, you can set the dial to do that as Steve advocated earlier in this thread. (Rather than at high speed.)

Lowest theoretical RPM would occur if the Idler sheaves were closed to max diameter AND Motor sheaves were opened to min diameter. With all of the things that vary, it is highly unlikely that we would ever get both conditions at the same time. So lowest speed achieved should be when either of those conditions are met, because the speed control will stop responding at that point. Unless you only adjust to the detent or to some dial indication. I agree that Motor sheaves fully open (min diameter) is the less desirable of the two, but I think it happens sometimes.

- David

dusty

This is the best I can do to depict my sheaves. The sketch shows (in the far right side) the diameters of the sheaves at high-mid-low speeds. For this sketch I define high as when the belt is flush with the outside perimeter of the motor sheave.

Thusly, low speed would reposition the sheaves with the belt flush with the outside edge of the idler sheave.

On my machines the belts do not come flush with the sheaves. They are inset at least 1/8" which I believe at least compensates for PD in my spread sheets (used for calculations.

If explanation is required just say so. I am must willing to make changes or answer questions about this or anything else I post on this forum.

Screenshot 2022-07-25 123058.png

SteveMaryland

In the interest of clarity, another post. See attached jpgs.

It helps to draw a picture. I have learned a lot by doing so during this thread. I am lucky to have a very good CAD program.

To address the OP's and Dusty's problem, they want lower min RPM, and it appears to me that the only way they will get that is to use a shorter belt length. See table. Shorter belt length results in lower min PD - which in turn results in lower min RPM.

It also results in higher max RPM, but that's how it is. Shorter belt = lower min speed, higher max speed. Longer belt = higher min speed, lower max speed. It is just geometry. Please review the attached materials and advise if unclear.

I also learned that there is a minimum PD under which we cannot go - because to go lower would result in the belt not fully engaging the sheaves and possibly falling into the gap between the sheaves. To be avoided. This makes the min practical PD = 1.88".

And if 1.88" is our practical min PD, and 5.06" is our practical max PD, then the lowest output speed we can get is - see table. That lowest speed is obtained by using, not the Official Shopsmith 26.57" long belt, but a shorter belt.

Likely such a belt of that length is available, or close - I have not shopped for one. I would try a link-belt instead.

I really am surprised that the Official Shopsmith belt will not allow 700 RPM output speed - isn't that a claim?

So as far as I am concerned, a remedy for the OP's and Dusty's issues were found, and we all learned.

dusty

Thank you very much, Steve.

The information that you have added is very informative and much appreciated. Your CAD documents are great.

We still are not on exactly the same page. I am out of step because I have not made the transition n to PD. Also, I have not been able to find official documentation on the Shopsmith belts (there have been several from different vendors); all sold by Shopsmith at one time. The numbers that I am working with are all from belts that I have and can measure.

It would be fair yo say that my numbers (other than PD) are rough averages of the belts I can measure.

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?

For this specific application .47" makes a significant difference.

Unfortunately, we have not heard from the OP and there is no way to know if his concerns have been answered. Hopefully they have.

edma194

SteveMaryland, Are you saying longer belts have too much slack to separate the motor sheaves far enough apart to reach minimum pitch diameter?

The upper sheaves are held to a fixed pitch diameter by the speed control so the lower sheaves have to absorb changes in speed created by changing loads. Shouldn't the minimum PD still leave a little uncompressed spring on the motor shaft to allow for that?

I don't recall where I learned of the following, not really sure it applies here. I thought the belt would push further between the spring loaded motor sheaves by some tension on the belt created from it being driven around around the sheaves. That would allow a slightly longer length belt than calculated above to still achieve the minimum PD.

dusty

edma194 wrote SteveMaryland, Are you saying longer belts have too much slack to separate the motor sheaves far enough apart to reach minimum pitch diameter?

The upper sheaves are held to a fixed pitch diameter by the speed control so the lower sheaves have to absorb changes in speed created by changing loads. Shouldn't the minimum PD still leave a little uncompressed spring on the motor shaft to allow for that?

I don't recall where I learned of the following, not really sure it applies here. I thought the belt would push further between the spring loaded motor sheaves by some tension on the belt created from it being driven around around the sheaves. That would allow a slightly longer length belt than calculated above to still achieve the minimum PD.

Would running with the belt on the upper sheaves set proud (at low speed) make your point.

See the sketch that I posted in this thread with the red ring around the sheave, The numbers that go with that sketch show what low speed was achieved.

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