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]

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]

as opposed to hitting one of the two stops.
- David

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

[dusty]

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]

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]

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 (43.04 KiB) Viewed 987 times

[DLB]

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]

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]

...
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