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Mark V Speed Control

DLB

dusty wrote ...

For comparison, what is the dimension of your fully compressed spring. I measure mine to be 1 27/64".

Not terribly close to that, so I should ask where/how you are measuring?

I used two machines in this, one I was working on from an estate sale that already had the belt cover off and is the source of the measurements I provided earlier in the thread. Then after Dennis' post I decided to check my Mini, source of the pics.

Machine A: 1-19/64"

Machine B (Mini): 1-1/4"

I am not 100% convinced the spring is fully compressed. Though it looks it. There is a second hard stop dictated by how close the Idler sheaves come together. On both of the machines I looked at the belts ran slightly proud of the rim on the Idler, suggesting, also visually, that this stop was in play. I cannot distinguish visually on either machine which stop is actually the limiting factor.

How I measured - The floating sheave has two spring seating surfaces, one for old style springs and one for 'new.' I don't know when the spring was changed, maybe when the HP was changed.(?) These are 'new.' It is easiest for me to measure between the 'new' flat washer and the 'old' spring seat then correct by the difference in spring seats. If I don't correct, my numbers are much closer to yours, changes to exactly the same and 3/64" less. (Saying all this because if you didn't know about this change you'd likely not correct for the spring seat surfaces. And I wish I could find my spare floating sheave to add a pic...)

- David

BigSky

I used calipers to span the area of interest but I am not surprised that there are differences. Our machines are undoubtedly of different vintage.

JPG

Candidates for factor limiting speed adjustment at slow.

Full compression of the motor pulley tension spring.(sheaves cannot open further)

Full closure of the control sheaves.

Candidates for limiting speed adjustment at fast.

High speed stop.(prevents further opening of the control sheave)

Full closure of the motor pulley sheaves.

From a practical standpoint, only the high speed stop should come into play. The low speed limits should not come into play unless belt wear or some other abnormal effect comes into play.

DLB

JPG wrote ...

From a practical standpoint, only the high speed stop should come into play. The low speed limits should not come into play unless belt wear or some other abnormal effect comes into play.

For me it's kind of opposite. I rarely get anywhere near the high speed limit, or to "S" for that matter, except for maintenance, like lubricating sheaves or removing the belt. I've even considered setting it somewhat higher for ease of maintenance, but have not done so. But I'm at or near the low speed limit regularly, and if the speed went lower I'd use it lower.

- David

dusty

I'm lost. Where is the "low speed limit".

DLB

dusty wrote I'm lost. Where is the "low speed limit".

AKA Stop or limiter. Meaning Idler sheaves closed to their limit and/or motor spring fully compressed.

- David.

dusty

DLB wrote
dusty wrote I'm lost. Where is the "low speed limit".
AKA Stop or limiter. Meaning Idler sheaves closed to their limit and/or motor spring fully compressed.

- David.

OK Now I understand and I fully agree (not that that means anything).

SteveMaryland

After much going up the learning curve, the following:

Contrary to my previous assumption, the Official Shopsmith Belt actually has a pitch length of 25.4", not 26.57" (675mm). I also incorrectly assumed a belt thickness (not width) of 3/8". Actually it is 5/16" and that makes some differences from previous. Belt width is still officially 15mm (.59") but others have reported lower. Per careful measurement, my spare belt measures 26.38" outside. Others report 27", so I think the Shopsmith belt length varies widely over the years and maybe from lot to lot.

Contrary to my previous material, the Official Shopsmith Belt can indeed achieve sub-700 RPM - but at sub-700 RPM, the smaller PD approaches 1.44" diameter. At this small PD the belt may be overstressed and heated, and worse, the belt will no longer be in full contact with sheave traction surfaces (see attachment), and the belt may drop inbetween the sheaves. Wear will aggravate the drop-inbetween tendency. So OK sub 700 RPM is possible but avoid it.

Shopsmith sold this belt as 504193 and currently as 521682. Unknown to me if there is any dimensional difference between these numbers.

Note in the attached table that for each output speed we have two PD's. Each of these PD pairs are linked because they must "add up" to the same constant belt pitch length. When we adjust the Shopsmith speed dial, we are changing BOTH PD's concurrently. It is the PD ratio which defines the speed, not one or the other PD separately.

It is the PD ratio range that gives us speed range. At one end we may have a 1/3 ratio but at the other end it will be 3/1. The ratio at one end is just the reciprocal of the ratio at the other end. Which is why low min speed and high max speed are concurrent.

This is also why, if we use a longer belt, we restrict speed range. In the extreme case (see attached), a belt of 30.8" would restrict speed range to zero, the PD ratio would be 1/1 and unadjustable, and the Shopsmith output speed would be motor speed/1.6.

I still cannot understand how the high speed adjustment screw can also affect low speed, as others have claimed. It sets a limit on highest speed only, as far as I can see. The low speed limit is set by the motor spring going solid. It would be desirable to have a more positive low speed limit.

Somebody on this thread said that in the event of an overload, the belt drive somehow adjusts in an attempt to maintain speed. I do not think so. The drive ratio stays where it is set regardless. The motor may change speed but that is the motor not the drive.

GRAPH AND SPEED TABLE FOR SHOPSMITH OFFICIAL BELT.pdf

jsburger

Given the above and all the apparent incorrect assumptions in this thread, does that mean that Magna Engineering really did know what they were doing when they invented the MK 5?

dusty

jsburger wrote Given the above and all the apparent incorrect assumptions in this thread, does that mean that Magna Engineering really did know what they were doing when they invented the MK 5?

Be specific, please. Which assumptions do you believe were/are incorrect AND WHY.

The only problem I have with anything stated herein is belt length. I have three (two new) 504193 belts and their ODs are all a tad over 27". I also have the very first belt that came with my 510 when purchased new. It is a Perelli carrying the 504193 PN. It is only .48" wide but it too is 27".

BUT. I still do not achieve anywhere near 700rpm.

Charts will not convince me, even my own. I can make the numbers work by manipulating ratios but that doesn't guarantee a functional machine that runs the speed chart. Tell me you have one that does this and you became convinced by seeing the numbers on a tach and I'll become a believer.

DLB

dusty wrote ...

The only problem I have with anything stated herein is belt length. I have three (two new) 504193 belts and their ODs are all a tad over 27". I also have the very first belt that came with my 510 when purchased new. It is a Perelli carrying the 504193 PN. It is only .48" wide but it too is 27".

You are getting a consistent measurement over multiple samples, as am I. The most likely explanation is that we are measuring differently. In measuring circumference of a belt, how we measure introduces an error. I'm using a thin flexible measuring tape normally associated with sewing. If I use a metal tape that I would use for just about anything else, I get a tad over 27". Both are actually a little bit higher than actual circumference, but a thin flexible measuring device comes closest.

BUT. I still do not achieve anywhere near 700rpm.

Nor did Dennis. What do you think is wrong? At the low speed stop, your belt positions in the sheaves should be similar, or not, to the pics I gave of mine. Measuring ratios, calculating ratios, and measuring RPM should all give the same approximate answer or we should be able to explain why. So far we seem to have resolved why we were getting different answers between the first two.

Charts will not convince me, even my own. I can make the numbers work by manipulating ratios but that doesn't guarantee a functional machine that runs the speed chart. Tell me you have one that does this and you became convinced by seeing the numbers on a tach and I'll become a believer.

Two things I know of are going to give us variation:

1) Actual motor speed Vs. rated. Test conditions are about as close to no-load as we can get on the Mark V headstock. Motor speed should be much closer to 3600 RPM than to 3450 RPM. Tachometer measurements are likely to be about 4% high compared to calculated.

2) Distance between shafts varies based on Idler eccentric adjustment and is not consistently 7.250". The effect will be the same as small variation in belt length. This will cause variation, one machine to the next, in how close we get to the theoretical minimum achievable speed.

Taken together, I'm not seeing how those get us anywhere near the 1055 RPM number that Dennis reported. As one example.

- David

dusty

I do understand the variability between machines but (for me) that is not a factor. I am comparing my results with SS documented numbers (the chart).

SteveMaryland

Belt pitch length = belt outside length - (belt thickness X 3.14). The actual belt thickness (not belt width) comes into play.

So for example if anyone has a belt with a 27" outside length and a 5/16" thickness, the pitch length of that belt is 26".

Traditionally the "centroid" of the belt section has been used to define pitch length. What I have been using is not the "centroid" point but rather the "geometric" point. The geometric point is easier to locate and differs only 0.016" from centroid point. Attached shows these two points.

Using centroid point instead of geometric point does slightly increase min RPM and slightly reduce max RPM from my table values. So how we measure the belt does matter.

The centroid is where the "neutral plane" of the belt section is located. Neutral plane is where, when the belt bends, it sees no stress, either tensile or compressive. Now when a belt is in operation and under tension, there really is no zero-stress plane but for speed calc purposes we are concerned about the geometry of the section not the stress distribution across the section. The centroid location is based on section geometry.

If I had 2 belts, identical in geometry but different in reinforcing-wire distribution - would they differ in pitch length? Or pitch diameter when on a sheave?

I have looked at the Gates material discussing this issue, and per the above I am not convinced that relocating the belt length-measuring point to near the belt top surface (where the steel wires are concentrated nowadays) would be correct for speed ratio calculation purposes.

Gates reading material attached. Gates is basing everything on fixed- diameter sheaves, and they do not apply to Shopsmith sheaves.

DATUM DIAMETERS - GATES V BELT PRIMER.pdf

DLB

SteveMaryland wrote Belt pitch length = belt outside length - (belt thickness X 3.14). The actual belt thickness (not belt width) comes into play.

So for example if anyone has a belt with a 27" outside length and a 5/16" thickness, the pitch length of that belt is 26".

According to some sources, the pitch or datum line is much closer to the outer diameter of the belt, depending primarily on the location of reinforcement. This has changed over time, so for any given belt it evidently depends on actual belt construction. Which, for the modern SS belt, we don't really know.

Here is a pretty good article on pitch/datum of pulleys and belts, includes "...So the diameter through which the pitch line passes is nearly equal to the outside diameter for most belts...": http://gatesbeltsandapplications.blogspot.com/2012/07/what-is-datum-diameter.html

So I'm not trying to suggest right or wrong, I don't think we know with certainty. I'm suggesting we are estimating. I can see what I presume is the primary reinforcement band in the side of a lightly worn SS belt, and estimate it is ~0.08" from the OD. (I used a slightly smaller estimate earlier in this thread, but the difference in pulley ratio is trivial.)

- David

dusty

DLB wrote
SteveMaryland wrote Belt pitch length = belt outside length - (belt thickness X 3.14). The actual belt thickness (not belt width) comes into play.

So for example if anyone has a belt with a 27" outside length and a 5/16" thickness, the pitch length of that belt is 26".
According to some sources, the pitch or datum line is much closer to the outer diameter of the belt, depending primarily on the location of reinforcement. This has changed over time, so for any given belt it evidently depends on actual belt construction. Which, for the modern SS belt, we don't really know.

Here is a pretty good article on pitch/datum of pulleys and belts, includes "...So the diameter through which the pitch line passes is nearly equal to the outside diameter for most belts...": http://gatesbeltsandapplications.blogspot.com/2012/07/what-is-datum-diameter.html

So I'm not trying to suggest right or wrong, I don't think we know with certainty. I'm suggesting we are estimating. I can see what I presume is the primary reinforcement band in the side of a lightly worn SS belt, and estimate it is ~0.08" from the OD. (I used a slightly smaller estimate earlier in this thread, but the difference in pulley ratio is trivial.)

- David

I too have done this and my amateur observations are the same as yours. The reinforcement seems to be very close to the top of the belt (negligible for my non-scientific analysis). I'm just attempting to determine if the reason for my not being able to attain 700 rpm is the fault of either me or my machine). Not how I got into this in the beginning.

JPG

I consider the high speed stop's only function is to preclude the control sheave from interfering with the speed control parts by opening the pulley too far.

There might well be another purpose, but every thing else in the drive train is mushy.(wear, belt stretch. . . .)

dusty

JPG wrote I consider the high speed stop's only function is to preclude the control sheave from interfering with the speed control parts by opening the pulley too far.

There might well be another purpose, but every thing else in the drive train is mushy.(wear, belt stretch. . . .)

I agree but that does have an effect on speed determination (calculations or otherwise).

DLB

I caved on the tachometer. Placed tape on the flat washer that helps retain the motor spring, arbor portion of a sanding disc for the upper, similar location on a 3-3/4" faceplate on the Idler. Raw numbers:

Motor 3580 RPM (about what I'd expect for minimal load).

Idler 1246 RPM

Main 790 RPM.

This is the machine I've provided measurements for earlier in the thread. I'll be repeating some of those, as RPM are higher than I expected even adjusted for actual motor RPM. 790 RPM at 3580 Motor RPM would be 761 at 3450.

I checked the sanding disc and tach on my PowerPro. Everything worked as advertised.

- David

JPG

Loaded motor rpm will drop taking the other two down as well.

The loaded at 700 rpm motor rpm is extremely low(<3200 rpm) Not a likely scene.

DLB

DLB wrote ...

This is the machine I've provided measurements for earlier in the thread. I'll be repeating some of those, as RPM are higher than I expected even adjusted for actual motor RPM. 790 RPM at 3580 Motor RPM would be 761 at 3450.

...

The most suspect measurement was radius of the belt on the Motor sheaves. I made a gauge out of a T-Nut, to rest across the sheave rim, and a length of threaded rod. This allowed me to measure more accurately and at bottom dead center. I had also changed my estimated correction for pitch/datum. Between the two, my estimated pitch/datum radius changed from 0.8" to 0.916". Overall resulting in a calculated Idler speed of 1232 RPM based on my best measured/estimated numbers (now using 3580 RPM) Vs. 1246 RPM measured. A difference of 14 RPM, or barely over 1%. Good enough.

I conclude that this machine, as configured, will not get below 790 RPM unloaded. Assuming the speed chart is based on 3450 RPM (fully loaded), the machine won't go below 761 RPM. Not bad compared to speeds reported by some other members, but not great. The belt already runs slightly proud on the Idler sheaves and with the spring at or near full compression.

- David

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