Mark V Speed Control

[DLB]

...
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]

...
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]

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]

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.

[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]

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.