Schedule a Meeting · Talk to a real woodworker: +1 937-890-5197 · 8:00AM – 4:30PM ET

Mark V Speed Control

BigSky

Help me please.

I am working on a Mark V after having it totally disassembled.

It was in terrible condition due to negligence of a previous owner.

My problem is with the pulley settings.

IIUC when the pulley rotation speed is equal to the motor speed (3450) the pulleys should equal in size and rotating at the same speed.

If I get that condition to exist - at what speed should the saw blade be rotating.

How deep into the pullets should the belt be riding.

edma194

With the pulleys at the same diameter they and the idler shaft will be running at motor speed. However, the main spindle does not run at the same speed as the idler shaft. There is 1.6:1 ratio between idler shaft speed and main spindle speed. So the blade should be running somewhere around 2160RPM. Remember that the speed of the motor will not be 3450RPM, it typically is something under that, somewhere between 3000 and 3450RPM depending on load conditions.

BigSky

That is good news. I knew there was a spindle difference but was not taking it into consideration.

Until I understand better, I will use only the pulley speeds for my calculations.

My confusion comes when I try to track the differences in pulley size. It seems that if one pulley gets 1/4" larger the other should get the same only smaller. That does not seem to be the case.

DLB

edma194 wrote With the pulleys at the same diameter they and the idler shaft will be running at motor speed. However, the main spindle does not run at the same speed as the idler shaft. There is 1.6:1 ratio between idler shaft speed and main spindle speed. So the blade should be running somewhere around 2160RPM. Remember that the speed of the motor will not be 3450RPM, it typically is something under that, somewhere between 3000 and 3450RPM depending on load conditions.

I don't want to derail the conversation, but I think the motor will run between 3450 at max rated load and higher RPM at reduced load. Up to 3600 RPM theoretical, but never realized. This, in fact, is the great think about induction motors, they have excellent speed control independent of load without an external speed controller.

- David

edma194

Yes, that makes sense. 6000RPM should be the no load speed based on the number of poles.

edma194

BigSky wrote Until I understand better, I will use only the pulley speeds for my calculations.

What are you calculating?

JPG

edma194 wrote Yes, that makes sense. 6000RPM should be the no load speed based on the number of poles.

3600 makes more sense!

edma194

Where did 6000 come from? Yes, 3600 does make sense. I got no idea how I ended up writing 6000.

BigSky

Thanks for the input. Yes, I do believe that I have confirmed mathematically that the two pulley sets change in equal and opposite direction.

edma194

BigSky wrote Thanks for the input. Yes, I do believe that I have confirmed mathematically that the two pulley sets change in equal and opposite direction.

There are some good drawings of the mechanism somewhere on the forum that show the range of movement of the split sheaves and some measurements. They move in concert like that because the lower sheave is moved by a spring that will equalize the diameters of the pulleys.

BigSky

It looks to me as though the spring attempts to keep the lower sheave at maximum diameter and that is overcome by the belt and the upper sheave as the operational speed is dialed down.

I think!

BigSky

edma194 wrote
BigSky wrote Until I understand better, I will use only the pulley speeds for my calculations.
What are you calculating?

I am attempting to make sense of this chart. I think the content of this chart is from Shopmith.

700 1120 Slow

750 1200 A

850 1360 B

950 1520 C

1050 1680 D

1150 1840 E

1300 2080 F

1450 2320 G

1600 2560 H

1750 2800 I

1900 3040 J

2050 3280 K

2200 3520 L

3680

2400 3840 M

2600 4160 N

2800 4480 O

3000 4800 P

3250 5200 Q

3500 5600 R

3800 6080 S

4100 6560 T

4400 7040 U

4700 7520 V

5100 8160 W

5250 8400 Fast

The numbers are, at best, close approximations of my real world numbers.

I would expect the ratio of motor speed to idler speed (3450) to be consistent but it certainly is not.

BigSky

I probably should not be responding to my own post but I feel compelled to do so.

I now have concluded that by expectations of a linear speed change across the spectrum is absolutely in error.

By placing a straight edge on the face of the sheaves I have become aware of a slight (so slight) crown. This explains why the ratios across the speed range are inconsistent.

SteveMaryland

The motor spring and the "porkchop" each push against the other in a never-ending battle of wills, linked together by the drive v-belt. Sorta like the Bickersons, but in normal op the "pork chop" is supposed to dominate the spring.

When the "porkchop" is dialed in the direction of higher letters (by operator), the motor compression spring is allowed to expand, which "closes up" the motor sheave halves, which increases the pitch diameter of the motor sheave - which in turn forces the idler sheave halves to "open up" thus reducing their PD. Which has the net effect of increasing the RPM of the spindle (output) shaft.

When "porkchop" is dialed the other way, motor compression spring is forced to compress further, the above geometry reverses, and RPM decreases. "Infinite' speed range but the relationship with the speed dial is nonlinear.

This is why, when something goes wrong with the "porkchop" and/or speed dial, the tendency is for RPM to increase on its own - because the motor spring is then dominating and back-driving the "porkchop". This however is the tail wagging the dog - normally the "porkchop"/speed dial dominates the motor spring but when we have a speed dial failure, the spring dominates and pushes the drive into uncommanded higher RPMs.

DLB

BigSky wrote I am attempting to make sense of this chart. I think the content of this chart is from Shopmith.

...

The first column of numbers and dial indications are consistent with Shopsmith's published speed chart for the conventional headstock. The second column appears to be 1.6 times the first, therefore consistent with Idler shaft speed as far as I know.

The numbers are, at best, close approximations of my real world numbers.

I would think a close approximation is the best one would expect. I think they are estimates, making some assumptions about load, belt wear, and high speed alignment.

I would expect the ratio of motor speed to idler speed (3450) to be consistent but it certainly is not.

I don't understand this one. Under some specific conditions? This ratio is what the speed control changes.

- David

BigSky

I think I fundamentally agree with you regarding the "pork chop" and the spring.

I still have questions about the linearity of speed change AND why it does not coincide with the Shopsmith chart.

Note that there is a point in that transition when motor speed and idler shaft speed are the same. This is at a point about half way through the transition that you describe. Both the upper and lower pulleys are equally open/closed thus a 1:1 ratio of the speed control pulleys. Speed can be altered either up or down from that point.

HopefulSSer

For my own education I'm trying to follow this conversation but I'm having a little difficulty....

BigSky -- what are you seeing that you say doesn't coincide with the SS chart? Can you give a specific example? And what is the end-goal of your investigation? What are you trying to determine?

BigSky

HopefulSSer wrote For my own education I'm trying to follow this conversation but I'm having a little difficulty....

BigSky -- what are you seeing that you say doesn't coincide with the SS chart? Can you give a specific example? And what is the end-goal of your investigation? What are you trying to determine?

This all started when I was given a device to measure rpm and I discovered that I could not achieve 700 rpm on the main shaft.

Then I was working to understand the ratios of the motor and idler shaft with the motor speed. Just an arithmetic operation.

motor to quill motor to idler across the entire range while understand that main to idler is a fixed 1.6.

Primarily objective NOW is to clear my own confusion.

HopefulSSer

Gotcha

SteveMaryland

Plot of above speed chart submitted by OP. Shows a (moderately) nonlinear relationship between dial marker (equally spaced letters) and actual speed.

The geometry of variable belt drives can be tricky to predict. Exact linearity of output speed across the dial is not to be expected, but the deviation from linear appears to be not much. Why the '"real" speed differs so much from the chart speed is not clear. The 1.6 multiple does not explain it.