Shopsmith Forums

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!

edma194 wrote: ↑Wed Jul 13, 2022 5:32 pm

BigSky wrote: ↑Wed Jul 13, 2022 3:21 pm 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.

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.

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.

BigSky wrote: ↑Sun Jul 17, 2022 10:19 am 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

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.

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?

HopefulSSer wrote: ↑Sun Jul 17, 2022 12:33 pm 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.

Gotcha

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.