# Thread: Need help with pullet diameter ratio math, please

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## Need help with pullet diameter ratio math, please

Can someone look at my speed chart, and walk me through the speeds. I am believing the chart is way off. Supposed to be a 12 speed lathe and yet only 4 speeds are useable.
Here are the givens: motor 1725rpm
motor 2 step pulley 2 1/4 and 2 3/4 dia
countershaft pulley 6" and 6 3/8"
slow speed step is 2 1/4 & 6 3/8
fast step is 2 3/4 & 6"
Spindle countershaft steps are 3"-4"-5", this yeilds ratios of 1.66:1, 1:1, .6:1 ,correct?
I have no interest in accuracy out to decimals places to right of the decimal point nor perfected pitch diameter of V groove. All of these are V-groove
I am suspect that the chart is off by 3 places left of decimal point. (hundreds of rpm)

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You said: "Spindle countershaft steps are 3"-4"-5""

Are you saying the countershaft cone pulley and the spindle cone pulley are identical, but reversed?

allan

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You have two ratios on the motor pulleys, and three ratios on the counter shaft, for a total of six ratios. When you use the backgear inside the lathe, you double the ratios for a total of twelve. Gates has how to figure r.p.m.s of pulley arrangements on their website.

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You can get close enough by figuring using the diameter of the pulley where the outside of the belt runs. Usually that is the pulley OD.

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I used this when modifying a wood band saw; https://www.blocklayer.com/pulley-belteng.aspx

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Your slowest speed, not using back gear, should be approximately 365 rpm at the spindle, assuming a 1725 rpm motor, and just using the pulley OD ratios. That agrees very well with the chart.

If by chance the motor is actually turning 3450 rpm, then the speed would be double, or about 730 rpm, which would appear to be about 400 rpm faster. There really is no way to get around pulley ratios. They just plain work. So that seems to come back to the motor rpm.

ARE YOU SURE the motor is actually turning at 1725 rpm?

How did you determine the ACTUAL motor rpm? Have you used some form of a tach? The label on the motor might not be correct, due to rewinding, or possibly due to a fault within the motor, depending on how the motor was wound.

At this point, I would suggest you fall back on the simple direct method of checking.... count the motor turns per one full spindle turn with the machine set up for the slowest speed that does not use back gear. You should find that the motor turns about 4.7 turns per spindle rpm.

If so, then the spindle speed pretty much has to be 0.211 x motor rpm, and if there is any large error error, the motor must NOT be going at the speed you think.

If not, if you get some significantly different number of motor turns, then something is "off" in the pulley measurements.

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Originally Posted by J Tiers
You can get close enough by figuring using the diameter of the pulley where the outside of the belt runs. Usually that is the pulley OD.
yes, that what I'm trying to do, and asking for someone to double check my math, please.
I'm getting speeds of 367, 609, 1015 in slow motor position

and speeds 476, 791, 1318 in high motor position,
if my math is correct, then this chart is trash

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Originally Posted by Ringo
yes, that what I'm trying to do, and asking for someone to double check my math, please.
I'm getting speeds of 367, 609, 1015 in slow motor position

and speeds 476, 791, 1318 in high motor position,
if my math is correct, then this chart is trash
Since I come up with 365 for your slow position, that would agree with your math. And it suggests there is no motor issue.

it looks like the actual problem is elsewhere..... and I think we know where it is...... NOT in the table of speed, that is perfectly fine.

Your set of pulleys on the countershaft and maybe the spindle seems to be the issue.

Take a good look at the drawing above the table.... note that the pulleys shown appear to be rather close in ratio..... THAT WOULD MATCH THE TABLE.

Your problem is that the pulleys are wrong..... they are too wide a ratio, someone seems to have changed them somewhere in the distant past. They are not the pulleys that came with the lathe. The only thing that is right about the pulleys is the one ratio at the slowest end. All the others are too large as to steps, and so the speeds are wrong.

Essentially, you probably have a "Frankenlathe", and the only way to fix it right is to get the right pulley set for the countershaft and maybe the spindle. Check with Scott Logan to find out what SHOULD be on the machine.

We'd probably want more info to be sure that is the case. And frankly, it would be a bit surprising to me if it WAS modified by some prior owner. But the speeds make little sense otherwise.
Last edited by J Tiers; 07-22-2019 at 12:18 PM.

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Originally Posted by Ringo
yes, that what I'm trying to do, and asking for someone to double check my math, please.
I'm getting speeds of 367, 609, 1015 in slow motor position

and speeds 476, 791, 1318 in high motor position,
if my math is correct, then this chart is trash
Are you sure you didn't get something mixed up? Here are the chart speeds and your speeds in ascending order:
Code:
```353 459 645 831 1157 1450
367 476 609 791 1015 1318```
They look pretty close to me.

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I noticed that also.... but they are in the wrong places...... the chart has them in ascending order without bouncing back and forth from High speed to low speed, and back every change.... it states that one set are available in low speed, and the other in high speed. And that would be true with closer ratio pulleys.

On the other hand, the ratios of the speeds are not agreeing..... 459/353 gives 1.3 as a ratio, and 1157/831 gives 1.39 as a ratio. Likewise, 645/459 gives 1.4, while 1450/1157 gives 1.25. Those ratios should agree, because going from low to high speed on the motor pulley portion should not affect the countershaft pulleys.

the ACTUAL ratios from the countershaft and spindle pulleys seem to be about 1.66 for both steps, a very different ratio.

That does not explain why the chart gives them as sequential, but the machine has them as a completely different sequence.

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