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View Full Version : [Math, I hate math] Calculating the RPM of a wheel for an application.

WhatTheFlux!
10-09-2013, 09:49 AM
Ok, I *know* a stepper would be easier but this is what I have on hand, and I have to make it work.

I have a DC gear motor, a 3" wheel and a pot for speed control. Assume I can reach any speed from infinity down to zero.

I need to figure out how many RPM I need to move that 3" diameter wheel .280" in two seconds.

Like I said, a stepper would make this far easier but, I don't have one.

schoolie
10-09-2013, 09:56 AM
You're trying to convert in/sec to RPM. First convert 1/sec to 1/min by multiplying by 60, then convert inches to revolutions by dividing by the perimeter of the wheel (pi*D).

0.280" / 2 sec * 60 sec / 1 min * 1 rev / (pi*D")

where D is the diameter of your wheel.

I get 0.891 RPM.

vincemulhollon
10-09-2013, 09:58 AM
I need to figure out how many RPM I need to move that 3" diameter wheel .280" in two seconds.

To one sig fig (hey you said you had a speed controller...) C=3*D so your wheel is around 9 inches around. You wanna go about a quarter inch in about two seconds another way to say it is there's 30 "two seconds" in a minute during which you'd go 30 quarter inches or about 7.5 inches. You're trying to go about 7 inches per minute using a wheel about 9 inches around so you want "about" one RPM to one sig fig. Tune to precision using your potentiometer speed control.

Thats the no math, no calculator, 10 second off the top of my head estimate. You could put 10 sig figs into my math above and get 10 sig figs but you have a speed controller so it doesn't matter anyway.

You could test this estimate by picking up a liquor bottle 3 inches across (best empty it first) then roll it 1/4 inch in 2 seconds and keep going constant speed I suspect it'll take about a minute for a full rotation. If you send me a liquor bottle I could test it for you. Maybe I best try it a couple times for a decent sample size, send me a couple bottles of rum.

10-09-2013, 10:32 AM
Both of those answers are correct (0.8917 RPM) but I like the test methodology of the second one better. ;-) If you need some more sample data, you could send me a bottle or two to test. You should send the same bottles to make sure your data is consistent.

You may have a small problem in that there may be a minimum speed your motor will run at unless you have a very large gear reduction.

Black Forest
10-09-2013, 10:41 AM
I think you also should send two bottles to Germany. Everyone knows about the good German engineers. I don't have a clue how to figure it out but I would call Nick and he would tell me. So one bottle for Nick, one for me as the middleman! Make it a fine single malt Scotch for best test results. I don't drink alcohol but I do serve it to my friends.

WhatTheFlux!
10-09-2013, 11:34 AM
Ok thanks for the equation. Now I have another problem.

DR
10-09-2013, 11:40 AM
Gee, the math test you must pass to graduate from high school in my area has harder problems than that.

Bob Fisher
10-09-2013, 11:53 AM
And you said in school,"I don't need to learn that, I'll never need it". Surprise! Bob.

EddyCurr
10-09-2013, 01:09 PM
Easy now, fellows.

I have every reason to believe that more than a few HSM members are
of an age such that whilst in grade school, they never anticipated there
would be a personal benefit to be derived from participation in typing
classes. Yet, here they are racking up their post counts ...

"Education is what remains after one has forgotten
what one has learned in school."
Albert Einstein

"Education would be much more effective if its purpose
was to ensure that by the time they leave school every
boy and girl should know how much they do not know,
and be imbued with a lifelong desire to know it."
William Haley

WhatThe, ask away -it is how one learns ...

.

Paul Alciatore
10-09-2013, 01:21 PM
Yes, there really were reasons for teaching you all that "useless" stuff in school.

I hate it when people say they hate math. I hated it too in grammar school. Just endless addition, subtraction, multiplication, and division problems. A four banger calculator, for \$2 will do them all. And word problems. At least some of them were somewhat interesting. I think my interest in math started when a teacher, who thought differently, offered extra points to anyone who solved a math problem. It was a trick one and you had to think. I got it wrong, but it started me thinking. It wasn't just endless, crank them out problems. It really was about thinking in a critical manner.

Stern
10-09-2013, 02:29 PM
Scrap the pot (DC otors dont like voltage changes much) and replace it with a PWM (cheap on Ebay, \$5.00-\$10.00), then you can use the pot on the PWM to set it to exactly what speed you need (they run from 10%-100%, but above 50% you dont loose any appreciable torque)

WhatTheFlux!
10-09-2013, 02:51 PM
Gee, the math test you must pass to graduate from high school in my area has harder problems than that.

Not much call for that sort of calculation in my previous career. I'm having to ask all sorts of questions now that I 'should have' learned long ago. :)

mklotz
10-09-2013, 04:28 PM
Yes, there really were reasons for teaching you all that "useless" stuff in school.

They spent an equal amount of time trying to teach one how to write his native language. That should have been a clue that literacy is as important as numeracy but, unfortunately, it seems to have escaped the notice of most Americans.

Bob Fisher
10-09-2013, 07:14 PM
And you said in school,"I don't need to learn that, I'll never need it". Surprise! Bob.
No offense intended, I need a tongue in cheek symbol. I am such an age to have never anticipated a need to type, have to do it the hard and slow way. Bob.

Paul Alciatore
10-10-2013, 12:01 AM
I attended an academic high school. No trade classes and that included or more precisely excluded typing. I had to teach myself one summer. Fortunately my parent's Underwood came with a teach yourself typing booklet. But I really wasn't very good at it until computers came along.

Evan
10-10-2013, 01:10 AM
The speed it runs at without a stepper or a feedback circuit will depend entirely on the load. That is the case with or without pulse width modulation. PWM is better but still subject to load. The only way to get accurate speed control is a stepper or better is to use a feedback speed sensor that tells the motor controller what speed it is turning.