Formula for converting FPM to RPM

fr = (V/3.142D)Hz


Why do you have "Hz" multiplied by rpm?

FPM divided by 3.1416*diameter equals rpm doesn't it. Then divide by 60 to get Hz?

Might be a little clearer if I said divide the FPM by 3.1416 and then divide the answer by the diameter(in feet) on the roll or felt length, depending on what speed you are looking for.

FTPM = Feet Per Minute
C = Circumference of Roll in Feet or Length of Felt in Feet

RPM = (1/C)*FTPM

To get the Circumference of the roll take the roll diameter (usually in inches) multiply by Pi 3.142 and divide that answer by 12 to get the answer into feet.

D = Diameter in Inches

Circumference in Feet = (D*3.142)/12

If you are using SKF and the speed point setup, 1/C will give you the ratio you need in your point setup.
Originally posted by Rick Sheldon:
Thanks for all of the input. After playing with all of the formulas I found another one that works:



I can't get this formula to work.

Of course it is late in the day, and I'm tired and old.
I think that Rick got fpm and rpm switched.

The long formula is:

feet/minute=(revolutions/minute)* (Dia. (in.) * pi)/12)

The simplification factor that I used of .262 is pi/12 to convert diameter in inches to feet.

It is easier to understand the practicality if you think about it as this:

fpm=rpm x circumference

It's an old power transmission formula that was used all the time to chech on the maximum rim speed of v-belt sheaves and it was ground into my head from 15 years in that field. BTW it is 6500 fpm for standard off the shelf sheaves. Higher rim speeds require dynamic balancing and perhaps series 50 ductile iron as a meterial rather than series 30 grey iron.

Thanks for the trip down memory lane.

Using the formula the way you put it with 1000 fpm and a 36" dia roll we would have:

rpm= ((1000 fpm)(3.14))/(36*12)=7.27 rpm

Using the other way:

1000 fpm=(rpm*.262*36)=106.0 rpm.

I'm pretty sure that you have fpm and rpm switched, but check me out again if you don't mind.

I think the fog has lifted a little. A shower and something to eat helped. Alec's formula works, just looking at it different than I think about it. Danny's simplied version makes since when I substitute RPM and FPM. If
FPM=RPM*dia*.262 then RPM=FPM/dia/.262 Whew!! Eight hours sleep and I might get this.

It's all in the grouping and keeping the units straight. If you want an answer in feet per minute, make sure you're dealing with feet and minutes before you put it in the formula

fpm = rpm * circumference

I think that's the easiest one to remember. And like BamaVib mentioned, the 1/circumference factor is the same one used in CSI in the equipment setup (it's the same math).
Here I am Ralph. Cool
I posted the formula, then realized that it wasn't very clear and there may be an error. I can do quick calculations on the floor in m/min with roll diameters converted to meters. I just didn't have time to clarify so I deleted it and went home. It's funny how I do this all the time but ask me to explain it and it gets all fuzzy Confused I don't think I'm getting old yet............
OK Vibbase, We'll let you off this time. Cool

Lot of ways to do the same thing here, so one should choose the easiest to remember.

All this brings up a good point, one who struggles with math is going to have a rougher road to travel in this field than normal.
Just my opinion and I could be wrong.
I hear you Ralph, I'm writing the CatIII exam this October and the math is unbelievable. When was the last time you used an oscilliscope to measure vibration and convert the volts to velocity?

So, I looked at the formula as written again;

Fr = V/(3.142D)

(divide by 60 to get Hz instead or cpm)I'm not sure why they added the Hz to the end of the formula without specifying division or multiplication, it's probably a typo on my sheet.

It's keeping the units the same that is important so if you're in feet per minute "D" must be in feet.

Would any of you like a copy of a formula page as it applies to vibration? It's the sheet supplied with the Vib Institute exams (CMVA is the affiliate here in Canada) I don't think there is any copyright problems. Let me know.
Originally posted by Ronnie Lynn:
I think the fog has lifted a little. A shower and something to eat helped. Alec's formula works, just looking at it different than I think about it. Danny's simplied version makes since when I substitute RPM and FPM. If
FPM=RPM*dia*.262 then RPM=FPM/dia/.262 Whew!! Eight hours sleep and I might get this.



I think you got it right, just typed it wrong.



You got that right. I would never have thought there would be this many post on this formula.
But like you said, "Lot of ways to do the same thing..".

Time to go back to work.

Let me see naught plus naught equals.......

Thanks Matt. 12 hours 7 days a week and I feel lucky I understood any of it. I only worked 8 hours today and I feel like I almost had the day off. I plugged the numbers into it the way you wrote it and the way I wrote it and I come up with the same answer? Not sure why that is but like what Ralph said,"so one should choose the one that's easiest to remember". I never really put it into a formula, just knew I needed circumference in feet and then divide into FPM. Works for me.

I've added this calculation to my VibCon vibration conversion program, which is available for free from CTC's web site:

As always, it converts between common vibration units. Enter a speed and the unit you know, and it calculates the rest. Very handy.

A new item has been added to the menu bar, "Belt Speeds" there and a screen appears asking for the roll diameter and either the roll speed in RPM or the belt speed in FPM. Enter one and the other is calculated.

Hope this helps!

Spintelligent Labs
Thanks Ronnie!

I'm thinking it's time for a major revision to include:

belt drive calculations (speed vs. pulley diameters)

Gear calculations

VFD speed calculations (if the power frequency is 42 Hz and it's an 1800 RPM motor, how fast is it turning)

Anything else to put on my wish list?

Spintelligent Labs
How major of a revision? I like the new look. Do you want to tackle bearing frequencies? At least from the geometric side (x diameter, y number of rolling elements, etc.)? Or CSI's bearing info is easily eported to a text file. How about resolution, LOR, Fmax, time to collect z number of revs. Or converting the AFBMA bearing numbers to SKF numbers? You know, in your spare time.
Oh no...what have I done! <g>

Patrick, I like the bearing geometry calculator and some of the FFT calculations too. Some vendors may not appreciate me providing an easy way to calculate just how long that 100,000 line FFT at 100 Hz is going to take!

No promises on when....but I'll keep the good ideas in mind.

Spintelligent Labs

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