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.

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.

Some how I am missing something here? Would we say RPM= FPM/(pi*dia/12). Danny in your example: 1000 FPM/(3.14*36/12)= 106 RPM. Ya'll help me if I am wrong. I use to think this was easy but now I am confused.

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. 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 I don't think I'm getting old yet............

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.

Last edited by vibbase September 21, 2005 11:06 AM

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.

Thanks, Ronnie

Ronnie,

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

RPM=FPM/(dia*.262)

Ralph,

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..".

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.

Matt, Please don't worry over something that trivial. Besides, you made me stop and think and exercise my brain. And trust me, my brain needs all the exercise it can get.

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"...click 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.

Last edited by lawrencep September 23, 2005 10:51 PM

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.

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.

Thank you in advance.