Inches per second to Mills 1

[Rpsfinest]

I understand the simple inches per second measurement, but what exactly is a Mill? I am talking axial vibration of a shaft. What does a Mill stand for? Millimeters per second? Thanks for your help in advance folks...

 

[Hatch]

A mil is a thousandth of an inch, (0.001 inches).
Or... 1000 mils = 1 inch.

 

[electricpete]

In the US, a mil is a thousandth of an inch and for rotating equipment vib refers to the displacement usually expressed as a pk/pk value.

IPS is vibration usually expressed in the US as pk/0 value.

If displacement is sinusoidal x(t)=X0*sin(w*t)
then velocity will be
v(t)=d/dt (x(t))= w*X0*cos(w*t).

Hence the conversion between mils and ips is frequency-depdendent.

The conversion I use to convert between displacement magnitude in mils pk/pk (X) and velocity magnitude in ips pk/0 (V) assuming single frequency sinusoid at frequency F (cpm) is follows:

X = V * (18,000/F)
V = X * (F/18,000)

Those are round numbers... it's actually closer to 19,000 but 18,000 is a good round number when you are converting to speeds like 3600, 1800, 1200 etc.

You can verify the result for yourself using first principles v(t)=d/dt (x(t))= w*X0*cos(w*t) taking using suitable unit conversions and the fact that V is expressed as PK/0 while X is expressed as PK/PK (in the US).

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[electricpete]

Also mils are widely used to measure shaft vibration displacement while ips are commonly used to measure bearing housing vibration velocity (except in slow speed equipment where bearing housing is sometimes treated in mils).

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[electricpete]

Correction to my first post:
"IPS is velocity usually expressed in the US as pk/0 value."

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[sms]

Are you looking at axial vibration or axial position? In a thrust monitor the display is in mils, which is the position of the shaft realtive to a zero position in thousandth of an inch. Often that zero position is the center of the cold float between the thrust bearings. The zero may also be with the thrust runner hard against the active shoe.

If you are measuring mils peak to peak (pp), then it is vibration, and the measurement is thousandths of an inch from the maximum positive excursion to the maximum negative excursion in the signal. Mils pp can be obtained either with a proximity probe directly measuring the displacment of a shaft, or with a casing transducer where the signal has been integrated from acceleration to velocity, and then integrated again to displacement.

When you say you are measuring axially, I suspect that you are looking at thrust position. What kind of measurement device are you using for this?

-The future's so bright I gotta wear shades!

 

[dgallagher]

Here is an old application note of mine regarding comparison of vibration readings:

http://www.reliabilitydirect.com/appnotes/cvr.html