Critical sped for vertical shaft

[aftabhanifee]

Hello,
I have a problem as shown in attachment,here a vertical shaft is powered by a motor.
at the top of the shaft a circular disk is fixed weigh approximately 3 kg.
the shaft runs at 3000rpm and motor power is 0.75 hp.

Iam intrested to know the critical speeds within the working rpm range.( first ,second and may be more critical speeds).

i would be obliged if somebody helps out.
regards,
aftab

 

[itsmoked]

It's based on too many variables to calculate usually. It's found by empirical methods that hopefully arrive ahead of destruction. If you are running this system with a variable speed motor setup you set the system up to sweep the speed range and watch or monitor with vibration instruments what occurs as the sweep happens. If at some point vibration appears then you'd bracket that speed with a "prohibited speed range" for normal operations. The setup you specified could have a between a couple and no resonant speed areas.

You want the sweep to be fast enough to not linger at an area that is resonant and who's amplitude will grow to destruction but you also want the sweep to be slow enough to see the effects of resonance to find the speeds. Start with a fairly fast sweep and if nothing is seen slow it slightly. Step and repeat.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Artisi]

Do you have a critical speed problem or this just an academic exercise?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[aftabhanifee]

thanks Keith for useful information,
the motor is DC motor and for this application i need to vary the speed.

By varying the speed, the resonance seems to appear at 600-700 rpm.After sweeping these speed range motor runs smoothly.
as the speed reaches near 2700 rpm, vibrations are too much but not as much as near 600 rpm range.

practically i know what may be the critical speed range but I need to know the results in theory.
it's better you state what assumption Iam lagging or data which I didn't specify for the calculations.

regards,
aftab

 

[aftabhanifee]

so far about eccentricity of disk,i have to go for CMM test.
thanks.

aftab

 

[Artisi]

Think you need to check balance etc of the disk and shaft before anything else.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[aftabhanifee]

@artisi
i had that in my mind but here i don't have equipment to check balancing.
is there any other method to check balancing.

aftab

 

[Artisi]

Yes - take it to an engineering facility that can balance.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[aftabhanifee]

@Artisi
thanks for help

 

[mmamini]

you can also use experimental modal analysis.
or if you have a complete, updated finite element model, you can use SSD to find critical speeds.

 

[aftabhanifee]

SSD means solid state drive. is it??

 

[mmamini]

SSF: Steady State Dynamic

this method generate critical speeds and critical domain after calculating eigenfrequencies.
This is Finite Element and you need experimental investigations to validate and update your model.