Vibration of a recip. air compressor
Vibration of a recip. air compressor
(OP)
I have a 2cy recip. air compressor directly coupled to a 1800 RPM/60 Hz motor. The air compressor is mounted on a frame which is suspended on an underfloor of a rail car. When the air compressor operates there is a significant vibration felt on the car floor.
The compressor is 350 lbs suspended on a frame with three lord isolators (283P-185) rated to 185 lbs max. and 740 lb/in spring rate. It seems that the disturbed frequency by the air compressor at 30 Hz is causing some of my other components mounted on the compressor frame to vibrate under its natural frequency. Can anyone suggest a better isolator or idea as to how I can improve the resonance of my compressor frame? Shall I reduce the stiffness of the isolator in order to reduce the natural frequency of the isolated compressor?
The compressor is 350 lbs suspended on a frame with three lord isolators (283P-185) rated to 185 lbs max. and 740 lb/in spring rate. It seems that the disturbed frequency by the air compressor at 30 Hz is causing some of my other components mounted on the compressor frame to vibrate under its natural frequency. Can anyone suggest a better isolator or idea as to how I can improve the resonance of my compressor frame? Shall I reduce the stiffness of the isolator in order to reduce the natural frequency of the isolated compressor?





RE: Vibration of a recip. air compressor
Life isn't that simple, since you do not have an SDOF system, you actually have 6 rigid body modes to worry about. It is anywhere between possible and likely that one of those is near 30 Hz, thereby ruining your isolation. Also your mounts may be stiffer than 740 in the off-load directions.
What you really need to do is to optimise the location of the mounts and their stiffness. You either need to know a great deal baout the system, or some test data.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vibration of a recip. air compressor
Fe
RE: Vibration of a recip. air compressor
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vibration of a recip. air compressor
RE: Vibration of a recip. air compressor
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vibration of a recip. air compressor
See: http:/
Just make sure that you select the proper size isolator (low natural frequency).
RE: Vibration of a recip. air compressor
My calculated natural frequency based on the disturbing frequency of 25-35 Hz is 8.3 - 11.7 Hz. so are you saying that the rope isolators I select should provide a natural frequency of less than 8.3 Hz?
See attached calculations I conducted for the HERM series mounts from Enidine. Based on my number, I selected HR12-400-BHMY.
RE: Vibration of a recip. air compressor
I had a similar problem earlier with my rolling rigid body mode at 39Hz close to a second order excitation of a 4S,4cyl engine while idling at 1200rpm. This was solved merely by orientation of the mount so that the stiffness changes in the lateral & transverse direction help to shift the rolling mode down to 25Hz. With recip compressors, you always need to ensure your rolling mode is away from your low torsional orders. (this would vary based on the torque-crake angle behaviour of your configuration).
Jeyaselvan
RE: Vibration of a recip. air compressor
Is the "direct coupled motor" a C face motor, so the motor and compressor are both "isolated"? or, Is the compressor on the lord mounts, and the motor hard mounted?
Passing shaft Horse power to a flexibly mounted component is tricky.
What is the piping like near the compressor?
RE: Vibration of a recip. air compressor
RE: Vibration of a recip. air compressor
If there's no budget for vibration analyst or vibration measurement gear capable of discerning frequencies it may be time to glue weights on a small loudspeaker and feed the output into the soundcard's microphone port for processing with a freeware FFT program. Could also be used for "bump test" to ID actual mounted frequencies
RE: Vibration of a recip. air compressor
"glue weights on a small loudspeaker and feed the output into the soundcard's microphone port"
I don't see how this can attain the vibratory frequencies in the structure. What if it makes almost no noise when it resonates?
Fe
RE: Vibration of a recip. air compressor
By gluing weights on the speaker's cone, you can create an accelerometer sensitive (tuned) to any range of frequencies, from the normal high tweet down to nearly seismometer levels. It is left as an excersize to the reader on how much weight to add, and what the calibration of such a device would entail...
RE: Vibration of a recip. air compressor
Fe
RE: Vibration of a recip. air compressor
DO you know what rates the mounts are in the radial as ooposed to axial directions?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vibration of a recip. air compressor
RE: Vibration of a recip. air compressor
You only included rate curves for the isolators, no accelerometer plots.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vibration of a recip. air compressor
Attached are the accelerometer plots.
Murtaza
RE: Vibration of a recip. air compressor
Were the "as built" measurements made while operating on the same rig?
With the drastic change in frequency content ( many more spikes at higher frequencies) my hunch is something other than just the isolator stiffness changed between tests.
I'd make some measurements while striking the (power off) compressor at the CG with a semi soft faced hammer. If my analyzer has a peak hold function I watch to see how the spectrum fills in during the striking sequence. Also I like to capture a time trace to see if the response to each strike resembles a decaying sinusoid.
RE: Vibration of a recip. air compressor
Then you need to start building a system model, at least in your mind if not on paper. As Tmoose implies, a modal approach is the most straightforward way. You can use forced response (ODS) data instead but it will lead to puzzling results like softer mounts giving more vibration.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vibration of a recip. air compressor
I have attached a sample ODS to illustrate the understanding. In this partiular case, the expected deflection shape (you would know this from your designs) is a second order vertical, while I measured an ODS corresponding to the rolling deflection shape, which is nothing but the rigid body roll mode shape coinciding with the second order excitations.
Jeyaselvan
RE: Vibration of a recip. air compressor
RE: Vibration of a recip. air compressor
The sum of unbalanced inertial forces of (+) crank (and any other) counterweights, (-) whatever the equivalent "piston" inertia is, wiht offsets and ???.
Crank counterweights have to be substantial to cancel 1/2 of the reciprocating hardware.
h
And 1/2 is close to the compromise chosen for many 1 and 2 cylinder engines,as anything else starts to favor vertical at the expense of horizontal, or vice versa.
Is the motor face mounted to the compressor? Or, is the frame required to resist the (pulsing) torque required to spin the compressor?
I'm still interested and concerned about the actual floor mounting vs test rig.
Dan T