harmonic
harmonic
(OP)
Can anyone out there explain for me why the bearing defect frequency and its harmonics amplitude decreased with frequency. Thanks.
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RE: harmonic
What I think you are asking is why amplitudes of bearing defects decrease with a decrease in machine speed?
There is a rule of thumb for that when the speed of a machine is doubled the vibration squares. The inverse would thus hold true
RE: harmonic
Well, firstly it depends what you are measuring, if you keep on differentiating then you can change that round. However, if we limit ourselves to velocity and acceleration it is generally true.
I'd answer it by saying that the response to an impact can be calculated as the sum of the modes at that point (modal superposition), and that by its nature a low frequency mode represents a bigger proportion of the signal than HF modes. But I'm not wildly happy about that.
Another observation that I don't necessarily agree with is that damping of high frequency modes tends to be greater than for low frequency modes.
Probably the real reason this is hard to answer is that it is a general trend, rather than a truth, and so any theory can be knocked down by referring to a specific case.
Cheers
Greg Locock
RE: harmonic
RE: harmonic
One thing that should be thrown into the mix of course is a consideration of which unit we are talking about: displacement, velocity or acceleration.
The exact same signal can be described as either
decreasing with frequency (in displacement)
constant with frequency (in velocity)
OR
increasing with frequency (in accelration)
RE: harmonic
Regards,
MICJK
RE: harmonic
Thread384-34375
Addressing normal spectrum (not demod), I'll repeat my statement that the linear model (as interpretted by electricpete) predicts the spectrum of periodic impacts at a fault frequency is the product of three other spectra:
#1 - Fourier transform of a single impact measured as force at the point of impact
#2 - Transfer function of the system reflecting attenuation of certain frequencies as they travel through the damped/mass spring system from point of impact to transducer
#3 (obvious) - A function which is 1 at harmonics of fault frequency and zero everywhere else.
#1 and #2 will both decrease toward zero as frequency approaches infinity for any real-world signals and systems. For #1 no signal has infinite frequency content unless it has zero rise time. For #2 I guess I was under the impression that most real-world linear systems have limited bandwidth and will provide increasing attenuation as frequency approaches infinity.
I think vanstoja was addressing some kind of attenuation as well in his comments about waterborn vibs. I didn't quite understand the comments about structureborne noise.
Sorry to ramble.
RE: harmonic