4 x bevel set frequency only 1

[GregPerry]

I have a 2 stage gearbox and a vibration consulatnt has measured a 4x bevel set frequency (first or high speed stage of the gearbox)in the vertical direction only. All other frequencies 1x, 2x and 3x are all very low. The 4x has a a RMS velocity of 11.23 mm/s.

Any idea as to what this could be.

Greg.

 

[Hatch]

I am quite puzzled by your info. What do you mean by 4X bevel set frequency. Is it the 1st gearmesh frequency which would be a very high frequency of around 2800 Hertz assuming an 1800 rpm input and 50% speed reduction. Or do you mean the 4X (4 times) the input shaft frequency which would be about 120 Hertz?

C. Hugh (

http://www.Hatch.ca)

 

[GregLocock]

Well, whichever it is , the reason that one particular harmonic stands out n the casing is often that there is a casing resonance at that frequency, or a resonance of the transmission components themselves.

Typical resonant frequencies for the body of an automotive alternator are 1400 Hz, so the frequency you estimate seems high, but not ridiculous.

On the other hand if you assume an 11 tooth pinion and 1800 rpm motor then 4th harmonic is 1320 Hz.

Cheers

Greg Locock

 

[GregPerry]

Thanks for the input.
Some additional information:

Input speed = 1480 rpm
Bevell pinion has 14 teeth
Bevel Pinion frequency (measured)= 349 Hz
4x (measured) = 1396 Hz

If this is a resonance frequency it will obviously not result in any damage in the long term?

Greg

 

[Hatch]

Where was the vertical measurement taken? If it was on a bearing, I would be very concerned. 11.23mm/s rms @ 1396 Hz is about 10g's which is a critical fault on most charts.

On the other hand 10g's on the gearbox casing indicates a very noisy gearbox and if the casing is in resonance I would expect to see cracking at some point.

You should establish if the 4X component is steady or changes with speed. Shut down the driver and monitor the 4X freq. If it shifts, you don't have a resonance.

A frequency of 1390 Hertz could be related to the bearings. Calculate all the bearing fault frequencies and compare them. Bearing faults will be related to the operating speed.
C. Hugh (

http://www.Hatch.ca)

 

[Yats]

I have a similar problem with 4X spike at 0.68 ips but generally in the horizontal readings. I got some 0.5X harmonics as well, 4X, 4.5X, 5X and 5.5X at lower amplitudes.My gearbox is a simple speed increaser, single. What I've noticed is that 4X has always been a prominent spike in readings whether the overall was 0.1 ips or 0.8 ips. I suspect its an inherent frequency. There are no components rotating at 4 times the incput or output speeds to my knowledge. It is highly unlikely the bearings would produce a frequency at exactly 4X but I have unclear bearing data to verify this for sure. I somewhat agree with Greg Locock on the reasonance theory. I started a thread on my problem. I invite you to check it out.