Westek Cooling Tower Gearbox Vibration

[vibtech]

We have a double reduction gearbox with an input speed of 1790 rpm,intermediate speed of 447 rpm and output speed of 96 rpm. Gearbox Parameters are as follows: HS pinion 14 teeth, Inter gear 56 teeth, Inter.pinion 21 teeth and LS gear 97 teeth. It has 8 blades and 4 struts.
I experienced an increase in vibration within the last couple of months. From July to the beginning of Oct. the vibration went from .05in/sec, to .177in/sec. There was a dominant peak at 644 rpm that raised the overall vibration. This particular peak did not show up before, but as the vibration increased, so did the peak. I notified the mechanics and they did a visual inspection on the gearbox. They told me that they scraped some scale off the jack shaft and tightened a couple of base bolts. It seemed to help because vibration went down to .08in/sec. Now my question: Was the loose base bolts causing the gearbox to flex which in turn caused the peak in question (I believe its interm.gearmesh?) to increase? If so, there is another gearbox that is showing the same peak dominating,but with higher vibration. I would greatly appreciate any suggestions. Thanks in advance.
Rafael

 

[micjk]

vibtech,
I don't think that internediate gearmesh was your culprit. Input mesh on drive pinion = @417.6 Hz where {(1790/60) X 14 = 417.66 = (447/60) X 56 = 417.2} Output mesh = @156.3 Hz where {(447/60) X 21 = 156.3 Hz = (96/60) X 97 = 155.2Hz}. I think what your are dealing with a combination of looseness, misalignment? and blade pass.
Bp = 96 X 8 = 768/60 = 12.8 Hz. Your dominant frequency was 644/60 = 10.7 Hz. These frequencies are much closer than the input/output Gm's were.
Any other input, I'm still learning,
Regards,
MICJK

 

[vibtech]

Thanks for your reply micjk.
Mechanics did an inspection on the fan and found that there was a couple of gearbox base bolts loose. They tightened them and also scraped off scale off of the blades and shaft. The 10.7 peak went away, and the vibration went down. It seems that the peak was coming from possible loose base bolts. There is another fan that is showing the same peak, I let you know if they find the same problem.

Thanks again

Rafael