What collection specification does a balancing kit use?

[mcdm]

Recently I flagged up a fan unit for balancing

Motor 01 h = 3.7 mm/sec rms
Motor 01 v = 2.8 mm/sec rms
Motor 02 h = 5.7 mm/sec rms
Motor 02 v = 5.4 mm/sec rms
Motor 02 a =1.2 mm/sec rms
Fan 03 h = 7mm/sec rms
Fan 03 v = 5.1mm/sec rms
Fan 04 h = 7.2mm/sec rms
Fan 04 v = 8.1mm/sec rms
All the above were not overall values but taken at the units rotational speed main peak

I happened to be on site when our balancing technician arrived; I set my data collector parameters to collect 60000 cpm 800 lines resolution velocity rms the levels I recorded were very similar to the above. However the levels recorded on the balancing technician’s equipment were different, he didn’t take any on the motor but on the fan, mind you it was a struggle to get him to take vertical measurements on the fan bearings. they were as follows.

Fan 03 h = 4mm/sec rms
Fan 03 v = 4.5mm/sec rms
Fan 04 h = 4.7mm/sec rms
Fan 04 v = 5.3mm/sec rms

I know I am collecting data over a vast frequency range 0-60000 cpm and that there can be a difference between transducers, lengths of cables, etc

I can understand that if a machine is vibrating at 8.1 mm/sec rms the results between two types of data collectors under similar conditions can be close but not exact.
But why is there such a vast difference between a data collector and a portable balancing machine.
What collection specification does a balancing kit use?
Just to throw a spanner in the works both sets of equipment were recently calibrated

 

[GregLocock]

Could be that you are using a different window, or a different bandwidth, or a different window compensation factor. 1000 Hz is not an unusually high Fmax, we commonly go to 10 kHz (not for balancing admittedly). For instance, his unit might be peak picking (which makes sense for balancing), while yours is measuring the total energy between two cursor lines, which is often a better approach.

HP had a good free booklet "Introduction to Signal Processing", maybe Agilent still give it away.

Or either instrumentation chain could be defective!



Cheers

Greg Locock

 

[mcdm]

thanks greg Valid points, as soon as I can I am going to set up a test rig and compare data levels between a data collector and a portable balancing kit. I will set the data collector in balance mode copying the portable balancers parameter settings. the figures quoted in my question were simplified so as not to bore everyone to death. thanks for your input on this issue. As I am a novice within the vibration field, feel free to comment as I definitely need help by the way who are HP/AGILENT have you got a link
regards mike cooney