Doubt in Noise Vibration and Harshness (NVH)

[k.udhay]

What does a first / second / nth order frequency mean physically?

 

[462chevelle]

first order would be 1 vibration per cycle, 2nd would be 2 per cycle. a v8 combustion engine will have a normal 4th order frequency. think of a tire with a lump on it for first order then think of a tire with 2 lumps on it for second order.

 

[k.udhay]

Hi 462chevelle. Thanks for your reply. But can you kind of explain me in detail pl.? You can assume me as a person completely new to NVH.

 

[xxChrisxx]

There really isn't anything more to it than that.

An order is a frequency that changes proportional to the reference speed. The number of the order is how many events occur per cycle.

For example:

A vibration that occurs once per rev would follow this frequency. It is said to be first order. 1st order vibration is typically an out of balance.

A shaft rotating:
600rpm would have a frequency of 10Hz
1200rpm would be 20Hz
2400rpm would be 40Hz

Because 600 revs per minue / 60 seconds = cycles per second

A vibration that occurs two times per rev is 2nd order and would have
600rpm = 600*(2/60) = 20Hz
1200rpm = 1200*(2/60) = 40Hz
2400rpm = 2400*(2/60) = 80Hz


Lets suppose that the shaft we are referencing against is an input chast of a gearbox, with input teeth:

1st gear: 13 teeth
2nd gear: 21 teeth
3rd gear: 33 teeth

And we are detecting a whine of
1000rpm = 216.66Hz
2000rpm = 433.33Hz
4000rpm = 866.66Hz

We can do the same calculation in reverse, to work out the order of the whine, and thus what gear is causing the whine.

216.666/1000*60 = 12.9999 = 13th order

The whine is occurring 13 times per rotation of the shaft, which corresponds to the 1st gear driving teeth.

 

[k.udhay]

Thanks a lot, Chris. I am clear about order of frequency.
I am just extending this question to a different topic. What is Gear whine by the way? Does it have a definition? What are the conditions for whine to occur? Thanks.

 

[462chevelle]

If you have an improper set up rear differential in a vehicle, whenever you are accelerating on the drive side if it can whine. When you let off of the throttle it will quit whining, because it is on the coast side. You can also refer to manual or automatic transmissions. Manuals are usually the worst when it comes to whining. Whenever you set up a differential. The pinion depth must be precise, so that when you install the carrier the teeth mesh correctly. You also have to consider the backlash(distance between the gears.)

 

[k.udhay]

Thanks, 462chevelle.