Frequency Range for Vibration Testing

[rkuchi]

What is the frequency range normally employed for vibration testing of diesel engine components (10Hz to 2kHz) normally for providing PSD specification (Shaker Testing)? This is mainly for checking durability/reliability of a component.

 

[GregLocock]

Depends on the configuration of the diesel, its usage cycle and the location of the component.

I'd expect you'd need to go to at least firing frequency at the red line, and probably 3 times that.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CESSNA1]

RKUCHI:
Electrodynamic shakers typically have a range of about 5 to 2000 Hz, but can go to about 3000Hz as a limit. What exactly do you want the test to do? Is there a problem with a particular component? I have not heard of doing this on a component level for reliability/dependability. Usually the entire engine is tested to identify problems and concerns and then they are remedied (hopefully). Usually there is so much going on in reciprocating machinery it is difficult to identify what components see what loads.

Regards and good luck
Dave

 

[rkuchi]

Thanks guys for your answers.

What we normally do is we collect acceleration data on a component while sweeping an engine through its speed range and determine the worst speed condition (high acceleration in low frequencies). I am interested to know as to what is a good frequency range to collect this measurements, say a V6 engine has a max speed of 3600 (max 3rd order freq is 180 Hz). What is normally the criteria.

Say If I take measurements till 2000 Hz, what is the useful frequency range of the vibration data? If a part undergoes any failure what should be the normal frequency range that we should be concerned about - just from experience. How can I make sure that I am not overdesigning the part, say by giving a PSD profile with high overall RMS value and with most of it in the high frequency range (above 500 Hz)

 

[GregLocock]

Well, the proper way to do it is to collect the vibration data from the mounting point on the engine, then use that to set your PSD.

In the automotive world I have never seen a fatigue failure on an engine above 400 Hz, so I'd say 600 would be a reasonable limit. Having said that no doubt next week we'll see something fail at 620 Hz.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rkuchi]

What is the best way to come up with a good PSD based on the acceleration response of the part (at some point). What is an allowable delta dB range on the PSD spectra?

a. When I specify a PSD profile taking into consideration acceleration responses in all three orthogonal directions, where I can specify my PSD profile (vs. Frequency). What is/are the criteria that can be used to come up with a PSD profile?

b. When somebody tests a part with a given PSD profile, how
much can they deviate from this profile

 

[CESSNA1]

RKUCHI:
Unless you have a defined PSD profile by a customer, you are going to have to come up with one. One of the most commonly used is the so-called "Willoughby profile". It goes from 5 to 2000 hz and has a maximum value of .04 G^2/Hz in the flat mid range from 70 to 350 Hz. The up and down slopes are 6 db/oct. You can start with this and adjust it as necessary. You may also want to look through MIL-STD 810. This is more of an environemnt spec than a reliability spec, but you may find something. The next suggestion is to contact the Reliability Analysis Center (RAC) in Rome, New York and talk to them.

As for part b if the deviation is on your part, then it is betweeen you and the customer. The deviation should be based on discussions between you and the customer and the results of trials using various profiles. The shaker control system are usually set to allow plus or minus 3 db on each side of the desired profile. They typically are set to shutdown at a 6 db variation.

Regards
Dave

 

[vanstoja]

A classic paper on diesel engine vibration response is
Marples,V., 1977, "On the Frequency Content of the Surface Vibration of a Diesel Engine", J. Sound and Vibration,Vol.52,No.3,pp.365-386. He tested an indirect injection, 4-cycle engine and took 1/3 Octave Band vibration spectra out to 16KHz but focuses on the frequency range from 0-5Kz which shows the highest level wideband peak in the 2500-3500Hz range. He says "the apparently broadband spectra in the region 500-5000Hz has been shown to be composed of many closely spaced, pure tone components". Narrowband (5Hz filtered bandwith) signatures of engine sidewall vibration show discrete engine-ordered peaks out to 2000Hz with humps at higher frequencies apparently due to structural resonances. For PSD modeling for shaker tests, a significant conclusion is "In broad terms the 0-5KHz spectrum of sidewall vibration is similar in shape for all speeds and loads. The maximum level increases by about 4dB/Octave with engine speed. There is relatively little additional increase with load, the change manifesting itself more by a shift of energy from high frequency(greater than 3.5KHz) to lower frequency".