Vibration criteria for machinery. 4

[3doorsdwn]

I'm a structural engineer (who occasionally designs some frames and foundations for vibrating loads), so I thought I'd pose my question here: In a lot of literature (on dynamic loading), a lot of charts are given for acceptable vibration criteria for humans and machinery.......Question: where does that criteria come from for machinery? What is the criteria? Controlling wearing of internal parts? Cracked housing? All of the above?

Also, some people have indicated to me that this criteria originates from the UL listing or manufacturer for the machine determined through failure testing. In the case of the UL: do they figure it for individual machines or for machine categories (i.e. turbines, compressors, etc.)? I've never seen this info in a UL handbook before.

Thanks in advance.

 

[micalbrch]

Vibration standards are made for individual machines. A good overview can be found in the attached file.

 

[3doorsdwn]

Thank you for that info.

A quick follow up question here: I've read through and I'm still not certain what the controlling criteria typically is. Is it displacement? Wear of parts? proper function of equipment? All of the above? I'm not certain if I got that (my apologies if I missed it).

 

[MintJulep]

Typically the criteria are about a machine transmitting vibration into the mounting structure or building.

 

[3doorsdwn]

Typically the criteria are about a machine transmitting vibration into the mounting structure or building.

I always thought that was covered by charts like the Reiher-Meister plot (i.e. that measured human perception to vibration).

In a lot of literature (that give "Criteria for vibrations of rotating machinery") there will be plots that graph frequency [on the x-axis] vs. amplitude [on the y-axis]..........where the lines intersect will be zones that fall into several categories: "Dangerous. Shut it down now to avoid danger." (or) "Faulty. Correct within 10 days to save maintenance dollars." And that just leaves me wondering [as a non-mechanical guy]: what is the danger, or what is faulty? Can the machine not run effectively with that kind of shaking? Is stuff wearing out inside?

 

[electricpete]

A typical equipment standard specification will specify limits for all three: displacement, velocity, acceleration. Displacement is limiting in the low frequency ranges, velocity in the mid-ranges, acceleration in the high frequency ranges.

For example NEMA MG-1 (going from memory)
< 2 mils pk/pk < 1200 cpm *
< 0.15 ips pk/0 1200 < f < 24,000 cpm *
< 0.8 g's pk/0 > 12,000 cpm *
* frequency ranges assume single-frequency sinusoidal vibration... often not the case.

A vibration analyst naturally would look not only at the "overall values" as above, but the specific frequency content and what it says about what is going on inside the machine. (0.1 ips due to rolling bearing defect is probably a concern... 0.1 ips due to unbalance is probably not). Also they look at trends and comparison to similar units. All of which is to say that predicting failure is not an exact science, so they look for deviations from what is normal. The goal is managing equipment reliability.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Figure 3 of attachment linked above resembles a limit on displacement, velocity, acceleration. The displacement limit applies below fx (and gives a linearly increasing appearance on a velocity chart), the velocity limit applies between fx and fy (gives a constant limit on a velocity chart) and the acceleration limit applies above fy (gives a linearly decreasing appearance on a velocity chart).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rmw]

Sorry, I don't know much about buildings, but I do about the rotating machinery. The vibrations causes repetitive movement of the parts which causes fatigue in the parts. Fatigue leads to failure. Most rotating machinery has some mighty expensive parts and when they fail it is a lot of drama. Speaking of buildings, I have seen rotating equipment failures do some real harm to buildings too.

rmw

 

[3doorsdwn]

Sorry, I don't know much about buildings, but I do about the rotating machinery. The vibrations causes repetitive movement of the parts which causes fatigue in the parts. Fatigue leads to failure. Most rotating machinery has some mighty expensive parts and when they fail it is a lot of drama. Speaking of buildings, I have seen rotating equipment failures do some real harm to buildings too.

So do they typically just test equipment (to failure) to establish this criteria (and just basically sees what goes first)?

Yeah, the building (and human perception) side I know fairly well......but being structural I wasn't too sure about the charts for equipment. Thanks.

 

[Tmoose]

I believe its all pretty empirical. There are plenty of machines for which "excessive" vibration is a symptom something is mechanically wrong internally, rather than really doing much harm by itself.

Exceptions include auxilliaries like switches and wiring, which can suffer from vibration originating exterally

Dan T

 

[vpl]

I've just been looking at some pictures of a destroyed main generator turbine at a 1000+ MWatt power plant. There are missing turbine blades, bent blades, sheared blades. Luckily this one only went partially through the turbine casing -- I've heard about them going through buildings. The turbine had higher-than-normal vibration, that plant management thought they could live with for a couple weeks
-- the equipment eventually told the plant management otherwise. Unfortunately, the pictures are proprietary and I can't share them. Wish I could as they answer pretty effectively what happens when high vibration occurs.


Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.