Reduce pitching motion with engine-compressor assembly 2

[jeyaselvan]

I am working for a compressor design & manufacturing company, wherein we use disel engine driven compressors(screw compressor). Usually the drives are mostly 6cyl and few cases of 4 cyl engines. Recently we had a new application with 3 cyl engines.

We are observing a lot of pitching vibration in the 1X order and also rolling vibrations in the 1.5X. I understand with the 3cyl engines, the primary moments are unbalanced inherently to its design. The pitching vibration is also dominant since the acceleration-forces of the end cylinders will not cancel each other. But this is very dominant and am concerned. The vibration levels on compressor are almost twice than the levels we normally observe.

Since I have limited knowledge on engines, I wish to know how to mitigate this situation. Is there anything I can demand from the engine manufacturer on this? Is it possible to minimise these forces?

Thanks

 

[ivymike]

you could pick up an engine with an internal balancing mechanism. aside from adding a mechanism, you're kinda stuck with the accelerations of the pistons being what they are.

 

[romke]

there is not much the engine builder can do, apart from using some kind of counterbalance mechanism that cancels out most but not all of the vibration - at a rather high cost. i do not think he is willing to do that.

another solution might be the use of a fluid coupling between engine and compressor. that way you can isolate the vibration from your compressor and also, if you want to, regulate the startup characteristics by varying the fill percentage of the coupling. voith is one of the major suppliers of that kind of startup couplings.

 

[ivymike]

another possibility might be to add considerable mass to the engine, or if it's a constant-speed application, add a tuned vibration absorber (frahm or similar) to counter the pitching.

 

[Tmoose]

Are the motor and gen hard mounted to a common frame? Do you have a means of measuring the resonant frequencies of the 6 DOF? If a mounted frequency is not <.5 X the forcing frequency, the a certain amount of amplification may be occuring. Also Depending on where the mounts are located, and their tuned frequencies, the rolling motions might be reduced by relocating the mounts higher, up by the CG. Then again, 1.5 orders sounds like 4 stroke firing frequency, and that might be the torque reaction to the widely spaced power strokes vs flywheel inertia

 

[GregLocock]

What problem is the pitching and rolling causing? Noise and vibration in the mountings, or what?







Cheers

Greg Locock

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

 

[jeyaselvan]

Thanks all of you for your feedback

The engine compressor assembly is mounted on antivibration mounts and is analyzed for the vertical mounted frequency (a little bit coupled) and tuned to 21Hz. I did measure (through force/response and response alone)the mounted resonant rigid body modes (in vertical direction) and found that the mounted resonant frequencies on engine is about 21Hz and on compressor is about 12 Hz. I expected that same to be 21 Hz through out,(for an assembly mode) but is not.Probably the stiffness (even though rigidly connected) between the engine block and compressor housing has an influence on it as well.

I even tried to increase the stiffen the mounts, but still that could not influence the 12Hz. I was thinking of compromising on the isolation and was expecting the vibration on engine & compressor to come down. But this yields no benefits.

The coupling is a resilient rubber coupling (Vulkan) and this selection has been done after doing a TVA.

Greg : The problem in pitching is that the vibration levels on the compressor are more than twice that of the compressors we are familiar with, the 4 & 6 cyl engines.
The dominant frequencies of vibration on the compressor corresponds to the engine 1X (pitching, strong),1.5X (rolling strong) and 2X (not as high as 1X & 2X). Usually we see only the engine excitations with 4 & 6 Cyl. Hence I expected only 1.5,3 & 4.5X.

 

[jeyaselvan]

Ivymike

The application is a varying speed from 1600 to 2800 RPM. Hence this limits the use of vibration absorbers.

Greg : To clarify you more on the vibration locations, conventinally we measure vibration at bearing locations (typically in compressors, we have 8/6 bearings depending on the gear / direct drives. I am worried since the vibration levels are more than twice than our regular machines. I am worried, since during pitching at 1X, the deflection shape indicates that the non drive end bearings are prone to severe vibration levels.

I have seen earlier cases with 4cyl engines, couplings more prone to failures, attributing to the bouncing due to the secondary forces.

Tmoose
I see you have a point there. the mount selection guy considered only the 1.5X as the minimum excitation for 3cyl engines. Probably with 21Hz mounted frequency, I could well be in the amplification zone of the typical isolation curve.
My problem here is not about the transmitted vibration, still I need isolation to protect the rest of the parts. I am more worried about source (meaning engine - compressor) vibration.

Thanks
Jeyaselvan

 

[SomptingGuy]

You can remove the 1.5x torsionals with a heron balancer, but balancing an I3 is always going to be a compromise.

- Steve

 

[TDIMeister]

Mahle demonstrated a pretty elegant and simple mass balance system for their I3 downsized engine demonstrator. More about it can be read in MTZ's 01|2009 Volume 89 article entitled, "The Mahle Downsized Engine as Technology Demonstrator - Concept, Layout and Design" obtainable from

http://www.all4engineers.com

If re-engineering the engine to accept balance shafts is impractical, 1st order mass imbalance can be reduced by minimizing reciprocating masses where possible; implementing mean-value balancing and reshaping the conrod to move the inertial C.G. closer to the big-end, while 1.5th order excitations are caused by combustion pulses, which can only be dealt with with an appropriate torsional vibration damper. Proper Diesel combustion development with a reduced geometric compression ratio could also help attenuate gas pressure peaks which lead to those excitations.

 

[TDIMeister]

It could also be that for your particular design at 2800 RPM as in your AVI file, you're coming to an eigenfrequency. You should measure accelerations some RPMs above and below this point with accelerometers in a test engine or in a multi-body simulation and take design measures as appropriate.

 

[Tmoose]

Does the deflection shape indicate a rigid body mode at the frequencies of interest?

Transmitted vibration and the vibration or motion of "isolated" component go hand in hand. If a mounted resonant frequency is close to one of the various forcing function frequencies, amplification of the device's motion and force transmitted to ground via mount stiffness will occur simultaneously.
I looked for an online image to show this, but only found the familiar "transmissibilty" curves with the transmitted force being very low when 3x supercritical. What those curves do not show is that the motion of the isolated object is essentially constant > 3x supercritical, as the item rotates around the CG (if unbalance is the driver), which is an ampltude about twice higher than when subcritical.

I don;t know yet what your "severe vibration" levels are.
Rules of thumb like "vibration over 0.3 ips pk cause bearing damage" are derived a bit backward in my opinion. The cause and the effect are switched. I believe Those kinds of limits came from comparing measurement on good running machines and machines with mechanical issues. For instance, bearing housing vibration over 0.3 ips pk at an outer race ball pass frequency would indicate a severely spalled race. The vibration is the result of the damage, not vice versa.

Plenty of machines have bearings sized such that load increases equivalent to 1 or 2 rotor weights ( 1 or 2 g vibration/motion acceleration) don't even exceed the fatigue or endurance limit of their ball/roller bearings.

That is not to say vibration by itself can not cause problems with wires and relays and mounted components. And of course customer perception of a vibration as excessive is a problem all by itself.

 

[jeyaselvan]

Tmoose
The deflection shape indicates to that due to the excitations. I have confirmed this by measuring the deflection shape at two speeds 1600 & 2800 RPM and in both cases I have the 1X deflection shape as pitching and 1.5X as rolling. Also my transmissibility to the base structure is pretty much the same.

As rightly said by you, it is pretty difficult to vye on numbers of vibration velocity measured. I am only trying to ensure that the vibration levels do not exceed the numbers, we are familiar with adn used to. With the 3cyl engine drive, this is 2 -3 times higher and hence the concern.
Also the spectral analysis (and also from the deflection shape) indicates that excitation components on the compressor are primarily correponds to the excitations of the engine.

By the way, If someone can inform how the engine industry in automotive segment works in this kind of issues? Do they declare the NVH levels of their engines? Is it possible & feasible for a user to specify the levels?.Will the engine industry takes this in the right spirit? Can I specify I need the torsional damper & the inertial balancer so that the vibration levels be within the levels I need?

 

[GregLocock]

" If someone can inform how the engine industry in automotive segment works in this kind of issues? Do they declare the NVH levels of their engines? Is it possible & feasible for a user to specify the levels?.Will the engine industry takes this in the right spirit? Can I specify I need the torsional damper & the inertial balancer so that the vibration levels be within the levels I need?"

You certainly can try. I suspect the likely answer will be that if you want low vibration levels then use a sensible engine layout, it will be more expensive to make an I3 as good as an I4 (not very good in its own right) than it would be to fit the I4. As an example Perkins Diesels used to define the maximum vibration at each mounting point.

Secondly you seem to misunderstand how engine mounts work. They really won't have much influence on the vibration of the engine/compressor once you get significantly above the rigid body resonances (12 Hz and 21 Hz in your case (there are 4 others, I suggest you find them)) - in effect the entire unit is floating in air and its acceleration is controlled by F/m.

So, if you were to fit VERY stiff engine mounts, and directly couple the mass of the engine to the bedplate, you might suppress some of the vibration.

Things you can try

1) a better engine layout (ie an opposed twin cylinder, H4 or I4+balancer, or a V6 or I6 or I5)
2) decoupling the compressor from the engine completely except for a suitable drivehsaft
3) a really big mass tied into the compressor

First order in an I3 comes about because the pistons are rocking the engine, and the piston mass is pretty much untunable usually, it has already been minimised.

So, can you tell us your actual vibration limits, or do we just have to guess?

Having said that there are plenty of I3 engines in operation, and so far as I know 1 and 1.5 order are just NVH issues, not durability issues, on them.

Cheers

Greg Locock

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

 

[jeyaselvan]

Greg

Thanks for your feedback.

Regarding the mounts, as you rightly said, I was trying to see whether I can influence the source vibration (may be to an extent possible)by increasing the mount stiffness, something close to rigid installation. I was trying to see whether the impedance of the base/chasis could help in reducing the source (engine-compressor) vibration. In the process, I found that I have no scope in increasing the same, since I will get into the resonant amplification (while idling)rather than at the isolation zone. Of course I do need isolation to protect rest of the parts in the compressor. Thanks Greg for making it more clearer. Yes, I recollect that the impedance of the mounts just do not exist when we go to high frequencies & the whole system floats and respond at its free vibration level, decoupling the engine-compressor from the base.

We work around 8-10 mm/s (upto 1kHz)velocity limits on our compressors driven by engines (mostly they are six and very few with 4 cyl engines). In this case, with 3 cyl engines, it is as high as 25-30 mm/s and the compressor designer guy is a bit panic to clear this machine.

Also as your suggestions on the configurations, in this particular machine, the compressor housing is coupled directly to the flywheel housing and this becomes a good transmission path for vibration. The concern is that I seelarge amplitudes of 1X and 1.5X on the compressor & bearing housing locations, which is making things difficult. Eventhough the pitching does not cause any out of phase movements between the engine & the compressor, the vibration levels on the compressor (lighter) are quite higher compared to that on the engine (heavier)

 

[GregLocock]

OK. we are singing the same song. So, unless you are (un)lucky and you have a rigid body mode within (say) 50% of your running speed*order number, retuning your mounts won't help.

I don't know how you can qualify your compressor for the low acceleration/high velocity regime you'll get from an I3, but it might well be a relatively benign forcing function compared with the second order forcing from an I4, sorry, you will have to do the maths on that.



Cheers

Greg Locock

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