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OLi

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Reply with quote  #1 
Anybody having some guides or documents on how to reduce vibrations from a small 2 cylinder 4 stroke engine in a light structure? Levels are impressive and one level rubber mount isolation is applied as a original solution but they are in sort of shear mode so they work a bit problematic. It will be added another layer in more conventional compression style isolators next month and maybe it will be enough but if not I am looking for any other ways. I normally look at industrial diesels a few reciprocating compressors and hydraulic pumps but this is a different case. I was thinking a more effective flywheel but it may give other problems and adding some dampers but it may transfer vibration to the frame and that was not wanted. I have balanced 1MW diesels but I believe those flywheels were requiring balancing so it was not reducing firing impulses. I am only providing the wireless measurement system in this case so I am not implying to be the magic explosion motor whisperer part from discussing normal isolator application but maybe it is a different problem to discuss if you seen some solution. If you look at the mount of a engine in a car there seems to be some common designs but I don't know what is good and this is another kind of vehicle that have other restraints. 
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Dan Timberlake

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Reply with quote  #2 
Hi Oli,

What is the frequency of vibration?
What is the orientation of the cylinders?
90 °, inline, opposed ?

What is the orientation of the crank throws?

Are you looking to reduce the vibration on the engine, or the vibration transmitted across the isolators to the vehicle?
How is the power transmitted from the engine to the drive wheels etc?
Constant rpm, or varying over a large range like most cars and motorcycles ?

Got some pictures?

regards,

Dan T

OLi

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Posts: 1,915
Reply with quote  #3 
Cylinders are inline, it looks like a very common engine layout. It looks like 1 and 2xRPM engine speed dominate. A tooth belt drive a gearbox. Special is that it is a small helicopter. We have measured on occasion on jet engine driven more full size helicopters like Hughes and my brute description is that from my view they behave like a flying fan, bladepass dominate. That is not the case here, engine dominate in the frame. I have not seen so much data yet and not from flying, we have 5x3-axxis sensors. Anyway it is the isolation of the engine from the frame that is the goal w/o having the engine shaking to pieces and no modifications that mess with certificates of the engine or reducing the motor vibration to achieve the same lower frame vibration. So when operating, speed is pretty much constant controlled by a governor. It looks like this but maybe smaller.

frontimg2.jpg 


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Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
Walt Strong

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Posts: 889
Reply with quote  #4 
Olov,

A few search notes:

First order Second order vibrations 

From <http://www.eng-tips.com/viewthread.cfm?qid=366049>

 

Engine Mount Analysis Methodology

http://altairatc.com/india/previous-events/2011/presenters/RL_07_Engine_mount_analysis_methodology_Piaggio.pdf

 

Search: small 2-cylinder 4-cycle engine vibration mounts

 

LORD SMALL ENGINE MOUNTS

From <http://rubberpartscatalog.com/home/products/Vibration-Control/Small-Industrial-Engine-Mounts>

 

http://www.compactradialengines.com/

 

Search: ultralight 2-cycle engine vibration mounts

LORD ENGINE MOUNTS FOR ULTRALIGHTS

From <http://www.aircraftspruce.com/catalog/eppages/lordMount.php>

 

https://www.bing.com/images/search?q=ultralight+2-cycle+engine+vibration+mounts&qpvt=ultralight+2-cycle+engine+vibration+mounts&FORM=IGRE

I believe your engine photo came from the reference above and indicates the engine is a 2-cycle. The 2xSS vibrations from a 2-cylinder 2-cycle engine is caused by mechanical reciprocating forces and from pulsation at firing frequency. A few tips that you probably already know:

1) Do not use typical/catalog transmissibility data based on infinitely rigid foundation -- it is not a rigid structure!
2) Select fail-safe vibration mount/isolator design -- helicopters do not fly well after engine falls off!
3) Try to locate vibration mounts so linear and rotation forces act on center of engine gravity
4) Tune vibration mounts so forcing frequencies do not match/excite natural frequencies
5) Have fun, but let someone else (youngster) take the first few flights!!

Walt


OLi

Sr. Member
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Posts: 1,915
Reply with quote  #5 
Thank you Walt! It is luckily not human grade ;-)
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Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
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