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Molding your own engine mount isolators

Flow

Member
Hi folks,
I am investigating materials selection for this at the moment.

Quick question on dynamic materials analysis;

For a given tan delta and impulse energy the mount with the lower durometer absorbs the greatest energy from the system as its compressive modulus is lower and thus movement is over a greater range?

So tan delta / compressive modulus is equivalent to energy absorption potential for an elastomer given for a given impulse, preload compression, frequency and temperature?

Is this correct?
 
What time is it?

Ever heard that phrase "Don't tell me how to build the watch, I just want to know what time it is!" ? My brother says it a lot

I personally think you might be posting on the wrong forum, but knowing this crowd you will likely find a Material Science Engineer who will answer your question!

Looking forward to learning more about Tan delta and the Compressive modulus ;) (heard their sound really rocks... )
 
Engine mounts

I am sure that the manufactures have many years of experience and and a lot of trial and errors and failures. For a few hundred dollars you get the benefit of all this.

Steve
 
As a long-time powertrain noise and vibration engineer specializing in mounting systems, I can tell you this is a very deep and complex technical subject. For some light bedtime reading, try "Automotive Vibration Control Technology: Fundamentals, Materials, Construction, Simulation, and Applications by Trelleborg/Vibracoustics." Although, I would not personally undertake designing and manufacturing my own engine mounts* for something meant to leave the ground with me or anyone else in it. :rolleyes:

(*Engine "mounts," in automotive terms, are the rubber bits. The supporting structures are referred to as the frame, subframe, engine-cradle, and off-engine brackets.)
 
Hi folks,
I am investigating materials selection for this at the moment.

Quick question on dynamic materials analysis;

For a given tan delta and impulse energy the mount with the lower durometer absorbs the greatest energy from the system as its compressive modulus is lower and thus movement is over a greater range?

So tan delta / compressive modulus is equivalent to energy absorption potential for an elastomer given for a given impulse, preload compression, frequency and temperature?

Is this correct?

I once had an engineer at Hartzell explain to me that in one particular case, I believe it was a Cessna 210 with a bed mount, they found that solid aluminum mounts translated the least amount of vibration to the airframe in that particular test. Given this is not practical, one could think that a harder mount may improve vibration characteristics. Others have tried pouring their own urethane mounts with mixed success so I encourage you to experiment and share what you find. I believe there is little risk in experimenting with this and much of it can be done on the ground.
 
Given this is not practical, one could think that a harder mount may improve vibration characteristics.

This is an oversimplification and broad generalization that only holds true in specific circumstances. The installed powerplant will have 6 rigid body modes which are a function of the MMOI of the powerplant, the location of the mounts, and the stiffness of the mounts. If any of these modes is in the frequency range of engine/propeller operating excitations, then the system will be in resonance and likely cause some objectionable vibrations at those frequencies. If you stiffen the mounts and thus move those modes above the operating range, then yes you could avoid that resonance... but at the expense of reduced isolation to all other frequencies. That may or may not be the right answer for a given system. Isolation occurs only at frequencies above the resonant modes, so the best isolation would be with a softer mount that puts all rigid body modes below the operating range. That solution comes with other tradeoffs and considerations. As I said, designing a mounting system is complex.

Others have tried pouring their own urethane mounts with mixed success so I encourage you to experiment and share what you find.

I know this is common for hotrods and the like, but have people really done this for aircraft engines?

I believe there is little risk in experimenting with this and much of it can be done on the ground.

I have to respectfully disagree. I would not attempt this without input from the airframe design engineer. Beefy steel street racers may have the strength to survive stiff urethane bushings for a while, but before long something is likely to break. I can't imagine our airframes would stand a chance against a hard-mounted engine, but again, I'm not an airframe designer.

Please just use extreme caution whatever you do.
 
I look forward to hearing some as well. Sounds like some serious Acid Jazz. :D

Acid jazz, also known as club jazz, is a music genre that combines elements of funk, soul, hip hop, as well as jazz and disco. Acid jazz originated in clubs in London during the 1980s with the rare groove movement and spread to the United States, Japan, Eastern Europe, and Brazil.

Lol, I had no idea that was a thing! Now I gotta find some samples...

Talk about thread drift. :D
 
The idea of spending $500+ for a set of rubber blocks does not easily resonate with me. Even considering the engineering that goes into their design, that was presumably for a given engine/airframe certified application that was not on an RV. What is the objective of the OP? Is it to develop a more economical DIY set or to fine tune the characteristics of the mount to his specific engine/airframe?
 
Rubber isolator compounds are not trivial. To the OP:
- Durometer will effect the resonant frequency, not the storage or dissipation of energy.
- Tan Delta is a measure of the energy absorbed or dissipated.
- the softer it is (lower durometer) the lower the natural frequency. The lower the natural frequency, the better isolation and the easier it is to formulate a rubber that will not dissipate energy, but will rather just isolate. The downside is this is the condition with the highest strain and, the higher the strain (deflection) the higher the stress, and the more material (size) you will need to have a mount that will last and not come apart. and if it does crack or split, that it has a fail safe design so it doesn't just come apart, but loses its isolation ability while keeping the engine on the plane.

There is a LOT that goes into mount design. It includes the rubber formulation, but also the hard (metal) parts around the mount and their geometry and how they influence the movement and overall stiffness of the mount. the formulation of the rubber itself is a recipe that is developed by elastomer specialists and is what they get paid for. They are like witches in the back room over a cauldron putting in a pinch of this and a dab of that to get just the right combination of durometer, tan delta, strength, temperature resistance, chemical resistance, etc., that they are looking for. It would be an interesting learning exercise, but of all the things that you can experiment with on an airplane, this is one where I would just get the production mounts.

Tim
 
Acid jazz

Lol, I had no idea that was a thing! Now I gotta find some samples...

Talk about thread drift. :D

Sorry about the thread drift.
I'm a big audiophyle. Not crazy about acid jazz but I have some. Sounds kinda like a band of jazz musicians all playing lead on different keys! :D
If you love guiter, look up Alexander Misko.
https://alexandrmisko.com/
 
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Hi folks,
I am investigating materials selection for this at the moment.

Quick question on dynamic materials analysis;

For a given tan delta and impulse energy the mount with the lower durometer absorbs the greatest energy from the system as its compressive modulus is lower and thus movement is over a greater range?

So tan delta / compressive modulus is equivalent to energy absorption potential for an elastomer given for a given impulse, preload compression, frequency and temperature? Is this correct?
I don't know your back ground but In a previous life I was an aerospace engineer and have some experience with vibration isolation (Lockheed F35). Do yourself a HUGE favor and buy some LORD mounts. The design, testing and experience of vulcanized elastomeric "rubber" is not something you can make at home in the kitchen oven... It requires very specialized analysis, design, materials, manufacturing not DIY land. Sorry. Trust me, LORD mounts with the silicone inner isolator are amazing (and expensive but worth it). LORD has the secret sauce over decades (since 1940) of experience and making vibration isolation for everything. I worked with LORD on a project and was impressed, amazing company.
 
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Ever heard that phrase "Don't tell me how to build the watch, I just want to know what time it is!" ? My brother says it a lot

I personally think you might be posting on the wrong forum, but knowing this crowd you will likely find a Material Science Engineer who will answer your question!

Looking forward to learning more about Tan delta and the Compressive modulus ;) (heard their sound really rocks... )

What a great way to end the day. I’ll now go laugh myself to sleep.
 
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In construction, Hockey pucks are used extensively to mount transformers that vibrate. They are also used to support floors of sound booths and recording studios. I wonder if they are any good for engines? They do well in cold, but I dunno about heat, oil, yadda yadda yadda :p
 
When I was in elementary school we had a contest to mount an egg in a cardboard box and have it survive a drop from the roof of the school. My solution was to suspend the egg in the middle of the box inside a section of pantyhose stretched tight and fastened to either side, with foam lining the box as a travel limiter. Worked a treat! Maybe you could try something like that? :p
 
In construction, Hockey pucks are used extensively to mount transformers that vibrate. They are also used to support floors of sound booths and recording studios. I wonder if they are any good for engines? They do well in cold, but I dunno about heat, oil, yadda yadda yadda :p

Actually hockey pucks get much harder when cold. They respond differently when cold then when first put in play. There is a very noticeable difference in response from a slapstick shot.
 
Thanks folks, great comments but no one answered my original question!! :)

Have a look at the DMA results of a couple of materials on the short list.

My biggest concern with using low rebound compounds is that they might cook them selves.

I have a full set of 800 bin FFT spectrum analysis for every 50 RPM of the BEFORE with the basic natural rubber engine mounts supplied by the manufacture.

So:
For a given tan delta and impulse energy the mount with the lower durometer absorbs the greatest energy from the system as its compressive modulus is lower and thus movement is over a greater range?

So tan delta / compressive modulus is equivalent to energy absorption potential for an elastomer given for a given impulse, preload compression, frequency and temperature?
 

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  • WR58518 DMA Summary.pdf
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Thanks folks, great comments but no one answered my original question!! :)

So tan delta / compressive modulus is equivalent to energy absorption potential for an elastomer given for a given impulse, preload compression, frequency and temperature?
Ans: Yes

Now make them and test it. Report back with your data. Good luck.
 
Sorry about the thread drift.
I'm a big audiophyle. Not crazy about acid jazz but I have some. Sounds kinda like a band of jazz musicians all playing lead on different keys! :D
If you love guiter, look up Alexander Misko.
https://alexandrmisko.com/

I somehow just saw this reply. I asked Alexa to play his music... she started with Careless Whisper, then Billy Jean. So far so good!

Reminds me of a couple months ago when I came out of Kroger, looking for those elusive sun shades ;), and there was some guy set up on the sidewalk with an electric fiddle, or violin maybe, not sure.. anyway he was playing along to acoustic versions of all kinds of popular songs and it was amazing. It was a really heart-lifting experience after living in such an isolated world for so long and just coming out of another cold grey Michigan winter to boot. I stayed for a few songs, recording on my phone, tossed him $20, and then had to get my groceries home.
 
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