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  #21  
Old 09-29-2015, 07:17 AM
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Most urethanes exhibit high hysteresis...when compressed, they do not quickly return to the previous dimension. This is not a problem given a low frequency cycle, i.e. plenty of time for dimensional recovery. At higher frequencies, it either maintains the compressed shape (which can put freeplay in an oscillating system, see next post),or if forced to follow the vibration, it heats rapidly.

In terms of vibration isolation, hysteresis makes the spring-mass system highly damped. Although that can be a useful property when the goal is mass control while operating at a resonant frequency, it also makes the material a poor isolator at non-resonant frequencies. This is the issue being discussed in previous posts. Given that resonant operation is not a factor in selecting a Lycoming mount, there isn't much reason to accept the compromise.

Because urethanes are engineered compounds, it is possible that the issue can be minimized...but I suspect the nice folks at the vibration control companies have already been there and done that. Cruise the Lord Aerospace catalog for illustration. In addition to 50 pages of vibration tutorial, you'll notice that the mount materials are are natural rubber, neoprene, and a variety of silicone compounds.

http://www.lord.com/Documents/Produc...torCatalog.pdf

Switch to a torsional coupler catalog, and I think you'll find that urethanes are limited to high stiffness couplings, i.e. applications that will not actually deform the urethane disk, diaphragm, or donut very much under load. It's not because they can't make a softer urethane. It's to limit heating due to hysteresis, engine output frequencies being high enough make it a problem.

Summary? Urethane motor mounts are probably a case of reinventing the square wheel.
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Last edited by DanH : 09-29-2015 at 07:25 AM.
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  #22  
Old 09-29-2015, 07:23 AM
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Quote:
Originally Posted by rv6ejguy View Post
This is what I did on mine. I machined the molds allowing for contraction, specified the durometer and they turned out great.
Me too, circa 1997.

And being one of my more stubborn friends, Ross still refuses to accept that urethane is a truly wrong material for a pin-and-hole torsional coupler
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Last edited by DanH : 09-29-2015 at 07:25 AM.
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  #23  
Old 09-29-2015, 08:19 AM
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Quote:
Originally Posted by DanH View Post
...Summary? Urethane motor mounts are probably a case of reinventing the square wheel...
Except for the fact that the Lord mounts sag significantly within months of installation under the weight of a 540, flying or not, and the hard core acro guys are replacing mounts inside 50 hours of use. The Lord technical literature calls out very specific inspection criteria concerning the shear deformation, and it's pretty tight. Dropping $400 bucks for mounts every 12 months is one good reason to look for alternatives.
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  #24  
Old 09-29-2015, 09:22 AM
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Quote:
Originally Posted by Toobuilder View Post
Except for the fact that the Lord mounts sag significantly within months of installation under the weight of a 540, flying or not, and the hard core acro guys are replacing mounts inside 50 hours of use. The Lord technical literature calls out very specific inspection criteria concerning the shear deformation, and it's pretty tight. Dropping $400 bucks for mounts every 12 months is one good reason to look for alternatives.
No argument there. The discussion is about the choice of alternative. Perhaps consider harder neoprenes and other synthetic rubbers, or silicones?
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  #25  
Old 09-29-2015, 09:52 AM
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Point taken. However, considering the conflicting requirements pointed out at the start of this thread, and your point that Lord has probably exhausted every possible option, a direct replacement is a tall order.

The poly I'm using is a reasonably hard durometer of 85 (selected at random), and it is reasonably close to being acceptable from a seat of the pants standpoint. Engineered geometry and durometer selection will likely yield improvement from this initial foray, but even in my wildest dreams of success with poly, I can't imagine it being acceptable for the vast majority of pilots for something like the RV-10 mission.

I'm going to play with it, but I'm going into the experiment expecting some downsides in exchange for "no sag".

It appears that poly can be cast quite easily, but I do welcome opinions concerning other materials that are capable of "home manufacturing".
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  #26  
Old 09-29-2015, 11:03 AM
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Quote:
Originally Posted by DanH View Post
Me too, circa 1997.

And being one of my more stubborn friends, Ross still refuses to accept that urethane is a truly wrong material for a pin-and-hole torsional coupler
Nope. Agree, still using rubber there. I am using urethane for my engine mounts though with no easy way to change to rubber since they are not a stock size.
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Last edited by rv6ejguy : 09-29-2015 at 11:08 AM.
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  #27  
Old 09-29-2015, 11:07 AM
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Quote:
Originally Posted by Toobuilder View Post
Ross (and Larry)

You poured the mounts yourself, or had them cast? I'm aware that they can be built to spec, but the idea of building a mold and pouring them at home is intriguing. How dimensionally accurate are they (contracion)?
I happened to have a urethane molding place across the street from my old shop. Settled on 55 durometer and they poured them into my molds, leaving the technical aspects up to them. I believe the contraction amount was 2-3% for the material they used but that was about 14 years ago now.

http://s1105.photobucket.com/user/rv...brary/Urethane Third photo shows the mold I machined.
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW- 413.5 hrs. on the Hobbs,
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http://www.sdsefi.com/aircraft.html
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Last edited by rv6ejguy : 09-29-2015 at 03:47 PM.
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  #28  
Old 09-29-2015, 03:19 PM
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Default Compression set

I am humbled by Dan's unending depth of knowledge. Not mentioned so far is compression set. It seems tool builder is on a potentially fruitful path to solve sag.
From my work experience, silicone is bad for compression set even compared to rubber. Urethane is very good for dimensional stability under load. If the points Dan makes about heating potential don't come into play as motor mounts, Toolbuilder could be onto something !
The off road folks have a lot of urethane bushings molded. Perhaps they could be redimentioned in the lathe with simple fixturing to avoid the castig step?
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  #29  
Old 09-29-2015, 04:42 PM
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Some people have the capability to actually machine polyurethane, but I'm not one of them. My process to make these prototypes is effective, but far from "production quality". I CAN build a perfect mold however, and that will allow me to experiment with various durometer material easily. The casting process does not look any harder than my current efforts, so that's a distinct possibility for the future.
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  #30  
Old 09-29-2015, 04:55 PM
RVDan RVDan is offline
 
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I keep wondering if this material http://www.earsc.com/HOME/products/D...ex.asp?SID=152
would work. The materials have a pretty wide temperature range, but EAR has never made a move to aircraft engine mounts.
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