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-   -   Polyurethane For Dynafocal Isloators? (http://www.vansairforce.com/community/showthread.php?t=129002)

Toobuilder 08-26-2015 11:40 AM

Polyurethane For Dynafocal Isloators?
 
Looking at a spare polyurethane automotive suspension bushing the other day and I thought I could chuck it in the lathe and turn out a dimensionally accurate replica of the Lord mounts on my Rocket pretty easily. Yes, it's a lot harder than the Lord mount, but if the vibe and shaking is acceptable, it should last about forever and will never sag.

Has anyone done this?

If so, any comments pro/con?

Toobuilder 09-20-2015 08:41 AM

OK no experience with this anywhere on the web that I can find, so I did it myself. I purchased a 24 x 3 inch stick of polyurethane for about $100 bucks (2 complete airplanes worth) and put my lathe to work on some mount pucks, spacers and washers.







Currently, they are installed on the upper ears only just until I decide if they are acceptable. After installation and ground test yesterday, I went ahead and flew it for about 30 minutes. As expected, it transmits more movement to the airframe, but I'm undecided if it is "too much". It is certainly not going to hurt any structure or instrumentation- it's just a bit more than I'm used to with this airplane. In fact, I've flown 4 banger Lycomings with similar vibration levels.

Overall I expect that these mounts will completely eliminate the "shear sag" that the rubber mounts have and last the life of the airframe, at the expense of being stiffer overall and very sensitive to temperature. These warmed up on yesterdays flight and got smoother, so I expect a cold start after sitting overnight in sub zero temps is going to be very rough for a while. We shall see. I'll keep you all posted.

Radioflyer 09-24-2015 07:27 AM

Michael, thanks immensely for doing the experiment. I will be on the lookout for further reports. The standard Lord or Barry rubber mounts are way too expensive, even the generic aircraft spruce ones and I've often thought about trying to find cheaper substitutes. I wonder if it is possible to do the same (I.e., machine or fabricate) mounts from a stick or stack of high Shore number rubber?

Toobuilder 09-24-2015 06:39 PM

It will be a couple weeks before the second set goes in but I will let you know how it goes of course.

I would like to point out that while the Lord (and equivalent) isolators are way more expensive than I'd like to see, they are quite a bit more sophisticated than what I'm doing here. Lords hang the entire engine on a rubber bellows which has the conflicting requirements of allowing a lot of engine movement, but keeping that same heavy lump located in one position in space. They need to produce consistent results over a wide range of temperatures, to boot.

All I'm doing is building oversized conical mounts, and they only have the requirement of keeping the engine located (forever, I hope), and provide an "acceptable" level of vibration isolation. I have some experimenting to do, but I doubt these will ever be acceptable for a family cruiser like a -10. OTOH, it will likely be "OK" for a hot rod like a Rocket, but a slam dunk for a hard core acro machine which only flies for 45 minutes at a time. Time will tell.

Mel 09-24-2015 07:05 PM

One thing you should consider is that additional vibration transmitted to the airframe is also transmitted to your instruments and avionics.
There's more than a good chance that their life will be shortened considerably.

David Paule 09-24-2015 08:01 PM

Since the real isolators are designed for a certain stiffness, at least measure the deflection required to compress them at some arbitrary force, and attempt to match that with your new ones. Just messing around by feel is not going to do the job.

Of course the real isolators have considerably more engineering poured into them than just that, but at least you'll get in the ball park.

Dave

hgerhardt 09-24-2015 08:46 PM

The stock-style rubber units have a steel spacer in the center which bottoms out as the rubber is compressed during assembly and allows the bolt to be properly torqued. Do you still have that same mechanism in place, or are you simply torquing against the polyurethane? If the former, I'd guess you had to do a trial-and-error method of machining a little plastic away at a time until the spacer bottomed, since I'd expect polyurethane at ~3" diameter to be too stiff to compress under only a 7/16" bolt torque load. Nonetheless, interesting experiment!

David-aviator 09-24-2015 08:53 PM

It is an interesting experiment.

Seems to me urethane is not good at high temps, at least the urethan foam used in fiber glass construction melted easy.

Perhaps the density of this product makes it ok at engine compartment temps.

rv6ejguy 09-24-2015 09:33 PM

I had custom polyurethane bushings poured for my Subaru installation at a low durometer and have used them since day one. Almost 400 hours now. I wouldn't do it again. They work on mine only because the Sube has way less vibration than a Lycoming but if you look at the characteristics of the material vs. rubber at higher frequencies (I didn't research that first), it reacts very differently and may not be the best idea.

Toobuilder 09-25-2015 03:11 AM

Quote:

Originally Posted by hgerhardt (Post 1016708)
The stock-style rubber units have a steel spacer in the center which bottoms out as the rubber is compressed during assembly and allows the bolt to be properly torqued. Do you still have that same mechanism in place, or are you simply torquing against the polyurethane? If the former, I'd guess you had to do a trial-and-error method of machining a little plastic away at a time until the spacer bottomed, since I'd expect polyurethane at ~3" diameter to be too stiff to compress under only a 7/16" bolt torque load. Nonetheless, interesting experiment!

I have the spacer - see post #2. It's machined about 0.010 short of the static stack height and definately bottoms under the torque load. I can feel this sharp increase as the poly compresses down to the spacer.


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