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Polyurethane For Dynafocal Isloators?

Toobuilder

Well Known Member
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?
 
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.

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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.
 
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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?
 
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.
 
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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.
 
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
 
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!
 
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.
 
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.
 
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.
 
The general comments of caution are appreciated and I may even instrument the engine, mount and airframe to see how much vibration is passing through the isolators. That said, there is a wide variety of "stiffer" engine isolators available for aerobatic aircraft - and in some cases, the very stiff "compression" side of the Lord isolators are recomended by Lord in all 8 locations for the hard core acro airplanes. And finally, solid polyurethane isolators are available from Spruce today for conical engines, so I'm not going too far off the reservation with this experiment.
 
Conical poly

I had poly conical in my -3 and you could feel every engine quirk. It wasnt bad but there was a difference. I did notice more vibration on the panel.

I wonder if you could machine out a 1" core and use a softer rubber material there and poly on the rest?

The general comments of caution are appreciated and I may even instrument the engine, mount and airframe to see how much vibration is passing through the isolators. That said, there is a wide variety of "stiffer" engine isolators available for aerobatic aircraft - and in some cases, the very stiff "compression" side of the Lord isolators are recomended by Lord in all 8 locations for the hard core acro airplanes. And finally, solid polyurethane isolators are available from Spruce today for conical engines, so I'm not going too far off the reservation with this experiment.
 
Of course it works and sag is much reduced. With everything, there are usually tradeoffs and it depends what's most important to you. Only time will tell if the avionics or airframe suffers more problems or not. Engine sag is annoying and ugly for sure.
 
...I wonder if you could machine out a 1" core and use a softer rubber material there and poly on the rest?

I am contemplating machining an air gap between the spacer and the body of the poly to try emulate the action of the Lord mounts, but I have to balance that with the shear loads on the remaining material. It will be an iterative process, for sure.
 
Why not switch to hard rubber. More compliant. If you can't source a "rod" to cut the shape from, maybe laminated sheet circles would work? Just an idea.
 
Laminated sheet might work if we were only dealing with compression loads, but the mounts have to deal with shear as well. Anyway, rubber is more compliant, but has a memory. Poly can handle billions of cycles and will retain original shape. That's the theory, anyway.

Flew it yesterday and hardly notice the vibration anymore. It's really not that bad unless you jump from rubber directly to poly and have the opportunity to compare the two.
 
Poly springs

There are different density polyurethane springs available too. We use them in the metal stamping industry for springs. They are large cylinder shapes, look like they could work for your raw material size. Maybe I can find someone at work to give me a source. Mitsumi comes to mind.
 
Urethane options

Urethane can be cast from liqued components just like epoxy. The durometer can be varied similar to to selecting slow and fast catalyst. Since tool builder has a lathe, simple molds can be formed for the task.

I did this for a Cont A40 mount to avoid the crude rubber sheet stock used in 1936. Just another option if you want to tinker.
 
Urethane can be cast from liqued components just like epoxy. The durometer can be varied similar to to selecting slow and fast catalyst. Since tool builder has a lathe, simple molds can be formed for the task.

I did this for a Cont A40 mount to avoid the crude rubber sheet stock used in 1936. Just another option if you want to tinker.

This is what I did on mine. I machined the molds allowing for contraction, specified the durometer and they turned out great.
 
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)?
 
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/Product Catalogs/PC6116_AerospaceandDefenseIsolatorCatalog.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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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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...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.
 
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?
 
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".
 
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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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/rv6ejguy/library/Urethane Third photo shows the mold I machined.
 
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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?
 
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.
 
I have had some experience with polyurethanes - we learned a lot - plenty can go wrong. I do applaud your pragmatic approach and will watch for your progress.

The term 'polyurethane' is about as generic as 'fibreglass'. Rudimentary differences in the chemistry has significant impact on properties. Suppliers have a broad catalog of products and properties to choose from. Era http://www.erapol.com.au/products/cast-elastomers/ in my country is one such company, usually they will supply samples for serious potential customers.

The usual supply format is as a multipart (most commonly 2) resin. Mixing - simple pot mixing with stirrers will require de-gassing in a vacuum pot so requires a longer pot life, the next easiest is to use viscostatic (tubes with a series of mix blades inside) dispenser.

Pre-cast PU's such as sheet can be a mixed bag - pure PU will have a liquid clear appearance in an amber colour - be suspicious of a milky appearance or coloured stock.

Some boutique suppliers of PU formulations exist although I no longer have contact details for them. Tell them your requirements and application and they can provide formulations to suit. Tell them something can't be done in PU is like a red rag to a bull.
 
PU Mounts

I happened across this post looking for an alternative use.

What was the verdict here Tool Builder; effective or otherwise?

My panel is lord mounted to reduce vibration to it, so not much of an issue with excessive vibration to the panel.
 
I tried running all 4, then just two, and eventually went back to the standard. Poly is effective at controling the spinner/cowl alignment and sag as expected but just a bit too harsh for a long cross country ship in my opinion. If it was a 30 minute per day acro ship, then I'd use them.

There are different formulations of poly that might work better, but its going to be a while before I have the time to machine up a mold and experiment.
 
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