Rubber Discs for Shock Strut

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karmarepair

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this should cut round rubber washers cleanly....

A mat board circle cutter might be worth a try, a LOT cheaper than the tools sold by McMaster Carr.
 

challenger_II

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My bet would be that die cutting would work better than fly cutting: well-lubed, the die would make a cleaner cut, with fewer imperfections that would need to be addressed.

It still appears that casting the doughnuts is the simplest, least expensive option.
 

Mad MAC

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Update: Found a source of 2 1/2" x 7/8" bore x 1/2" 60 duro rubber doughnuts. Research has determined that the Mooney M20 pucks are 65 duro. I ordered a batch, so we shall see how this goes.
You couldn't give the source for those rubber doughnuts, just for everyone's future reference.
 
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challenger_II

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Show and Tell time:

I really should have done this, early on. It would have simplified the discussion.

This is my gear arrangement, as built. I want to replace the coil spring with rubber discs, as the original plans show.
Looking closely, you will see that the spring is mostly compressed, and the aircraft is at rest, and empty. Add my 230#, and the springs are close to bottomed-out. Ergo, not much spring left, when landing.
 

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Vigilant1

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Show and Tell time:

I really should have done this, early on. It would have simplified the discussion.

This is my gear arrangement, as built. I want to replace the coil spring with rubber discs, as the original plans show.
Looking closely, you will see that the spring is mostly compressed, and the aircraft is at rest, and empty. Add my 230#, and the springs are close to bottomed-out. Ergo, not much spring left, when landing.
With the arrangement you have, is there sufficient >damping< (absorption of energy so there's little/no rebound)? It looks like the only damping you get now is from the scrub of the main wheels against the pavement/ground as the gear splays out on touchdown, and it's entirely possible that this is sufficient (or would be sufficient if you had more gear travel).
 

Tuneturkey

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Your solution with the rubber spacer is a great fix for me. My nose wheel has a space of 4" for insertion of a spring or the rubber grommets. the nose wheel strut is 3/4" OD is perfect. Question though: How viscous is the 60 duro rubber. Max weight on the nose wheel is close to 100# TOW. Will the rubber compress with three of these in line on the strut? What is the deflectiion/lb of load for 60 duro?
 

challenger_II

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That is the $64,000 question. It is why I am looking for 30, 40, and 50 duro discs so as to make a comparison, and get the right "ride".
The original plans, which date to 1932, called for "20-38" rubber. I have found no correlation of that spec to modern rubber specs.
 

tr7v8

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Various transport in the UK has used rubber in compression for suspension. Classic Mini used rubber with the dry suspension. And the Moulton cycle has used it for years & still does. Alex Moulton designed the Mini suspension and also then started Moulton cycles. Moulton Bicycle - Wikipedia
 

challenger_II

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Update #2:

Received the pack of 2" OD x 7/8" ID x 1/2" 60 duro pucks. Made up a 6" stack (the unloaded gap of my existing shock strut, flange to flange) on a 7/8" tube. Set this rig up in the arbor press, and measured the compression in inches under various loads.
At 325# (bear in mind, my plane weighs 750# gross), the compression was 5/8". That figure is only slightly less than the current coil spring, at gross.
At 1.25" compression, we were measuring 585#.
The current coil spring system has only 3/4" before it is bottomed-out.

Come Tuesday, the 2 1/2" OD x 7/8" ID x 1/2" 60 duro pucks will arrive, and I will test those items and compare numbers.

The real proof of the pudding will be when I mount them on the plane, and go see if they cure the "pogo stick" behavior.
 

cluttonfred

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Sounds like those 2" pucks are right on the money in terms of hardness for your purpose. I have often seen spacers used with this sort of setup, perhaps just to keep the rubber pucks from deforming and/or sticking to each other. I wonder if it would be useful to replace your 12 x 1/2" pucks with 11 x 1/2" pucks + 10 x 0.05 in aluminum spacers or 10 x 1/2" pucks and 9 x 1/8" hard plastic (nylon, delrin, etc.) spacers? Using one or two less pucks might get you a little more travel as well.
 

challenger_II

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There is to be a .020" ali disc between each rubber puck, once installed: as of time of test, I haven't fabbed up but 4 ali discs. The purpose of the ali disc is to allow the pucks to compress evenly.

As far as travel, I am already gaining 3/4 inch, over the current spring installation.

As for alternative spacer materials, all the rubber puck shock systems I have observed in service have used either aluminum, or steel, as the spacer. I have contemplated Delrin, fiberglass, and phenolic, but am concerned with fit, form, function, and longevity. Ali has proven to work, so that is the material I will use.
 
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cluttonfred

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Understood on the spacers and choice of material, I guess I was confused by your previous post when you said 5/8" compression for the rubber pucks and 3/4" for the spring. I see now that the first was at rest and the second was bottomed out.

As far as travel, I am already gaining 3/4 inch, over the current spring installation.
 

challenger_II

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Um, yah: I haven't been exactly clear in my communications... :)

Total travel, with the spring, is 3/4", from "Free" to bottomed-out.
With the rubber puck stack, total travel is 1.25" from "free" to a load of 585#, or 1/2 of a theoretical 1.5G landing. As the strut travel is 2.5", I have ample stroke before the rubber pucks mechanically bottom out.
Not very scientific, I admit, but it does give me reference on what to expect when I test-fly the rig.

On another note, the 2 1/2" OD pucks arrived, today: the center bore is over-sized, and does not "interference-fit" the strut, and will not exhibit the desired preload snubbing, so I will wait to see how the 2" OD pucks perform on the aircraft.

So, its off to make a stack of aluminum washers, I am! Good thing its windy, here: will help overcome the itch to go fly the Beast, rather than work on it! :)
 

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