Landing Gear Donuts

[Scott Royall]

Hi Guys,
I thought you would be interested in the original specifications and drawings for the Mite donuts. The part number called out in the M18-X Parts Manual is Firestone CRE 714-1. I actually found the company that bought the molds and the rights to manufacture the donuts from Firestone. It’s called Pacific Molded. They didn’t know that they had airplane parts and they hadn’t made any of them in 20 years. The really sad thing is that they disposed of the molds about 18 years ago due to lack of demand. The drawings, redrawn from earlier versions, are dated from December, 1963 and redrawn as late as 1982. By 1963 the donuts were made with durometer values from 30 to 70. By 1982 they were made from 28 to 85. We don’t know the range back in 1948 and we don’t know the durometer of the original mite springs but I think we can deduct that they weren’t as hard as 90, and they could have been as soft as 30.

[Scott Royall]

So I should update my speculation about the hardness of landing gear donuts. I found a supplier that will cut donuts of any hardness for $6 each, so Mike Magnani and I decided to experiment. They are not conical and they are just Solid Rubber so we expected differences in performance and had to restack the parts to allow the flat rubber disk to fit into the suspension. Neither Mike nor I had a plane ready to put landing gear on so we recruited Keith Mackey. I think that Keith will write about his experience, and maybe post some pictures from his test with 40 durometer donuts. In the end his conclusion is that 90 is a good hardness and he is very happy with the 90 durometer donuts that were the new-old stock that he bought from either Fred Schmidt or Paul Workman, I can’t remember which one it was. So, any of you that buy the new donuts that Paul has available can rest assured that they will work very well. Being an experimenter, I plan to try a couple of other test to see if I can come up with something that will work reasonably well and that is not so expensive. Right now, Mike has assembled his Mite with 60 durometer donuts and plans to see how they work when he finally has his Mite sitting on it’s suspension.

[Keith Mackey]

I installed the 40 duro donuts that Scott & Mike supplied on my aircraft nose gear as a test to see if they would be of the proper hardness. Since these donuts are flat and not tapered as were the originals, it was necessary to move the washer 17-2 (see first picture) from the top of the stack to the bottom. This allowed the flat donuts to be stacked between two flat surfaces.

The installation was easy enough as it was not difficult to compress the 40 durometer donuts, but they proved too soft. They needed an extra washer on the top to prevent the rubber from oozing out at the top. Once the weight of the aircraft was applied, the AN24-25 bolt was spaced too high to effectively limit the slack in the travel of the stack. (See second picture). It was determined that the 40 duro donuts were too soft to be a practical alternative to the originals.

Next test was with 60 duro donuts. With the factory donuts installed, the retaining tube stuck up 13 mm above the top of the donut/washer stack (see third picture). With the 60 durometer donuts, the stack was thicker and only 6 mm of the tube was exposed. (See fourth picture) It was impossible to compress the assembly sufficiently to effect the installation without either shaving the donuts or removing the gear from the airplane (Didn't attempt this). The result is that the usability of the 60 duro donuts is unknown.

During this process, it was discovered that at some time in the past when the airplanes original dounts had deteriorated, "U" shaped 1/8" shims had been installed probably to compensate for the worn out rubber. They looked like they might have been factory and since no M-18C 55 parts catalogue exists, they were installed in the same manner as they were when the airplane was stripped for overhaul.

One on my few complaints about the airplane is that the gear was too stiff. After my experiments with the new donuts, I decided to remove these shims as they probably were not original, but were used to compensate for worn out donuts. With the shims removed, the stiff gear problem was solved. The gear is now very flexible, the brakes work better perhaps due to a change in geometry of the gear trailing arm and an odd tire wear pattern now appears corrected.

I got the present donuts from Fred Schmidt in the lot he bought from the factory supply. These are to my knowledge 90 durometer. I would suggest that the supply Paul Workman has are the same and should work perfectly.

Thanks to Scott and Mike for helping me find the solution to a vexing problem.

Keith

[Scott Royall]

I think this will be the last entry that I make on this thread.
I have been looking for a way to verify that my mite springs are good, and/or a way to replace them if needed. I decided that I needed to get a good reading of the hardness of the rubber in my springs. So I have been carrying them around for a while looking for someone that could measure them. I found a place a few days ago that had an old durometer and we measured my springs. Some measured as hard as 100 and others as soft as 85. What a dilemma. No consistency. I couldn’t believe that there was that much variation.
Yesterday, I met a guy named Grant that has been working with rubber most of his long life (he is proud to be a great grandpa). His company is Grip Rubber in Salt Lake City, Utah. We talked about the landing gear springs and how to replace the old ones. Grant suggested several possible ways to create new springs, some more labor intensive and some less. He estimated that he could make a mold for about $600 and each part would cost between $20 and $40. If anybody is interested in a cheaper more labor intensive way to make them give me a call.
While I was talking to him he pulled out his Calibrated, Certified, Shore A, durometer to show me some examples of different hardness’s of different materials. So I grabbed my landing gear springs and had him measure them for hardness. I have 3 sets of the gear springs that I have collected over the years (18 springs total). We measured each of them and they all measured within 3 points of 80 durometer (77 to 82). Surface irregularities can account for small variations in durometer readings. I can tell the springs I have came from at least three different manufacturing runs because of the markings on the face of the spring. Some are old and some are newer.
Rubber that is exposed to oil can soften over time but if it is dry and hot it will harden over time. Based on my new friend's description, all of the Mite springs I have seen are neoprene rather than natural or crude rubber and neoprene is very stable over time. Grant's opinion is that all 18 of my springs would have been manufactured as 80 durometer.
I am now comfortable that the springs I have are good springs and that the hardness is what it should be.
Check off one more task! I only have about a million to go.