Bush gear failures


63 posts in this topic

Posted

One thing you could try is to put a couple of pucks in the lathe and machine a bit of an angle into the face of one end on a couple of the pucks say to produce an included angle of 150 deg. instead of perfectly flat.... for say 50% of the face of the puck. put them top and bottom of the puck stack and it might soften initial travel a bit. 

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Posted

I haven't drilled the pucks yet, this is my next job. The reasoning behind this is to give the material an additional path to displace to and maybe soften the rate. If you can add extra pucks to your build this step may be unnecessary,the current setup works fine but I feel just a little softer/more wheel travel may be an improvement. I was thinking 5 x 5mm holes worth an option to encrease up to 10 mm?

Dusty, 

Have you considered using 6 units of 1.5"(height) of the 90 durometer Poly? I know that would require building a new strut rod, but I though that may be a good starting place. If not, is that about where you would start if you had to do it all over?

 

Here is the raw material for one of my springs for my landing gear. They are 1 1/2" long 90 durometer polyurethane pucks. The pucks are cut from a tube with 2" OD and 1" ID. 

 

Inspired by Dusty....

poly stack.jpg

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Posted

Nice. My polyurethane tube arrived today from McMaster Carr, and the 4130 tubing for the new struts is on the way from Wicks Aircraft. This weekend I plan to knock the new struts out. Maenwhile, here is what the project is looking like right now.

20210303_154758.jpg

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Posted

Nice Bird! Which poly tube did you end up buying?

 

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Posted (edited)

Nice Bird! Which poly tube did you end up buying?

 

I bought the 2" OD, 1" ID, x 36" long, 90a durometer black tube from Mcmaster Carr that I had linked earlier in the thread.

 

edit: how did youyou cut yours? I'm debating on whether to use my bandsaw or was thinking my miter saw might work better

 

Edited by 109jb

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Posted

I used a Chop saw with a much slower speed (modulated by my trigger finger), a low duty cycle (not cutting constantly, so as not to melt the poly), and using a thin kerf, brand new metal cutting blade. Using the chop saw, and a nearby vice, to hold the tubing steady, seems to give me a safe way to make nice 90 degree cuts. Then I took the pucks to the belt sander to smooth them out and make sure there are not areas that might promote a future split. I need to say, I do think it is important to watch the temperature of the poly when cutting. I just use my fingers and some common sense. It is easy to melt the material. To be candid, an appropriate lathe setup would be a better choice in my opinion, but I can't see anything terribly wrong with my approach. If anyone looking on has any input on this, please speak up. I listen...

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Posted

I do have a lathe but not with a 2" spindle bore. I will likely cut them a bit long and then put them in the lathe to clean them up.

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Posted

If I were to start over I would use the 6 pucks. There is the chance of combined sag but a little preload will negate this. I had a local engineer cut mine in a lath after freezing them. The suggestion of machining an angle  is a solid idea .

It's great to see so much enthusiasm for this project .

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Posted

Makes sense Dusty. As part of the strut system, I am thinking to have a 1" pipe going through the 1" ID of the 6 pucks. I would liberally coat the pipe, and the puck separation discs, with Prothane grease. I know that doing this will only allow the outer perimeter to bulge under compression, but I think the damping result of having the tight fit, and the very viscous Prothane grease, may work out nice on rebound. I am hoping that using six pucks will help with softening the jounce; considering the compromise of having a tight fit on the ID of the Poly tube. If things are too stiff, I can always drill small holes near the ID wall of the Poly tube to give a little extra room for bulging.  However, I would rather account for the stiffness, due to the llack of bulging on the inner perimeter, by adding more pucks if need be. We'll see what happens. Anyone feel free to jump in with any thoughts.

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Posted

I did some trigonometry on the gear dimensions and angles in your sketch, which seems to agree with the basic dimensions of my Highwing llc (Fitt) gear, shown below. The force on the diagonal rod with the disks on it is about 2400 lbs of stretch on the rod during a reasonably hard landing (3 g's, about 8 feet per second sink rate).

That means the poly pucks will see a compression of about 2400 lbs during that event. For my springs on the Highwing gear, that compresses the steel by about 1.5",  and at 3.8 G's, maybe 12 feet per second, it compresses the spring completely.

It might be great to see what the compression on the poly pucks is at that load, 2400 lbs, just as a comparison. the durometer doesn't measure spring rate at all, it is a measure of how the material resists being dented by a sharp pressure. My guess is the stuff is very stiff and would work well as a stiff spring.

analysis4.jpg

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Posted (edited)

I think at 2400 pounds of force, I would be deflecting the Poly 2.29". I need to verify my math though.

2400 pounds/2.355 square inches = deflection per inch of thickness *Young's modulus of 4000*(1 + 2*(2.355 square inches receiving the force/(9 inches of length* diameter of 2 inches*3.14)^2)) 

Solving for X, you get X = .254402 inches of deflection per inch of thickness. 

With a thickness (height in this case) of 9 inches, this gives a total deflection of 2.29 inches, and a wheel travel of about 3.34"

 I need to test this in my shop.

 

Update: I think my math could be missing something. I need to recheck it tomorrow. I think I would have a little more deflection; leading to about 4.5 inches of wheel travel.

Edited by Supermotive

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Posted

For my gear pictured below, the strut force will be given by:

(Strut tension) = (Upward load at wheel) * 19.66/13.51. Assuming a tooth jarring 3g load @ 1200 gross that is 1200 * 3 / 2 = 1800 pounds upward at the wheel, so the strut tension will be 1800 *19.66/13.51 = 2619 pounds tension in the strut

 If I do like I guessed and use 5 pucks each 1.5" long for a total of 7.5" of polyurethane length.

F = E * A * (delta L) / L0 = 4400 * 2.356 * (-1) / 7.5 = 1382 pounds at 1 inch deflection

looking at the equation it will be linear, so 1382 pounds per inch deflection with a 7.5 inch starting length, so 2619 / 1382  = 1.9 inches of deflection

Looking at an additional puck for 9" total yields 1152 pounds per inch equating to 2.27 inches of deflection

The second picture shows what my gear would do at 1.9" extension of the strut, which would be an upward travel of about 3.2 inches of the wheel, not counting any tire deflection.

Thinking I may switch to 9" of pucks so I can preload them a little. I'm thinking no sag at empty weight would be good and then once loaded will sag a little. My old Kitfox sagged a tiny bit when empty with the stock gear and stock bungees but they were old I'm sure. I would think new bungees would not sag. So at 9" of polyurethane I would give it about 0.26" preload which would support 300 pounds per side without sag, and that would give me 2 additional inches of extension before 3 g @1200.

gear geometry.png

extended strut.png

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Posted

109jb,

I think your math looks great and we agree!  

I think an easy way to check the poly disks is to just put the diagonal strut together, and put 2400 lbs of tension on it to see the deflection. A simple rig with a hydraulic cylinder and a strong mount should do the trick. Or you could pick up a VW beetle with it!

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