Bush gear failures


63 posts in this topic

Posted (edited)

He has either changed the design or changed his mind on the subject in the year since we spoke.

Your plane, your choice.  I wouldn't run this design gear with steel springs, that's my choice.

Edited by Av8r3400

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Posted

He has either changed the design or changed his mind on the subject in the year since we spoke.

Your plane, your choice.  I wouldn't run this design gear with steel springs, that's my choice.

He added a cross bar that ties in the two sides, parallel to the belly. Early models didn't have that, and some hard landings caused the side frame to buckle in. The new bar considerably strengthens the frame against side loads (now it is much like the High Country gear, for example).  I think that answers many of the concerns posted. We shall see.

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Posted

Any updates to this blog? I am building a bush gear, and I want to be sure I build the most robust gear within reason.

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Posted

Regarding springs,my choice would be acme aero springs but my budget (read wife) says no way. I am experimenting with eurethane pucks with some success. 2 kg weight saving,no chance of bottoming out and a degree of rebound damping.some more hard testing is required but results are favourable.

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Posted

Regarding springs,my choice would be acme aero springs but my budget (read wife) says no way. I am experimenting with eurethane pucks with some success. 2 kg weight saving,no chance of bottoming out and a degree of rebound damping.some more hard testing is required but results are favourable.

Dusty, would you please share details about your plane and the pucks you chose and you testing results (or opinions)

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Posted

I have had several landing gear failures since going to the spring "bush gear". I have been researching the subject and have noted quite a few similar failures of that type gear, all in about the same location and of the same nature, Inward bending of the forward strut leg. I have been reluctant to re enforce that leg for fear of promoting a more serious failure elsewhere. The gear is a Chinese copy of the gear used by Piper from the 1930s onward but the Piper gear does not seem to suffer the same failure. Today I made a cardboard model of the gear geometry and I believe I have located the problem. The bottom anchor bolt for the spring leg on the "bush gear" is fastened to a metal plate welded between the front leg and the axle. When the spring strut is fully extended and bottoms out, or the spring becomes coil bound, that point then becomes a rigid fulcrum with the axle as a lever. The upward motion or the axle is then translated into an inward rotary motion on the front gear leg. The failure always seems to occur at the point halfway between the axle and the fuselage, which makes sense. I have never bent a rear strut, nor the spring strut, always the front one.

Looking at photographs and real life examples of Piper, Hatz, and Pietenpol gear shows that the anchor point on those gear is at the inboard end of the axle. My model shows a significant difference in the movement of the middle of the forward gear leg when the anchor point is moved to the end of the axle. As a matter of fact, the farther inboard the pivot is, the lesser the rotary movement at the front gear leg center.

I have yet to figure a force analysis on the gear, but I think i am onto something. I'll post more as I find more.

On a positive note, my welding is getting better.

Bob McCaa

 

All of the "bush gear" I have seen has a straight pull from the axle point through the vee to the upper mount.  By lowering the intersection of the Vee like you see on the piper gear you will better distribute the forces.  This along with changing the springs to ones with more travel before they bind is the key to an easy fix.  The gear I now have on my plane had been bent more than once and was bent when I got it.  I straightened the tube and lengthened the slots as well as changing out the springs.  Since that time I have flown the plane a a weight much greater than the published numbers and have dropped it in pretty hard with no ill effects.

The gear legs that I have were custom built, but if one would supply me with measurements on the gear they have I can mock it up and make replacement vee and struts that would take care of the issues without having to completely swap the gear legs.  Would be a quick matter of lifting the plane, pull a few bolts, pop the new struts and vee in and lower it back down. 

:BC:

 

Leni, would you share your configuration, springs, etc? You seem like a guy in the know, and I want to learn a little before I start cutting pipe.

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Posted

Regarding springs,my choice would be acme aero springs but my budget (read wife) says no way. I am experimenting with eurethane pucks with some success. 2 kg weight saving,no chance of bottoming out and a degree of rebound damping.some more hard testing is required but results are favourable.

Dusty, would you please share details about your plane and the pucks you chose and you testing results (or opinions)

The pucks are definitely firmer than the springs., which I believe is a good thing as my spring setup had been bottoming out.

20201012_165729.thumb.jpg.3c26ed435ad064

This is the top of the rod that goes out to the wheel. The bolt was partial chopped through from the hammering. I fly off field a lot and on some pretty rough hillsides.

I have tried different urethane shore hardness. Currently the red (90) is the best. I tried a 75, which was too soft and sagged. 

Screenshot_20201016-114307_Chrome.thumb.

The next step is to machine the pucks to have grooves around them, or vertically drill them. Like a pistol magazine. Urethane can't be though of as a spring as it doesn't compress.

Think more like water where it just displaces and absorbs energy in the process. 

20200926_131752.thumb.jpg.d455ecd79bc73d20200926_141422.thumb.jpg.7caf7f046797d4

The piece of wire is my high tech device for measuring suspension travel. This is from a normal landing, so there is plenty of travel, but I would like more.

 

If rebuilding a suspension I would use 5 or 6 pucks to effectively change the rate. My weight saving calculation was out a little, approximately 1.4kg weight saving. 

We get the urethane here from an industrial supplier as a one piece tube. We freeze it,and part it off in a lathe. This is extremely cheap so not a huge cost for experimenting.

I have a wee way to go, but even at this stage I see a huge improvement over the die spring. 

I will post updates as I go, but am pretty busy with work at the moment, and rebuilding another kitfox.

 

 

 

 

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Posted (edited)

Awesome Dusty! What is the name of the supplier you buy the urethane from. I would like to buy some and do a little experimenting myself.

Edited by Supermotive

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Posted

I use LEP Engineering plastics who may only  be  a NZ company .you are bound to have a similar company near you. The item I use is from a stock mold 50x25   Code#PT405025

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Posted

When I googled the part number you give, I get no results.  Are there any other ways to identify it so we can search for it here in the USA?  Thanks,  JImChuk

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Posted

I've been thinking about making new struts for my gear and using snowmobile springs and then saw this. My setup currently would use bungees. Here in the USA, Mcmaster Carr has the 90A durometer tubes for sale, here are some links:

https://www.mcmaster.com/87235K58 6" long tube 2" OD, 1" ID 90A durometer

https://www.mcmaster.com/87235K32 Same thing but 36" long and much better price per inch.

I may have to give this a go.

 

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Posted

I've been thinking about making new struts for my gear and using snowmobile springs and then saw this. My setup currently would use bungees. Here in the USA, Mcmaster Carr has the 90A durometer tubes for sale, here are some links:

https://www.mcmaster.com/87235K58 6" long tube 2" OD, 1" ID 90A durometer

https://www.mcmaster.com/87235K32 Same thing but 36" long and much better price per inch.

I may have to give this a go.

 

This really has my attention. I like the "no spring bind" feature. I guess the trick will be figuring out the best durometer/length configuration. I have a MKIV with a Jab 2200. I am thinking 8-10" of 90A, but I am not certain yet. Anyone know more than I do on this?

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Posted

Regarding springs,my choice would be acme aero springs but my budget (read wife) says no way. I am experimenting with eurethane pucks with some success. 2 kg weight saving,no chance of bottoming out and a degree of rebound damping.some more hard testing is required but results are favourable.

Dusty, would you please share details about your plane and the pucks you chose and you testing results (or opinions)

The pucks are definitely firmer than the springs., which I believe is a good thing as my spring setup had been bottoming out.

20201012_165729.thumb.jpg.3c26ed435ad064

This is the top of the rod that goes out to the wheel. The bolt was partial chopped through from the hammering. I fly off field a lot and on some pretty rough hillsides.

I have tried different urethane shore hardness. Currently the red (90) is the best. I tried a 75, which was too soft and sagged. 

Screenshot_20201016-114307_Chrome.thumb.

The next step is to machine the pucks to have grooves around them, or vertically drill them. Like a pistol magazine. Urethane can't be though of as a spring as it doesn't compress.

Think more like water where it just displaces and absorbs energy in the process. 

20200926_131752.thumb.jpg.d455ecd79bc73d20200926_141422.thumb.jpg.7caf7f046797d4

The piece of wire is my high tech device for measuring suspension travel. This is from a normal landing, so there is plenty of travel, but I would like more.

 

If rebuilding a suspension I would use 5 or 6 pucks to effectively change the rate. My weight saving calculation was out a little, approximately 1.4kg weight saving. 

We get the urethane here from an industrial supplier as a one piece tube. We freeze it,and part it off in a lathe. This is extremely cheap so not a huge cost for experimenting.

I have a wee way to go, but even at this stage I see a huge improvement over the die spring. 

I will post updates as I go, but am pretty busy with work at the moment, and rebuilding another kitfox.

 

 

 

 

Dusty, I really like what you did here. You inspired me to start experimenting on my end. With that said, I need a starting place. I am looking to start with the Avid bush gear drawing, but substituting the urethane. A couple of questions: did you cut your urethane to 2"od and 1" id? Are you using 4 x 1.5" tall pucks with aluminum 2"od by 1" id washers? I am thinking of 8-10" height for mine. What size pipe and wall thicknesses are you using to fit inside the id of the urethane pucks? By the way, I personally prefer the Prothane lubricating grease. It is water resistant, plays real nice with urethane and metal, and provides a nice viscous friction damping feature.

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Posted

The inside and outside diameter is stock,we are metric here but I suspect they were supplied in imperial which is fairly close. The internal diameter felt a little tight so I wrapped a bit of tube in sandpaper and clearances it, I don't think this is really necessary as   friction is an advantage. I used grease more for corrosion control. 4 pucks fitted the high tech ( looks about right) philosophy . The distance between washers however is an area worth experimenting with(3x50mm v's 4x38mm?As is extra length of the puck assembly(your 8-10' sounds good).I used Standard washers however they could be fibreglass or CF . I am keen to see others experimenting with this setup as at the moment rebuilding a fox 4-1200 for my brother is taking most of my non flying spare time cheers

Dusty

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Posted

I'm guessing the 50x25 is 50mm by 25mm for us 'merican folks that don't speak metric unless under duress :lmao:

I found these online:

 

https://www.riogrande.com/searchresults#t=products&sort=relevancy&layout=card&numberOfResults=36&f:categoryfilter=[Tools %26 Equipment,Metal Forming,Hydraulic Forming,Urethane]

Holy Cow are those expensive.  Why not just use the McMaster Carr urethane tubes I posted above? 50mm is close to 2" and 25mm is close to 1"

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Posted

1. I'm not sure Polyurethane is the same as urethane - probably is but don't know that for sure.

2. Might want the purty red color ones :rolleyes:

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Posted

Correct. Polyurethane is the product in discussion (not urethane)

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Posted

Would it be beneficial to have 1 or 2 pucks in the stack with a lower shore rating to allow a small amount of softer travel?

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Posted

I am tinkering with this design right now. I am trying to understand the optimal angles for the intersection of the members to minimize bending moments. Thoughts?

Napkin.jpg

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Posted

Dusty: So I have a 36 inch long (914 mm) piece of the McMaster Carr polyurethane tube on its way.  So I gather from your posts that the 4 pucks that are pictured are 50 mm OD (about 2 inches), 25 mm ID (about 1 inch), and 38 mm long (about 1.5 inches long. That would be 152 mm total length ~6 inches).  My thought is to make struts that allow up to 200 mm total length, start with 4 50mm long pucks and see how it goes. This way, if too soft or too much travel I could insert a solid spacer to replace one of the pucks. What do you think of this plan, being the resident expert?  Also, a question. Does your setup sag at all with the airplane empty.

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Posted

Yes I used 4 pucks,1 inch x 2inch 1.5 deep.if I were to start from scratch. I would use more pucks as you suggest.You will loose some material when parting off  but you have plenty of spare. I am not sure on going to 2 inch deep pucks, it may work in your favour to effectively soften your rate which is possibly a good thing. The diameter to depth ratio I assume will change the  characteristics of the setup. Regarding sag ,there is a small amount over the last 6 months which seems to have stabilised 10mm?(similar to the die Spring)  If you need to preload you can make a simple Spring compressor. I can post a photo later today . I am interested to know how this works out for you 

cheers

Dusty

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Posted

Yes I used 4 pucks,1 inch x 2inch 1.5 deep.if I were to start from scratch. I would use more pucks as you suggest.You will loose some material when parting off  but you have plenty of spare. I am not sure on going to 2 inch deep pucks, it may work in your favour to effectively soften your rate which is possibly a good thing. The diameter to depth ratio I assume will change the  characteristics of the setup. Regarding sag ,there is a small amount over the last 6 months which seems to have stabilised 10mm?(similar to the die Spring)  If you need to preload you can make a simple Spring compressor. I can post a photo later today . I am interested to know how this works out for you 

cheers

Dusty

Dusty, you have been a real help for the community with this. This seems like a cutting edge idea. Thank you for supporting the research here.

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Posted

Dusty, One more question for you. You said that you drilled the pucks like a revolver cylinder. Do you recall how many holes and what diameter you used?

Thank you so much for your help and inspiration.

 

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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?

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