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Maybe i'm overly cautious, but the idea that a piece of plastic with some hand drilled holes and saw marks on it being the main interface between my control stick and my airframe isnt exactly what I look at as the pinnacle of safety. Soooo..... I machined some aluminum control column bearings with a surface roughness control of 8 at the internal interface. Installation and function report will be following shorty.
Never heard of that piece of plastic breaking or causing a control problem, but your machined blocks sure have a nice look to them.
A surface finish of 8 is very impressive. Nice work.
I think that plastic block is glass filled nylon which is pretty tough stuff. I agree with Jim, I don't think there's any history of failure.
Avid Flyers used an oak piece of wood. Never had any issues with any of mine there either. Wonder which of the three types would wear out first? My Kitfox has 700 something with plastic, and the Avid has 600 something with the wood block. They both will go a long time yet I'm thinking. Your's look nice though. JImChuk
The aluminum does look nice, but the plastic is absolutely fine. There is more than enough strength, it's self lubricating, and with the small degree of travel it will almost certainly outlast the airframe, IMHO. :)
Wow. Nice!
My "just in case" isn't as pretty. I added a plate over the top of the plastic bearing. My thought was..... if the plastic did break, the "torque tube" is caged between the bolts and the plate.
Attachment 31329
I do agree with most of you that it shouldnt really be an issue depending on the level of attention to detail was aimed at that effort during installation - sadly whomever did the drilling and install on mine missed the center and then decided to remove as much plastic from the top piece as possible (for reasons I am not quite sure of) - ill post a picture of my stock one here in a bit. Since I am currently re-skinning portions of the plane and redoing the paint on the body, the floors were coming out already and that provided me the opportunity to make these little upgrades to the plane that I felt would help separate it from every other Model 4, while also looking good and increasing safety. When I removed the plastic one from my aircraft, the lower portion of the clamshell can be squeezed with two fingers and will basically fold in on itself - not something I want retaining the main controls of my aircraft. Yes - ill suffer a slight weight increase, but the added strength gives me a bit more piece of mind.
I do still have 3 remaining setups (ran a batch of 5) so if anyone is interested in swapping over to the aluminum please drop me a PM. Im not looking to profit on them so itll be cost to machine and base material plus shipping.
I believe that those "plastic" bearings are made of Teflon and will likely outlast the airframe.
Had to do a bit of research before I could make any statement - Seems these bearings are actually made from Delrin. Melting point of delrin is just under 200 degrees F, melting point of Teflon is just above 600 degrees F. Placed a 250 degree heated iron on the block I removed from the plane and sure enough it melted.
Now with that said, Delrin still has some amazing material properties :
- Color: White or Black
- Temperature Range:
- 6" Lengths: 0° to 180° F
- All other sizes: -20° to 180° F
- Tensile Strength: 7,690 psi (Good)
- Impact Strength: 1-1.5 ft.-lbs./in. (Poor)
- Hardness: Rockwell R120-R122 (Hard)
- For Use Outdoors: No
Some things to note about Delrin - it is not hydrophobic - so it is likely to take in small amounts of water and is typically not recommended for long term exterior exposure. While most of these aircraft are kept indoors, exposure to the elements is guaranteed and as such life expectancy can only be expected to be reduced. The big one to note from the above properties is the tensile stength of the material - due to how these parts are used and exposed to some form of radial forces as the control stick is moved during flight and run up, this tensile strength is of great importance.
Now lets consider some material properties of 6061-T6:
- Temp Range: Up to 900 degrees F (depending upon application)
- Tensile Strength: 42,000 PSI
With a maximum operating temperature 5 times greater and tensile strength of almost 5.5 times that of Delrin, I am confident that while it may not be ABSOLUTLEY necessary, from a safety standpoint, this upgrade makes sense and provides me a bit more peace of mind.
I don't want to be pedantic, but if there's >7,690psi being imparted on the control system, or it's >180°F in the cockpit, I'll have bigger worries than the material properties of that Delrin part.
Delrin has one property that's important in this application, but which is lacking in 6061-T6. Its slippery surface is inherently self-lubricating. In your application, how will the aluminum be kept lubricated where the powder-coated steel control system rotates against it?
I made new blocks when I wasn’t happy with the fit I had gotten on the originals, and I did indeed use Delrin. I believe that’s a great material for this application. There is really not a lot of force on that bearing and it is a relatively large contact surface. As far as outdoor exposure is concerned, the issue is long-term UV exposure. That’s a complete non-issue with floorboards in place. Your aluminum parts do look very nice, and I would recommend perhaps some bonded dry lubricant on contact surfaces. I just wouldn’t want builders to have unwarranted concerns about strength of that part. As I believe has been stated previously, I don’t think that there has ever been a reported catastrophic failure of that bearing from any of the thousands of Kitfox aircraft that have been built.
At the end of the day that’s the point of this. The increase in strength is merely a justification to ensure I’m not replacing a part with a weaker one. Second, the point of owning the plane is to make it “mine”. Small parts that have high “flash value” are a great means to attract eyeballs to an airplane at an airshow / fly-in. Again, I agree with the previous statements from those that have stated they have never seen a failure etc etc. - I haven’t either, but if the plane is completely stripped down for rebuild, why not take advantage of that situation to create custom finishing parts.
This is (was) the bushing in my Model 4. The plastic was very crumbly, though there was enough of it left so that the control column wouldn't pop out.
Attachment 31433
Well looks like we’ve all witnessed our first failure. Glad you made it back safe from that - I bet that was a pucker factor 5 moment.
Aluminum ones are gone or I’d offer you a freebee - I am running a few out of carbon fiber with a small inlaid groove for a delrin insert so it too will be self lubricating ( even though range of motion on this thing shouldn’t really need large amounts of looob).
whenever I finish those up and if they function as intended I’ll happily send you one for the fee of a bit of feedback if you at all need one still.
Quote:
Originally Posted by txmatt
It appears that your assembly may have been exposed rather than covered like most... is that correct? Delrin doesn't normally "age" and get brittle, except with long-term exposure to sunlight. Then again, it might not even be Delrin (just a possibility?). Glad you caught it... be kind of hard to miss. There is no real upward force on that bearing, unless you pull on the stick.
Quote:
Originally Posted by AvDES LLC
Thank you very much for the kind offer. I made my replacement out of UHMW polyethylene.
Attachment 31434
The manual I have just calls for polyethylene, which is what I assume the original was made out of and is what I used for the replacement. I did some googling but didn't find much info, I assume because these aren't typically a problem. This thread and all the talk of Delrin has me wondering though... Is that what is currently being used? Perhaps there was a change along the way.Quote:
Originally Posted by jrevens
I'm not sure what you mean by covered? Perhaps that's a post-4 thing? This assembly sits on top of the floor board in my Model 4. As far as I know the plane was always hangared, so it wasn't exposed in that sense.
The new Delrin bearing block has several advantages over UHMW polyethylene:
- It is a simple rectangular shape, not machined down in the bolt boss areas. This gives substantially more wall thickness in the areas where yours broke.
- Delrin is 3 times stronger in tensile strength; 10,000 psi vs. 3100 psi.
- Delrin is over 20 times stiffer; 420 vs. 110,000 flexural modulus.
Its one of the things that has definitely been improved over the early Kitfox models.
Quote:
Originally Posted by txmatt
It is under the rear floorboards on the newer series.
Interesting it cracked like that, probably as was suggested it was in the open and exposed to UV too long
and became brittle. The real Jesus bolt is the one holding the lift struts on, but I've never heard of one of
those shearing ...
Jeff
now let’s not put that into the universe. If one did shear, I’m sure we’d never hear a first hand accounting of the story.Quote:
Originally Posted by n85ae
That wing strut bolt was one of the changes in the early-production Series 5 kits that raised MGW from 1,400# to 1,550#. It changed from an AN4 (76,000psi shear) to an NAS1104 (95,000psi shear).
Attachment 31436
Quote:
Originally Posted by Eric Page
Has this question been answered? I see a comment about a Delrin inlay as well as a carbon on.
This has been addressed. I have reworked the design to an all carbon fiber body with a delrin inlay to allow for minimal friction between the control shaft and the bearing itself. This thread was more intended to get people thinking about that little lynch pin in the design of some of these planes. The older builds most likely still contain that brittle plastic piece and it should be addressed sooner than later by the owner.Quote:
Originally Posted by Podunc
Thus was a great post no doubt! It’s awesome to see everyone’s innovative ideas.
They make that plastic in white which is a virgin plastic, and in black which i believe is the recycled plastic. The white is slippery er than the black, but the sun will eat at the white and make it very brittle and cracks very easy. The black doesn't get attacked by the sun as bad, so last alot longer.
We use 4x8 sheets of white to cover the inside of the hopper on the gold plant to keep dirt from stickin and also on the bottom of our skis. Have never tried the black on those places
Could you be a bit more specific? "Plastic" is a rather vague terms since there are numerous types (White mentioned in this conversation is most likely teflon whereas black is delrin and the material properties between the two are not alike). Polyethene has also been mentioned here so a bit more information about the material you're mentioning would be useful.
Here’s the update for the base portion of the bearing. You can see the ridge that allows for delrin bearing retention and the foam of the ridge allows for 60 thou diametric gap between the ridge and the control tube to ensure all contact with the tube is through the delrin only.
Attachment 31933