bach33089

bach33089

Member
Well, I have sworn up and down that I want to remain within the context of the plans as much as possible, but the further along I get, the more room for MINOR improvements I see. I have NO intent on redesigning the airplane, but I have been contemplating a small change for some time and I wanted to see if anyone has tried it.

First some background, when my control sticks arrived, their bushing's inside diameter was insufficient to accept the correct bolt size as called out in the plans. No problem, drill press to the rescue, or so I thought. The drill bit broke off in the middle of the work, leaving a rather rough and abrasive surface, NOT GOOD for a bushing!!! Rather than order a new one from Van's I have been contemplating stealing an idea from a few friends of mine in the airshow business. The brass bushings just won't cut it with most of their hard pulls and they need to be lubricated often. The quick answer was to go to a Higher Performance plastic known in the trade as Nylatron. They have had tremendously good luck with it, as it is highly wear resistant and self lubricating. I am strongly considering machining my own Nylatron bushings for the controls. Has anyone done something similar? Also, does anyone see any problems or "gotchas" with this? The other good thing about this material is that its failure mode is NOT catistrophic, thus the likely hood jammed controls due to a bushing failure becomes almost non-existent.

Let me know your thoughts, this is a "too good to be true" scenario and some feedback would help me feel more comfortable going forward!
 
Nylatron is, like any nylon, hygroscopic and because of this is not particularly dimensionally stable so you will have to take this into account when sizing your clearances.
 
Order new brass bushings, and the appropriate chucking reamer. Works great. Hard to imagine a catastrophic failure mode for the installed brass bushing, either.
 
Brass, Bronze, Copper, Lead or any soft metals

can be difficult to drill. Boelube helps and step up your drill bit diameter in small increments until you reach the size you need. I used a reamer on mine as well as Boelube.

As far as the plastic, have not seen it used. For me just bushings, spherical balls and ball bearings are mostly what I am used to. Sorry, no ideas there.
 
I can't say if the Nylatron bushings would work or not, but it is important to realize that the brass bushings are held tight in the mounts by the bolt, and the stick (or bellcrank) rotates about the bushing. It is easy to tighten the bolt to hold the bushing stationary. If you did that to the Nylatron, would it "squish", increasing the diameter and causing the mounting ears to contact the stick or bellcrank?

A question to answer if you try and go this way.

Paul
 
Thanks everyone for the advice/feedback. I think I am going to machine up some Nylatron and see how it fits/feels. Being that Nylatron is much harder and has a much higher strength than the more traditional UHMW plastics already used in the kit, I have good feelings about its ability to resist fatigue loading and environmental impact. Nylatron's porosity is MUCH lower than traditional nylon plastics and is NOT prone to absorption, I ran a test on this by submerging a piece of Nylatron of similar size and shape to the proposed bushing in a warm water bath for 24 hours. After drying to a "saturated surface dry" condition, the amount of moisture retained was < 0.002% the mass of the bushing, and I could not measure and increase in size with the tooling I had available. I should have admitted that in a previous life I was a civil engineer specializing in material sciences. Thankfully, I am not building a target this time, or so I hope! I will update this post once I have had a chance to observe the bushing under a few load cycles in the control stick, Paul's comment has me wondering.... Gotta love science vs. reality!
 
Polymer creep flow

Please keep in mind that polymers will creep (change shape) slowly under continuous pressure. "Harder" plastics just take longer. I think this would apply to the pivot bolt load imposed on the nylatron bushing.
SO, from a practical standpoint, it will be important to check the bearing after some period of time to see if it is spinning on the bolt (undesireable per Vans design). The initial install will not necesarily be indicative of long term bushing geometry resultant after creep flow reduces compression load enough to arrest the flow.
 
Foster, to reiterate what Paul said, just make sure you're clear on the fact that this isn't a free floating bearing surface. The brass bushing is fixed by the clamping force of the bolt and the control stick weldament rotates around it...
 
You must log in or register to reply here.