Yoke Shaft bushing

[TimHicks]

My yoke bushings (the rectangular pieces of phenolic that the yoke passes through at the instrument panel) are worn.
The pilot side worse than the co-pilot side.

I'm planning to buy some new sheet teflon from McMaster to fab new ones.

The old ones measured 0.25" thick, 2.00" wide, 2.375 tall.

The hole in the middle seems to be centered.

The smallest dimension that I get on the hole (in the old parts) is in width on the co-pilot side 0.785". So lacking any better information, I'll probably make the holes around 0.780 to 0.785" diameter.

The same old hole (co-pilot) measured 0.810" tall. The old pilot hole measured 0.820 wide and 0.860" tall.

Any reason not to use PTFE (teflon)?

[Robert Lees]

Re: Yoke Shaft bushing

No reason at all, it seems a perfectly suitable alternative to me.

When I replaced mine (in 1992 or thereabouts) I used "Oilon", a green-coloured oil-impregnated nylon, and they're still in use today, with about 2500 hours on them.

The hole is slightly taller than it's width due to the vertical "swing" of the control shaft as the pilot pulls fore-and-aft. But I cannot tell you by how much. I guess a little experimentation is required (and I assume I did the same all those years ago). In fact I think it's a case of bevelling or chamfering the vertical axis of the hole to allow for that slightly swinging up-and-down movement of the shaft.


I think I'll call it a day at that, otherwise regular readers here will think this is a sex site.
Rob

[Hank Jarrett]

Re: Yoke Shaft bushing

The engineering reason not to use Teflon is it can "cold flow" if there is a small load applied for a long time (like sitting in a hangar with the weight of the control shaft on it). The real world answer is that the Teflon would flow so little in our lives you probably couldn't measure it, but the factory shows phenolic and an retentive inspector could give you a hard time. Electronics suppliers can provide the phenolic if you want to go that way.
Hank

Is anyone looking?

[Ray36048]

Re: Yoke Shaft bushing

I am also working on making a set for my project. I don't think the hole is perpendicular to the surface. I believe it is something like 12*. The diameter looks like 0.750 for a start. I have not been able to locate the ¼ inch rubber channel that was used. The width and height mat have to be changed dependent on what is used in place of the rubber channel. I am thinking about using felt. So far, I have made a couple out of plywood to determine the correct setup. I plan to use nylon for the final part.

[rammon3]

Re: Yoke Shaft bushing

Keep in mind what Hank said. A lot of IAs will not look favorably on substitute materials like this. Unfortunately, we live in a litigious society such that these guys have no choice but to cover their asses. If you want to make a change remember "In God we trust, all others must have approved data".

[Hank Jarrett]

Re: Yoke Shaft bushing

If i was going to modify mine I would go all the way. I saw one where the owner put in a floating ball with a linear bearing for the control shaft. The linear bearing allowed the wheel to rotate and translate fore and aft with almost no friction and the ball rotated to align with the shaft, again for almost no friction. With roller bearing pulleys, flexible cables and well lubricated hinges the difference was noticeable, but not worth the huge cost in my opinion.
Sometimes a new material is un-arguably better, but not actually justified for the cost.
Hank

With the control shaft bearing material, it isn't the cost of the material, it's the cost of doing it legally.

[astjp2]

Re: Yoke Shaft bushing

I have some 1/4" phenolic you can have, if the chome is real nice, the shaft will flow like a hot knife through butter. I drilled mine to .750 then used a flap wheel to open it up until the shaft was real smooth moving back and forth. Rechroming the shaft really helps. Dont lube them with a wet lube, use something dry like graphite. Tim