Polymer Bearings

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DaveD

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Given their light weight and suitability for high load & low speed applications it looks to me like oil filled nylon, delrin, or PTFE filled polymer Bearings would be a good fit for a variety of homebuilt applications, especially in the control and trim systems, but they don't seem to be in common use. Is this just conservative designers preferring to use tried and tested ball Bearings, or is there some more fundamental limitations that I'm missing? I guess for primary controls the absolute minimum of friction is desirable?
 

Dan Thomas

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Given their light weight and suitability for high load & low speed applications it looks to me like oil filled nylon, delrin, or PTFE filled polymer Bearings would be a good fit for a variety of homebuilt applications, especially in the control and trim systems, but they don't seem to be in common use. Is this just conservative designers preferring to use tried and tested ball Bearings, or is there some more fundamental limitations that I'm missing? I guess for primary controls the absolute minimum of friction is desirable?
You can easily overbuild a light airplane and install a lot of unnecessary stuff. Plain hinges are common in TC'd airplanes, with ball bearings showing up in heavier versions like the Cessna 18X airplanes and the 206. The Champs and old Citabrias used bolts running through bronze bushings; later versions used bolts running through needle bearings. Guess which one was more hassle? The needle bearings. There was very little rotation, and dirt, vibration and flight loads caused those needles to Brinell the bolts and make for rough operation. The old bronze bushings just kept going for years and years.

In the control systems, cable pulleys often have ball bearings. They work well inside the airplane, but after 40 years the grease in the bearing dries out and it gets stiff. Cessna uses plastic, probably glass-filled nylon, for their rudder bar bearings. In their lighter airplanes, the elevator and rudder are hinged using a steel sleeve inside a bronze bushing, with a bolt through the surface's mounting brackets and the sleeve, clamping that sleeve between the brackets so that the sleeve and bushing take all the wear. Some mechanics make the mistake of leaving the bolt loose, thinking that it' supposed to rotate in the sleeve and brackets, and that gets expensive as those thin aluminum brackets get wallowed out. Their ailerons often used piano hinges. They wear out, especially when mechanics or owners lube them with oil instead of the specified dry graphite. Oil attracts dust and dirt, which grind those hinges to nothing. Getting the dry graphite in where it needed to go was no fun; I mixed it with a bit of brake cleaner fluid and wicked that in; the fluid dries quickly and leaves the graphite all through the hinge.
 

proppastie

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the piano hinge or bolt through a bracket or tube....is quicker and easier as regard homebuilds....factory builds probably it is considered the metal or bronze bushings will wear better in the long run.....considering the age of the fleet most still having original bearings it was/is not an unreasonable assumption.
 

DaveD

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Thanks guys, I might try a mock up with both ball bearings and oil impregnated nylon to see if I can feel the difference. I was a bit concerned with stick-slip behaviour, which is horrible in a control circuit.
 

PMD

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Grumman used plastic (I think nylon) bushings for ailerons and flaps. Worked like a darn, but after 50 years, I think where they wear against the aluminum tubes is becoming an issue.
 

Dan Thomas

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Grumman used plastic (I think nylon) bushings for ailerons and flaps. Worked like a darn, but after 50 years, I think where they wear against the aluminum tubes is becoming an issue.
Plastic can get dust and grit embedded in it, and it then acts like sandpaper and starts eating stuff.

Storing an airplane outside is hard on an awful lot of things, with the wind constantly wiggling the controls just a bit, wearing the hinges and the control cables where they pass over the pulleys and fairleads. We replaced the control cables on a 40-year-old low-time (1100-hour) Cessna 180 for just that reason. The wind blows grit into everything. The wind rocks the airplane a bit, sloshing the fuel, moving the fuel gauge sender, and wearing it out. Birds and mice nest in the airplane. Mice eat it. The droppings of both animals corrode the metal. Rain and snow get into it and start corrosion and rot. The sun bakes everything including plastics and upholstery and seals and hoses, and the UV destroys plastics and paint. The heat bakes the grease hard in the control system pulleys, flap rollers, and so on. Airborne pollutants cause many problems. Neglected batteries go flat, freeze and burst, and release acid to eat the airframe. Ugh.
 

dcarr

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Van's Aircraft uses polymer (UHMW?) bearings in several places. Some I can think of off the top of my head are the rudder pedal bearings and the flap torque tube bearings. In both of those cases, the bearings are ~0.75" wide and ride on the OD of ~1" 4130 tubes.

Here's an example:

Screen Shot 2021-09-18 at 12.42.45 PM.png:
 

Dan Thomas

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Van's Aircraft uses polymer (UHMW?) bearings in several places. Some I can think of off the top of my head are the rudder pedal bearings and the flap torque tube bearings. In both of those cases, the bearings are ~0.75" wide and ride on the OD of ~1" 4130 tubes.
UHMW is soft and deforms under pressure. I'd bet they're Delrin. Or nylon.
 

rv7charlie

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I think there are a couple of designs that use sleeve type bearings over torque tubes for ailerons (BD4, and maybe Tailwind?), but Van uses Heim joints (rod end bearings) for almost all the moving parts of the control linkages. The Delrin bearings for the pedals & flaps in my RV7 kit were a bit of a pain to get working smoothly. The stuff is almost impossible to machine; just soft enough to move out of the way instead of being cut by any kind of grinding tool. And certainly nowhere near as smooth in operation as the Heim joints. Big question would be overall control forces. In a big 4 seat, highly stable plane like a C182, you'd probably never notice the breakout forces, but in an RV I suspect they would be very noticeable. They also may well introduce issues when the wing is loaded up, due to bending causing misalignment of two bearings.
 

Dana

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The stuff is almost impossible to machine; just soft enough to move out of the way instead of being cut by any kind of grinding tool.
I have to differ; in my experience Delrin is a joy to machine. Machine, with sharp tools; forget about grinding it. But yes, it is difficult to make a small cut and it does seem to "grow" a few thousandths after cutting so you have to cut it a hair undersized.

It's a great bearing material but it will never be as free moving as ball bearings.
 

Geraldc

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I use these bushes Make whatever housing you need. Easily replaced.
1632006782651.png
 

Dan Thomas

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The Delrin bearings for the pedals & flaps in my RV7 kit were a bit of a pain to get working smoothly. The stuff is almost impossible to machine; just soft enough to move out of the way instead of being cut by any kind of grinding tool. And certainly nowhere near as smooth in operation as the Heim joints
That sounds like UHMW. It's miserable to machine, and resists sanding. Delrin is hard and is easily machined and sanded and trimmed or ground.

UHMW is commonly used in industry as an anti-abrasion surface. Grain handling equipment, conveyor belt troughs, bottoms of jet riverboats. Soft but tough.
 

rv7charlie

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It may well be UHMW; I don't recall the manual calling out the material. I just know that as-delivered, the holes in the blocks were just enough undersized for the rudder pedal shafts and flap shaft that they needed to be enlarged. Took seemingly forever. Tried sanding, burrs in die grinders, whittling with a knife blade, etc.
 
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Map

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Sealed ball bearings are best, but there is no reason why other materials including plastic cannot be used. It depends on the allowable bearing stress. This can be very low for plastic, so the bearing surface has to be very large.

The bearing stress is the load divided by the bolt diameter divided by the thickness of the bracket material.

So for a given load, you need to have a large diameter bolt, but since this is usually limited by what is available and fits, the main choice is the thickness of the support bracket. In some cases this can be as thin as 1/8", for plastic it may need to be as wide as 0.5" or thicker.
 

Lendo

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I've used Delrin on a lot of things including bearings, and interestingly I used it as clamp material on metal as it it doesn't damage the substrate metal, like expensive chrome handlebars on motorbikes, when carrying accessories like Radio or GPS. As stated it machines well into any size or shape.
George
 

DaveD

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I have to differ; in my experience Delrin is a joy to machine. Machine, with sharp tools; forget about grinding it. But yes, it is difficult to make a small cut and it does seem to "grow" a few thousandths after cutting so you have to cut it a hair undersized.

It's a great bearing material but it will never be as free moving as ball bearings.
I'll second this. Delrin is a delight to machine although oil or molybdenum filled nylon may give smoother operation. I'm leaning this way for the rudder pedals and flap torque tube. I'm in two minds over elevator and aileron as any stick-slip behaviour here would be horrible for control feel.
 
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