Pushrod Bushings/Bearings

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Aerowerx

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I have read where bushings or linear bearings can be used on long pushrods to prevent buckling.

Since the pushrods do not move in a straight axial fashion, how are the bushing used? Are they slightly over sized to accommodate the motion of the pushrod?

In the same light, how do the bearings work. Or are they also over sized?

Then there are idler arms, which would solve all these problems.

I was looking at one build log, for a Sonex IIRC, where the guilder removed the bushings and installed idler arms because he was unable to ge the full elevator range of motion.
 

TFF

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Im not sure where your bushings are? The ones in the ends of the pushrods or are you talking about something going through the bulkhead?
 

Aerowerx

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First, I do not have any bushings. Not yet anyway. Just trying to decide---bushings (which are simpler and lighter), or idlers.

And then, since I was talking about "long pushrods to prevent buckling"....Won't do any good at the ends, will they?
 

Victor Bravo

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If you use linear bearings or plain bushings, you of course will have to oversize them as mentioned. This leaves you with the potential to have "soft spots" or mush in the controls. Depending on the type of aircraft, you can get into flutter, or just not have it as crisp as it could be. Even without these catastrophic problems, there is the chance of troublesome wear, or having excessive friction built into your aircraft.

I have a friend with a Cirrus VK-30 (one of the few), and he has very stiff ailerons in flight What he wouldn't give to be able to get into the wing and fix it now!

It thus seems to my feeble mind that any small amount of extra effort or cost while you are building it is worth it in the long run. So better quality parts, or spherical bearings where you might have gotten away with a flattened tube and a clevis bolt, and having large enough clearance holes in ribs or bulkheads, etc. seems like a good investment.

Besides, this is the textbook scenario that idler arms were invented for. IMHO, if you can afford the room and the mounting braceket and the few ounces of weight....
 

TFF

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There tends to be bushings at the end of pushrods on some designs too, ya know. Your answer is they let the pushrod defect so much and then arrests it. Not tight around the pushrod. Simple and light instead of perfect.
 

Birdman100

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Oops. :nervous: Not a good idea, but I do wonder if glass would stiffen the pushrod easily, lightly, and sufficiently.


BJC
Glass has low E (stiffness) thus is a bad choice for stiffening. It should be done with CF, not with braided sleeve but with unidirectional fibers (plies or rovings). But, then it is much better not to stiffen existing metal tube but to design in composite (CF) from scratch or, at least to use pultruded carbon tubes which have highest stiffness to weight ratio.

If you use linear bearings or plain bushings, you of course will have to oversize them as mentioned.
No, I dont think so.

Here it is a nice example:
IMG_0268.JPG

IMG_0396.jpg
 

bmcj

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What about a Pulltruded rod through the center, set off (held in center) with nylon discs evenly spaced along the length? It would add some stiffness and come into full play if the rod did start to buckle fully.
 

Birdman100

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However, the concept is not that trivial as it may look from images, first I would do contact surfaces in metal (not bad idea nylon discs as bmcj sad; but never leave bare composite to rub on something - it can be overheated due to friction - not likely scenario but why to take any risk).

Second, the rod will deform together with the structure (case with aileron push/pull tubes in wings, not the issue with tail tubes in fuselage), which means there is additional bending, stresses and friction.
 

Birdman100

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Hey people, am sorry! In the pictures in post #9 this is actually hydraulic line! My bad, I figured it up later. Nevertheless its pretty similar to how push/pull is derived in some designs.
 

ScaleBirdsScott

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I like the idea of idlers as it just reduces column lengths all around, and if need be opens up a chance to use it for leverage if it makes more sense to do ti with an idler than with a bellcrank.
 

Blue Chips

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Wasn't there a discussion here about filling tubes with expanded foam to add rigidity, that would be simple and light.
There is a big difference between the commercial expanding foam and the small DIY cans bought at the lumber yards.
 

BoKu

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I have read where bushings or linear bearings can be used on long pushrods to prevent buckling.

Since the pushrods do not move in a straight axial fashion, how are the bushing used? Are they slightly over sized to accommodate the motion of the pushrod?..
Standard practice for European sailplanes, which I use in the HP-24, is to use close-tolerance guides (rollers or linear ball bearings) and just let the tube bend to accommodate the arc of the bellcrank arms. Clearly, this isn't going to have universal applicability--combining large OD tubes, short distances between the guide and the bellcrank, and short bellcrank arms, will probably result in short fatigue life. However, 5/8" OD tubes, ~24" between the bellcrank and the guide, and ~3" bellcrank arms, seems to be within the envelope of established practice for many European sailplane designs.

For the linear ball bearings, I use a version of this that I make myself. However, mine have 6 balls instead of the 4 that the ACS parts have:

PUSHROD BALL ROLLER GUIDES from Aircraft Spruce

This is one of the detail design practices that I learned by picking apart broken European sailplanes. I strongly recommend visiting as many aircraft repair places as possible and looking at crashed, broken, and under-repair aircraft. You can learn some amazing things about how various aircraft are built.

Thanks, Bob K.
 

BJC

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I had an idea today about this....

Would it work to put carbon fiber over a PVC pipe (schedule 40 type---the white stuff)? Or would it be too heavy?
It might be lighter to use aluminium covered with a thin layer of glass followed by CF, or just CF over aluminium, then chemically remove the aluminium.


BJC
 

Dana

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I had an idea today about this....

Would it work to put carbon fiber over a PVC pipe (schedule 40 type---the white stuff)? Or would it be too heavy?
It would be too heavy; the PVC would contribute nothing structurally. Though I suppose you could put release agent on the PVC to use it as a form, then pull it out.

Dana
 

Tiger Tim

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I would think if you were going to put carbon over anything as a push rod, either foam or balsa wood would be best. Both are light and easy enough to shape into the ideal form which IIRC would be tapered at both ends.
 

Aerowerx

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Then you could dissolve out the foam with a solvent, leaving a carbon tube.

I thought I saw something like this on some web page, but don't remember where.
 
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