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Building Tips and technical articles. => Building techniques => Topic started by: Motorman on July 13, 2018, 11:11:23 AM
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How much friction between the center pin and the bellcrank is relative to diameter of the pin? I've always used 1/8" music wire but what if the pin was 1/2" dia? More area but less pounds per square inch.
First, on my bigger models I use 5/32" wire instead of 1/8. That's mostly out of general paranoia, but it's what I do. It works fine.
Second, if I'm working this out right in my head, and if you use the high-school physics model of friction, it all works out to the same thing. But -- that's two 'if's. The Sig nylon bellcranks use bearings that are about 3/8" in diameter, and they're fine in that regard. All in all, I don't think that the bellcrank pivot friction has a chance of dominating the overall control system friction, but it's what the entire plane hangs off of in flight. I'd go ahead and make it stout.
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Big pin would have more friction to due more contact area in the larger circumfrance, in my estimation. Material used needs to be taken into account.
Type at you later,
Dan McEntee
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The question is which would have less friction big pin or little pin, in both examples materials are the same.
- Simple theory says "no difference."
- My intuition says "difference" (think of your half-inch hole, or a one-inch hole. If this were an engineering project, a good manager would send someone off to find out, unless...
- My experiences says "it's minuscule, stop worrying" -- at which point a good manager would say "OK, finding out is a waste of time, just do it, get to work."
If you're still concerned, why don't you grab some scrap material and do some testing?
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Big pin would have more friction to due more contact area in the larger circumfrance, in my estimation. Material used needs to be taken into account.
Type at you later,
Dan McEntee
I was going to point out that you're wrong, then I realized that my "in the head" calculations were wrong.
Note that according to high school physics, the friction between two materials is only proportional to the coefficient of friction and the force pressing them together -- not the area. If you increase the area, you decrease the force per unit area, so the friction per unit area goes down -- yadda yadda. This works in absolutely every case where you have two materials rubbing against each other that studied high-school physics. (Actually, as far as I know it's mostly right, but doesn't apply to everything).
However, area aside, even if you do have materials that follow the above relation, the friction force acting at the periphery of the pivot will be the same regardless of diameter, but the moment arm over which it works will be proportional to the diameter -- so my earlier statement was wrong, because I was doing the math in my head.
But I still claim that for any reasonably-sized pivot, you're OK. The ball links, hinges, etc., are going to add up to way more friction than the bellcrank pivot, so if you just feel bound and determined to sweat bullets over something, sweat bullets over those.
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The question is which would have less friction big pin or little pin, in both examples materials are the same.
Little, if friction was the only issue.
Brett
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Something I always wondered. If you have 2 bushings, one is large diameter and short and the other is smaller diameter and long but they both have the same bushing contact area on each shaft, would they have the same friction?
Friction is one of the smokiest areas within mechanical engineering (or perhaps materials science) to pursue knowledge. The A #1 most important part of the answer is what materials you're using -- and you're just saying "bushing", as if all materials are alike.
Once again, if the materials in question have studied high-school physics, all that matters is the diameter, not the area (so, not the length).
That doesn't bear on how they wear.
Why don't you do some experiments, advance the state of the art, and then let us all know what you found?
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Use grease.
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Use grease.
For small areas, maybe. For large areas the shear from the grease might easily exceed the friction. When I lubricate something on a stunt plane control system, I use the lightest of light machine oil. Note that plastic ball links should generally be left dry because any sort of oil might cause them to stick.
Brett
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For small areas, maybe. For large areas the shear from the grease might easily exceed the friction. When I lubricate something on a stunt plane control system, I use the lightest of light machine oil. Note that plastic ball links should generally be left dry because any sort of oil might cause them to stick.
Brett
Agreed - if it was that important i'd recommend a graphite dry lubricant or make the bush out of PTFE!
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Agreed - if it was that important i'd recommend a graphite dry lubricant or make the bush out of PTFE!
Straight teflon is not a great idea, because you also care about the potential for wear or cold-flow. Try "slippery delrin" instead
https://www.mcmaster.com/#catalog/124/3720/=1dpmpu1
Which is delrin with Teflon embedded in it.
Brett
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Good point - and Delrin is nicer to machine as well!
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Long ago, I turned some low friction plastic bushings on a Hardinge "chucker". I can't recall the name of the plastic, but it was said to have graphite incorporated and was black. Seems to me like the name of the stuff was something like "Viton", which isn't right, but similar name.
There is also a "slippery nylon", which is a whole lot cheaper than the "slippery Delrin", like 10% of the cost. I have not been real impressed with oilite bronze, but in this case, think it might be a better material for temperature and humidity considerations. Who is going to be the first to put a zerk fitting or oil hole on the bottom of their plane? :o Steve
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Who is going to be the first to put a zerk fitting or oil hole on the bottom of their plane? :o Steve
Way ahead of you on that one, about 25 years.
Brett
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As Tim alluded, tribology can get tricky. First order things get swamped by something that you forgot to account for, but more often people worry about second order things and forget to factor a really important item.
A few things to think about in the Motorman quiz:
The usual--but not exclusive--reason for a large journal is to increase load carrying capability with a given material combination. In general, this is not the way to reduce friction unless something else is being changed as well.
--local contact area (and resulting stress) after shaft deformation. The small shaft may carry the load, but if it deflects very far it will reduce operating clearance. A long, small diameter shaft that you attempt to run at close tolerances is a potential binding problem. Rarely an issue for a bellcrank installation using the sloppy fits of most ready-made parts!
--deflection and hugely non-precision parts result in the actual area in bearing being far less than the first simple view will use in a calculation. Hence, you get horrible correlation from these flawed calculations and actual results. This makes people think of this type of engineering work as "black magic." This one is knowable and should not be part of the “magic” grouping.
--when a material cold flows, the precision goes out the window, and the contact area is "adjusted" but just for the geometry/position when it sees the load. (Imagine a Teflon bearing that got pull-tested with the controls at neutral every time.) So now the area is even more unpredictable and will continue to change.
--Contact stress for galling or material transfer. Shouldn't be an issue for most things in a model airplane. But I wouldn't use K&S aluminum tubing for guides on solid steel leadouts, either. The tiny diameter and contact just at the rim of the tube where the lines may come out raked makes for a very small contact area. There are tables for galling stresses for various material combinations. It would be more likely to see this on an aluminum collet for an engine drive hub, especially if it was put together squeaky clean with no oil at all.
--Be aware that tables showing coefficients of friction make assumptions about surface quality and wear-in. If you do not duplicate these assumptions in your actual parts then your results will not seem textbook and you will be mumbling about black magic again. For a precision machine, you might need to burnish mating parts during assembly, or operate it thru a certain number of cycles to wear off any surface asperities to get a stable friction number. Kind of like we did with old technology engines. Wear-in for plastic parts is a whole different deal, and I would not assume friction will go down any time soon. So if the bellcrank/pushrod joint is too tight now, it might stay that way right up to the point you are fed up with the plane and either crash or get rid of it.
I just finished reworking a Brodak white nylon(?) bellcrank so that I was comfortable using it. The molded contact diameters were super loose to allow it to work since the diameters are not even close to round, and they have mold draft/shrinkage in them too. The wobble was so bad that I would have needed huge cutouts in an I-beam spar for a mid-sized plane. Not good. So I drilled out the crank and bushed it with K&S thin wall tubing, pressed in. The nylon(?) bushing was not going to help me because the hole in it was not a close fit to the 1/8" music wire shaft I planned to use instead of the 6-32 screw. So I replaced it with a metal bushing. Now have close tolerances and precision (well, relatively!) fits but have to provide lubrication because these materials are not self lubricating. This combination, in new condition, has low drag using a 3/8" (?) diameter journal.
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How much friction between the center pin and the bellcrank is relative to diameter of the pin? I've always used 1/8" music wire but what if the pin was 1/2" dia? More area but less pounds per square inch.
The school solution, as I recall, is that the inverse relationship between area and PSI makes the total friction the same. But a bigger bearing area will last longer and be more tolerant of softer materials. Thus a very small bearing area would need to be made of heavy, expensive, and hard-to-work materials, like ball bearings.
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I used oilite bronze for the exits for leadout slider. And bearings for the bellcrank. We'll see how that works.
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The school solution, as I recall, is that the inverse relationship between area and PSI makes the total friction the same.
But to get moment (torque), which is what you want to minimize, you multiply that friction force by the radius of the axle, don't you? Hence the wee pin, rather than a structurally better and lighter tube.
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I never forget the influence of vibration by designing these junctions.
We used to say: "The little invisible Green Gremlin is sitting on the pushrod, bellcrank etc. and knocks always it with a big hammer..."
So I never use metal-metal connection.
Formerly we used highly polished 3 mm ( 1/8" ) dia. spring steel as pin, with textile-bakelite, now with ZX-100 type self-lubricating plastic. Textile-bakelite had got one drop of oil by building, ZX-100 does not require that. No friction was observed practically, and the 8-9 mm (1/3") thickness of bellcrank was quite enough to bear all the forces appearing by a fullsize plane. After some 12-13 years service time no wearing could be observed.
https://plus.google.com/photos/117790355930193335731/album/5715088163776972593/5715089616610138770