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HelicopterGasser Model RC HelicoptersOther › Linkage design 101
11-22-2006 09:36 PM  11 years agoPost 21
AceBird

rrElite Veteran

Utica, NY USA

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I'm thinking that a push pull linkage has the lateral forces cancelled out as one side pushes and the other pulls. The only thing left to counteract is the rotational torque of the servo.
Steve, you can’t be serious.

One side pushes as the other side pulls, so you want to say each link splits the load, half is on one and half is on the other, fine. My single link is bigger in diameter, weighs the same as your two links do but will carry more load because it is stiffer (look at the buckling load of a column)

Now whether I have two links or one link “the only thing left to counteract is the rotational torque of the servo”.

I have drawn two diagrams depicting a dual link and a single link arrangement. It doesn’t matter what units you use but each will have the same load on the servo. It this case it amounts to 10 units of torque. On the left hand side of each diagram I put in some arrows with a value of 0 because there is no force at the center of the servo trying to push or pull the servo shaft longitudinally. The forces are torsion forces acting on the 4 rubber grommets of the servo which are more up and down rather than in the direction of the link.
……….and has the added benefit of not having to worry about getting perfect ball spacing and link angles to prevent binding.
You completely lost me here. Dual links require perfectly equal length links or you will not only bind you will break the link. It is like a toggle press where the forces are multiplied 100 times. The 5 units of force in each link go to 500 units of force between each other trying to tear each other apart. I think you may have broken your links because they were not the same length. And the link that popped of probably happened because they were stretched oversize over time and use.

Two things make a ball link come off:
1. they are too loose (oversize to the ball)
2. exceed the angle of the ball where the stem cams off the link (a link that buckles under column load is more apt to do this) Normally the more you push or pull the more the link wants to center on the ball and be retained. However when you start off with the link at an angle to the ball stem you have less allowable angle to work with before the caming does occur.
I've lost two helis in my illustrious career when the link rods have broken and had another link fail on a push pull setup that I only caught when doing some routine maintenance.
Sounds to me like you should be willing to rethink what you are doing.

Ace
What could be more fun?

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11-24-2006 02:57 AM  11 years agoPost 22
steve9534

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yakima, wa.

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Ace
Here's one more go. Try setting a servo on the bench and hook it up with a wheel and two balls on either side of the wheel with two links attached. Connect the servo to a receiver and battery pack so you can control it from the Tx. If you restrain one of the links simulating the resistance a single linkage would encounter and then use the Tx to control the servo, any control input to the servo will cause it to move laterally. Try the same thing again while you hold both links and the servo will only rotate.
The single link rods need to be twice as big as the push pull link rods because they have to resist both compresssion and tension forces. In compression the rod has a tendency to buckle and a single link must be much larger than the dual rods in a push pull setup because the push pull rods are primarily loaded in tension where you don't have to worry about them buckling. Only if one fails, would the remaining rod have to deal with compression loads. You've acknowledged that your single rods have to be larger and thus save no weight. Whether or not the push pull rods are stiff is irrelevant so long as they remain intact - witness the pull-pull cables used for many rudder controls on aircraft. The cables need not be stiff at all because they only have to pull.
As for your diagram, it is flawed. To use another illustration look at the wheel on a car. The power of the motor is translated into lateral motion when the wheel is rotated against the resistance on one side only of the tire against the road.
The quote about not having to worry about perfect ball spacing and link angles was in reference to MA's single link controls on the Fury.
I'm not sure what the part about the links coming off has to do with the current discussion. I did mention link rods failing, but the failures I've experienced have been with the rods breaking, not with the links coming off. Two of the three failures have been with single link, not push pull systems, so binding of the linkage was not the issue in those cases. Hope this helps. steve.

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11-24-2006 03:19 PM  11 years agoPost 23
AceBird

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Utica, NY USA

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As for your diagram, it is flawed.
I am sure that is a possibility. If so, what is flawed?

If your dual link did not see tension in one link and compression in the other link AT ALL TIMES then it would be identical to a single link that you think has these lateral forces, wouldn't it?

I'll give you something to think about. If I rotate the case of the servo 90 degrees to the way they are normally mounted and the four grommets are still preventing rotation is there still this lateral force or did it go away? If it didn't go away then the upper grommets would be pushing while the lower grommets are pulling. That might be hard to explain. Mathematically you get yourself in trouble trying to explain where the mysterious lateral force is coming from.

Ace
What could be more fun?

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11-24-2006 04:53 PM  11 years agoPost 24
steve9534

rrKey Veteran

yakima, wa.

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Ace
I'm about to sign off here, but one last reply. The diagram is flawed because it doesn't show the lateral force generated by a single link system. You can draw a diagram that shows anything, but that doesn't necessarily make it so.
Yes, a push pull system sees tension on one side and compression on the other and that is why the lateral forces are reduced. It's just that the compression load on one side is reduced by the amount of the tension load on the other side. If you use cables or very flexible link rods then the lateral forces on the servo would be the same as a single link system. So far, I haven't seen a heli that doesn't use relatively stiff link rods. Since no one does what you're describing it seems a mute point.
I don't understand the last point so I can't comment. Maybe best just to agree to disagree. Happy flying. steve.

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HelicopterGasser Model RC HelicoptersOther › Linkage design 101
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