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Old 04-14-2016, 10:37 AM   #1
dominator
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Default push pull geometry question

Attached is an example. Im trying to figure out if push pull rod length will affect the geometry of lever arms. If my servo arms are 18mm and swash balls are 21mm from main shaft. Does the length of my push pull link change the geometry at all?
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Old 04-14-2016, 01:23 PM   #2
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Yes but very little.

push pull geometry.pdf
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Old 04-14-2016, 01:26 PM   #3
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Ignore the 67.2 dimension at the bottom it snapped to the wrong point.
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Old 04-14-2016, 02:29 PM   #4
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Thank you
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Old 04-20-2016, 02:42 PM   #5
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for proper push/pull that doesn't bind you must have all pushrod connections at 90 degrees to the output arm. Otherwise, as pictured in the first post, you'll have binding anywhere but at a single point in rotation. this angle changes depending on the distance appart the two levers are located, and also the difference in length of the arms. The easiest way is to draw two simple circles in cad the diameters you need for the increase or decrease in ratio you need, and then plot two tangential lines between the circles. The point where the tangent lines intersect the circles is where you want the points. Attached is an image showing your arm lengths, with distances between pivot points of 20 and 30 to show the different "offsets" you'd need for the ball locations. i hate that term offset, as it's not offset, it's proper and 90 degrees,.. but offset sort of gets the point across based on the image the third image is of a much smaller wheel to show how the "offset" changes. In other words, you'd have to re-plot to get perfect geometry any time you change the diameter of either bellcrank, or the distance between them. Push/pull with a straight across arm as the first poster shows is just a recipe for advanced servo wear and tear. If you have that sort of geometry and don't want to re-drill some good servo wheels to place the balls appropriately, I would highly recommend running the servo mounting screws very loose to let the rubber servo mount dampers compress in either direction as you move the controls and torture the servos
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