How does an articulated gate arm work?

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How does it pivot at the elbow, and the front part stays level to the ground while the back half pivots up?

In: 2

If you have a parallelogram with pivots at the corners, the sides will stay parallel to each other, so you can have one segment not move at all, and the segment opposite will move but not rotate. If you have an image of a specific gate you’re confused about, that would help.

edit: Is this what you’re talking about? https://imgur.com/PvzpdrZ

You’re talking about the gate arms in parking garages?

I’ve worked in the industry but have barely touched that variety. But I’ve been told about them by colleagues.

There’s gearing hidden in… let’s call it the “shoulder” joint of the arm, which attaches to a rod which moves the “elbow” joint.

The gear ratio is effectively 1:1 but reversed, so that for every degree the shoulder joint turns, the elbow joint turns a degree in the opposite direction, keeping it level.