# How to make pivoting arms move symmetrically towards each other?

I have a basic design of two arms which need to be brought together. But I tried the design using a test setup and it did not work successfully. Here is a diagram of my design -

What I observed is that arm 2 moves up before arm 1. I want both of them to move towards each other to cause a compression, and away from each other to cause expansion. Is there any way in which that could be achieved using minimal design requirements?

• Have the arms connected with gears at their pivot. Apr 3 '20 at 12:28
• @SolarMike: That's the same solution I was thinking of. Even if short and sweet it belongs in an answer, not a comment. Apr 3 '20 at 13:26

I can think of two options:

1) have the arms connected by gears at the pivot point - when one is moved so will the other, in fact one could be fixed...

2) connect the arms with a threaded rod (needs left & right hand threads) and use a pulley to rotate the rod - then the arms will move according to the pitch of the thread - actually 1 turn will move twice the pitch.

• in both these cases, how will the rotation in any one direction, cause the arms to move in opposite directions? Could you explain that? Thanks Apr 3 '20 at 13:48
• Think of a pair of calipers, some have a threaded rod (with left & right hand threads) which means they open and close with rotation of the rod. With gears if one move then the other moves... Apr 3 '20 at 13:52
• Thanks a lot! Makes sense, but just be sure, it will cause both the arms to move right? I.e none of the arms are stationary, because I want it to compress a material kept in the middle equally on both the opposite faces. Apr 3 '20 at 14:03

There are many ideas which have worked for different gears

like this drafting compass with a threaded shaft and a roller:

And this pantograph.

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You could try 2 bar linkages, it's basically what you're already trying but without a pulley and unequal displacement.