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I have the following setup, consisting of a bar attached to another bar via a ball joint (indicated by a circle, and attached to the wall over a pulley with a weight at the end (the small "kg" symbol). I want to pull on one side of the bar. This is sort of like a one-handed cable row at a gym.

Top view: Side view:

But when I pull, indicated by the black arrow, the angle (in red) between the force and the bar gets wider than the 90 degrees before the pull.

Top view:

I thought of adding a blocking element:

Top view:

But this would not help, as the pulley always aligns with the pulling force vector.

Top view:

How can I pull, while keeping everything (the angle between the two bars, but at least the angle between the back bar and the force vector) at 90 degrees? I hope this makes sense. I am thinking about something between the two bars, such as a spring.

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  • $\begingroup$ Put an extra handle on the bar so that your hand is inline with the pulley - making a C shape. $\endgroup$ – Solar Mike Nov 2 '20 at 7:30
  • $\begingroup$ Unfortunately, I cannot pull from the middle, I need to somehow tweak this existing configuration which is part of a larger project. $\endgroup$ – berndibus Nov 2 '20 at 7:35
  • $\begingroup$ Well you are applying an offset force. It will twist. I suggested a tweak that overcomes that and you did not say no changes in the question. $\endgroup$ – Solar Mike Nov 2 '20 at 7:37
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    $\begingroup$ a) Can you use guide rails on either side? B) is the force applied variable or constant? $\endgroup$ – NMech Nov 2 '20 at 7:42
  • $\begingroup$ @NMech I need the degrees of freedom, but something like guide rails might be the only choice. I hope there is be another way, some construction between the two bars. $\endgroup$ – berndibus Nov 3 '20 at 7:23
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Idea 1: Adjust the pulley attachment position so that it is in line with the pulling force. If this is a rowing machine, then maybe use 2 pulleys like this:

modified diagram

Idea 2: Use a linear bearing to prevent the bar from twisting:

enter image description here

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