I am trying to design a mechanism that puts tension on a string. The purpose is to dip this string in a solution for a stipulated amount of time, and once the time is up, the string is pulled back up! (only the middle of the string needs to be submerged) and yes i have access to electrical power.

The hinge is angled at 40 degrees. Giving Slack to the string.

enter image description here

The hinge is closed. There is tension in the string.

I am unsure of how to automate the process of opening and closing the hinge. I can't think of any similar applications online. I have thought of using a pulley and gear system, but the string that I am using is extremely fragile. Any other design or help will be greatly appreciated! Thank you in advance

  • 1
    $\begingroup$ Kind of need more detail. Do you have access to electrical power? What is the overall purpose of this device? $\endgroup$
    – RC_23
    Sep 24, 2021 at 6:08
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    $\begingroup$ Sounds like you need a servo $\endgroup$ Sep 24, 2021 at 6:49
  • $\begingroup$ How much linear motion would you need? What force would be required to tension the string adequately? Would a hobby "remote control" servo mechanism do the job? See en.wikipedia.org/wiki/Servo_(radio_control). They're easy to control with, say, an Arduino and a 5 V supply. $\endgroup$
    – Transistor
    Sep 24, 2021 at 7:16
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    $\begingroup$ Does the string have to be under tension? Because if not, change your setup so the servo drops the string vertically (string hangs free at the far end). $\endgroup$ Sep 24, 2021 at 11:44
  • $\begingroup$ @JonathanRSwift, I think your comment should be converted to an answer, perhaps with images and references. It seems precisely what would accomplish the task, including addressing the fragility of the string. $\endgroup$
    – fred_dot_u
    Sep 24, 2021 at 14:30

1 Answer 1


An obvious solution is to design a "punch" that fits the groove, which you drive with a linear actuator. There are many ways to do that, a lead screw is an obvious one.

enter image description here

you could use two small rounded grooves on the "punch" to fit the string (so that it doesn't get out and doesn't get squashed or mangled).

Additionally I've put a small hole in the middle that can be used to pull the punch toward the base of the hinges.

  • $\begingroup$ I don't quite get what you mean by "punch" ! @Nmech $\endgroup$
    – Dugong98
    Sep 24, 2021 at 6:15
  • $\begingroup$ I am not in front of a computer so I can't draw it ( I will do later). But what I mean is a "thing" coming from the top, trapping and pushing the string downwards. $\endgroup$
    – NMech
    Sep 24, 2021 at 6:26
  • $\begingroup$ Hydraulic or pneumatic actuator might be a good cheap alternative too $\endgroup$
    – joojaa
    Sep 24, 2021 at 7:21

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