Twisted String Actuators

[Travis] tells us about a neat actuator concept that’s as old as dirt. It’s capable of lifting 7kg when powered by a pager motor, and the only real component is a piece of string.

The concept behind the twisted string actuator, as it’s known to academia, is as simple as putting a motor on one end of a piece of string, tying the other end off to a load, and putting a few twists in the string. It’s an amazingly simple concept that has been known and used for thousands of years: ballistas and bow-string fire starters use the same theory.

Although the concept of a twisted string actuator is intuitively known by anyone over the age of six, there aren’t many studies and even fewer projects that use this extremely high gear ratio, low power, and very cheap form of linear motion. A study from 2012 (PDF) put some empirical data behind this simple device. The takeaway from this study is that tension on the string doesn’t matter, and more strands or larger diameter strands means the actuator shrinks with a fewer number of turns. Fewer strands and smaller diameter strands take more turns to shrink to the same length.

As for useful applications of these twisted string actuators, there are a few projects that have used these systems in anthropomorphic hands and elbows. No surprise there, really; strings don’t take up much space, and they work just like muscles and tendons do in the human body.

Thanks [ar0cketman] for the link.

39 thoughts on “Twisted String Actuators

      1. I made up the “Ethiopian string ratchet” descriptive name a long time ago when I saw one in use to make grass rope in Ethiopia on some old documentary video. The operator alternately pulls opposite ends of a string down, and both pulls turn a central shaft (stick) the same direction, which spins a wooden flywheel. I called it a ratched because pulling either end of the string increases the speed of the flywheel. Mutliple searches in Google over the years have found nothing on this. I will try to describe it: The string winds a few turns around the central shaft, up and over and behind, then down to another fixed shaft (acting like a pulley), then up over the central shaft from behind and down over the front a few turns. The loose ends tighten when pulled, gripping the central shaft and turning it downward toward the operator, no matter which string end is pulled. The central shaft can have a flywheel attached, made from wood or stone. Descriptive enough? Is there a better name for this device?

        1. Sounds like a very interesting mechanism, but I don’t quite get it. How many shafts are there and how are they placed? Your description suggests two shafts, but I can’t seem to get how the rope is wrapped to produce continuous rotation in just one direction. Sorry to be dense, but I’d really like to understand this.

          1. It’s got two separate ratcheting mechanisms to apply force to flywheel. The flywheel has a clutch like in a bicycle so it only spins one way allowing a person to pull on one ratchet lever while resetting the other. There are ropes attacthed to each ratchet lever to make it more convenient for the user.

          2. The only “clutch” is the tightening of the strings, which grip the shaft like a clutch when tension is applied to one end or the other. The key is that the winding direction of each end assures that they both increase the speed of the shaft when pulled.

          1. Thank you for that information. I thought the old documentary mentioned Ethiopia, but perhaps it was Bangladesh. It is a spinner, and as I interpreted pulling the strings to operate it, it seemed to be “ratcheting” the shaft to spin faster. Now we have a better name for such a primary and interesting low-tech tool.

          2. One picture is indeed worth a thousand words in this case. Amazing, and simpler than imagined by the other commenters. Just rope and two shafts, no clutch, no gears, no other mechanism.


        2. That sounds like many of the old treadle lathes, but modern enough to have a clutched flywheel. Only instead of a pedal to power it you’ve got some levers with pull-strings on them.

          1. +1 The verbal description failed to fire the right neurons, but the website did the trick. Very simple, but equally, very smart…. .. I too learned something new.

            That idea would also work for a hand powered pole lathe, power to a drill bit, a spinning wheel for cotton/silk/wool etc, in fact anything that requires continuous rotation in one direction… a very neat idea.

          2. Actually, I was remarking on the verbal description (I just carefully imagined it and it made sense), since I replied on a page that had not been refreshed (hence lacked the link to the web page).

            But the web page definitely helped to confirm that I had it right, and I am amazed that someone was able to find such a great reference so quickly.

            The twisted string actuator and the string-clutched-spindle-thingy are now part of my design vocabulary.

            And, just to give back a little .. Is anyone familiar with reciprocal frame structures? These are another high/low tech jewel.

  1. Here’s a video put out by a Korean group

    The examples in the article aren’t really related to string actuators. Twisted string actuators (TSA) are a force transmitter. In seige engines such as the ballista skeins of rope are used to store and rapidly release energy. That’s why they’re called torsion engines. Bow drills have more in common with a belt and pulley than they do with TSAs though neither pullies nor TSAs convert linear movement into rotational movement, So I guess rack-and-pinion arrangements would be a more apt comparison.

  2. But I assume if your using this to lift a weight and the weight doesn’t have any method of resisting the torque from the twisted string all you’ll get is a spinning weight instead of a lifted one.

    1. Yes, the actuator needs a “slide” (perhaps cabinet or drawer rails, or a pair of rods) or other constraint method to prevent payload rotation. It also actuate a hinged joint, such as on a robot arm.

  3. A commercial application: makes a towed propeller+alternator for yachts. The propeller spins 100 or so feet behind the boat (so it stays submerged). The simple rope line transmits the rotational force to a gimballed waterproof alternator on the aft deck of the boat. Alternator output fed to batteries…wala! Small amount of drag exchanged for DC volts!

    (I know units would be amp-hours.)

    Happy customer, no affiliation:

  4. The interesting thing is it isn’t listed as one of the “simple machines” (lever, screw, inclined plane, pulley etc.). The rollamite was a new one, but I don’t think I’ve seen a wound string.

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