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Experiment: Pure twist

If you use a spanner to undo a nut, the twist you generate comes from the force you apply.
This system of links will apply a pure twist to the larger cardboard shape. Pure twist means twist without a resultant force, like the twist you get with this old wheel brace. Download the drawings of the links if you want to do the experiments yourself. We cut these shapes from corrugated cardboard with the corrugations running along them. We made holes in the links, positioning them as precisely as we could. The drawings show you where. The next part was fiddly. We laid the links out on the table and connected them with string, like this. Then we suspended the complete linkage assembly from the support beam, like this, using the force transducers on the larger shape.
We bent three hooks out of identical paper clips and installed one in each of the two holes in the main shape under each transducer, and one in the middle of the lowest link, like this. Now for the experiment. We’ll load each of the hooks on the main shape with the same number of washers. We used 15 on each. Now, we’ll adjust the links of the top mechanism to make them roughly horizontal. It shouldn’t be too critical, but do the best you can. This is our datum condition. We will measure the height from the top of the main shape to the bottom of the support beam and record it. Next, we’ll load the bottom link of the twist mechanism.
We’ll use 10 washers. This applies a pure twist to the main shape. You can see it rotate. It’s what we expected. We will measure the height again, from the top of the main link halfway between the transducers to the bottom of the support beam. It’s not changed. The next part is neat. Take 5 washers off the right hand hook on the main shape and put them on the left hand hook of the main shape. Look at that. We’re back to our datum condition. The shape is horizontal again, and the height is as before. The transducers are the same length as before, so the total force on the main cardboard shape is as before as well. But the loads aren’t symmetrical.
Somehow, our linkage has applied a twisting effect with no net force. It’s time for some analysis.
If you use a spanner to undo a nut, the twist you generate comes from the force you apply.
The twist and the force come as a package.
If you use a screwdriver to undo a screw, the twisting effect comes without a net force (apart from the force along the axis to keep the blade in place, but that’s another story).
Two parallel, equal, opposite forces generate twist without unbalanced force. This is a ‘couple’ or ‘couple moment’. It has many applications in engineering mechanics.
In this experiment you’ll build a system out of cardboard and thread that generates a pure couple. You can use the force transducers to check it out.
You can download instructions to the experiment in the Downloads section below.

Talking points

  • What everyday situations involve pure twist?
  • I think the mechanism is elegant, see what you think? (Robin Ford)

Share your experiment

If you attempt the experiment, take a photo and upload it to our Through Engineers’ Eyes Padlet wall. You can include a link to your photo in the comments for this step (click on your post on the Padlet wall and then copy the web address).
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Through Engineers' Eyes: Engineering Mechanics by Experiment, Analysis and Design

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