Experiment: Pure twist

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).