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Odd Little Things · entry 30 of 52
The screw is one of the oldest simple machines, turning rotation into a strong pulling force.
The longer version
A screw is an inclined plane wrapped around a cylinder. Each turn of the thread advances the fastener by one pitch, so a small twisting force applied over many turns becomes a large pulling force over a short distance. Nothing is gained for free: you trade distance for force, which is why driving a screw takes many turns to move it a little.
The same account covers screws used to raise water, press fruit and close a vice jaw. Friction is what lets a screw stay put, since without it the load would simply unwind the thread. Ancient writers described the device in terms of the wedge and the lever, and the traditional list of simple machines grew out of those early mechanical descriptions.
Where this entry sits
Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.
More From Odd Little Things
Eight more entriesA wedge splits material by concentrating a pushing force into a narrow leading edge.
Read 32Keys work by lifting small pins inside a lock to the exact height that frees the cylinder.
Read 33Safety pins stay closed because the spring in the bend keeps pressure on the clasped head.
Read 34Paper clips hold sheets together through spring tension in their bent loops of wire.
Read 35Staples grip paper because their legs are bent inward after passing through the sheets.
Read 36Adhesive tape sticks because a soft layer flows into surface texture and resists being pulled away.
Read 37Hook and loop fasteners join by catching tiny hooks in a dense field of soft loops.
Read 38A ballpoint pen deposits ink from a tiny rotating ball that rolls the ink onto the page.
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