Screws turn twisting into holding — Engineering, 7–10 years
A screw’s sloping thread helps it enter a material and hold parts together. Engineering, 7–10 years.
A screw is a wrapped slope
Look at the ridge winding around a screw. It is like a long, sloping ramp wrapped around a narrow cylinder. Turning the screw makes that ramp move forward, so a small twisting motion can pull the screw into wood and hold two pieces together.
Why screws were useful
Nails can join pieces quickly, but pulling them out or taking a thing apart can be difficult. People needed fasteners for furniture, machines and tools that could be tightened and removed. The screw solved this problem: its thread grips the material, and its head gives a tool somewhere to push or turn.
Joining two pieces of wood
Place one wooden strip over another and line up a 30-millimetre screw with the hole. Turn the screwdriver clockwise six times until the head is level with the surface. The thread pulls the upper strip down and grips the lower one. Turn anticlockwise to loosen the same joint.
The reasonable mistake
It is tempting to think that any screw will hold anything if you turn it hard enough. A very short screw may not reach the second piece, while a very long one can come through the other side. Engineers match the screw’s length, width and thread to the materials and the load.
Screws around us
Screws hold door hinges, shelves, bicycle parts and the handles of tools. Small screws join the case of a toy or pair of glasses, while large ones can anchor a wooden deck. Some bottle tops use a thread too, so turning the cap moves it down and seals the bottle.
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