The Physics of the Screw: From Archimedes to the Modern Bolt
Of the six classical simple machines, the screw is the one that hides its underlying idea most cleverly. Unwrap a screw's thread and lay it flat, and it's simply an inclined plane — the same ramp geometry behind a loading dock, just wound helically around a cylindrical shaft instead of laid on the ground. That single insight explains both why a screw can generate such enormous force from a small hand and why threaded fasteners hold their position instead of sliding back out.
The mechanical advantage of a turn
Turning a screw with a lever arm — a screwdriver handle's radius, a jack handle, a wrench — sweeps your hand through a circular distance of 2π times that lever-arm length for every full turn. Meanwhile, the screw itself advances along its axis by only its pitch: the linear distance it travels per turn, often just a millimetre or two on a machine screw. The ratio of those two distances is the screw's ideal mechanical advantage, and because a small pitch divided into a comparatively large circular sweep produces a big number, screws routinely have mechanical advantages in the hundreds — turning a modest wrench with modest force can develop a clamping force that would otherwise take a strong adult's full-body weight to apply directly.
A longer arm or a finer thread — both help, equally
Doubling the lever-arm length doubles the mechanical advantage; halving the pitch (a finer thread) also doubles it. Real designs balance the two: a longer handle is limited by what's comfortable to grip and swing, while a finer thread is limited by manufacturing precision and the reduced strength of a shallower thread under heavy load, so engineers trade the two against each other depending on the application.
Why screws don't unwind on their own
A wedge or an inclined plane offers little resistance to sliding back down once you stop pushing, but a screw thread is different: the same friction between the tightly pressed male and female threads that seems like pure loss during turning is also what keeps the screw from spinning back out under load. This self-locking behavior is why a bolted joint holds without a separate locking mechanism, and why a car jack stays raised even after you let go of the handle — the friction that eats into the ideal mechanical advantage is doing useful work of its own.
Archimedes and the water-lifting screw
The device now called Archimedes' screw is traditionally credited to the Greek mathematician Archimedes of Syracuse in the 3rd century BC, though similar water-lifting devices may have existed earlier. It uses a helical surface turning inside a tube, tilted at an angle, to carry water steadily upward as the whole assembly rotates — irrigation and drainage devices built on the same principle are still used today, sometimes essentially unchanged in their basic geometry after more than two thousand years. It stands alongside the lever as one of the clearest illustrations of how a simple machine, properly understood, can outlast the specific culture and century that produced it.
From ancient irrigation to the modern bolt
The same helical-inclined-plane idea scaled down and refined over centuries into the screws, bolts, and threaded fasteners that hold together nearly everything built today, and scaled up into screw jacks, presses, and vises that can develop tons of force from a hand crank. A drill bit is a screw thread cutting its own path into a material as it turns; a corkscrew uses the same geometry to bite into a cork and pull it free with a fraction of the force a straight pull would need.
Estimating your own screw's advantage
If you know roughly how far around your wrench or screwdriver handle sweeps and the pitch printed on the fastener's packaging (or measured with calipers), a screw calculator will show you both the frictionless ideal mechanical advantage and, once you factor in a realistic efficiency for a threaded connection, a much more honest estimate of the effort you'll actually feel turning it.