A screw thread is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.
Screw thread, used to convert torque into the linear force in the flood gate. The operator rotates the small vertical bevel gear in the center. Through mechanical advantage this causes the horizontal bevel gears (at far left and far right, with threaded center holes) to rotate. Their rotation raises or lowers the two long vertical threaded shafts - as they are not free to rotate.
Different (and incompatible) threads including (from left) M12 left hand, standard M12, M12x1.5 (fine), M12x1.25 (fine), 1/2" UNF, 1/2" UNC, 1/2" BSW, and 1/2" BSF
Camshaft cover stud threaded 1⁄4-20 UNC (left, for aluminium cylinder head) and 1⁄4-28 UNF (right, for steel nut; from a 1960s Jaguar XK engine)
The three diameters that characterize threads
A helix is a shape like the cylindrical part of a coil spring or machine screw. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helices, and many proteins have helical substructures, known as alpha helices. The word helix comes from the Greek word ἕλιξ, "twisted, curved".
A "filled-in" helix – for example, a "spiral" (helical) ramp – is a surface called a helicoid.
(l-r) Tension, compression and torsion coil springs
A machine screw in macro view
Crystal structure of a folded molecular helix reported by Lehn et al.
A natural left-handed helix, made by a climber plant
