screw

1. a mechanical device for fixing one object to another, characterized by a spiral ridge cut on the external surface of a cylindrical rod of decreasing diameter towards its point. This is a male screw. In a female screw the screw is cut on the inside of a cylindrical cavity.
2. a colloquial term for a worn out or inferior horse.

• AISF bone s. — see buttress thread bone screw.
• buttress thread bone s. — the thread is not a simple V as in the standard thread but has one side of the groove at right angles to the direction of the screw and the other side at a 45° angle. Called also AISF, Howmett compression, Richards–Bechtol.
• cancellous bone s. — has a coarse thread; threaded to only the first third of the length of the screw.
• cortical s. — threaded the full length of the screw; fine thread to hold in dense bone.
• Howmett compression bone s. — see buttress thread bone screw.
• lag bone s. — used as a compressing unit between two fragments with the first half of the screw near the point threaded, with the diameter of the ridges greater than that of the unthreaded half near the head. As the ridged part of the screw bites in the walls of the drill hole in the distal fragment, the unthreaded part is free to move within the drill hole in the proximal fragment, thus compressing the two pieces of bone together.
• Richards–Bechtol bone s. — see buttress thread bone screw (above).
• standard bone s. — with a V thread the full length of the screw, with a single slot, cruciate or Phillips head.
• transfixation s. — orthopedic screw used to reattach bone fragments to a solid skeletal part. The proximal part near the head is not threaded and needs to be of a smaller diameter than the threaded portion nearer the tip.

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A screw is one of the six simple machines. A simple screw is a helical inclined plane. A screw can convert a rotational force (torque) to a linear force and vice versa. The ratio of threading determines the mechanical advantage of the machine. More threading increases the mechanical advantage. A rough comparison of mechanical advantage can be done by taking the circumference of the shaft of the screw and divide by the distance between the threads.

Examples

• Lead screw and ball screw are specialized screws for translating rotational to linear motion.
• Automated garage door, where a motor drives a long finely threaded shaft at relatively high speed and lifts the heavy door at a slower rate.
• Archimedes' screws and worm gears are examples of this machine.

See also

• Screw (disambiguation)
• Screw - the threaded fastener
• Archimedes' screw

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