The screw is an amazing invention among the basic machines. It's great at turning rotational motion into linear motion and torque into force. Out of the six fundamental machines, the screw is super versatile and useful in many situations.

A typical screw looks like a rod with ridges or grooves spiraling around it. These ridges are called threads. When you put a screw into something like a hole, its threads twist together with the ones inside the hole. This twisting action allows the screw to move forward or backward.

Screws can work in two main ways: either the screw turns into a hole in something that doesn't move, or a nut turns around the screw. Both ways use the screw's design to get things done efficiently.

From a simple view, a screw is like an inclined plane wrapped around a cylinder. This comparison helps us understand how screws work. They're compact and powerful tools for directing force and motion. So, the next time you use a screw, remember how it cleverly turns one kind of motion into another, making tasks easier and more efficient.

History of the Screw

The screw, with its simple yet ingenious design, has become an indispensable fastening device and mechanical tool.

Throughout history, the screw has evolved from a simple tool for food processing to a versatile fastening device and mechanical component. Its thread design provides secure and reliable connections, making it essential in various applications across industries.

Mechanical Advantage (MA)

The mechanical advantage MA of a screw is defined as the ratio of axial output force Fout applied by the shaft on a load to the rotational force Fin applied to the rim of the shaft to turn it. For a screw with no friction (also called an ideal screw), from conservation of energy the work done on the screw by the input force turning it is equal to the work done by the screw on the load force:

Work is equal to the force multiplied by the distance it acts, so the work done in one complete turn of the screw is, W(in) = 2πrF(in), and the work done on the load is W(out) = lF(out). So the ideal mechanical advantage of a screw is equal to the distance ratio shown to the left.

It can be seen that the mechanical advantage of a screw depends on its lead, l. The smaller the distance between its threads, the larger the mechanical advantage and the larger the force the screw can exert for a given applied force. However, most actual screws have large amounts of friction and their mechanical advantage is less than given this equation. 

Because of the large area of sliding contact between the moving and stationary threads, screws typically have large frictional energy losses. Even well-lubricated jack screws have efficiencies of only 15% - 20%, the rest of the work applied in turning them is lost to friction. When friction is included, the mechanical advantage is no longer equal to the distance ratio but also depends on the screw's efficiency. Our final equation for a screw's true mechanical efficiency is as shown on the right where n is the screw's efficiency. 

Common Uses

Because of its self-locking property, the screw is widely used in threaded fasteners to hold objects or materials together: the wood screw, sheet metal screw, stud, and bolt and nut. The self-locking property is also key to the screw's use in a wide range of other applications, such as the corkscrew, screw-top container lid, threaded pipe joint, vise, C-clamp, and screw jack.

Screws are also used as linkages in machines to transfer power, in the worm gear, lead screw, ball screw, and roller screw.

Rotating helical screw blades or chambers are used to move material in the Archimedes' screw, auger earth drill, and screw conveyor.