Machines are made up of machine elements, which are individual parts of the machine that move and impact other parts to make the machine work. Although a machine element may consist of smaller structural parts, the entire setup that performs the function is considered the element. For example, a valve consists of several components, including a handwheel and a shaft or stem, but only the entire valve would be called a machine element. There are a wide variety of different machine elements, both general-purpose and special-purpose, and their optimal design is a major field of study in engineering.
General-purpose machine elements can be used in any application, and include items such as gears, bearings, and shafts. Gears are rotating parts with interlocking teeth that turn against other gears, causing them to turn and transmit rotational force, or torque. Bearings are used to carry force — an example is the ball bearing, which allows for reduced friction as it rolls freely beneath a moving part. Shafts and couplings, like gears, also transmit torque and are used to connect parts, especially in vehicles such as cars and motorcycles. Although many variations and other types of general-purpose elements exist, they all work based on the same principles of force and motion and can be incorporated into almost any machine as necessary.
The other main category of machine element is the special-purpose machine element. These devices also use the basic principles of force and motion to function, but unlike general-purpose elements, they can be used only in specific applications. Pistons and valves are examples of such parts. Valves control the flow of fluid or gas through a pipe by obstructing the pipeline, and can be opened or closed using an external switch or wheel. Pistons are moving components used in engines and pumps to transmit force from the pressure of gas or fluid in a cylindrical chamber, or to force out that gas or fluid.
In designing a machine element, engineers seek to optimize several factors. The machine element must have enough strength and durability to perform its intended function reliably, but should also be as small and light as possible. Ideally, it should also be easy and inexpensive to manufacture and maintain while still conforming to industry standards. Since the basic function of the machine element is the most important consideration in design, the process begins by suiting the element to its function and then incorporating other factors.