However simple, any machine is a combination of individual components generally referred to as machine elements or parts. Thus, if a machine is completely dismantled, a collection of simple parts remains such as nuts, bolts, springs, gears, cams, and shafts—the building blocks of all machinery. A machine element is, therefore, a single unit designed to perform a specific function and capable of combining with other elements. Sometimes certain elements are associated in pairs, such as nuts and bolts or keys and shafts. In other instances, a group of elements is combined to form a subassembly, such as bearings, couplings, and clutches.

The most common example of a machine element is a gear, which, fundamentally, is a combination of the wheel and the lever to from a toothed wheel. The rotation of this gear on a hub or shaft drives other gears which may rotate faster or slower, depending upon the number of teeth on the basic wheels.

Other fundamental machine elements have evolved from wheel and levers. A wheel must have a shaft on which it may rotate. The wheel is fastened to the shaft with a key, and the shaft is joined to other shafts with couplings.. The shaft must rest in bearings, may be started by a clutch or stopped with a brake. It may be turned by a pulley with a belt or a chain connecting it to a pulley on a second shaft. The supporting structure may be assembled with bolts or rivets or by welding. Proper application of these machine elements depends upon a knowledge of the force on the structure and the strength of the materials employed.

The individual reliability of machine elements becomes the basis for estimating the overall life expectancy of a complete machine.

Many machine elements are thorughly standardized. Testing and practical experience have established the most suitable dimensions for common structural and mechanical parts. Through standardization, uniformity of practice and resulting economies are obtained. Not all machine parts in use are standardized, however. In the automotive industry only fasteners, bearings, bushings, chains, and belts are standardized. Crankshafts and connecting rods are not standardized.