Studs, also called double-headed screws or double-headed studs, are used to connect machinery for fixed connections. They have threads on both ends, and the middle screw can be thick or thin. They are generally used in mining machinery, bridges, cars, motorcycles, boiler steel structures, tower cranes, large-span steel structures, and large buildings. Bolts specifically refer to screws with larger diameters, and can also be headless, such as studs.