Shaft collars are circular collars, or rings, that fasten around a shaft. They can serve a variety of purposes, including spacing or stops (such as on a flagpole), or they can be used for mechanical processes which require positioning or the limiting of motion relative to the shaft. They can also be used for coupling machine components to the shaft. Shaft collars clamp to the shaft as either a one-piece or two-piece design, generally using set screws or bearing locknuts to secure onto the shaft. They are typically made of aluminum or various types of steel, depending on the application, rigidity and stress resistance required. For some applications, a plastic shaft coupling will suffice. Shaft collars come in various sizes and are measured by internal, or bore diameter, outside diameter, and width.