Whether it is paper manufacturing, plastic packaging printing, chemical processing, or operating complex machine tool systems, transferring fluids between stationary and rotating components is a constant engineering challenge.   What exactly is a rotary joint? A rotary joint, also known as a rotary union, is a precision mechanical sealing device that allows the transfer of a fluid (liquid or gas) from a stationary supply line to a rotating machine component.Consider a heated roll in a paper mill or a machine tool spindle requiring coolant: the fluid must enter the component while it rotates at high speed, without leakage and while maintaining constant pressure. The rotary joint is the interface that makes this possible. Common industry terms include: