Also known as rotary joints, they are used to permit sealed fluid flow from a stationary supply to a rotating component.

Rotary unions consist of inlet and outlet connection(s) with separate flow passages. An internal seal arrangement (often dynamic or mechanical) used occasionally with a ball bearing, permit rotation without fluid leakage within the union housing.

Various designs exist to suit many different fluids, such as oil, water & air.

Speed of rotation, fluid flow rate and fluid pressure are the key criteria for selection. Port sizes are chosen to match mating components.

Rotary unions are used throughout industry and are often located on the end of a rotating roll shaft or rotary cylinder, where cooling is required for the removal of heat during a manufacturing process. A fluid such as water can be forced into the roll via the rotary union inlet port and passage where heat is absorbed and carried away with the fluid as it exits the rotary union through the outlet passage and port. High fluid flow rates and lower fluid inlet temperatures will be more effective at removing heat from the application.

The seals and bearings are often replaceable items within a rotary union because long operating durations cause these parts to deteriorate.

A typical example for a rotary union is shown on the picture.