A rotary union is the fitting used to transfer the pressurized fluid from a fixed inlet to rotating outlet, without obstructing the flow of the fluid or air. This fitting is sometimes referred to as a swivel joint, rotating manifolds, or rotary coupling.

Figure 1 shows a swivel fitting containing a single circuit to transmit fluid. These fittings pay for themselves many times over by reducing stress on a hose, thereby extending its life. This describes one end of the swivel spectrum.

At the other end are rotary unions that transmit fluid for multiple circuit lines through a single manifold that rotates continuously. In general, fluid enters one or more ports in the stationary portion of the manifold and exits through one or more ports on the other portion, which rotates with the machine. A rotary seal between the two halves contains the pressurized fluid, yet allows relative rotation between the halves. For simplicity of discussion, the term rotary union will be used here as an all-inclusive term to describe swivel fittings and rotating manifolds.

The rotary seal is probably the most critical part of the device, whether a swivel fitting or rotary manifold. This is because the seal between the rotating and stationary halves must be tight enough to prevent leakage of pressurized fluid, while introducing as little torque drag as possible. Torque drag is a measure of the swivel joint’s resistance to rotation.

These seals vary in complexity depending on the application. For simple swivel fittings undergoing less than 360 deg. of rotation, the seal may be little more than two machined surfaces loaded against each other. Rotating manifolds, however, may require ball bearings and spring-loaded seals with auxiliary loading by fluid pressure. If the seal is not pressure balanced (fluid pressure acting on opposite sides of the seal), torque drag may increase with fluid pressure.

As with any custom-engineered product, manufacturers can supply a swivel joint to meet virtually any specification. However, a variety of standard swivels is available to keep costs reasonable.

Read more: Rotary Unions & Swivels