Hydraulic Rotary Union:
Pressure on rotating components with the hydraulic rotary union

The hydraulic rotary union is a component that connects stationary and rotating parts of a hydraulic system. A distinction is made between one-way systems and two-way systems.

How the Hydraulic Rotary Union Works

The hydraulic rotary union essentially consists of a shaft and a housing. Both components are mounted to rotate relative to each other. There is a cavity between the shaft and the housing. Both components have one (one-way systems) or at least two (two-way systems) bores. On the side of the cylindrical housing, the bore is located on the outer surface. In the case of the wave-shaped shaft, the bore is located on the underside.

In one-way systems, the medium flows through the rotary union and is then discharged into the atmosphere. This is, for example, the standard design for pneumatic systems. Hydraulic rotary unions with a one-way system can be found, for example, in lawn sprinklers.

In a two-way system, the medium is returned. The circuit is thus closed. This is the standard design for hydraulic systems.

Defects in the Hydraulic Rotary Union

The weak point of the hydraulic rotary union is its seal. Due to the rotating bearing, it is difficult to create a reliable seal. The sealing surfaces must both withstand high pressure and be mounted to rotate relative to each other. Rotation under high contact pressure leads to friction losses and gradual wear. The seal of a hydraulic rotary union is therefore designed as a wear part that must be replaced at fixed intervals.

The problem with leaking seals in a rotary union is not only escaping oil. Ingressing water can also cause severe damage to the interior of this component. This occurs, for example, with the rotary spindles of lathes and milling machines with hydraulic drives. In this case, corrosion damage to the inner shaft can quickly occur. In both cases, strict adherence to the replacement intervals for the seals provides the desired remedy.

Specialized manufacturers now offer seals with new material properties that enable a long service life.

Hydraulic Rotary Union – Potential Areas of Application

The hydraulic rotary union is used wherever a fixed and a rotating component share the same hydraulic system. This is found, for example, in excavators.

The drive, pump, and hydraulic power unit are housed in the upper carriage. The undercarriage, whether with a wheeled or tracked chassis, is operated hydraulically. The rotation of the upper carriage is carried out by a ring gear with an electric or hydraulic motor. To supply the chassis with hydraulic pressure, a rotary union is installed as standard in excavators.

Hydraulic Rotary Union – Available Alternatives

The installation of a hydraulic rotary union requires that it can be maintained regularly. This cannot always be guaranteed. As an alternative to this solution, the rotating and stationary components can be designed with two independent hydraulic systems. Another idea is to operate one part purely electrically and only the other part hydraulically. While this increases the costs for design, procurement, and assembly, such a design can increase the operational readiness of the system in particularly difficult environments (radiation exposure, underwater, toxic atmospheres).