What are rotary unions and how are they used in motion applications?

There are many rotating components in motion applications – robot arms, winders, spools, spindles, and rotary tables, for example. Cooling, heating, lubricating, or transmitting fluid power becomes more difficult when air or liquid must be transferred from a stationary source to a rotating component. This is where rotary unions come into play.

Rotating unions allow fluid media – air, steam, or liquid – to be transferred between rotating and static components. The devices are designed to handle a wide range of temperature, flow, and pressure conditions, including vacuum applications. In addition, rotary unions allow one-directional rotation in any combination of rotation angles, unlike other types of fluid transfer mechanisms.

While rotary unions come in a variety of shapes and sizes – they’re often custom-designed for specific applications – they all contain four basic components: the housing, one or more bearings, the shaft, and a seal.

In most cases, the housing is the stationary part and is connected to the media supply. The shaft rotates with the connected rotary equipment independent of the housing. Ports in the housing allow media to flow radially or axially through the union. Radial bearings support the rotating part, usually one or more fully sealed deep groove ball bearings.

Among the most important components of rotary union manufacturers are the sealing mechanism, which prevents or reduces leakage between rotating and stationary components with minimal friction and wear. The types of seals vary from simple lip seals to spring-loaded mechanical seals that minimize friction and wear by automatically adjusting pressure on the seal faces.

Although fluid transfer should ideally be leak-free, in some applications, a leak-free transfer is not feasible. A collection system recovers the leaked media and prevents it from entering the environment in these cases, protecting surrounding equipment and personnel.

When designing a custom rotary union or choosing an off-the-shelf product, it is important to consider the following factors:

  • Media type being transferred
  • Rotation speed
  • Media flow rate
  • Media pressure
  • The temperature of the media
  • Leakage allowed

A union’s housing, shaft, and seal materials must be compatible with the fluid flowing through the union. The fluid media can range from water to highly corrosive liquids and steam, and manufacturers offer a wide range of materials, not just for housing and shafts, but also for seals.

In addition to single- and dual-flow designs, rotary unions can also have multiple independent flow paths that allow different media to be transferred simultaneously without mixing. The passage of liquids and electrical or fiber optic cables can also be accommodated by slip rings or fiber optic rotary joints.