Choosing the Right Rotary Union

A rotary union is a mechanical connection device that allows a fluid to be transferred from a fixed part of a machine to a rotating part and vice versa. The fluid conveyed in the system can be liquid or gaseous, under pressure or under vacuum.

A rotary union consists of a shaft, one or more seals, one or more bearings and a body. It keeps the fluid moving. It must be completely sealed: the mechanical, lip or O-ring seal is therefore a key element of the connection.

Some manufacturers use the terms “rotary union”, “rotary distributor,” or “rotary joint.”

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How to choose a rotary union?
GAT rotary union
Many parameters related to the rotating system will help you choose a rotary union.

The first criteria it’s important to consider is the media the union will be used for. It can be used for gas, air, vacuum, water, oil, steam, cooling liquid etc. Fitting materials should be selected according to the possible presence of abrasive or corrosive agents.

The number of passages necessary for the union, the rotational speed, the pressure in your installation and the operating temperature will allow you to calculate the size of your rotary union.

Lastly, if you need to transmit an electrical signal or current in your installation, you will need to use a rotary union equipped with a slip ring.
- media
- number of passages
- speed
- pressure
- temperature
- electrical signal or current to be transmitted
How to calculate the dimensions of your rotary union?
SMC rotary unions
There are many sizes of rotary unions available. Outlined below you will find the essential characteristics for calculating the size of your product.
- The nominal diameter (DN) of your installation: this is the inside diameter of a connecting pipe.
- The operating pressure of your installation: the rotary union is used in both vacuum and pressure systems. You will need to know the usual pressure of the media, generally between 0 and 1,380 bar (0 to 20,000 psi), and the maximum operating pressure. If the system operates at variable pressure, choose the maximum pressure as the operating pressure.
- The operating temperature of your media as well as the ambiant temperature: you will need to determine the minimum and maximum temperature. They generally vary between -100°C et 200°C.
- The necessary rotational speed and direction: the speed is generally from 0 to 3,600 rpm. A higher speed, higher than 10,000 rpm, is also possible (depending on the manufacturer).
- nominal diameter
- operating pressure or maxiumum pressure
- temperature
- rotational speed
Rotary union or slip ring?
A rotary union and a slip ring both facilitate the rotation of machine components. A rotary union allows you to transmit a fluid, while a slip ring transmits an electrical current or signal.

In a slip ring, currents are transmitted from a stationary inlet, often a brush, to a rotating outlet, often a ring. In a typical installation, the slip ring is mounted on the moving part. The wires that pass through the hollow shaft conduct the electrical charge. Slip rings are found in many applications such as HD video systems (surveillance cameras) or renewable energies (wind turbines).

If you need an electrical signal in your application, it is possible to combine the use of a rotary union and a slip ring to pass both fluids and currents. These systems combine fluid transmission with electrical power or signal transmission. They are called “combined rotary unions” or “rotary unions with integrated slip ring.”
What other characteristics are important to consider when choosing a rotary union?
The material of the union is important. You can choose one in steel, aluminum, brass, cast iron or bronze depending on the nature of the fluid to be conveyed. Then you will need to choose the type of connection: male or female, screw-in, flange, welded on or directly connected.

There is a wide choice of standard rotary unions, however many manufacturers also offer custom rotary unions, with or without manifolds.