Rotary union or rotating unions are mechanical devices applied in transfer of fluid from a stationary source into a rotating machine piece. Rotating union application is in various manufacturing processes for cooling, heating, or transferring of the required fluid.

The joints also go by other names like:

  • Rotary Swivels
  • Rotary Coupling
  • Hydraulic Coupling
  • Through Bore Rotary Joint
  • Pneumatic/hydraulic Rotary Union example Moflon rotary union
  • Electrical Rotating Union
  • Rotating Joints
  • Vacuum Rotating Union

Moflon Rotary Union

It is a type of hydraulic rotary union for use with rotating apparatus. Moflon rotary union ensures safety and stability in the transmission of fluid and air pressure. The union is also known as the pneumatic rotary union, and it can support pressure ranging from -10MPa to 90MPa.

Moflon Rotary Union GP (1)

Moflon is a leader in the design and manufacture of the slip ring and rotary unions. Moflon rotary unions can are used in sectors ranging from commercial, industrial, military, agriculture, oil& Gas, airplanes and in other uses.

Types of Moflon Rotary Union

The types vary according to:

  • Size of the bore measured in millimeters.
  • Supporting passages from 1 to 16
  • Port size
  • Media used the transfer. The media can be compressed air, vacuum, steam, coolant, hot or cold water, oil, hydrogen, among others.
  • Speed of operation either as high- speed, low-speed or medium-speed
  • The operating pressure from low to high pressure

How Does A Rotating Union Function?

It locks into valve inputting a fluid to an outlet. It holds and allows the flow of gas or liquid from the source into the rotating joint. The fluid transfer from the union occurs once the valve open ends meet during the rotation enabling the flow of more liquid or gas into the swivel from the source.

Moflon Rotary Union GP (2)

Hotshot6902 [CC BY-SA 4.0 (]

The joint can work even in high-pressure conditions and when there is continuous rotation on an axis. Design of rotary joint depends on the required psi according to rpm and valves present.

Rotary Union Key Components

A rotating union of any size, shape, or configuration incorporates the following four main components. The main material used in their construction is stainless steel which helps them to become resistant to corrosion and rust. Other metals used include aluminum.

The main components of a rotary union are:

  • Housing which has the inlet port. The port attaches to the medium supplying hose. All the other elements of the rotating union are in the housing.

In some cases, the housing has an outlet opening when it is used for both supplying the roll fluids and removing fluids. Large rotary unions use a moving housing which rotates at par with the drum. Smaller unions have stationary housing.

  • Shaft: Its main purpose is to transfer the fluid from the rotating union to the drum. The shaft turns as the drum in most cases although large flanged rotating unions have a stationary shaft. The shaft also incorporates the assembly of the seal and the bearings.
  • Bearing: They are very important in a rotating union assembly to allow the rotating part to move with ease and efficiency. There are two types of bearings in rotating unions, roller and non-roller type. The non-roller bearing uses graphite or bronze material.
  • Mechanical seal: it is the main component in a rotating union. The main use of a seal is to prevent fluid leakage during operation. Type of seals include
  • Pusher type
  • Lip seals
  • O-ring
  • Non-pusher type

In a situation where the leakage is an avoidable a system incorporated in the design to protect the environment and the operators.


Rotating union are classified according to the:

  • Ports
  • Materials handled
  • Port design either straight or elbowed

Uses of Rotating Unions

In many assemblies, they use rotating unions with some having multiple unions. They appear in machines and vehicles that require the supply of lubricants, air, and liquids in their moving parts.

  • Agriculture in all areas in the field and during the processing of harvested products.
  • Automotive industry during manufacturing and in the products like brake system in a car.
  • Oil industry: During drilling in rigs and refinery process. The rigs use clutches, brakes and mud flashers which all require the rotating union. Batch mixers, blenders and flaking mills in a refinery also use rotating unions.
  • Mining equipment in shuttle cars, drill heads, backhoes, dragline among others all use rotating unions.
  • The steel industry uses a rotating union for continuous casting machines (CCM) to cool the many rolls supporting the molten slabs.
  • Rotating union use is practically in any machine that requires fluids and air in its operation. The machine used in mixing liquids also requires the use of a rotating union in their assembly.


There is a difference between a slip ring, and a rotary union is that a slip ring passes electrical signals from a stationary part to a rotating part. Rotary union on the hand passes fluid or air from a stationary part to a rotating part.

In machines using hydraulic a rotary union is a must to enable the passing of compressed air and the hydraulic fluid between the fixed and moving part.

Moflon offers a wide range of rotating unions depending on the client requirement, which contributes to the wide range of design, size, shape, and functionality of different pneumatic unions in the market.

Moflon company is well established and has a vast experience of over thirty years in the rotating union industry. The company may have in its stock a hydraulic rotating union which can fit your specification.

If you are looking for a rotary union solution, you should be more specific on the use, type of fluid to use, pressure required, and size of the fitting as these contribute to the specification of the design.

When giving the specifications for the design you should also take into consideration whether the rotary union will be capable of withholding the medium, and if not, the design should include a collector for the excess medium. If you have any question about rotating union, leave a comment below, and we will get back to you.

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