Rotary motion is essential in countless machines—manufacturing robots, printing equipment, winders, conveyors, packaging machines, and high-speed spindles all rely on components that can rotate while transferring fluids or signals. But how do these machines manage to move continuously while still delivering hydraulic oil, coolant, steam, air, or data through a rotating interface?
The answer is a specialized device known as a rotary union.

1. What Is a Rotary Union?

A rotary union—also called a rotary joint or rotary union joint—is a mechanical device that allows the transfer of fluids between a stationary source and a rotating machine part. The fluids can include:

• water

• hydraulic oil

• coolant

• steam

• compressed air

• vacuum

• chemicals

In simple terms, a rotary union enables a machine to rotate 360 degrees continuously while still maintaining a leak-free internal passage for fluid transfer.

If you are wondering “what is a rotary union?”, the simplest definition is:

A rotary union is a sealing device that connects a rotating component to a stationary supply line, enabling fluid, gas, or media to pass through without leakage despite continuous rotation.

2. What Is a Rotary Union Used For?

Rotary Unions serve hundreds of engineering applications. Understanding what is a rotary union used for helps clarify why they are so critical.

Common Applications Include:

1. Hydraulic Systems

Machines that require rotating cylinders or actuators often use a hydraulic rotary union to transfer pressurized oil.

Typical examples:

• excavator swivels

• hydraulic presses

• injection molding machines

• mobile cranes

• robotics wrists and joints

2. CNC Machinery & Machine Tools

High-speed spindles use rotary unions to transfer:

• coolant

• lubrication

• air for tool clamping

3. Printing & Packaging

Rotary unions deliver ink, adhesives, or air to high-speed rollers while maintaining consistent pressure.

4. Food Processing

Steam or heated water is delivered to rotating drums such as dryers, cookers, and blanchers.

5. Aerospace & Defense

Rotary joints deliver hydraulic pressure, cooling liquid, or signals in turrets, radar systems, and missile launchers.

6. Plastics & Rubber Processing

Rotary unions transfer temperature-controlled water or oil in film extruders and calender rollers.

In short, rotary unions are used wherever rotating components require media transfer under pressure, temperature, or speed.

3. Internal Structure of a Rotary Union

Understanding the internal components helps explain how a rotary union works.

A typical rotary union contains:

1. Housing (Stationary Part)

This connects to the fixed supply line and holds the internal sealing system.

2. Rotor (Rotating Part)

This attaches to the rotating equipment—rollers, spindles, or drums.

3. Bearings

These support smooth rotation and alignment between rotor and housing.

4. Sealing System

The most critical part. Common seal types include:

• mechanical seals

• carbon graphite seals

• O-rings

• lip seals

• tungsten carbide seals

The seal keeps fluids inside the passage while allowing rotation.

5. Passageways / Ports

Channels that guide media through the rotary union joint.

6. Springs (for Mechanical Seals)

Maintain sealing pressure even at high speed or variable load.

4. How Does a Rotary Union Work?

The basic operation is simple but highly engineered.

Step-by-Step Explanation

Step 1: Fluid Enters the Stationary Housing

Hydraulic oil, air, steam, or coolant flows into the inlet of the housing.

Step 2: Sealing System Creates a Leak-Proof Barrier

Mechanical seals or carbon graphite seals create a low-friction seal between the stationary housing and the rotating shaft.

The seal is the heart of the system.

Step 3: The Rotor Begins to Rotate

Attached to a shaft, drum, or spindle, the rotor rotates at varying speeds—sometimes up to 20,000 RPM.

Because the seal is designed for friction reduction and heat dissipation, it stays leak-free.

Step 4: Fluid Passes Through the Rotor

Fluid continues through internal passages and exits into the rotating equipment.

Step 5: Continuous Rotation Without Leakage

The rotary union maintains:

• sealing integrity

• pressure stability

• smooth rotation

even as the machine runs continuously.

Simplified Explanation

A rotary union works by maintaining a high-precision, low-friction seal between stationary and rotating parts, enabling fluid to pass through without leakage despite movement.

5. Different Types of Rotary Unions

There is no single type of rotary union. They vary by fluid, pressure, speed, and application.

1. Hydraulic Rotary Union

A hydraulic rotary union is designed specifically for hydraulic oil under high pressure (up to 5000 PSI or higher).

Features:

• heavy-duty steel or stainless steel

• advanced mechanical sealing

• multiple channels for multi-function circuits

Used in:

• excavators

• lifting equipment

• wind turbines

• heavy robotics

2. High-Speed Rotary Unions

Used in CNC spindles and machine tools.

Characteristics:

• high RPM capability

• balanced rotor

• coolant-through-spindle support

3. Multi-Passage Rotary Unions

These allow multiple independent media channels such as:

• oil + air

• vacuum + water

• coolant + hydraulic oil

Each channel is sealed separately.

4. Rotary Slip Ring (for Electrical Signals)

A Rotary Slip Ring is similar in purpose but transfers electrical signals or power, not fluids.

Used in:

• wind turbines

• surveillance cameras

• radar systems

• aerospace systems

5. Fiber Optic Slip Ring (for High-Speed Data)

A fiber optic slip ring transfers optical signals through a rotating interface without interrupting data flow.

Benefits:

• no electrical noise

• ultra-fast communication

• zero interference

Used in:

• medical imaging

• military radar

• satellite communication systems

6. Rotary Union vs Rotary Slip Ring: What’s the Difference?

Although they serve a similar purpose, their functions differ.

Many advanced systems combine fluid transfer and electrical data transfer using hybrid assemblies.

7. Key Design Factors That Affect Rotary Union Performance

Engineers must consider:

1. Fluid Type

Coolant, steam, hydraulic oil, or air require different seal designs.

2. Pressure Rating

Hydraulic applications require stronger seals than low-pressure air systems.

3. RPM (Rotational Speed)

High-speed CNC unions use special balancing and cooling methods.

4. Temperature

Steam and hot oil need thermal-resistant materials.

5. Multi-Passage Requirements

Robotics and automation often require several channels for different media.

6. Material Selection

Common materials include:

• brass

• stainless steel

• carbon steel

• ceramic

• graphite

8. Common Industries Using Rotary Unions

- CNC machining

- Plastics and rubber processing

- Paper production

- Textile manufacturing

- Food processing

- Robotics

- Aerospace

- Automotive

- Marine applications

- Renewable energy systems

Rotary unions are essential in any machine requiring rotation plus fluid transfer.

9. Advantages of Rotary Unions

• Continuous 360° rotation

• Leak-proof transfer of media

• Long lifespan with low maintenance

• Stable pressure control

• Compatibility with high temperature and high pressure

• Support for multi-passage channels

• Can be integrated with slip rings for hybrid use

10. Conclusion: Why Understanding How a Rotary Union Works Matters

A rotary union is a deceptively simple yet technologically advanced component that keeps modern machinery running. By providing a rotating, leak-free interface for transferring liquid, gas, or steam, it enables high-performance automation across many industries.

In summary:

• If you need to know what is a rotary union, it is a mechanical device that transfers fluids between stationary and rotating components.

• If you are asking what is a rotary union used for, the answer is: any application where fluids or gases must pass through a rotating machine element.

• Rotary unions are available in many forms, including specialized types such as hydraulic rotary union, multi-passage units, and hybrid forms combined with rotary slip rings or fiber optic slip rings.

• Understanding how a rotary joint works helps engineers choose the right design for pressure, temperature, fluid type, and rotational speed.

With the right rotary union joint, machines achieve higher productivity, greater reliability, and longer service life.