how does a rotary joint work ？

November 21, 2025

How does a rotary joint work ？

In modern industrial systems, transferring fluids to rotating equipment efficiently and safely is critical. Rotary unions, also known as rotating unions, are the devices that make this possible. These components allow liquids or gases to flow from stationary piping into rotating or reciprocating equipment and back out, all while maintaining a sealed connection. Whether in CNC machine tools, steam-heated cylinders, or cooling rolls, rotary unions are essential for heat transfer, lubrication, or fluid circulation.

Understanding the types, working principles, materials, and applications of rotary unions can help engineers and plant managers choose the right solutions for their equipment.

What is a Rotary Union?

A rotary union is a mechanical device that enables the transfer of fluid between stationary supply lines and rotating or reciprocating equipment. Its main purpose is to maintain a sealed flow path while allowing rotation, minimizing leakage, and sustaining system pressure.

Typical components of a rotary union include:

Housing – Supports the entire assembly and connects to stationary piping.
Shaft – Transfers the fluid from the supply to the rotating element.
Bearings – Support rotation while maintaining alignment.
Seals – Prevent fluid leakage and withstand operational pressures.

Common materials used for rotary unions include stainless steel, carbon steel, aluminum, and brass, chosen according to fluid type, temperature, and system pressure. All parts belong to cnc turning parts and could cnc machining by cnc lathe machine .

How Rotary Unions Work

Rotary unions operate by providing a sealed passage for fluid while accommodating rotation. Fluids can include water, steam, thermal oil, compressed air, or other process liquids. The key principle is maintaining a continuous, leak-free connection between stationary and rotating parts.

Depending on the system, rotary unions can be designed for single-pass or dual-pass operation. Each design ensures efficient fluid transfer while accounting for condensate removal, pressure, and rotational speed.

Types of Rotary Unions

Single-Pass Rotary Union

Single-pass rotary unions are designed for simpler fluid transfer applications.

Type 1: Installed at both ends of a drum; fluid enters from one end and exits from the other. No inner tube is used.
Type 2: Installed at one end of a drum; fluid is both introduced and discharged at the same end. The inner tube remains stationary relative to the drum.

Applications: Cooling rolls, small-diameter cylinders, or low-pressure steam systems.

Advantages: Simpler construction, easier maintenance, suitable for low-to-medium pressures.

Dual-Pass Rotary Union

Dual-pass rotary unions provide more complex flow paths for applications such as steam heating, condensate removal, or multi-channel fluids.

Inner Tube Fixed: The drum contains siphons or tubes that remove condensate. This type is widely used in steam applications, where efficient condensate removal is essential.
Inner Tube Rotates: The inner tube rotates together with the drum. Fluid is introduced through the inner tube and discharged via the outer tube.
Synchronized Siphon: A siphon inside the drum rotates with the drum to remove condensate, often used for steam-heated rolls.

Applications: Paper mills, textile dryers, printing presses, and any system using steam-heated cylinders or rollers.

Advantages: Better condensate handling, higher efficiency in heat transfer, suitable for higher-pressure and high-temperature applications.

The Role and Benefits of Rotary Unions

Role:
Rotary unions are critical for transferring fluids into rotating equipment safely and efficiently. Without these devices, it would be impossible to maintain pressure and flow while the equipment is in motion.

Benefits:

Efficient Heat or Fluid Transfer – Ensures proper cooling or heating in rotating machinery.
Leak Minimization – High-quality seals prevent fluid loss and system contamination.
High-Speed Compatibility – Designed for systems with fast rotation or reciprocation.
Pressure and Temperature Handling – Can withstand steam, thermal oil, or high-pressure fluids.
Versatile Material Options – Can be manufactured in stainless steel, carbon steel, aluminum, or brass to suit fluid compatibility and corrosion resistance.

Materials and Construction

Choosing the right material is crucial for longevity and performance:

Stainless Steel: Corrosion-resistant, ideal for steam, hot water, and chemical fluids.
Carbon Steel: High strength for industrial mechanical systems; suitable for low-to-medium corrosion fluids.
Aluminum: Lightweight, often used in low-pressure or mobile equipment.
Brass: Good corrosion resistance for water, air, and moderate-temperature fluids.

Sealing Technologies:

Mechanical seals for high-speed or high-pressure systems
Elastomer/lip seals for low-to-medium pressure systems
Hydrostatic seals for precision and minimal wear

Applications Across Industries

Rotary unions are widely used in industries requiring fluid transfer to rotating equipment:

Printing and Paper Industry: Steam-heated cylinders, cooling rolls
Metal Processing: Rolling mills, heat treatment cylinders
Textile Industry: Steam-dyeing machines, drying cylinders
Chemical, Food, and Packaging: Thermal oil heating, water circulation
CNC and Machine Tools: Coolant supply to rotating spindles and tool holders

Their adaptability ensures they are essential wherever heat, lubrication, or fluid transfer to moving parts is required.

Choosing the Right Rotary Union Manufacturer

Selecting a reliable rotary union manufacturer is critical for performance and safety. Consider the following:

Type of Rotary Union – Single-pass or dual-pass, based on fluid, pressure, and condensate requirements.
Material Compatibility – Choose stainless steel, carbon steel, aluminum, or brass based on media.
Sealing Technology – Mechanical, elastomeric, or hydrostatic depending on rotation speed and pressure.
Operational Conditions – Pressure, temperature, and rotational speed limits.
Customization – For unique drums, cylinders, or multi-channel requirements.
Maintenance Support – Manufacturer should provide seal replacements and maintenance guidance.

Maintenance and Best Practices

Inspect seals and bearings regularly to prevent leakage
Use proper lubrication to reduce wear
Replace high-wear components on schedule
Follow manufacturer guidelines for fluid compatibility and safety

Conclusion

Rotary unions are indispensable in modern industrial equipment, providing safe and efficient fluid transfer to rotating or reciprocating machinery. Understanding their types, working principles, materials, and applications ensures optimal performance, longer lifespan, and safer operations.

Whether using single-pass or dual-pass rotary unions, and materials such as stainless steel, carbon steel, aluminum, or brass, choosing the right rotating union manufacturer and following proper maintenance practices is key to operational success.