What is the difference between a hydraulic rotary union and a swivel joint?

Introduction: Hydraulic Rotary Unions and Swivel Joint

In the world of fluid and gas transfer systems, hydraulic rotary union and swivel joint play pivotal roles. Both of these components allow for the smooth and efficient transmission of media from a stationary part to a rotating or moving part of the machinery. While they may seem similar in function, hydraulic rotary unions and swivel joints have distinct characteristics, uses, and operational mechanisms. Understanding these differences can help engineers and maintenance professionals select the right component for their specific needs.
In this article, we will compare hydraulic rotary unions and swivel joints in-depth, exploring their design, application, materials, and performance factors. We will also highlight the benefits and limitations of each type, providing a clearer picture for industries involved in fluid handling, automation, and heavy machinery.

1. What is a Hydraulic Rotary Union?

1.1 Definition and Function

A hydraulic rotary union, also known as a rotary joint, is a device used to transmit hydraulic fluid or other media between a stationary pipe or system and a rotating component. Hydraulic rotary unions are designed to allow high-pressure fluid flow while preventing leakage, even under rotational motion.

1.2 Key Features of Hydraulic Rotary Unions

Sealing Mechanism: Rotary unions have high-performance seals that keep the fluid contained while allowing for continuous rotation of the attached machinery.
Application: These unions are primarily used in applications where the system needs to rotate under high pressure, such as in robotics, CNC machines, and manufacturing equipment.
Material Options: Hydraulic rotary unions are made from materials like stainless steel, brass, or other corrosion-resistant alloys that ensure durability under pressure.
Multi-Port Design: Many hydraulic rotary unions support multiple fluid passages, enabling simultaneous transfer of different media (e.g., oil, air, coolant).
High-Pressure Ratings: They are built to handle extreme pressures, often exceeding 10,000 PSI.
Precision Sealing: Advanced sealing technologies (e.g., mechanical face seals, O-rings) prevent leaks and ensure long-term reliability.
Customization: Available in various materials (stainless steel, brass) and configurations to suit specific applications.

1.3 How Hydraulic Rotary Unions Work

Hydraulic rotary unions consist of a stationary housing and a rotating shaft. The housing connects to stationary fluid lines, while the shaft rotates with the machinery. Seals between these components maintain a leak-free barrier, allowing fluids to flow through the union during rotation.

1.4 Applications of Hydraulic Rotary Unions

Hydraulic rotary unions excel in scenarios requiring reliable fluid transfer under demanding conditions:
Robotics: For rotating robotic arms, allowing them to function without compromising the fluid or gas transfer.
Manufacturing: In CNC machines and other automated manufacturing equipment that require fluid delivery while rotating parts move.
Hydraulic Systems: Used in industrial hydraulic systems where continuous fluid transfer is necessary despite rotational movement.
Metalworking: Transfer coolant to rotating cutting tools.
Packaging Equipment: Enable continuous motion in filling and labeling machines.
Wind Turbines: Lubricate rotating shafts in nacelles.
Aircraft Fuel Systems: Rotate fuel lines in engines and landing gear.
Hydraulic Actuators: Power moving parts in flight control systems.
Dynamometers: Transmit fluids to rotating wheels during performance testing.
Articulated Arms: Supply hydraulic power to rotating joints in industrial robots.

2. What is a Swivel Joint?

2.1 Definition and Function

A swivel joint, on the other hand, is a type of mechanical coupling designed to allow the movement of connected equipment while maintaining a sealed connection for fluid, gas, or air transfer. Swivel joints are more general-purpose components and are used in systems where less pressure is involved compared to hydraulic rotary unions.

2.2 Key Features of Swivel Joints

Flexibility: Swivel joints are designed to withstand angular misalignment, providing flexibility for connected hoses or pipes to move and rotate freely without leaking.
Application: These joints are typically found in applications like hydraulic cranes, hose reel systems, and pressure washing equipment.
Material Options: Swivel joints are made from materials like aluminum, stainless steel, or carbon steel, depending on the specific application and environmental conditions.
Single-Port Design: Most swivel joints support a single fluid passage.
Lower Pressure Ratings: They typically handle pressures up to 1,000 PSI.
Basic Sealing: Simple seals (e.g., rubber gaskets) prevent leaks but may require frequent maintenance.
Cost-Effective: Cheaper than hydraulic rotary unions due to their simpler design.

2.3 Applications of Swivel Joints

Hose Reels: In hose reel applications, swivel joints provide the necessary rotation while allowing water, oil, or other fluids to flow freely.
Hydraulic Cranes: Swivel joints are often used in cranes, where the movement of the boom requires flexible fluid connections that can rotate without restriction.
Fluid Transfer Systems: Common in systems that transfer water, oil, or compressed air, swivel joints ensure that the connections can rotate without creating pressure points or fluid leakage.

2.4 How Swivel Joints Work

Swivel joints consist of two threaded ends connected by a rotating pivot. One end attaches to a stationary pipe, while the other connects to a rotating hose or arm. The pivot allows rotational movement without tangling or damaging the hose.

Key Differences Between Hydraulic Rotary Union and Swivel Joint

1. Design and Construction

1.1 Rotary Union Design

Hydraulic rotary unions are typically built to handle high-pressure hydraulic systems, with multiple sealing points designed to prevent leakage at high speeds. They often feature a more complex internal structure designed to handle media like water, oil, or steam under pressure.

Sealing Mechanism: Uses dual balanced seals (e.g., carbon-silicon carbide pairs) to prevent leakage under high pressure. For example, the HS-GF hydraulic union employs CNC-machined seals with a surface roughness of ≤Ra 0.4μm for zero leakage.
Bearings: High-precision bearings (e.g., ABEC-5 or NSK bearings) ensure smooth rotation at speeds up to 1,000 RPM while minimizing friction.
Materials: Stainless steel (316L or 321 grade) or hardened alloys dominate, with corrosion-resistant coatings for harsh environments like marine or chemical industries.

1.2 Swivel Joint Design

Swivel joints are designed for more general-purpose applications, with simpler internal structures that focus on providing flexibility and preventing twisting of hoses or pipes. They are typically less complex and suitable for lower pressure environments.

Versatility: Designed for multi-channel applications. For instance, dual-path swivel joints (e.g., dual-circuit models) can simultaneously transmit oil and coolant in CNC machinery.
Sealing: Utilizes O-rings or lip seals compatible with diverse media. Steam swivel joints (e.g., QD series) use graphite-impregnated seals to handle temperatures up to 220°C.
Materials: Cast iron, brass, or PTFE-lined bodies for cost-effective solutions in low-to-medium pressure systems.

2. Pressure and Flow Capacity

2.1 Hydraulic Rotary Union

Designed to handle high-pressure applications, hydraulic rotary unions can manage pressures of up to several thousand PSI. They are built to withstand the demands of industrial machinery and systems requiring stable fluid transfer despite rotational movement.

2.2 Swivel Joint

Swivel joints, on the other hand, are designed for lower to moderate-pressure applications. Their primary purpose is to provide rotation and flexibility to piping systems that do not require the high pressure that hydraulic rotary unions are built to handle.

3. Sealing Mechanism

3.1 Hydraulic Rotary Union

Hydraulic rotary unions use advanced sealing technologies such as lip seals or O-rings, which are specially designed to withstand high pressure and rotating motion without compromising fluid containment.

3.2 Swivel Joint

Swivel joints use a simpler sealing method, such as one or two seals, to prevent leakage during rotation. These seals are typically less sophisticated than those used in hydraulic rotary unions and may not be suitable for high-pressure environments.

4. Applications and Use Cases

4.1 Hydraulic Rotary Union

These best suited for applications that involve high-pressure hydraulic fluids and require constant rotation, such as industrial machinery, manufacturing, and robotics.
Heavy Equipment (Excavators/Cranes): Rotary Unions: Enable infinite slewing in excavators through central hydraulic swivel;
Manufacturing Systems: Rotary unions cool rotating dies in plastic injection molding;
Energy Sector: Rotary unions in wind turbine hydraulic pitch systems.

4.2 Swivel Joint

Swivel joints are more suited to general-purpose applications that require flexible hose or pipe connections that can rotate freely, such as hose reels, hydraulic cranes, and pressure washing systems.
Heavy Equipment (Excavators/Cranes): Swivel Joints: Provide boom articulation in mobile cranes via multi-axis hoses;
Manufacturing Systems: Swivel joints power oscillating robotic welders;
Energy Sector: Swivel joints for offshore fuel loading arms.

5. Durability and Maintenance

5.1 Hydraulic Rotary Union

Due to the high-pressure nature of their applications, hydraulic rotary unions built for longevity and heavy-duty use. However, they require regular maintenance to check for wear on seals and internal components.

5.2 Swivel Joint

Swivel joints are typically easier to maintain because they used in less demanding environments. While they still need regular checks for leaks and seal wear, the maintenance requirements generally lower compared to hydraulic rotary unions.

When to Choose a Hydraulic Rotary Union or Swivel Joint

1. Choosing a Hydraulic Rotary Union

If you are working with high-pressure systems that require consistent fluid or gas transfer despite rotational movement, a hydraulic rotary union is the best choice.
Common applications include hydraulic systems, robotic arms, and CNC machines where maintaining pressure and fluid flow is critical.

Continuous 360° rotation required;
High-pressure (>3000 PSI) operation;
Multiple fluid circuits needed;
Precision speed control critical;

2.Choosing a Swivel Joint

If you are dealing with lower-pressure systems and need a flexible, rotating connection for hoses or pipes, a swivel joint may be the more suitable option.
Applications such as hose reels, low-pressure cranes, and fluid transfer systems benefit from the flexibility and simplicity of swivel joints.

Articulated movement with <300° rotation;
Space constraints limit component size;
Moderate pressure (<2500 PSI) suffices;
Cost sensitivity outweighs performance needs;

Advantages&Disadvantages of Hydraulic Rotary Union and Swivel Joint

1. Hydraulic Rotary Unions

Advantage: Can handle high-pressure environments.
Advanced sealing mechanisms ensure leak-free operation.
Suitable for continuous, high-performance applications.
Disadvantages: More complex and costly to maintain.
May not be necessary for lower-pressure applications.

2.  Swivel Joints

Advantage: Simpler design and easier maintenance.
Less costly than hydraulic rotary unions.
Ideal for applications requiring rotation but not high pressure.
Disadvantages: Limited pressure handling capacity.
Not ideal for systems that require high precision and sealing.

FAQs

Can a swivel joint replace a hydraulic rotary union?
No, swivel joints lack the durability and pressure resistance required for industrial applications.
What causes leaks in hydraulic rotary unions?
Worn seals, misalignment, or excessive pressure are common culprits.
How often should I replace swivel joints?
Depending on usage, replace every 1–3 years or when leaks occur.
Are hydraulic rotary unions waterproof?
Yes, most are design to handle water and other corrosive fluids.
Can I use a swivel joint in a robot arm?
Only for lightweight, low-pressure tasks; hydraulic rotary unions are better for industrial robots.
Can swivel joints handle continuous rotation like rotary unions?
No. Prolonged 360° rotation in swivel joints causes accelerated seal wear due to asymmetric loading.
What lubrication recommended for high-speed rotary unions?
Synthetic PAO-based greases with PTFE thickeners provide optimal performance at >500 RPM.
How do temperature extremes affect component choice?
Rotary unions better withstand thermal cycling through their multi-seal designs, while swivel joints may require special elastomers below -20°F.

Conclusion

Hydraulic rotary unions and swivel joints serve distinct purposes in fluid power systems. Hydraulic rotary unions engineered for high-pressure, high-speed environments, while swivel joints excel in low-risk, low-pressure tasks. By understanding their differences and applications, you can optimize system efficiency and reliability.

This technical comparison equips engineers with the knowledge to optimize hydraulic system designs. By understanding the distinct capabilities of rotary unions and swivel joints, professionals can improve equipment reliability, reduce maintenance costs, and enhance operational safety across industrial applications. As motion control requirements evolve, selecting the appropriate rotational interface remains critical for achieving peak hydraulic system performance.

While hydraulic rotary unions and swivel joints serve similar functions in terms of allowing rotation and fluid transfer, they differ significantly in terms of pressure capacity, application suitability, and design complexity. Hydraulic rotary unions built for high-pressure, high-performance environments, whereas swivel joints are ideal for more flexible, low to moderate-pressure applications. By understanding the specific requirements of your system, you can choose the right component to ensure efficiency, safety, and reliability in your operations.