Rotary Union Design for Rotating Equipment

Understanding Rotary Unions: The Critical Link in Rotating Machinery

Rotary unions, also known as rotary joints or rotating unions, represent one of the most critical yet often overlooked components in industrial rotating equipment. These precision-engineered devices enable the transfer of fluids—whether liquids, gases, or steam—from stationary supply lines into rotating machinery without leakage or contamination. For industries across Atlantic Canada, from paper mills in Nova Scotia to offshore petroleum operations, proper rotary union design is essential for operational efficiency, safety, and equipment longevity.

At their core, rotary unions must accomplish what seems mechanically paradoxical: maintaining a sealed connection between components moving at vastly different speeds. The stationary housing connects to fixed piping systems while the rotating element spins with the machinery, sometimes at speeds exceeding 10,000 RPM. This fundamental challenge drives the sophisticated engineering principles that govern rotary union design.

Core Design Principles and Engineering Considerations

The design of a rotary union begins with a thorough analysis of the application requirements. Engineers must consider multiple interdependent factors that ultimately determine the configuration, materials, and sealing mechanisms employed.

Operational Parameters

Before selecting or designing a rotary union, engineers must establish the following critical parameters:

• Rotational Speed: Measured in RPM, this determines bearing selection, seal type, and heat generation characteristics. Low-speed applications (under 100 RPM) allow for different design approaches than high-speed applications exceeding 3,000 RPM.

• Operating Pressure: System pressures can range from vacuum conditions (-0.1 MPa) to high-pressure applications exceeding 35 MPa (5,000 PSI). Pressure ratings directly influence seal design and housing material thickness.

• Temperature Range: Operating temperatures affect material selection, thermal expansion calculations, and lubricant requirements. Maritime applications in Nova Scotia must account for ambient temperature variations from -30°C to +35°C in addition to process temperatures.

• Media Compatibility: The fluid being transferred—whether water, hydraulic oil, steam, compressed air, or aggressive chemicals—determines seal materials and internal component specifications.

• Flow Requirements: Volume flow rates, typically measured in litres per minute, influence passage sizing and pressure drop calculations.

Structural Configuration Types

Rotary unions are manufactured in several fundamental configurations, each suited to specific applications:

Single-passage unions transfer one medium and represent the simplest design. They are commonly used in machine tool spindles, textile machinery, and simple heating or cooling applications. These units typically range from 6 mm to 150 mm in passage diameter.

Multi-passage unions accommodate multiple independent fluid circuits through concentric passages or parallel channels. These designs are essential for complex machinery requiring simultaneous transfer of hydraulic fluid, coolant, and compressed air. Some sophisticated designs accommodate up to 12 independent passages.

Integrated unions combine the rotary union function with additional components such as slip rings for electrical power transmission or fibre optic rotary joints for data transfer. These hybrid solutions reduce overall system complexity and footprint.

Sealing Technologies and Material Selection

The sealing interface represents the heart of rotary union performance. Seal failure leads to fluid leakage, contamination, and potential catastrophic equipment damage. Engineers must carefully select sealing technologies based on application demands.

Mechanical Face Seals

Mechanical face seals, operating on principles similar to mechanical shaft seals in pumps, provide excellent performance across a wide range of conditions. These seals feature two precision-lapped faces—one stationary and one rotating—held together by spring force and system pressure. The sealing faces maintain a microscopic fluid film (typically 0.25 to 0.75 micrometres thick) that provides both lubrication and sealing.

Common face seal material combinations include:

• Carbon-graphite against tungsten carbide: Excellent for high-speed applications with good chemical resistance

• Silicon carbide against silicon carbide: Superior for abrasive media and high-temperature applications up to 260°C

• Carbon against ceramic (aluminium oxide): Cost-effective solution for moderate conditions

Balanced vs. Unbalanced Seal Designs

Seal balance refers to the ratio of hydraulic closing force to opening force across the seal faces. Unbalanced seals, where the full system pressure acts to close the faces, work well for low-pressure applications (under 1.7 MPa). However, for higher pressures, balanced seal designs reduce face loading and heat generation, extending service life significantly.

A properly balanced seal typically operates with a balance ratio between 0.65 and 0.85, meaning 65-85% of the system pressure acts as closing force. This careful engineering reduces face wear while maintaining reliable sealing.

Elastomeric Seals and O-Rings

Secondary sealing elements, typically elastomeric O-rings or custom-moulded seals, provide static sealing between components and allow for thermal expansion. Material selection follows established compatibility guidelines:

• Nitrile (Buna-N): General-purpose applications with petroleum-based fluids, -40°C to +120°C

• Viton (FKM): Excellent chemical resistance for aggressive media, -20°C to +200°C

• EPDM: Ideal for water and steam applications, -50°C to +150°C

• PTFE: Universal chemical compatibility, -200°C to +260°C, but requires spring energising

• Kalrez (FFKM): Premium performance for extreme chemical and temperature conditions

Bearing Systems and Rotational Support

Rotary unions require precision bearing systems to maintain proper alignment between rotating and stationary components while supporting applied loads. Bearing selection directly impacts service life, friction losses, and maximum operational speeds.

Bearing Types and Applications

Ball bearings provide excellent performance for high-speed, low-load applications. Deep groove ball bearings are standard in rotary unions operating above 1,000 RPM, offering low friction coefficients (typically 0.0015 to 0.003) and good axial load capacity.

Needle bearings suit applications with significant radial loads and space constraints. Their high load capacity relative to cross-sectional area makes them ideal for compact rotary union designs.

Plain bearings manufactured from self-lubricating materials such as carbon-graphite composites or PTFE-lined bronze offer advantages in corrosive environments or applications where contamination sensitivity precludes rolling element bearings.

Lubrication Considerations

Bearing lubrication in rotary unions presents unique challenges. For many applications, the transferred media provides lubrication to internal components—a design approach that eliminates external lubrication requirements but demands careful material compatibility analysis. When external lubrication is required, sealed-for-life greased bearings or oil mist lubrication systems are commonly employed.

Application-Specific Design Requirements

Different industries across Atlantic Canada present unique challenges for rotary union design. Understanding these application-specific requirements ensures optimal equipment specification.

Pulp and Paper Industry

Nova Scotia's forestry sector relies heavily on paper machine dryer sections where rotary unions, often called steam joints in this context, transfer high-temperature steam into rotating dryer cylinders. These applications typically involve:

• Steam pressures of 700-1,000 kPa (100-145 PSI)

• Temperatures reaching 180°C

• Rotational speeds of 50-300 RPM

• Siphon systems for condensate removal

• Service life expectations exceeding 5 years

Design considerations include thermal expansion management, condensate drainage efficiency, and resistance to cycling conditions during machine start-ups and shutdowns.

Marine and Offshore Applications

The Maritime provinces' significant marine industry utilises rotary unions in winch systems, crane slewing mechanisms, and propulsion components. These harsh environment applications demand:

• Corrosion-resistant materials (316 stainless steel or duplex stainless steel housings)

• Enhanced sealing systems for salt spray exposure

• Accommodation of vessel motion and shock loads

• Compliance with marine classification society requirements (Lloyd's, DNV, ABS)

Machine Tool Applications

Modern CNC machining centres use rotary unions for through-spindle coolant delivery, enabling high-pressure coolant (up to 70 bar) to reach the cutting zone directly through the tool. These precision applications require:

• Minimal runout (less than 0.01 mm TIR)

• High-speed capability (10,000+ RPM)

• Compatibility with water-soluble and synthetic coolants

• Compact designs to minimise spindle length

Installation, Maintenance, and Troubleshooting

Proper installation and maintenance practices significantly impact rotary union service life. Engineering specifications must include comprehensive installation guidelines and maintenance protocols.

Installation Best Practices

Critical installation considerations include:

• Alignment: Misalignment between the rotary union and the rotating shaft accelerates bearing and seal wear. Maximum permissible angular misalignment typically ranges from 0.5° to 2° depending on the design.

• Mounting: The stationary housing must be restrained against rotation while allowing slight axial and radial movement to accommodate thermal expansion and minor misalignment.

• Piping connections: Flexible hose connections to the stationary port prevent transmission of piping loads to the rotary union housing.

• Filtration: Upstream filtration (typically 25-50 micrometre) protects seal faces from abrasive particles that cause accelerated wear.

Preventive Maintenance Protocols

Establishing regular inspection schedules extends equipment life and prevents unplanned downtime. Recommended practices include:

• Monthly visual inspection for external leakage

• Quarterly verification of mounting torque and alignment

• Annual measurement of internal clearances and bearing play

• Seal replacement at manufacturer-recommended intervals or upon detection of increased leakage

Common Failure Modes and Diagnostics

Understanding failure mechanisms aids in both troubleshooting and design improvement:

Excessive leakage typically indicates seal face wear, scoring, or contamination. Analysis of leak rate trends helps predict seal replacement timing—sudden increases often indicate acute damage requiring immediate attention.

Elevated temperature at the seal interface suggests inadequate lubrication, excessive face loading, or improper balance ratio for the application pressure.

Vibration and noise point to bearing degradation, misalignment, or cavitation within the fluid passages.

Future Trends and Advanced Technologies

Rotary union technology continues evolving to meet increasingly demanding industrial requirements. Emerging trends include:

Smart monitoring systems incorporating sensors for real-time measurement of temperature, vibration, and leak detection. These Industry 4.0 compatible solutions enable predictive maintenance strategies that minimise unplanned downtime.

Advanced materials including diamond-like carbon (DLC) coatings and ceramic matrix composites offer enhanced wear resistance and extended service intervals.

Additive manufacturing enables optimised internal flow passages that reduce pressure drops and improve thermal management—geometries previously impossible with traditional machining methods.

Partner with Sangster Engineering Ltd. for Your Rotary Union Requirements

Designing and specifying rotary unions for demanding industrial applications requires deep expertise in mechanical engineering, materials science, and fluid dynamics. At Sangster Engineering Ltd., our team brings extensive experience in rotating equipment design and analysis, serving clients throughout Nova Scotia, Atlantic Canada, and beyond.

Whether you require assistance with new equipment specification, failure analysis of existing rotary unions, or custom design solutions for unique applications, our professional engineers provide the technical expertise needed to optimise your rotating equipment systems. We understand the specific challenges facing Maritime industries—from harsh coastal environments to the demanding requirements of resource extraction and processing operations.

Contact Sangster Engineering Ltd. today to discuss your rotary union design requirements and discover how our engineering expertise can improve your equipment reliability and operational efficiency. Our Amherst, Nova Scotia office serves as the base for providing comprehensive mechanical engineering services throughout the region.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.