Passage count
Specify the number of independent flow paths. Identify which rotor-side port connects to which stator-side port and whether different passages carry different media or pressures.
Gas and liquid through rotation
Pneumatic and hydraulic rotary unions for gas, vacuum, and liquid transfer
A rotary union transfers gas or liquid between stationary and rotating equipment without winding the connected hoses or piping. Applications may involve compressed air, vacuum, water, coolant, hydraulic fluid, or another specifically defined medium.
Slip Ring USA configures the passage count, media, pressure, vacuum, flow, port and thread, sealing system, speed, mounting, environment, leakage criteria, torque requirements, and validation plan around the application.
Application fit
Define what must cross the rotating boundary
Pneumatic and hydraulic rotary unions are considered when a gas, vacuum, or liquid service must pass between stationary and rotating parts during continuous rotation, oscillation, or intermittent movement.
The exact medium must always be stated. Terms such as “gas,” “oil,” or “liquid” are not sufficient for evaluating materials, seals, pressure, temperature, leakage, and service life.
- Compressed air
- Vacuum
- Inert gas
- Water
- Coolant
- Hydraulic fluid
- Lubricating fluid
- Multiple separately controlled media
- Media at different pressures
- Fluid passages combined with electrical power, signals, or data
Four essential inputs
Start with passage count, port, medium, and pressure
These four parameters are required before a pneumatic or hydraulic rotary-union configuration can be evaluated.
Exact port and thread
Provide the complete thread or connection standard—not only the nominal size. For example, 1/4-inch NPT and G1/4 BSPP are different interfaces and are not interchangeable.
Medium
Identify the exact gas or liquid in every passage, including relevant concentration, additives, contamination, compatibility, or safety concerns.
Working pressure
Provide the normal and maximum pressure for every passage. For vacuum service, provide the required vacuum level and operating conditions.
Pressure alone does not complete the specification. Flow, passage diameter, speed, temperature, materials, leakage, and mechanical requirements must also be defined.
Complete specification
Define the flow path and the machine around it
Flow requirement
Provide the required flow rate or minimum internal passage diameter for every path. Include acceptable pressure drop when it matters to machine performance.
Temperature
State the ambient temperature and the temperature of each medium. Identify startup, shutdown, cleaning, or process conditions that may create different temperature extremes.
Leakage
Define the acceptable internal and external leakage for every passage and the test conditions used to evaluate it.
Rotation
Specify RPM, direction changes, continuous or intermittent operation, duty cycle, operating hours, total revolutions, and start-stop behavior.
Mechanical envelope
Provide the required center bore, maximum outer diameter and length, mounting arrangement, orientation, port locations, hose routing, and surrounding service space.
Torque
State any machine-level torque limitation and describe the available drive margin. Rotary-union torque is influenced by seals, contact force, bearings, fluid behavior, passage count, diameter, speed, and sealing requirements.
Environment
Describe dust, water, washdown, humidity, salt exposure, vibration, shock, hazardous-area requirements, and the actual exposure behind any requested IP rating.
Maintenance and life
Define the required operating period, maintenance interval, seal-service expectations, replacement strategy, and the performance limits that must remain within specification.
Port and thread selection
A nominal size does not identify the connection
Thread form affects installation, sealing, fittings, and leakage performance. The mating interface and sealing method must be specified together.
NPT
NPT is a tapered pipe thread commonly used in North America. Specify the complete size and the approved thread-sealing method.
G or BSPP
G/BSPP is a parallel pipe thread. The thread itself is not the sealing surface; an appropriate gasket, O-ring, bonded seal, or other defined sealing feature is required.
R or BSPT
R/BSPT is a tapered pipe-thread form. The exact mating thread and sealing method should be confirmed rather than inferred from nominal size alone.
Metric
Metric ports must include the nominal diameter and pitch. A parallel metric thread generally requires a separate sealing feature when used as a fluid port.
UN, UNF, SAE, and other interfaces
Unified and application-specific fitting standards may also be considered. Identify the exact standard, size, sealing surface, fitting, and mating component.
Imperial, metric, NPT, and BSP connections should not be treated as interchangeable. If an adapter is required, include it in the flow, envelope, sealing, and installation review.
Media and sealing
Match wetted materials and seals to the actual medium
Material compatibility cannot be inferred from a broad category such as “air,” “water,” or “hydraulic oil.”
Reactive, flammable, corrosive, oxygen-enriched, hydrogen, hazardous, medical, or personnel-safety applications require additional engineering and safety review.
The engineering review may consider:
- Exact chemical or commercial fluid name
- Concentration and additives
- Gas or liquid state
- Cleanliness and particulate exposure
- Lubricity
- Normal and maximum temperature
- Normal and maximum pressure
- Vacuum conditions
- Wetted metals and polymers
- Seal material and geometry
- Permitted internal and external leakage
- Cleaning or sterilization requirements
- Safety and regulatory requirements
Passage architecture
Treat every passage as a separate controlled path
Multiple passages may carry different media, pressures, or flow requirements within one rotary union. Each path must be identified and validated independently.
Passage count, bore size, port arrangement, and connection direction can affect the overall diameter, length, flow path, torque, and installation clearance.
- Rotor-side and stator-side port mapping
- Medium in each passage
- Normal and maximum pressure
- Vacuum level when applicable
- Required flow or internal diameter
- Port and thread on each side
- Sealing method
- Temperature
- Permitted leakage
- Isolation from adjacent passages
- Required flushing, draining, or cleaning behavior
Torque and mechanical behavior
Evaluate torque from the complete configuration
Pressure alone should not be used as a direct estimate of operating torque. Torque expectations and test conditions should be agreed for the complete configuration.
Operating torque can be influenced by:
- Seal material and contact force
- Fluid viscous shear
- Bearing friction and applied load
- Passage count
- Shaft and housing diameter
- Rotational speed
- Temperature
- Higher enclosure-sealing requirements
- Alignment and installation
- Startup after a stationary period
Integrated rotary transfer
Combine fluid passages with power, signals, or data
A pneumatic or hydraulic rotary union may be integrated with electrical power, control signals, encoders, Ethernet—including 10 Gigabit Ethernet—fiber optic, or RF paths when the complete assembly is engineered and validated together.
Each transfer domain is specified independently before the full assembly is reviewed for:
Review integrated slip rings and rotary unions- Mechanical packaging
- Sealing and material compatibility
- Electrical isolation
- Signal integrity
- Shielding and grounding
- Thermal load
- Torque
- Connection routing
- Serviceability
- Application-level validation
Validation
Test every passage against defined acceptance criteria
The medium, pressure, temperature, duration, speed, orientation, and permitted leakage used during testing should be defined before validation begins.
A configuration-specific validation plan may include:
- Dimensional inspection
- Port and thread verification
- Passage mapping
- Airtightness or leakage testing for each passage
- Pressure testing
- Vacuum testing
- Flow or pressure-drop verification
- Rotational testing
- Starting and running torque measurement
- Temperature evaluation
- Cross-passage isolation testing
- Environmental or enclosure testing
- Electrical, signal, or network testing for integrated assemblies
- Evaluation with the customer’s equipment when required
Application review
Bring us the complete fluid-transfer requirement
Send the application, every medium, passage count, normal and maximum pressure, vacuum level, flow or internal-diameter requirement, exact ports and threads, fittings, sealing method, leakage criteria, temperature, RPM, duty cycle, envelope, center bore, mounting, orientation, environment, maintenance expectations, test requirements, and expected quantities.
We will identify missing information and determine whether the application fits an existing rotary-union platform or requires a custom design.
Engineering questions
Pneumatic and hydraulic rotary-union FAQ
- What is a pneumatic or hydraulic rotary union?
- A rotary union is a mechanical interface that transfers gas, vacuum, or liquid between stationary and rotating equipment while allowing continuous rotation, oscillation, or intermittent motion.
- What is the difference between a pneumatic and hydraulic rotary union?
- A pneumatic rotary union carries gas or vacuum, while a hydraulic rotary union carries liquid. The exact medium, pressure, flow, temperature, leakage requirement, materials, seals, and connections determine the final configuration.
- What information is essential for selecting a rotary union?
- Start with four items: passage count, exact port or thread, medium, and working pressure. Final selection also requires flow or passage diameter, vacuum when applicable, temperature, speed, duty cycle, envelope, mounting, orientation, environment, leakage, maintenance, and validation requirements.
- Can one rotary union carry different gases or liquids?
- Multiple isolated passages may be considered, including passages with different media or pressures. Each path must be defined by its medium, pressure or vacuum, flow, port, temperature, permitted leakage, compatibility requirements, and acceptance test.
- Can a rotary union carry vacuum?
- Yes, depending on the required vacuum level, leakage limit, flow, medium, passage arrangement, seals, speed, temperature, mounting, and validation conditions. “Vacuum” alone is not a complete specification.
- What thread information should I provide?
- Provide the complete thread standard and size, such as 1/4-inch NPT, G1/4 BSPP, or M10 × 1.0. Also identify the mating fitting and sealing method. A nominal size without the thread standard is insufficient.
- Is G1/4 the same as 1/4-inch NPT?
- No. G1/4 BSPP is a parallel pipe thread, while 1/4-inch NPT is a tapered pipe thread. They have different geometry and sealing requirements and should not be connected as though they were interchangeable.
- What determines rotary-union torque?
- Torque is influenced by the sealing system, seal contact force, bearings, fluid behavior, passage count, diameter, speed, temperature, enclosure sealing, alignment, and installation. Pressure alone should not be used as a direct torque estimate.
- Can electrical power or Ethernet be integrated?
- Yes, when the fluid and electrical requirements are engineered together. Slip Ring USA also accepts integrated Gigabit and 10 Gigabit Ethernet applications for engineering review. Final feasibility depends on the complete fluid, electrical, data, mechanical, environmental, and validation requirements.
- Does an IP rating define compatibility with the internal fluid?
- No. An IP rating addresses defined external ingress exposure. It does not establish compatibility between the internal medium and the wetted materials or seals. Both requirements must be reviewed separately.
- How is leakage evaluated?
- Define whether the requirement addresses external leakage, leakage between passages, or both. The test medium, pressure or vacuum, temperature, duration, orientation, rotation, and permitted leakage rate should be agreed for the configuration.
- What information is needed for a rotary-union quote?
- Provide the application, exact medium in every passage, passage count, normal and maximum pressure, vacuum when applicable, required flow or internal diameter, exact port and thread, fitting and sealing method, temperature, permitted leakage, RPM, duty cycle, envelope, center bore, mounting, orientation, environment, maintenance expectations, test requirements, integrated electrical or signal needs, and expected quantities.