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Hydraulic Rotary Joints: Technical Description and Selection Criteria

Rotary joints for hydraulic applications safely and rotatably transmit hydraulic media between stationary and rotating assemblies. They are used in industrial plants, cranes, conveying technology, and special machinery where lines must be routed around angles or through rotating shafts. Typical requirements include leak-tightness at high pressure, long service life under cyclic loads, and compatible connections and materials for the hydraulic oil used.

Construction and Materials

Rotary joints essentially consist of a rotating inner body and an outer housing with static sealing. Housing materials are typically hardened steel or stainless steel (e.g., 1.4301/304 or 1.4404/316L) for corrosive environments. Running and sealing surfaces are often nitrided or honed to reduce wear. For high-pressure hydraulic applications, precisely turned internal parts with radial and axial seal packs are used.

Sealing Systems: In this category, rotary joints with NBR (nitrile rubber) seals are available. NBR offers good resistance to mineral oils, adequate temperature ranges from approximately -30 °C to +100 °C, and good abrasion resistance. For higher temperatures, aggressive media, or longer service life, FKM (Viton) or PTFE-supported seals are advisable. The choice of seal significantly determines leak-tightness, friction coefficient, and service life.

Connections and Interfaces

Hydraulic rotary joints are offered with a variety of connections: BSPP (G), NPT, Metric (M-form), SAE, and quick-coupling variants. Thread types are typically parallel or tapered; mounting adapters and transition pieces are necessary when connecting different systems. For low leakage rates and easy assembly, screwed pipe fittings and O-ring-secured flange connections are common. In rotating applications with high torques, axial clearance of the connections must be ensured to prevent hose lines from twisting or being crushed.

Designs and Performance Data

Rotary joints are available in single- to multi-channel versions, with through-bores ranging from a few millimeters up to >20 mm inner diameter. Typical pressure ranges in hydraulic systems are between 250 bar and 350 bar; standard rotary joints in this category are designed for 250 bar, with special versions available up to 350–420 bar. Permissible speeds are usually low (up to about 60 rpm), as high speeds reduce the service life of the seals. For true high-speed applications, other design principles or additional lubrication and cooling concepts are used.

Temperature and Media Compatibility

For standard hydraulic oils and mineral oil-based media, NBR seals are the economical choice. For alternative hydraulic fluids such as HLP, HETG, PAG, or biodegradable ester fluids, compatibility must be checked in advance. Temperature and media resistance influence both the seal selection and the housing material selection. For continuous contact above +80 °C, sealing materials with higher heat resistance are recommended; for frequent temperatures below -20 °C, soft NBR compounds should be avoided.

Application Practice: Installation Examples and Influencing Factors

Practical Example 1: A 2-channel rotary joint with NBR seal is used in a hydraulically driven swivel axis of a container handling device. Requirements: peak operating pressure 280 bar, max. speed 10 rpm, medium: HLP46. Installation: 3/8" BSPP internal thread on the rotating axis, 1/2" BSPP external thread to the stationary line. Result: Leakage level <0.5 ml/min at 250 bar, seal replacement interval after 18 months of daily use.

Practical Example 2: Chain conveyors with lateral adjustment use a single-pass rotary joint for continuous oil supply to the drive motor. Prerequisites: continuous flow 20 l/min, pressure 160 bar, temperature range -10 to +60 °C. Solution: Rotary joint with larger inner diameter (12 mm), reinforced rolling bearing to absorb axial loads, and NBR seal in high-strength compound. Advantage: Constantly stable oil supply without hose twisting and reduced downtime due to longer maintenance intervals.

Practical Example 3: In a hydraulically controlled robot axis, a multi-channel rotary joint for electrical lines and hydraulics operates in one component. Requirements: combined cable packages, low leakage requirements, moderate operating pressures up to 200 bar. Technical solution: Multi-channel rotary joint with integrated DN 6 and DN 4 channels, NBR seals for hydraulic channels, additional sealing for electrical channel. Installation note: Align channels before final assembly, tighten connections with torque according to manufacturer's specifications.

Maintenance, Testing, and Installation Notes

Before installation, connections and sealing surfaces must be cleaned; foreign bodies quickly lead to leaks and premature seal failure. Seals should be pre-lubricated with the recommended lubricant to prevent initial damage. Testing procedures include pressure testing at 1.1–1.5 times operating pressure and leakage measurement over a defined period. Replacement intervals depend on pressure cycles, medium, and speed; documented monitoring of the leakage rate enables planned maintenance. During assembly, the installation position must be observed: some designs require axial clearance for the rotating body; rigid pipelines must be decoupled by flexible connection hoses.

Selection Criteria for Procurement

Crucial for selection are: nominal pressure, flow requirement, number of channels, connection standards, sealing material, material, and environmental conditions. Sizing is based on maximum flow rate and permissible pressure loss; the larger the inner diameter of the channel, the lower the pressure drop at a constant flow rate. For critical applications with explosive or corrosive media, stainless steel and FKM/PTFE seals are mandatory. Validate specifications and drawings against the installation situation, especially the maximum rotation angle, thread types, and any adapter requirements. Further technical information and test protocols can be found on our technical page: https://maku-industrie.de/technik.

• Important selection points: Pressure/Flow, Channels, Connections, Seal, Material

Compatibility and Spare Parts

For Parker Rectus products, check original part numbers, as dimensions and tolerances may vary with replica parts. Replacement seals and repair kits are available for standard sizes; if required, we offer suitable adapters or customized solutions. When replacing, document the part identification and production batch to ensure traceability.

Scope of Delivery and Incoming Goods Inspection

The scope of delivery usually includes the rotary joint, including all factory-installed seals and, if applicable, mounting tools. Upon receipt of goods, check for: visible damage, correct connections/threads, rotary bearing function without rough running, and documentation such as pressure test or conformity certificate. Complaints must be substantiated with batch numbers and photos.

Further Information and Application Examples

In-depth application examples and practical cases are available on our application examples page: https://maku-industrie.de/anwendungsbeispiele. For tailored advice on integration into your system, you can request technical data sheets or have an engineering check performed.

FAQs

1. For what pressures are standard rotary joints with NBR seals suitable?
Standard versions with NBR seals are typically designed for operating pressures up to 250 bar; specified versions reach 350–420 bar. The specific pressure resistance is model-dependent and can be found in the manufacturer's specifications.

2. Which seal is better for high temperatures or synthetic hydraulic fluids?
For higher temperatures (>80 °C) or synthetic or biological hydraulic fluids, FKM- or PTFE-based seals are preferable. NBR is suitable for mineral oil-based media up to approx. 100 °C; for increased temperature or media requirements, a compatibility check is necessary.

3. How do I determine the correct size of the rotary joint for my flow?
Size based on maximum volume flow and permissible pressure loss; choose a channel cross-section that minimizes pressure loss at a given flow rate. Additionally, consider the number of channels and required connection sizes; technical data sheets provide flow characteristics and pressure loss tables for precise design.