Swivel joints for hydraulic applications allow more freedom of movement
With swivel joints, you can reduce the mechanical stress that causes high wear and tear on the hose.
Hydraulic Rotary Joints: Function, Types, and Application Criteria
Rotary joints for hydraulic applications safely guide hydraulic media through rotating or swiveling connections, thereby reducing hose tension, kinking, and premature wear. Unlike rigid pipe connections, rotary joints compensate for relative movements between units without twisting the hydraulic line. Key factors include the sealing system, material selection, media compatibility, and mechanical integration into the machine.
Structure and Operating Principle
A typical rotary joint consists of an inner body with through-bores, an outer body or housing, a rotating surface, and a sealing assembly. The through-bores are guided through concentric channels, which are separated by seals to allow for multiple independent circuits. The rotational movement occurs at the interface between the outer body and inner body; guiding tolerances and sealing compression are crucial here. Typical connection types include NPT, BSPP/BSPT, ORFS, SAE flange, as well as pipe or hose nozzles with crimp connections.
Materials and Coatings
The choice of material determines pressure resistance, corrosion resistance, and wear behavior. Standard materials include case-hardened steel, stainless steel (AISI 316/L), and brass. Steel joints are often nitrided or phosphated to increase wear resistance and provide an adhesion base for paints. Stainless steel joints are required for media with chemical aggressiveness or in food and pharmaceutical environments. For abrasive media, hardened through-bores or coated sealing surfaces (e.g., DLC) are used.
Seals and Sealing Concepts
Seals are the critical component regarding leakage and friction torque. For hydraulic rotary joints, O-rings made of NBR, HNBR, or FKM are typically used. HNBR offers improved temperature and oil resistance; FKM is suitable for high temperatures and aggressive additives. For high pressures or multi-circuit requirements, combinations of hydrodynamic seals and backup rings are used to prevent extrusion. Special labyrinth seals or compound seals are employed in dirty and dusty environments. The sealing material must be compatible with the hydraulic oil, additives, and operating temperatures.
Designs and Configurations
Rotary joints are available in single, double, or multi-channel versions. Single-channel joints are suitable for simple return or pressure lines. Double and multi-channel variants allow for combined pressure and control lines in a rotating connection. Compact inline joints, right-angle designs, and swiveling flange joints for large moment loads exist. Open designs with an external axis facilitate lubrication, while closed designs offer better protection against environmental influences. The choice of design depends on installation space, angle of movement, number of lines, and inertia requirements.
Hydraulic Parameters
Essential parameters include nominal pressure, flow rate, leakage rate, rotation angle, and torque. Most industrial rotary joints are designed for operating pressures of 250 to 420 bar; special versions reach 700 bar. Flow rates can range from a few milliliters per minute to several hundred liters per minute; limiting pressure loss and preventing cavitation are important. Rotation angles are specified as end stops or as continuous rotation (360° continuously). For applications with constant rotation, the lubrication and sealing design is optimized for low friction torques and long service life.
Connection and Mounting Interfaces
The choice of connection influences tightness, installation effort, and interchangeability. Threaded connections (BSPP, NPT) are widely used; for higher tightness and easier assembly, ORFS and SAE-16/SAE-24 connections offer advantages. Flange solutions are common for high moments and large pipe cross-sections. Mounting options: Fixed flange and rotatable bolt-on flange, sleeve with ball bearing, or integration into a component with axial screw connection. During installation, attention must be paid to alignment, axial compensation, and bearing preload to avoid uneven loading and increased leakage.
Operating Conditions and Service Life
Operating temperatures are based on the seals and lubrication used; typical ranges are -30 °C to +120 °C, depending on the sealing material. Cyclic loads, vibrations, and shock loads shorten the service life; relevant test parameters are the number of cycles until a defined leakage rate and torque increase over the service life. Maintenance consists of regular checks for leakage, lubrication intervals, and, if necessary, seal replacement. For critical systems, condition-based maintenance with measurement of leakage rate and torque trend is recommended.
Selection Criteria
The selection of a suitable rotary joint is based on the following parameters: operating pressure and temperature range, flow rate and number of media, required rotation angle or continuous rotation, mounting and connection conditions, industrial environment (corrosion protection), and permissible leakage risk. For safety-relevant applications, rotary joints with certifications for pressure vessels and high-pressure test protocols should be selected.
Practical Examples
Example 1 – Robot Swivel Arm in Manufacturing: On a swivel arm, two hydraulic circuits (drive and brakes) are guided through a two-part rotary joint. The joint is made of hardened steel with FKM seals and ORFS connections, approved for 350 bar and 60 l/min. The use of a multi-channel joint eliminates additional drag lines; mounting is done with axial flange mounting. After installation, leakage rates and torque curves were recorded to determine lubrication intervals.
Example 2 – Mobile Hydraulics Tipper Body: In a tipper, a 360°-capable rotary joint enables the supply of hydraulics for tipping cylinders and controls. Due to harsh outdoor conditions, a stainless steel housing with NBR seals and additional dirt protection was chosen. Connection is via BSPP threads; the joint is designed for 250 bar. The design significantly reduces hose lengths and increases the service life of the lines.
Example 3 – Test Bench with Rotational Speed Sensor Technology: In a test bench, several hydraulic test circuits are guided through a multi-channel rotary joint. Requirements were low leakage (<0.1 ml/min), temperature stability up to 100 °C, and flow rates of 0.5–20 l/min. Solution: Stainless steel version, HNBR seals, labyrinth attachment as secondary seal, and ORFS connections. The tightness was revalidated after 200,000 cycles.
Standards, Tests, and Quality
Rotary joints should be tested according to relevant industry standards; relevant are ISO 9001 manufacturing processes, pressure tests according to EN/ISO standards, as well as leakage and endurance tests depending on the application. For certain industries, additional certificates or documentation such as material certificates (EN 10204/3.1) and test protocols are important. Manufacturer specifications regarding pressure loss rate, permissible torque, and recommended lubricants are binding selection criteria.
Integration into Existing Systems
Integration begins with a technical specification of all relevant parameters: media type, temperature, pressure, flow, and motion profile. Connection dimensions must be matched to the existing piping system. When retrofitting, the axle mounting must be checked for play and runout; for rotating connections, attention must be paid to balanced masses and specified torques to keep bearing and seal frequencies outside the resonance range. For complex multi-channel requirements, preliminary sampling and functional testing under realistic loads are recommended.
Further technical information and application examples can be found under Technology and Application Examples on our website.
- Quick Overview: Materials: Steel/Stainless Steel/Brass; Seals: NBR/HNBR/FKM; Pressures: 250–700 bar; Flow Rates: ml/min to 200+ l/min; Connections: BSPP/NPT/ORFS/SAE/Flange; Rotational Capability: up to 360° continuous
FAQ
1. Which seal is suitable for 100 °C and hydraulic oil with additives?
For applications up to 100 °C and oil-containing additives, HNBR is the preferred choice; FKM is used for higher temperatures or aggressive additives. Check the oil manufacturer's material specifications.
2. Can a rotary joint carry multiple media (e.g., oil and air) in one housing?
Yes, multi-channel rotary joints separate media via separate channels and seals. Important factors are pressure differences, compatibility of the seals with both media, and the integrity of the separating walls.
3. How do I know when a rotary joint needs maintenance or replacement?
Indicators include increasing leakage rates, a significant increase in torque, visible external leakage, or metallic noises. A test interval based on the number of cycles and operating conditions is recommended; for critical systems, condition-based maintenance with measurement protocols is advisable.