How to Troubleshoot Hydraulic Rotary Union Leaks: A Comprehensive Guide

Hydraulic rotary unions serve as indispensable components in complex machinery systems where simultaneous rotational movement and fluid transmission are essential. These precision-engineered devices are widely deployed across diverse industries, including heavy construction equipment, automated manufacturing systems, and renewable energy infrastructure.

By facilitating uninterrupted fluid transfer while accommodating rotational motion between stationary and moving parts, they enable critical functions such as hydraulic power transmission, cooling system operation, and pneumatic control in applications like excavator booms, rotating tables, and wind turbine nacelles.

The integrity of these unions is paramount to operational efficiency and safety. Leakage presents multifaceted challenges: immediate production halts due to system depressurization, escalating maintenance costs from component replacement and cleanup operations, and environmental contamination risks from spilled hydraulic fluids infiltrating ecosystems. Beyond these direct consequences, chronic leakage can accelerate equipment degradation through lubricant loss and corrosion, creating cascading failure risks in interconnected mechanical systems.

The hydraulic system is a transmission system widely used in modern industry. It has the advantages of high power, fast response, and high precision. However, various faults may occur in the hydraulic system during use, among which leakage is the most common. Leakage will not only lead to a variety of faults, such as decreased system efficiency, low working pressure, and slow movement speed, but also pollute the working environment and waste resources. So how to eliminate the leakage of the hydraulic system? The following are some steps and precautions for eliminating the leakage of the hydraulic system.

Step 1: Identify the Source of the Leak

Diagnosing a leak requires a systematic approach. Start by visually inspecting the union and surrounding areas for:
Visible Fluid Residue: Look for oil stains, puddles, or buildup.
Unusual Noises: Hissing or clicking sounds may indicate pressure loss.
Temperature Changes: Overheating could signal friction from worn seals.

Tools for Diagnosis:

Pressure Gauge: Measure system pressure to identify drops.
UV Dye and Blacklight: Pinpoint leaks in hard-to-reach areas.
Thermal Imaging Camera: Detect temperature irregularities.

Step 2: Common Causes of Hydraulic Rotary Union Leaks

Leaks often stem from preventable issues. Here’s a breakdown of typical causes:

A. Seal Wear and Degradation

The seals are an important part of the hydraulic system. They can keep the hydraulic oil clean and sealed. If the seals are of poor quality or improperly assembled, leakage will occur. Therefore, the quality and assembly of the seals should be checked first to ensure that they are intact and installed correctly.

Lip Seals: Prone to abrasion from contaminants.
O-rings: Degrade over time due to heat or chemical exposure.
Mechanical Face Seals: Require precise alignment to avoid friction.
Over time, seals can harden, crack, or lose elasticity due to exposure to high temperatures, pressures, or incompatible fluids. For instance, elastomeric seals may deteriorate in high-temperature applications, while metallic seals may wear out under excessive vibration.

B. Misalignment or Loose Fittings

Check whether the size and design of the sealing part are correct, the processing accuracy is good, and the assembly is correct. Incorrect size design of the sealing part, poor processing accuracy, and poor assembly will cause hydraulic system leakage. Therefore, it is necessary to check whether the size design of the sealing part is correct, the processing accuracy is good, and the assembly is correct to avoid internal and external leakage.

Improper installation can strain seals or cause fittings to loosen. Always torque bolts to the manufacturer’s specifications.
The interface between the rotor and stator experiences friction, leading to wear on both components. This wear can create gaps where fluid can escape, especially in high-speed applications.
Misalignment during installation can cause uneven pressure on seals, leading to premature failure. Additionally, over-tightening or under-tightening fasteners can compromise seal integrity.

C. Contaminated Fluid

Particles in hydraulic fluid scratch seals and corrode metal surfaces. Regular filtration is essential.
Particulates in the hydraulic fluid can act as abrasives, damaging seals and surfaces. Water ingress can also corrode metal parts and degrade oil viscosity.

D. Excessive Pressure or Temperature

Another important reason for hydraulic system leakage is that the system operating pressure is too high. Excessive system operating pressure will cause the seal to fail, causing the hydraulic oil to leak. Therefore, it is necessary to check whether the system operating pressure is too high and ensure that it does not exceed the sealing pressure range of the seal.

Overloading the system beyond its rated capacity accelerates wear. Check pressure relief valves and cooling systems.
In some cases, the rotary union’s design may not adequately address the specific application’s demands, such as insufficient cooling channels for high-temperature fluids or inadequate seal redundancy.

E. Environmental Factors

Moisture, dust, or corrosive chemicals can compromise seals. Consider shield upgrades for harsh conditions.

In short, it is necessary to start from multiple aspects to eliminate the leakage of the hydraulic system, from seals to relative motion pairs, oil temperature, system operating pressure, etc., and they must be checked and monitored. At the same time, it is also necessary to pay attention to the correct use and maintenance to avoid leakage caused by incorrect use and maintenance. Only comprehensive inspection and maintenance can effectively eliminate the leakage of the hydraulic system.

Step 3: Repairing Hydraulic Rotary Union Leaks

Once the cause is identified, implement targeted repairs:

A. Seal Replacement

Disassemble the Union: Follow the manufacturer’s manual.
Clean the Components: Remove debris and old seal residues.
Install New Seals: Use compatible replacements (e.g., Viton for high heat).
Reassemble and Test: Pressurize the system to check for leaks.

B. Component Replacement

If the shaft or housing is damaged, replace the entire union. Upgrading to corrosion-resistant materials (e.g., stainless steel) may prevent future issues.

C. Alignment Adjustment

Use laser alignment tools to ensure the shaft and housing are perfectly aligned.

D. Fluid System Flush

If contamination is the root cause, flush the system with a compatible cleaner and replace the fluid.

Step 4: Preventive Maintenance Strategies

Proactive care reduces downtime and extends the union’s lifespan.

A. Regular Inspections

Schedule quarterly checks for leaks, corrosion, and wear.

B. Lubrication Best Practices

Apply manufacturer-recommended lubricants to seals to minimize friction.

C. Fluid Quality Monitoring

Test the fluid regularly for contaminants and replace filters as needed.

D. Operate Within Specifications

Avoid overloading the system to prevent premature wear.

E. Training for Maintenance Teams

Ensure staff recognize early leak signs and follow proper repair protocols.

Summary: Prevention is better than repair

3 Golden Rules:
Regular monitoring: Check the hardness of seals, bearing clearance, and connecting bolt torque every quarter.
Environmental management: Control temperature fluctuations to<± 10 ° C, humidity to<70%, and dust concentration to<5 mg/m³.
Data archiving: Record the type of medium, peak pressure, and maintenance measures for each leak, and establish a fault prediction model.

FAQs About Hydraulic Rotary Union Leaks

Q1: How often should hydraulic rotary union seals be replaced?

A1: Regular cycle:
Nitrile rubber (NBR): 6-12 months (standard operating conditions).
Fluoro rubber (FKM): 18-24 months (high temperature/corrosive environment).
Polytetrafluoroethylene (PTFE): 3-5 years (high pressure/low wear scenario).
Warning signal: If the sealing lip has hardened, cracked, or compressed with permanent deformation greater than 15%, it needs to be replaced immediately.

Q2: The stress test shows normal, but there is still leakage during operation. What could be the possible reason?

A2: Dynamic leakage triggers:
Vibration: Equipment resonance causes micro-motion wear of seals (requiring the installation of shock absorbers).
Thermal expansion: The expansion coefficient of the metal shell and seal does not match at high temperatures (replaced with graphite-filled PTFE).
Fluid pulsation: The flow fluctuation of the pump causes instantaneous high pressure (stabilized by installing an accumulator).

Q3: How to deal with leaked hazardous chemical media?

A3: Emergency procedures:
Immediate shutdown: Close the upstream and downstream valves and drain the residual medium.
Pollution control: Use absorbent cotton to block the leakage area and avoid skin contact (wear acid and alkali-resistant gloves).
Compliant disposal: The collected waste liquid shall be handed over to professional institutions for treatment by the EPA or GB 8978 standards.

Q4: What is the maximum allowable eccentricity for a rotary joint?

A4: General standards:
Low speed (<50 rpm): eccentricity ≤ 0.2 mm.
High speed (>200 rpm): eccentricity ≤ 0.05 mm.
Exception: Multi-channel connectors (such as MCR-12) require strict control within 0.03 mm due to their complex structure.

Q5: How to quickly locate small leakage points?

A5: Efficient method:
Ultrasonic testing: Use UE Systems UltraProbe to capture high-frequency leakage sound waves.
Infrared thermography: Identifying leakage areas through temperature differences (applicable to high-temperature media).
Soap water test: Apply to the surface of the joint and observe the location of bubble generation (limited to low-pressure systems).

Q6: Is the old rotary joint worth renovating and upgrading?

A6: Analysis of Transformation Benefits:
Economy: If the maintenance cost is less than 60% of the new part price and the expected lifespan is extended by more than 2 years, it is recommended to renovate.
Upgrade plan:
Install wireless vibration sensors (such as SKF Enlight) for status monitoring.
Replace with a modular sealing system (such as the EagleBurgmann Cartex series) for quick maintenance.

Conclusion

Hydraulic rotary union leaks are a multifaceted challenge that demands a blend of technical expertise, meticulous maintenance, and proactive planning. By understanding the root causes, employing systematic diagnostics, and implementing targeted solutions, operators can minimize leaks and maximize equipment uptime. Remember, prevention is key—investing in training, quality components, and regular inspections will pay dividends in the long run.

Troubleshooting hydraulic rotary union leaks demands technical expertise and a structured approach. By combining regular inspections, timely repairs, and preventive measures, you can keep the machinery running efficiently and safely.

For further reading, check out our articles on Hydraulic System Efficiency Optimization and Common Rotary Union Failure Modes.
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