Fault Analysis and Optimization Strategies for Rexroth A4VSG Axial Piston Variable Displacement Pumps in Tunnel Boring Machines

This article systematically analyzes common faults of Rexroth A4VSG series axial piston variable displacement pumps in TBM applications, covering working principles, typical failure symptoms, diagnostic methods, and preventive maintenance measures. The study focuses on five major failure mechanisms—insufficient flow, abnormal pressure, overheating, vibration/noise, and leakage—and proposes targeted solutions considering the unique operating conditions of TBM. By incorporating oil film load capacity datasets and refined management concepts, this paper provides practical guidance for improving the reliability and service life of A4VSG piston pumps in tunnel boring machines.

Critical Role of Axial Piston Pumps in Tunnel Boring Machines

Modern Tunnel Boring Machines, as core equipment for underground construction, rely on hydraulic systems to power critical functions such as cutterhead drive, thrust systems, and segment erection. Among hydraulic components, Rexroth’s A4VSG series axial piston variable displacement pumps stand out as the preferred power source due to their high-pressure/high-flow capacity, precise displacement control, and exceptional reliability. With a nominal pressure of 350 bar (peak 400 bar) and displacements ranging from 40 to 355 mL/rev, these closed-loop pumps are ideally suited for the continuous, stable performance demands of heavy-duty tunnel boring machines.

However, harsh tunneling environments, dynamic load variations, and prolonged operation pose significant durability challenges. Statistics indicate that approximately 35% of hydraulic system failures in tunnel boring machines are directly related to axial piston pumps, manifesting as flow fluctuations, pressure anomalies, overheating, and leakage. Such failures not only reduce efficiency but may also trigger cascading effects, jeopardizing overall system safety.

This paper examines the structural features of Rexroth A4VSG axial piston pumps, analyzes their typical failure modes in TBM applications, and proposes preventive strategies based on oil film behavior analysis and precision maintenance. Combining theory with practice, it offers a comprehensive fault diagnosis and maintenance framework to enhance tunneling quality and productivity.

Structure and Working Principle of Rexroth A4VSG Axial Piston Pumps

The Rexroth A4VSG series represents cutting-edge industrial hydraulic technology, with a design optimized for high-load, variable-duty applications like TBM. Utilizing a swashplate displacement mechanism, its core components include the cylinder block-piston assembly, port plate, swashplate adjustment mechanism, drive shaft, and bearing group. As the motor rotates the pump shaft, the interaction between the swashplate and slippers drives pistons in reciprocating motion, enabling fluid intake and discharge. Adjusting the swashplate angle infinitely varies pump displacement to meet diverse flow/pressure requirements during different tunneling phases.

In shield machine hydraulic systems, A4VSG pumps typically power thrust cylinders and cutterhead drive motors. The thrust system demands stable low-speed/high-torque performance, while cutterhead drives require rapid load adaptation. Such complex conditions impose extreme demands on the pump’s three critical friction pairs (piston-cylinder bore, slipper-swashplate, and cylinder block-port plate). Research shows that under transient conditions, oil film thickness in these pairs can drop abruptly by over 40%. Insufficient oil film load capacity leads to metal-to-metal contact, accelerating wear and precipitating failures.

Table: Key Technical Parameters of Rexroth A4VSG Axial Piston Pumps

From a tribological perspective, the reliability bottleneck of A4VSG pumps in tunnel boring machines lies in oil film stability. Data from the National Basic Science Data Center indicates that under step-load conditions, oil film thickness in axial piston pump friction pairs may instantaneously decrease by >40%, drastically increasing direct metal contact risks. Particularly when tunneling through heterogeneous strata, violent cutterhead load fluctuations transmit to the pump’s swashplate mechanism, destabilizing control piston movement and causing flow/pressure oscillations—a phenomenon especially pronounced in aging equipment.

Understanding the A4VSG pump’s design and operating principles is fundamental to accurate fault diagnosis. The following sections delve into five prevalent failure categories in TBM applications, offering actionable solutions.

Insufficient/Unstable Flow Output Analysis

Flow anomalies rank among the most frequent A4VSG pump failures in tunnel boring machines, typically presenting as sluggish or weak actuator movement (e.g., thrust cylinders or cutterhead motors), or even complete failure. Based on characteristics and root causes, flow issues divide into "inadequate flow" and "flow fluctuations," each with distinct origins and remedies.

Flow Deficiencies Due to Inadequate Inlet Supply

Insufficient hydraulic fluid intake is the primary cause of A4VSG pump flow reduction in tunneling. Confined tunnel spaces often necessitate compact hydraulic reservoirs, while high dust levels exacerbate:

• Low oil levels exposing pump inlet ports
• Clogged inlet filters from contaminant accumulation
• Air leaks in aging/vibrating inlet lines

These issues increase suction resistance, preventing adequate vacuum formation in the pump chamber and impairing piston retraction. One metro project found 42% of flow deficiencies traced to poor suction.

Solutions for intake issues:

• Maintain safe oil levels via twice-daily inspections
• Select high-flow, high-contaminant-holding inlet filters; shorten replacement cycles in harsh environments
• Detect air leaks via transparent hoses or ultrasonic testers; replace degraded seals promptly
• Install oil coolers for high-temp or continuous operations to prevent viscosity-related suction resistance

Internal Leakage-Induced Flow Losses

Extended operation causes friction pair wear, enlarging internal clearances and leakage—another major flow loss source. Prolonged high-load operation accelerates wear in three critical pairs (piston-bore, slipper-swashplate, block-port plate). Contaminated fluid introduces abrasive particles that score surfaces, worsening leakage paths. Additionally, fatigued center springs reduce block-to-port plate clamping force, degrading sealing.

Diagnosing internal leakage:

• Monitor pump housing temperature; abnormal rises often accompany leakage
• Compare no-load vs. loaded flow differences; significant gaps indicate leakage
• Oil analysis for metal wear particles predicts friction pair condition

For leakage repairs, prioritize port plate and block surface refurbishment. Light wear may be corrected via precision grinding (flatness ≤0.005 mm); severe cases require full assembly replacement. Field data show proper OEM parts and clearances restore pumps to >92% of original volumetric efficiency.

Displacement Mechanism Faults and Flow Instability

During tunneling, cutterhead load variations demand A4VSG pumps to rapidly adjust displacement. However, contaminated control circuits or worn components cause flow instability, evident in erratic actuator speeds or pressure gauge fluctuations.

Common displacement mechanism failures:

• Control piston scoring from contaminants, impeding motion
• Weakened regulator spring stiffness, reducing control force
• Servo valve stiction, preventing precise swashplate adjustment
• Clogged control orifices from fluid contamination

One tunnel project reduced flow oscillations from ±15% to ±3% by:

• Ultrasonic cleaning of control circuits
• Replacing all worn servo valve components
• Maintaining fluid cleanliness at ISO 4406 18/16/13 or better

*Table: A4VSG Flow Fault Quick-Reference Guide*

Systematic analysis of A4VSG flow anomalies enables rapid troubleshooting. Notably, ~70% of flow faults relate to fluid contamination, underscoring strict oil management as the foundation for reliable operation.

Pressure Anomalies and Overheating Diagnostics

Pressure irregularities and overheating are interrelated A4VSG failures in tunnel boring machines. As fundamental hydraulic parameters, abnormal pressure signals deteriorating pump health, while overheating reflects multiple issues that accelerate seal degradation and oil oxidation—creating vicious cycles. Accurate diagnosis ensures uninterrupted tunneling.

Root Causes of Low Pressure Output

Weak thrust or inadequate cutterhead torque often indicates low pressure output. Unlike flow deficiencies affecting speed, pressure loss prevents sufficient force/torque generation. System-wide leaks may also contribute, necessitating comprehensive checks.

Pump-specific low-pressure causes:

• Port plate/block mismatch: Severe wear or pitting shorts high/low-pressure chambers. One case showed 30% leakage increase from block misalignment dropped system pressure by 15–20%.
• Displacement mechanism misalignment: Non-zero swashplate angle at neutral (from improper adjustment or trunnion wear) reduces effective pressure.
• High-pressure seal failure: Vibration-induced housing or shaft seal damage causes external leaks.

Corrective actions:

• Precision-grind port plate/block surfaces (flatness ≤0.005 mm)
• Recalibrate displacement mechanism per OEM specs; replace worn trunnions
• Locate external leaks via fluorescent dye; install high-pressure fluorocarbon seals
• Install flow meters to quantify internal leakage

Hazards and Handling of Abnormal High Pressure

Conversely, unexplained pressure spikes also plague A4VSG pumps. While hard-rock tunneling increases loads legitimately, sustained high pressure under steady loads signals faults. Excessive pressure wastes energy and shortens component life.

Primary high-pressure triggers:

• Relief valve drift/sticking (60% of cases)
• Directional valve failure (dangerous pressure trapping)
• Line blockages (especially clogged return filters)
• One contractor reduced pressure spike incidents by 75% via:
• Regular relief valve recalibration
• Return filter cleaning
• Installing pressure sensors for early warnings

Multifactorial Overheating Analysis

Temperature excursions are composite A4VSG failure indicators. Housing temperatures >35°C above ambient warrant investigation. Overheating oxidizes oil, degrading lubrication and creating feedback loops. Heat sources divide into mechanical friction and hydraulic losses.

Mechanical friction hotspots:

• Bearings: Increased clearances from wear boost friction. Data show accelerated heating after 10,000 service hours.
• Slipper/swashplate interface: Oil film collapse causes metal contact.
• Piston/bore pairs: Incorrect clearances or contamination increase friction.

Hydraulic losses primarily stem from internal leakage, converting pressure to heat. A 5% volumetric efficiency drop raises temperatures by 8–10°C.

Overheating countermeasures:

• Replace bearings preventatively at 8,000–10,000 hours
• Maintain ISO fluid cleanliness
• Ensure cooler functionality; add auxiliary units if needed
• Select viscosity-appropriate fluids (synthetics for high temps)
• Limit return line backpressure to ≤0.3 MPa

Analysis of oil film load capacity datasets (Wuhan University of Science and Technology) reveals that optimized surface textures improve film stability, reducing slipper temperatures by >20% during transients—informing A4VSG thermal performance upgrades for tunnel boring machines.

Infrared thermography scans establish temperature baselines for predictive maintenance. One operator cut unexpected failures by 40% using this approach.

Vibration/Noise and Leakage Mitigation

Excessive vibration/noise are early A4VSG failure warnings, while leakage impacts efficiency and environmental compliance. In confined tunnels, vibrations harm operator health and mask other faults; leaks waste fluid and pollute. Holistic solutions enhance overall reliability.

Mechanical Vibration Sources and Remedies

A4VSG mechanical vibrations originate from rotational imbalance and widened clearances. External tunneling vibrations exacerbate pump looseness, creating feedback loops. Frequency-specific noises help diagnose issues.

Common vibration causes:

• Shaft misalignment: >0.1 mm motor-pump coupling misalignment causes noticeable vibration (25% of cases).
• Bearing wear: Excessive radial play induces shaft wobble.
• Loose fasteners: Tunneling vibrations loosen mounting hardware.
• Swashplate oscillation: Worn displacement mechanisms cause erratic movement.

Vibration reduction tactics:

• Laser-align shafts to ≤0.05 mm tolerance
• Monitor bearing clearances; replace if >0.15 mm
• Apply thread-locking compounds to critical fasteners
• Install vibration-isolating mounts
• Use spectrum analyzers for early fault detection

Hydraulic Noise Generation and Abatement

Fluid-borne noises (high-frequency whines/pulsations) differ from mechanical sounds, often tied to system design or settings.

Noise triggers:

• Cavitation: Air ingress or restricted intake collapses bubbles audibly (≥15 dB increase).
• Pressure pulsations: Pump flow ripples amplify with impedance mismatches.
• Water hammer: Sudden valve closures generate shockwaves.
• High viscosity: Increases flow resistance, especially during cold starts.

Noise reduction methods:

• Upsize inlet lines to reduce cavitation risk
• Install pulsation dampeners/accumulators
• Optimize valve transition rates
• Use temperature-appropriate viscosity grades
• Bleed air pockets routinely

Leakage Classification and Control

Leakage types divide into internal (efficiency loss) and external (fluid loss/environmental harm).

Common leak sites and fixes:

• Shaft seals: Account for 60% of external leaks. Replace seals and resurface shafts simultaneously.
• Joint faces: Degraded gaskets or uneven bolt torque cause seeping. Use high-density seals and cross-tighten per specs.
• Displacement mechanisms: Loose adjusters or O-ring failure leak control oil. Upgrade to high-pressure seals.
• Fittings: Tunnel vibrations loosen threads. Switch to flared or flange connections.

Precision maintenance programs dramatically reduce leaks. Guangdong Pipeline Project Phase II cut leakage rates by 80% via:

• Three-tier inspection system (operator, supervisor, specialist)
• Standardized checklists with clear criteria
• Visual leak risk tagging
• Seal life-cycle tracking
• Preventive seal replacement

Implementing vibration, noise, and leakage controls boosts A4VSG operational stability and environmental performance. Condition monitoring paired with preventive maintenance extends pump MTBF by 30–50%.

Preventive Maintenance and Precision Management

Preventive maintenance (PM) and precision management are pivotal for A4VSG reliability in tunnel boring machines. Compared to reactive repairs, systematic PM reduces failures by >40% and unplanned downtime by 60%. Tailored strategies extend pump life while improving project economics.

Oil Film-Based Predictive Maintenance

Oil film integrity dictates A4VSG friction pair longevity. Datasets like Wuhan University’s "Oil Film Load Capacity in Axial Piston Pumps Under Transient Conditions" enable predictive upkeep.

Oil film monitoring techniques:

• Oil analysis: Monitor wear metals and contaminants. Iron spikes signal metal contact.
• Temperature trending: Hotspots precede film failure.
• Vibration spectra: Film thickness changes alter frequency signatures.
• Pressure ripple: Declining film stiffness increases pulsations.

One tunnel project extended A4VSG overhaul intervals from 6,000 to 8,000 hours (35% cost savings) by aligning PM with film condition data—a model for data-driven hydraulic asset management.

Precision Management in Hydraulic Systems

Guangdong’s South China Sea Shield Tunnel Project demonstrated precision management’s effectiveness. Its "exact, meticulous, thorough, standardized" approach optimizes each maintenance facet.

Core precision management elements:

• Standard operating procedures (SOPs): Detailed pump checklists (e.g., 500-h bearing clearance checks, 1,000-h efficiency tests).
• Full lifecycle quality control: Document fluid selection, filter changes, and assembly tolerances.
• Risk preemption: Identify high-risk scenarios (e.g., hard-rock tunneling stresses displacement mechanisms).
• Performance metrics: Link maintenance quality to incentives (e.g., "Zero-Leak Team" awards).

Implementation examples:

• "One-pump-one-file" tracking for historical data
• QR-coded spare parts management
• "Zero Defect" benchmarking campaigns
• Failure case libraries and knowledge sharing
• Triple-check (self/peer/expert) quality assurance

Recommended Maintenance Intervals and Key Tasks

Rexroth guidelines and tunneling experience inform the following PM schedule:

Table: A4VSG Preventive Maintenance Plan

Operational best practices also extend pump life:

• Hand-rotate pumps pre-startup; idle ≥5 mins before loading (≥10 mins in winter)
• Avoid continuous peak-pressure operation (limit to 90% rating)
• Maintain ≥25Hz minimum VFD frequency for lubrication
• Post-shutdown oil level checks with approved fluids
• Log all startups, shutdowns, and anomalies

Precision PM programs improve A4VSG service life by >30% and reduce failures by 50%—ensuring reliable shield machine performance. This systematic approach defines modern hydraulic equipment management standards.

Conclusions and Future Perspectives

Rexroth A4VSG axial piston variable pumps are pivotal to shield machine hydraulic system performance, directly impacting tunneling safety and efficiency. This study’s findings yield critical conclusions while outlining future innovations. Evidence confirms that scientific fault analysis and preventive strategies significantly enhance A4VSG reliability and durability in harsh tunneling conditions.

Key Findings

Comprehensive A4VSG failure analysis reveals:

1 Fluid contamination dominates failures: ~70% of pump faults correlate with exceeding ISO 4406 18/16/13 cleanliness, especially port plate wear and valve stiction. Maintaining oil purity is the most cost-effective prevention.

2 Oil film stability is critical: Wuhan University data shows >40% transient oil film thickness reductions cause metal contact. Optimized surface textures improve film load capacity.

3 Temperature integrates multiple failure modes: Abnormal heating reflects bearing wear, leakage, or fluid degradation. Baseline trending enables early detection.

4 Precision maintenance delivers ROI: Guangdong’s South China Sea Project cut leakage by 80% and costs by 35% via standardization and data-driven decisions.

5 Prevention outperforms repair: PM extends A4VSG overhaul intervals by 30–50% versus reactive fixes. Condition-based predictive maintenance represents the future.

Technological Advancements

Future A4VSG developments for tunnel boring machines include:

1 Smart monitoring systems: Integrated temperature/pressure/vibration sensors with IoT enable real-time diagnostics. Zhejiang University studies show 48-hour advance prediction for 80% of pump faults.

2 Advanced materials: Ceramic coatings and nanocomposites (e.g., Wintone Z63 for valve housings) enhance wear resistance.

3 Active oil film control: Building on aerospace piston pump research (Project U1737110), next-gen pumps may feature surface textures or electric field modulation for film stabilization.

4 Energy-optimized designs: Duty-specific efficiency curves and response tuning could save 15–20% energy.

5 Remanufacturing expansion: Certified refurbishment processes for blocks, port plates, etc., restore pumps to 90% performance at 50% cost.

Recommendations for Industry

Best practices for TBM operators:

1 Implement comprehensive fluid management: Regular testing, fine filtration, and scheduled changes. Premium fluids triple pump life.

2 Develop specialized maintenance teams: Training reduces misdiagnosis by 60% (per industry data).

3 Equip with diagnostic tools: Particle counters, IR cameras, and vibration analyzers enable data-based decisions.

4 Leverage OEM technical support: Partner with Rexroth for complex issues and updates.

5 Participate in industry knowledge sharing: Learn from peer experiences via associations and forums.

As tunnels grow deeper, longer, and more complex, shield machine hydraulic systems face escalating demands. Rexroth A4VSG axial piston pumps, as core power components, directly influence project viability. Through rigorous failure analysis, scientific maintenance, and emerging technologies, their performance will continue advancing—ensuring efficient, reliable underground construction worldwide.