Hydraulic Pressure Issues in Heavy Equipment

[MikePhua]

Introduction
Hydraulic systems power many functions in heavy machinery—digging, lifting, pushing, rotating—everything depends on correct hydraulic pressure. When pressure drops, or fluctuates, performance suffers. To understand these issues, one must know what sets hydraulic pressure, what causes it to fail, and how to fix and prevent problems. This article covers common causes, diagnostics, solutions, and real cases to help operators and maintenance teams.

What Sets Hydraulic Pressure

• Hydraulic Pump: Generates the pressure and flow needed.
  
• Control Valves: Direct flow to the right actuator (e.g. cylinder or motor).
  
• Relief / Pressure Relief Valves: Prevent overpressure by opening when pressure exceeds set limits.
  
• Hydraulic Fluid: Correct viscosity, cleanliness, fluid level.
  
• Lines, Seals, Hoses: Integrity (no leaks), correct routing and secure fittings.
  

Hydraulic pressure is usually measured in psi (or bar), and flow in gallons per minute (or liters per minute). For example, a medium excavator might need 2500-3500 psi (≈170-240 bar) in its main circuit to operate properly.

Common Causes of Pressure Problems

1. Low or Incorrect Fluid Level
   If fluid is low, pump will draw air (aeration) which reduces pressure. Overfilling may cause foaming.
   
2. Fluid Contamination
   Particles, water, or degraded fluid lead to restricted flow, wear, and inefficiencies.
   
3. Leaky Lines, Seals, or Connections
   External leaks reduce flow, internal leaks (e.g. worn seals) allow fluid bypass, dropping effective pressure.
   
4. Clogged Filters or Blocked Hydraulic Paths
   Filters that are past service interval, or clogged passages in valves or hoses can starve parts of fluid, reducing pressure.
   
5. Pump Wear or Damage
   Worn internal parts, damaged gears or pistons in the pump leads to reduced ability to build pressure.
   
6. Relief Valve Misadjustment or Wear
   If relief valves open too early or are worn, they relieve pressure prematurely, so circuits don’t receive full pressure.
   
7. High Fluid Temperature
   When fluid gets too hot, its viscosity drops; thinner fluid leaks past seals more easily and delivers less pressure under load.
   
8. Aeration and Cavitation
   • Aeration: air entering the system, causing spongy or unstable pressure.
     
   • Cavitation: vapor bubbles forming in fluid then collapsing, which damages pump internals and disrupts pressure delivery.
     

Diagnosis Steps

• Check fluid level and top up if low. Use the fluid type recommended by manufacturer.
  
• Examine fluid condition: look for milky color (water), dark or burnt-smell fluid (overheating or degradation).
  
• Inspect all hoses, fittings, connections for leaks.
  
• Replace or clean filters.
  
• Measure pressure with a test gauge at key points: pump outlet, before big valves, and at actuator. Compare with manufacturer's specs.
  
• Check relief valve setting; test if it opens at right pressure.
  
• Monitor fluid temperature; ensure cooler/radiator (if equipped) is functioning.
  
• Listen for unusual noises: whining, knocking, hissing could indicate air in system, cavitation or damaged pump components.
  

Solutions & Preventive Measures

• Replacement of worn seals, hoses, fittings.
  
• Regular filter change intervals. Use OEM or high-quality filters with proper ratings.
  
• Use clean fluid; prevent contamination from dirt or water. Store fluid correctly, use proper venting to avoid water ingress.
  
• Bleed air out of system after maintenance or if low fluid was present.
  
• Maintain correct relief valve settings; adjust or replace valves when worn.
  
• Maintain cooling systems; ensure fluid cooling and ambient airflow are not blocked.
  
• Operator training: avoid overloading, idling in extreme heat, ensure proper warm-up.
  

Real-World Cases

• A fleet of excavators working in cold weather saw slow hydraulic responsiveness. After inspection, found fluid was wrong viscosity for low temperature; fluid replaced with correct spec and pressure restored.
  
• In mining equipment, high ambient temperature caused hydraulic oil overheating. Relief valves were opening prematurely due to thinned fluid. Installed larger coolers and better oil coolers; scheduled morning start ups to pre-cool, resulting in 40-50% fewer pressure-related shutdowns.
  
• Another case: a loader had jerky boom motion. Diagnosis revealed air entering through a loose hose clamp on suction line. Replaced clamp, bled system, and boom smoothness returned.
  

Key Terms

• Aeration: Air entrained in hydraulic fluid; degrades performance.
  
• Cavitation: Formation and collapse of vapor bubbles in fluid; damages components.
  
• Relief Valve: Safety valve that limits maximum pressure.
  
• Viscosity: Thickness or resistance to flow of fluid; changes with temperature.
  
• Internal Leak: Fluid bypass inside pump, valve, or actuator (not visible externally) reducing pressure.
  

Conclusion
Hydraulic pressure issues are a major source of performance loss and downtime in heavy equipment. Most problems trace back to fluid condition, leaks, worn components, improper settings or environmental factors like heat. By following diagnostic steps, employing preventive maintenance, and learning from real cases, fleet operators can reduce incidents and extend service life. Regular monitoring of fluid levels, filters, temperature and pressures is essential. Proactive maintenance saves not just repair costs but also lost productivity.