Why Hydraulic Tanks Are Pressurized in Modern Excavators

[MikePhua]

The Evolution of Hydraulic Tank Design
Hydraulic systems have long been the backbone of heavy equipment, powering everything from boom lifts to track drives. In earlier generations of excavators, hydraulic tanks were vented to atmospheric pressure, relying on gravity and pump suction to circulate fluid. However, as machine complexity increased and hydraulic demands grew, manufacturers began pressurizing hydraulic reservoirs to improve system efficiency and reliability.
John Deere, founded in 1837 and a global leader in construction and agricultural machinery, introduced pressurized hydraulic tanks in several models during the late 1990s and early 2000s. One such example is the Deere 160LC excavator, which features a sealed and pressurized hydraulic reservoir designed to optimize pump feed and reduce aeration.
Why Pressurize the Hydraulic Tank
Pressurizing the hydraulic tank serves multiple purposes:

• Prevents cavitation at the pump inlet by ensuring positive pressure
  
• Reduces foaming and air entrainment in the hydraulic fluid
  
• Improves cold-start performance by maintaining fluid flow
  
• Enhances filtration efficiency by stabilizing flow across filters
  
• Minimizes contamination by sealing the tank from external air
  

Typical tank pressures range between 15–45 PSI (1–3 bar), depending on the machine and application. This low-level pressure is sufficient to assist fluid movement without risking seal damage or overloading components.
Sources of Pressure in the Tank
Contrary to common assumptions, the pressure inside a hydraulic tank is not solely generated by mechanical pumps. Instead, it arises from a combination of factors:

• Thermal expansion of hydraulic oil during operation
  
• Volume displacement caused by cylinder movement (e.g., boom and stick actuation)
  
• One-way breather valves that allow air in but restrict air out
  
• Engine-driven air pumps or passive pressurization systems in some models
  

For example, when an operator raises the boom, hydraulic fluid exits the tank and fills the cylinders. As the boom lowers, fluid returns to the tank, increasing the internal volume and compressing the trapped air. The breather valve prevents this air from escaping, thereby maintaining pressure.
The Role of Breather Valves and Venting Systems
Breather valves are critical components in pressurized tanks. They regulate airflow into the reservoir and maintain a safe pressure range. Located near the filter housing or fill cap, these valves typically include:

• A spring-loaded diaphragm or check valve
  
• A pressure relief mechanism to prevent over-pressurization
  
• A filtered intake to prevent dust ingress
  

If the breather valve fails or becomes clogged, the tank may over-pressurize, leading to seal blowouts or oil spray during maintenance. Conversely, a stuck-open valve can cause pressure loss and pump cavitation.
Maintenance Precautions and Safety Hazards
One of the most overlooked risks in pressurized hydraulic systems is the danger of sudden oil release during service. Opening a fill cap or disconnecting a hose without venting the tank can result in high-pressure oil spray, posing serious injury risks.
Best practices include:

• Always vent the tank before opening any access port
  
• Use the manufacturer’s recommended procedure for depressurization
  
• Wear protective gear when working near hydraulic connections
  
• Replace worn or damaged breather valves promptly
  
• Monitor tank pressure during operation using onboard diagnostics or manual gauges
  

In one incident on a drainage site in Ontario, a technician attempted to top off the hydraulic oil without venting the tank. The result: two buckets of oil sprayed across the machine and operator, requiring a full cleanup and replacement of contaminated components.
Accessing and Removing the Hydraulic Cap
Some excavators use locking fill caps that require a hex key to disengage an internal pin. On certain Deere models, a 4mm Allen wrench is inserted into the cap slot and twisted to retract the locking mechanism. However, wear and corrosion can jam the pin, making removal difficult.
Troubleshooting tips:

• Insert the hex key and rotate while applying gentle upward pressure
  
• If the pin doesn’t retract, inspect for mechanical damage or bent components
  
• In extreme cases, remove the entire cap assembly by unbolting it from the tank
  
• Avoid forcing the cap, as this may damage the locking mechanism permanently
  

One operator in Houston reported that the locking pin had been bent due to prior misuse. They had to remove the entire stem and replace the cap assembly to restore functionality.
Thermal Expansion and Pressure Fluctuations
Hydraulic oil expands as it heats, contributing significantly to tank pressure. A temperature rise from 20°C to 80°C can increase oil volume by up to 4%, which in a sealed tank translates to a measurable pressure increase. This is why machines operating in hot climates or under heavy load often show higher tank pressures.
To mitigate thermal effects:

• Use hydraulic oils with stable viscosity indices
  
• Install temperature sensors to monitor fluid conditions
  
• Ensure the breather valve is rated for thermal expansion
  
• Avoid overfilling the tank, which reduces air cushion space
  

In Arizona, a contractor retrofitted their fleet with thermal relief valves after noticing frequent seal failures during summer operations. The modification reduced downtime and improved system longevity.
Conclusion
Pressurized hydraulic tanks are a subtle but vital advancement in modern excavator design. By maintaining positive pressure, they ensure consistent fluid delivery, reduce aeration, and protect system integrity. However, they also introduce new maintenance protocols and safety considerations. Understanding the mechanics of pressurization—from breather valves to thermal expansion—empowers operators and technicians to service these systems confidently and safely. As hydraulic technology continues to evolve, the pressurized tank remains a cornerstone of efficient and reliable machine performance.