Bleeding Hydraulic Systems in Heavy Equipment

[MikePhua]

Hydraulic systems are a critical component of many heavy equipment machines, including excavators, backhoes, and skid steers. These systems use pressurized fluid to transmit force, allowing operators to perform a wide variety of tasks, from lifting heavy loads to moving earth. However, a key issue that can arise with hydraulic systems is air becoming trapped in the hydraulic lines, which can lead to decreased performance, erratic operation, and potential damage to the system.
To restore the system’s optimal performance, it’s important to "bleed" the hydraulic system—removing the trapped air. This article will guide you through the process of bleeding a hydraulic system, common causes of air in the system, the importance of proper maintenance, and troubleshooting tips to ensure your system works as it should.
Understanding the Hydraulic System in Heavy Equipment
Before diving into the bleeding process, it's essential to understand the hydraulic system itself. Hydraulic systems rely on several key components that work together to create and transfer pressure. These components include:

1. Hydraulic Pump: The pump is responsible for moving hydraulic fluid from the reservoir and pressurizing it, enabling the fluid to flow to different parts of the system.
   
2. Hydraulic Fluid: The fluid, often oil-based, is the medium through which force is transferred. It also acts as a coolant and lubricant within the system.
   
3. Hydraulic Lines and Hoses: These carry the pressurized fluid to different parts of the machine.
   
4. Valves: Valves control the direction and flow of hydraulic fluid, determining which part of the system receives power.
   
5. Cylinders: Hydraulic cylinders convert the hydraulic pressure into linear motion, powering functions like lifting, pushing, or tilting.
   
6. Hydraulic Reservoir: This holds the hydraulic fluid when it's not circulating through the system. It also acts as a cooling chamber to dissipate the heat generated by the system.
   

When air enters the hydraulic system, it can cause several issues. Air reduces the efficiency of the system, prevents proper pressure buildup, and causes the system to behave erratically. This is why bleeding the hydraulic system is vital.
Causes of Air in the Hydraulic System
There are several ways air can enter a hydraulic system, leading to the need for bleeding:

1. Loose Fittings: If any of the hydraulic hose connections or fittings are loose, air can enter the system. Over time, vibrations or improper installation can loosen these connections.
   
2. Low Hydraulic Fluid Levels: Insufficient fluid levels can cause the pump to suck in air from the reservoir, leading to air being introduced into the system.
   
3. Leaks in the Hydraulic System: A punctured hose, cracked fitting, or worn-out seal can allow air to infiltrate the hydraulic lines.
   
4. Overheating: Excessive heat in the system can cause hydraulic fluid to break down, leading to bubbles forming in the fluid, which can then enter the system.
   
5. Improper Maintenance: Lack of regular maintenance and failure to check for signs of wear or damage in the hydraulic components can lead to air entering the system over time.
   

The Process of Bleeding the Hydraulic System
Bleeding the hydraulic system involves expelling air from the hydraulic lines to restore full pressure. The exact procedure can vary depending on the machine and its hydraulic system design, but generally, the process follows these steps:

1. Check Fluid Levels: Before starting, ensure that the hydraulic fluid levels are adequate. Low fluid levels are one of the most common causes of air entering the system. If the fluid is low, top it up with the recommended hydraulic fluid.
   
2. Locate the Bleed Valve: Most heavy equipment with hydraulic systems will have a dedicated bleed valve or screw located near the hydraulic pump or cylinder. Refer to the equipment’s manual to find the exact location of these valves.
   
3. Activate the Hydraulic System: With the bleed valve open, activate the machine’s hydraulic controls (such as moving the joystick or lever) to circulate the fluid. This step will force the trapped air to rise to the top of the system and out through the bleed valve. The fluid will typically come out in spurts as air escapes, and you may see bubbles in the fluid.
   
4. Tighten the Bleed Valve: Once you see a steady stream of fluid coming from the valve with no bubbles, the air should be fully expelled from the system. Tighten the valve securely to avoid any fluid leakage.
   
5. Test the System: After bleeding, test the hydraulic system by operating the machine to ensure that all functions are working smoothly. Check for any unusual sounds, reduced performance, or delayed responses.
   
6. Check Fluid Levels Again: After the bleeding process, check the hydraulic fluid levels again, as some fluid may have been lost during the process. Top off the fluid as needed.
   

Troubleshooting Tips After Bleeding the System
Even after bleeding the system, you may experience persistent issues. Here are some common problems and troubleshooting tips:

1. Air in the System Returns:
   • If air returns to the hydraulic system, it could be due to a persistent leak or loose connection. Inspect all fittings, hoses, and seals for damage.
     
   • Check the hydraulic fluid reservoir for any signs of contamination or foam. This may indicate that the fluid needs to be changed.
     
2. System Still Lacks Power:
   • If the hydraulic system still lacks power after bleeding, the problem may lie with the hydraulic pump. Over time, pumps can wear out and lose their ability to generate pressure. A professional inspection may be needed.
     
3. Erratic Operation:
   • If the machine operates erratically after bleeding, it could be due to contamination in the fluid or a problem with the hydraulic valves. Flush the system and replace the hydraulic fluid, and ensure the valves are clean and in proper working condition.
     

Preventing Air in the Hydraulic System
While bleeding the hydraulic system can solve the immediate problem, preventing air from entering the system in the first place is the best approach. Here are some preventive measures:

1. Regular Maintenance: Perform routine inspections of the hydraulic hoses, fittings, and seals to ensure that no air leaks are present. Tighten any loose fittings and replace worn components as needed.
   
2. Monitor Fluid Levels: Always maintain proper hydraulic fluid levels to prevent the pump from sucking in air. Make sure that fluid is topped up to the manufacturer’s recommended levels.
   
3. Replace Worn Components: Over time, hydraulic components such as seals and hoses will degrade. Regularly replacing worn components can help prevent air from entering the system.
   
4. Use Quality Hydraulic Fluid: Ensure that the hydraulic fluid you use meets the specifications recommended by the equipment manufacturer. Low-quality fluid or incorrect fluid can cause foaming and lead to air issues.
   

Conclusion
Bleeding the hydraulic system is a vital part of hydraulic system maintenance for any heavy equipment machine. By understanding the common causes of air in the system and the steps involved in bleeding, you can ensure that your machine operates at peak efficiency. Regular maintenance, including checking for leaks and monitoring fluid levels, can prevent air from entering the system in the first place. By staying proactive with maintenance, you can minimize downtime, extend the life of your equipment, and avoid costly repairs.