Boom Cylinder Repack and Synchronization Issues in Hydraulic Excavators

[MikePhua]

The Role of Boom Cylinders in Excavator Function
Boom cylinders are critical components in hydraulic excavators, responsible for lifting and lowering the boom arm. Most machines use dual boom cylinders mounted symmetrically to distribute load and maintain structural balance. These cylinders operate in tandem, receiving equal hydraulic flow and pressure to ensure synchronized movement. Any deviation in timing or speed between the two can result in uneven lifting, structural stress, and reduced operator control.
Repacking Cylinders and Post-Service Behavior
Repacking a hydraulic cylinder involves replacing internal seals, wipers, and wear bands to restore pressure integrity and prevent fluid leakage. While repacking is a routine maintenance procedure, improper reassembly or air entrapment can cause performance issues. After repacking, cylinders may exhibit:

• Slower extension or retraction
  
• Asynchronous movement between paired cylinders
  
• Jerky or hesitant response
  
• Audible cavitation or fluid hammering
  

These symptoms often stem from trapped air, unequal fluid volumes, or internal friction differences between the two cylinders.
A technician in Alberta repacked both boom cylinders on a mid-size excavator and noticed that one side lagged during lift. After cycling the cylinders under load and bleeding the lines, synchronization improved significantly.
Hydraulic Synchronization and Flow Balancing
Hydraulic systems rely on equal flow distribution to paired actuators. In excavators, boom cylinders are typically plumbed in parallel, meaning each receives fluid simultaneously from the same valve spool. However, small differences in internal resistance, seal drag, or fluid volume can cause desynchronization.
Key factors affecting synchronization:

• Cylinder bore and rod diameter mismatch
  
• Unequal seal friction after repacking
  
• Air pockets in one cylinder or line
  
• Valve spool wear or internal leakage
  
• Flow restrictors or check valves installed asymmetrically
  

Solutions include:

• Cycling the boom fully up and down 10–15 times to purge air
  
• Loosening hydraulic fittings slightly to bleed trapped air
  
• Verifying that both cylinders were filled with equal fluid volume during reassembly
  
• Inspecting valve block for spool wear or bypass leakage
  
• Installing flow dividers or synchronizing valves if persistent imbalance occurs
  

A contractor in New Zealand installed a flow divider between the boom cylinder ports on his aging excavator. The modification restored synchronized lift and reduced frame twisting during heavy loads.
Bleeding Air and Restoring Cylinder Timing
Air trapped in hydraulic cylinders or lines compresses under pressure, causing delayed or uneven movement. After repacking, it’s essential to bleed the system thoroughly. Recommended steps:

• Start engine and warm hydraulic fluid to operating temperature
  
• Extend and retract boom slowly under no load
  
• Hold cylinders at full extension and retraction for 5–10 seconds
  
• Repeat cycle with increasing load
  
• Monitor cylinder speed and listen for cavitation
  

If one cylinder remains slower, it may contain residual air or have higher internal drag. In some cases, removing the cylinder again and manually bleeding it may be necessary.
A fleet manager in Texas reported that after repacking boom cylinders on a D-series excavator, synchronization was off by nearly 3 inches. After manually bleeding both cylinders and replacing one worn piston seal, the issue was resolved.
Preventive Measures and Best Practices
To avoid post-repack issues:

• Use OEM seal kits matched to cylinder serial numbers
  
• Lubricate seals during installation to reduce initial drag
  
• Fill cylinders with fluid before reassembly if possible
  
• Torque gland nuts and rod ends to spec
  
• Inspect rod straightness and surface finish
  

During reinstallation:

• Align cylinder mounts precisely to avoid side loading
  
• Replace worn bushings or pins that may affect movement
  
• Check hydraulic lines for internal collapse or contamination
  

A technician in British Columbia found that a repacked cylinder was binding due to a bent rod. After replacing the rod and polishing the bore, synchronization returned and the boom operated smoothly.
Stories from the Field
In Alaska, an excavator used for slope work showed boom drift after cylinder service. The operator noticed that one side lifted faster, causing the bucket to tilt unexpectedly. After installing a pressure-compensated flow divider and flushing the system, the machine regained precise control.
In Thailand, a contractor repacked boom cylinders on a river dredging excavator. The machine became sluggish and out of sync. The issue was traced to air trapped in the pilot lines controlling the main valve. After bleeding the pilot circuit and recalibrating the joystick response, performance normalized.
Conclusion
Boom cylinder repacking is a vital maintenance task, but it requires attention to detail and post-service calibration. Synchronization issues often arise from air entrapment, seal friction, or hydraulic imbalance. By cycling the system, bleeding lines, and inspecting components thoroughly, operators and technicians can restore smooth, synchronized boom movement. In machines where precision and balance are essential, even small timing differences can ripple into big problems—making post-repair tuning as important as the repair itself.