Repacking Telescoping Boom Cylinders on Digger Derrick and Crane Rigs

[MikePhua]

Quick answer
Repacking telescoping boom cylinders requires partial disassembly of the boom sections, careful unpinning of internal cylinders, and strategic use of lifting equipment. Leaving the base section mounted and pulling the boom stages sequentially simplifies the process and reduces risk.
Telescoping boom cylinder function and wear
Telescoping boom cylinders are nested hydraulic rams that extend sequentially to increase reach. Common in digger derricks, service trucks, and cranes, these cylinders operate under high pressure and are prone to seal wear, especially in dusty or high-cycle environments.
Each stage includes:

• A hydraulic cylinder with internal seals and wipers
  
• A pin connection to the next boom section
  
• Wear pads or slide blocks to reduce friction
  

Over time, seals degrade, leading to fluid leaks and reduced extension force. Repacking involves replacing these seals and inspecting the rod and barrel for scoring or corrosion.
Disassembly strategy and boom section handling
To access the cylinders:

• Do not extend the boom before disassembly. Extended cylinders add unnecessary length and weight.
  
• Leave the base section mounted on the truck or crane. This provides a stable anchor and simplifies alignment.
  
• Unpin the rear cylinder mount and block it to prevent it from dropping into the boom.
  
• Pull boom sections sequentially using a forklift or crane. Measure the boom beforehand to identify center points for lifting.
  
• Use a spreader bar when lifting long sections to prevent bending or twisting.
  
• Remove or loosen wear pads before pulling sections to avoid binding.
  

One operator used a 40-ton P&H crane and a 25,000 lb Hyster forklift to handle boom sections on an F800 digger derrick. By leaving the base mounted and pulling sections slowly, he avoided damage and reduced labor.
Cylinder removal and repacking procedure
Once the boom sections are separated:

• Unpin the cylinder from the next stage. This may require reaching into the boom or using access ports.
  
• Remove the cylinder carefully, avoiding rod damage.
  
• Disassemble the cylinder using a seal kit matched to the OEM part number.
  
• Inspect the rod and barrel for scoring, pitting, or chrome flaking.
  
• Replace all seals, wipers, and wear rings. Use hydraulic assembly lube during reinstallation.
  
• Torque gland nuts and end caps to spec, and pressure test before reinstallation.
  

If the cylinder is heavily worn or bent, replacement may be more cost-effective than repacking.
Safety and equipment recommendations

• Use cribbing blocks to support boom sections during disassembly
  
• Avoid working under suspended loads
  
• Keep tension balanced between lifting equipment to prevent sudden movement
  
• Label hydraulic lines and pins during removal to aid reassembly
  
• Pressure test the repacked cylinder at 1.5× operating pressure before use
  

Conclusion
Repacking telescoping boom cylinders is a labor-intensive but manageable task with the right equipment and strategy. Leaving the base mounted, pulling sections sequentially, and using measured lifting points ensures safety and efficiency. With proper seal replacement and inspection, the boom can be restored to full function without costly downtime.