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The Lull 644 telehandler is a member of the well‑known Lull family of rough‑terrain forklifts and telehandlers originally produced by JLG Industries — a company with roots in the early 1960s that pioneered aerial work platforms and later expanded into material‑handling equipment. Lull machines are prized in construction, oilfield service, and industrial yards for their ability to lift heavy loads over rugged terrain. The 644 model, in particular, strikes a balance between lifting capacity (typically around 6,500 lb at a 44‑inch load center) and compact maneuverability, making it one of the best‑selling mid‑size telehandlers globally with thousands of units in service.
A critical component of the Lull’s boom extension and lifting system is the inner boom cylinder, a hydraulic actuator that extends and retracts the boom sections. Over time, wear, contamination, or seal failure can necessitate disassembly and rebuild of the inner boom cylinder. This article provides a detailed, practical guide to that process, combined with terminology explanations, recommended practices, and real‑world experience — all rewritten in clear, original language.
Inner Boom Cylinder Function and Terminology
Before diving into disassembly, it is useful to define key terms associated with hydraulic cylinders and telehandler booms:
• Hydraulic Cylinder — A mechanical actuator that converts pressurized hydraulic fluid into linear motion and force.
• Inner (or Telescopic) Cylinder — In a multi‑section boom, the inner cylinder is the most deeply nested section that extends last and retracts first.
• Rod and Piston Assembly — The internal parts that move within the cylinder barrel; the piston divides the internal volume into two pressurized chambers.
• Seal Kit — A set of O‑rings, wipers, and pressure seals that maintain fluid separation and prevent leaks.
• Pressure Side / Return Side — The pressure side receives hydraulic fluid to push the piston; the return side allows fluid to flow back to the reservoir.
• Bleeding Air — The process of eliminating trapped air in the hydraulic system that can cause spongy or erratic movement.
Disassembling the inner boom cylinder requires an appreciation of these parts and understanding how they interact under load.
Tools and Safety Preparations
Cylinder repair involves stored energy and heavy components, so safety is paramount. Recommended tools and preparations include:
• Hydraulic Hose Support — Support boom sections mechanically before disconnecting hydraulic lines to avoid sudden movement.
• Torque Wrench — For correct reassembly of cylinder end caps and retaining nuts.
• Seal Pick and Soft Mallet — For removing old seals without damaging bores.
• Calipers and Micrometer — To measure piston rod diameter and seal groove widths.
• Clean Work Surface and Rags — Because contamination degrades seal life, keep work areas tidy.
Stand a floor jack or boom support under the boom to prevent collapse when the cylinder is disconnected from the boom linkage. Never rely on hydraulic residual pressure to hold sections in place.
General Disassembly Sequence
• Depressurize the System — Before removing any lines, cycle the machine controls with the engine off and loader on solid ground to ensure minimal residual pressure.
• Disconnect Hydraulic Lines — Label each line carefully; use plugs to prevent fluid loss and contamination.
• Remove Mounting Pins — Use a drift punch and driver kit to push out pins securing the cylinder to the boom linkage.
• Support the Cylinder — With the cylinder freed on both ends, support it on a bench or stable cradle.
• Clean Exterior — Wipe off all dirt and grime; hydraulic systems are sensitive to contamination.
• Remove End Cap — Unscrew the cylinder head or gland nut. Be cautious — internal seals may spring out.
• Extract Rod and Piston — Pull the rod and piston assembly out of the barrel. Inspect for scoring, rust, or bent rods.
• Remove Seals and Wipers — Using soft picks remove all old seal elements; measure groove widths and record seal sizes for ordering replacement kits.
Inspection Points and Wear Assessment
Once inside, inspect several critical surfaces:
• Rod Surface — It should be smooth and free of scratches or pitting; even minor corrosion can destroy new seals rapidly.
• Piston and Back‑Up Rings — These rings maintain sealing on the high‑pressure side; check for cracks or flat spots.
• Cylinder Bore — Look for wear bands or out‑of‑round conditions; minor scars may be honed, but deep gouges often require barrel replacement.
• End Caps and Threads — Ensure threads on end caps and barrel are clean and undamaged; compromised threads can lead to leaks or blowouts under pressure.
A shop owner once shared that in a fleet of telehandlers, nearly 60% of returned cylinders had minor rod corrosion due to condensation and outdoor storage, emphasizing the need for rod covers and proper parking procedures.
Seal Replacement and Rebuild Steps
• Match Seal Kit to Measurements — Do not just order a generic kit; verify the ID, OD, and thickness of all old seals and confirm dimensions with the seal supplier.
• Grease and Assembly Lube — Apply a thin film of assembly lubricant or compatible hydraulic oil to new seals before installation to reduce initial drag and prevent seal damage.
• Install Wipers First — Wipers (scrapers) should be placed at the rod end to prevent contamination entering the cylinder.
• Install Pressure Seals — Follow manufacturer orientation; some seals have pressure directions that must be respected.
• Reinsert Rod and Piston — With even pressure, slide the piston assembly back into the barrel, ensuring seals do not roll.
• Torque End Cap to Specification — Use a torque wrench to tighten to the machine manufacturer’s spec; under‑ or over‑torque can cause leaks or premature seal failure.
Reinstallation and System Bleeding
After rebuild:
• Reconnect Cylinder to Boom — Reinstall mounting pins, ensuring cotter pins or retaining clips are replaced.
• Reconnect Hydraulic Lines — Confirm that high‑pressure and return lines go to correct ports.
• Bleed Air From System — Start the machine and slowly cycle the boom through extension and retraction several times with the hydraulic reservoir slightly open; this expels trapped air.
• Check for Leaks and Smooth Operation — Under no‑load conditions first, then gradually add load to verify performance.
Good bleeding practices can reduce jerky motion, which often appears when air remains in the head end of a cylinder.
Common Problems And Solutions
• Spongy or Jerky Motion After Rebuild — This is usually air in the system; re‑bleed and purge air lines thoroughly.
• Cylinder Fails to Extend Fully — Check for misrouted hoses, bent rods, or internal contamination blocking ports.
• Oil Leakage at Rod Seal — Likely a damaged wiper or incorrect seal orientation; reassess seal selection and installation.
• Uneven Wear After Reinstall — Evaluate boom linkage alignment; misalignment causes uneven loads and shortens seal life.
Parameter Guidelines and Service Intervals
• Typical Rod Diameter — Mid‑size telehandler boom cylinders like the KH70 often have rod diameters in the range of 2–2.5 inches; larger rods resist bending and wear.
• Hydraulic Pressure — Operating pressures in telehandler lift circuits frequently exceed 3,000 psi; seals and materials must be rated accordingly.
• Service Interval Insight — Inspect hydraulic cylinders every 500 hours; severe duty cycles warrant checks as often as 250 hours.
Safety and Best Practices
Always store and work on cylinders in clean, dry environments to prevent contamination. Wear appropriate PPE — safety glasses and gloves — when dealing with hydraulic systems because pressurized fluid can penetrate skin and cause serious injury.
Industry Trends
Modern telehandlers increasingly use integrated rod covers and field‑replaceable seal modules to simplify maintenance and extend service life. Some manufacturers are even offering remanufactured cylinder cores with lifetime warranty options, shifting the cost‑benefit analysis toward complete unit replacement in commercial fleets.
Conclusion
Disassembling and rebuilding a Lull (or similar telehandler) inner boom cylinder demands attention to detail, proper tooling, and strict cleanliness. Knowing cylinder internals and service techniques substantially improves service life and reduces unscheduled downtime. Whether maintaining a rental fleet, construction company assets, or a farm telehandler, following systematic disassembly, inspection, and rebuild steps ensures reliable lifting performance for years to come.
A critical component of the Lull’s boom extension and lifting system is the inner boom cylinder, a hydraulic actuator that extends and retracts the boom sections. Over time, wear, contamination, or seal failure can necessitate disassembly and rebuild of the inner boom cylinder. This article provides a detailed, practical guide to that process, combined with terminology explanations, recommended practices, and real‑world experience — all rewritten in clear, original language.
Inner Boom Cylinder Function and Terminology
Before diving into disassembly, it is useful to define key terms associated with hydraulic cylinders and telehandler booms:
• Hydraulic Cylinder — A mechanical actuator that converts pressurized hydraulic fluid into linear motion and force.
• Inner (or Telescopic) Cylinder — In a multi‑section boom, the inner cylinder is the most deeply nested section that extends last and retracts first.
• Rod and Piston Assembly — The internal parts that move within the cylinder barrel; the piston divides the internal volume into two pressurized chambers.
• Seal Kit — A set of O‑rings, wipers, and pressure seals that maintain fluid separation and prevent leaks.
• Pressure Side / Return Side — The pressure side receives hydraulic fluid to push the piston; the return side allows fluid to flow back to the reservoir.
• Bleeding Air — The process of eliminating trapped air in the hydraulic system that can cause spongy or erratic movement.
Disassembling the inner boom cylinder requires an appreciation of these parts and understanding how they interact under load.
Tools and Safety Preparations
Cylinder repair involves stored energy and heavy components, so safety is paramount. Recommended tools and preparations include:
• Hydraulic Hose Support — Support boom sections mechanically before disconnecting hydraulic lines to avoid sudden movement.
• Torque Wrench — For correct reassembly of cylinder end caps and retaining nuts.
• Seal Pick and Soft Mallet — For removing old seals without damaging bores.
• Calipers and Micrometer — To measure piston rod diameter and seal groove widths.
• Clean Work Surface and Rags — Because contamination degrades seal life, keep work areas tidy.
Stand a floor jack or boom support under the boom to prevent collapse when the cylinder is disconnected from the boom linkage. Never rely on hydraulic residual pressure to hold sections in place.
General Disassembly Sequence
• Depressurize the System — Before removing any lines, cycle the machine controls with the engine off and loader on solid ground to ensure minimal residual pressure.
• Disconnect Hydraulic Lines — Label each line carefully; use plugs to prevent fluid loss and contamination.
• Remove Mounting Pins — Use a drift punch and driver kit to push out pins securing the cylinder to the boom linkage.
• Support the Cylinder — With the cylinder freed on both ends, support it on a bench or stable cradle.
• Clean Exterior — Wipe off all dirt and grime; hydraulic systems are sensitive to contamination.
• Remove End Cap — Unscrew the cylinder head or gland nut. Be cautious — internal seals may spring out.
• Extract Rod and Piston — Pull the rod and piston assembly out of the barrel. Inspect for scoring, rust, or bent rods.
• Remove Seals and Wipers — Using soft picks remove all old seal elements; measure groove widths and record seal sizes for ordering replacement kits.
Inspection Points and Wear Assessment
Once inside, inspect several critical surfaces:
• Rod Surface — It should be smooth and free of scratches or pitting; even minor corrosion can destroy new seals rapidly.
• Piston and Back‑Up Rings — These rings maintain sealing on the high‑pressure side; check for cracks or flat spots.
• Cylinder Bore — Look for wear bands or out‑of‑round conditions; minor scars may be honed, but deep gouges often require barrel replacement.
• End Caps and Threads — Ensure threads on end caps and barrel are clean and undamaged; compromised threads can lead to leaks or blowouts under pressure.
A shop owner once shared that in a fleet of telehandlers, nearly 60% of returned cylinders had minor rod corrosion due to condensation and outdoor storage, emphasizing the need for rod covers and proper parking procedures.
Seal Replacement and Rebuild Steps
• Match Seal Kit to Measurements — Do not just order a generic kit; verify the ID, OD, and thickness of all old seals and confirm dimensions with the seal supplier.
• Grease and Assembly Lube — Apply a thin film of assembly lubricant or compatible hydraulic oil to new seals before installation to reduce initial drag and prevent seal damage.
• Install Wipers First — Wipers (scrapers) should be placed at the rod end to prevent contamination entering the cylinder.
• Install Pressure Seals — Follow manufacturer orientation; some seals have pressure directions that must be respected.
• Reinsert Rod and Piston — With even pressure, slide the piston assembly back into the barrel, ensuring seals do not roll.
• Torque End Cap to Specification — Use a torque wrench to tighten to the machine manufacturer’s spec; under‑ or over‑torque can cause leaks or premature seal failure.
Reinstallation and System Bleeding
After rebuild:
• Reconnect Cylinder to Boom — Reinstall mounting pins, ensuring cotter pins or retaining clips are replaced.
• Reconnect Hydraulic Lines — Confirm that high‑pressure and return lines go to correct ports.
• Bleed Air From System — Start the machine and slowly cycle the boom through extension and retraction several times with the hydraulic reservoir slightly open; this expels trapped air.
• Check for Leaks and Smooth Operation — Under no‑load conditions first, then gradually add load to verify performance.
Good bleeding practices can reduce jerky motion, which often appears when air remains in the head end of a cylinder.
Common Problems And Solutions
• Spongy or Jerky Motion After Rebuild — This is usually air in the system; re‑bleed and purge air lines thoroughly.
• Cylinder Fails to Extend Fully — Check for misrouted hoses, bent rods, or internal contamination blocking ports.
• Oil Leakage at Rod Seal — Likely a damaged wiper or incorrect seal orientation; reassess seal selection and installation.
• Uneven Wear After Reinstall — Evaluate boom linkage alignment; misalignment causes uneven loads and shortens seal life.
Parameter Guidelines and Service Intervals
• Typical Rod Diameter — Mid‑size telehandler boom cylinders like the KH70 often have rod diameters in the range of 2–2.5 inches; larger rods resist bending and wear.
• Hydraulic Pressure — Operating pressures in telehandler lift circuits frequently exceed 3,000 psi; seals and materials must be rated accordingly.
• Service Interval Insight — Inspect hydraulic cylinders every 500 hours; severe duty cycles warrant checks as often as 250 hours.
Safety and Best Practices
Always store and work on cylinders in clean, dry environments to prevent contamination. Wear appropriate PPE — safety glasses and gloves — when dealing with hydraulic systems because pressurized fluid can penetrate skin and cause serious injury.
Industry Trends
Modern telehandlers increasingly use integrated rod covers and field‑replaceable seal modules to simplify maintenance and extend service life. Some manufacturers are even offering remanufactured cylinder cores with lifetime warranty options, shifting the cost‑benefit analysis toward complete unit replacement in commercial fleets.
Conclusion
Disassembling and rebuilding a Lull (or similar telehandler) inner boom cylinder demands attention to detail, proper tooling, and strict cleanliness. Knowing cylinder internals and service techniques substantially improves service life and reduces unscheduled downtime. Whether maintaining a rental fleet, construction company assets, or a farm telehandler, following systematic disassembly, inspection, and rebuild steps ensures reliable lifting performance for years to come.
