How to Properly Purge the Track Tension Cylinder on the Caterpillar 955L Track Loader

[MikePhua]

Maintaining correct track tension on a Caterpillar 955L track loader is essential for optimal machine performance and track longevity. The track tension cylinder, a hydraulic component responsible for adjusting track tightness, occasionally requires purging to remove air trapped inside the system. Air in the cylinder can cause improper tension, leading to accelerated wear or track derailment.
Understanding the Track Tension Cylinder
The track tension cylinder is a hydraulic piston assembly located near the track idler. It applies force to the idler wheel to maintain proper track tension. Over time or after maintenance activities such as hydraulic repairs, air can accumulate in the cylinder, reducing its effectiveness.
Signs Air in the Tension Cylinder May Be Present

• Track feels loose or too tight despite adjustment attempts.
  
• Abnormal track noise during operation.
  
• Hydraulic fluid leaks or spongy pedal response.
  
• Difficulty in achieving or maintaining proper track tension.
  

Step-by-Step Purging Procedure

1. Prepare the Machine
   • Park the loader on a level surface.
     
   • Engage the parking brake and shut down the engine.
     
   • Ensure hydraulic system pressure is relieved before beginning.
     
2. Locate the Track Tension Cylinder
   • The cylinder is mounted near the front idler on each track.
     
   • Identify the purge valve or bleed screw on the cylinder or the hydraulic line.
     
3. Purge Air from the Cylinder
   • Open the purge valve slowly to allow trapped air to escape.
     
   • Operate the track tensioning system (manually or via hydraulic controls) to cycle the piston back and forth, facilitating air removal.
     
   • Observe the escaping fluid; once a steady stream of hydraulic fluid without bubbles appears, close the valve.
     
   • Repeat the process if necessary on both track tension cylinders (left and right).
     
4. Check and Adjust Track Tension
   • After purging, check the track tension according to manufacturer specifications.
     
   • Adjust if necessary using the track tensioning system controls or manual adjustment bolts.
     
   • Confirm the track runs smoothly without excessive play or binding.
     

Common Challenges and Solutions

• Persistent Air in the System
  • Inspect hydraulic lines for leaks or damage that may introduce air.
    
  • Verify that fittings are tight and seals are intact.
    
• Difficulty Locating Purge Points
  • Consult the service manual or diagrams for exact valve locations.
    
  • Some models have integrated purge ports requiring special tools.
    
• Hydraulic Fluid Contamination
  • Contaminated fluid can introduce air pockets or damage seals.
    
  • Perform a full hydraulic fluid change and filter replacement if contamination is suspected.
    

Preventive Maintenance Tips

• Regularly inspect hydraulic hoses and fittings for leaks.
  
• Check hydraulic fluid levels and quality frequently.
  
• Avoid rapid or abrupt track tension adjustments that may introduce air.
  
• Schedule routine purging after major hydraulic repairs or replacements.
  

Case Study
A contractor operating a Caterpillar 955L noticed frequent track derailments and uneven wear. Upon inspection, the track tension cylinders contained trapped air, reducing their effective force. After following a systematic purging procedure and replacing worn hydraulic seals, track tension stabilized, extending track life and improving machine reliability.
Technical Glossary

• Track Tension Cylinder: Hydraulic piston that controls the tightness of the track.
  
• Purge Valve/Bleed Screw: A valve that allows trapped air to escape from the hydraulic system.
  
• Hydraulic Fluid: Specialized oil used to transmit power within the hydraulic system.
  
• Hydraulic Pressure: The force exerted by hydraulic fluid in a confined system.
  
• Track Idler: A wheel that guides and tensions the track chain.
  

Conclusion
Proper purging of the track tension cylinder on the Caterpillar 955L is a vital maintenance task that ensures track stability and prevents premature wear. By following a careful procedure and addressing hydraulic system integrity, operators can maintain optimal track performance and reduce downtime. Regular inspection and maintenance not only enhance machine reliability but also contribute to operational safety and efficiency.