Caterpillar 345BL Swing Drive Failure and Dual Swing Motors

[MikePhua]

Background
The Caterpillar 345BL is a heavy-duty hydraulic excavator widely used on large construction and mining sites. It features a powerful swing system that rotates the upper structure smoothly and precisely around the undercarriage for optimal operating efficiency.
Issue Description
A 1998 model 345BL ceased swinging after a noise occurred during a right-side swing. Investigation revealed damage inside one of the two swing drive motors — specifically, the presence of broken bronze caps (likely cylinder barrel bushings or retaining caps) sitting below nine small pistons within the motor.
Function and Purpose of Dual Swing Motors
Some large excavators like the 345BL use two swing drive motors in parallel to distribute the torque load required for smooth rotation of the upper structure. This design ensures balanced force application and reduces wear on any single component. Both motors typically work simultaneously, but in some cases, one may have been inactive or underutilized.
Having two motors improves the power-to-weight ratio and reliability of the swing system, but makes diagnosing failures more complex.
Typical Root Causes of Broken Bronze Caps in Swing Motors

• Contamination: Dirt or debris in hydraulic fluid can cause accelerated wear or damage to piston components inside the motor.
  
• Hydraulic Pressure Issues: Overpressure or pressure spikes may stress internal components, causing caps or bushings to break.
  
• Lubrication Deficiency: Insufficient hydraulic fluid or contamination degrading lubrication can lead to excessive friction and fracture of metal parts.
  
• Improper Load Distribution: If one motor is inactive or functioning incorrectly, the other motor experiences disproportionate load, leading to premature failure.
  
• Operational Abuse: Sudden or heavy swing motions, or use of the bucket/stick to force rotation, places severe stress on motors.
  

Recommendations

• Carefully clean and flush the hydraulic system prior to installation of new motors or parts.
  
• Ensure all hydraulic filters are replaced and fluid meets Caterpillar specifications.
  
• Verify both swing motors receive consistent and proper hydraulic input; investigate controls, valves, and pressure sensors.
  
• Avoid mechanical loads on the swing system from attachments during swing motion.
  
• Consult Caterpillar technical support or authorized dealers for detailed troubleshooting specific to dual motor systems.
  

Glossary

• Swing Drive Motor: Hydraulic motor that rotates the upper structure of an excavator around its base.
  
• Bronze Caps: Likely bushings or piston component caps inside hydraulic motors, crucial for maintaining structural integrity and smoothing movements.
  
• Pistons: Cylindrical components in motors that convert hydraulic pressure to mechanical rotation.
  
• Load Distribution: The way force or torque is shared between multiple components or motors.
  
• Hydraulic Contamination: Presence of dirt, water, or particles in hydraulic fluid causing damage to system components.
  

Summary
The dual swing drive motor setup in Caterpillar 345BL excavators offers increased swing power and balance but requires proper maintenance and fluid cleanliness to avoid premature motor damage. Broken bronze caps inside swing motors can stem from contamination, pressure spikes, or uneven motor load. Meticulous system flushing, balanced hydraulic distribution, and operational best practices help prevent failure and ensure the excavator’s reliable swinging function for extended service life.