JCB 814 Final Drive Motor Underperformance

[MikePhua]

Noticed Weakness in One Direction
A clear symptom emerged when the machine struggled to move in reverse: one of the final drive motors was sluggish, barely turning, while that same motor performed noticeably better when slewing. In forward gear, performance improved but did not match the other side’s speed, suggesting a genuine motor inconsistency rather than a systemic issue .
Testing to Isolate the Fault
A reliable diagnostic approach involves using inline pressure gauges on both feed lines of the affected motor. By comparing forward and reverse pressures, you can isolate the issue:

• If pressures are consistent, the fault likely lies within the motor—possibly a malfunctioning internal valve like a shock valve.
  
• If discrepancies appear, the issue may lie upstream in components like the rotary joint's main valve .
  

Understanding Final Drive Motor Weakness
Weak performance in a final drive motor can stem from multiple system factors—many not originating in the motor itself:

• Hydraulic fluid level or pressure deficiency results in insufficient torque for the travel motor.
  
• Blocked case drain filters or problematic swivel joints can also degrade motor performance.
  
• High friction drive components, like tight tracks, also place extra demand on the motor.
  These problems can mimic motor failures, so it’s important to rule them out first .
  

Typical Failure Points in the Motor
If the motor itself is to blame, wear or damage typically centers on:

• Main bearings, which generate noise, heat, and vibration when failing
  
• Gears and gear bearings, where wear becomes evident as pointed or uneven surfaces
  
• Upper and lower shafts with splines, where worn splines may lose engagement
  
• Degraded seals, leading to fluid leakage
  Furthermore, clogged case drain filters or a weak charge pump often initiate failures and must be addressed to prevent recurrence .
  

Order of Troubleshooting Steps

1. Confirm equal pressure in both directions using pressure gauges.
   
2. Inspect swivel joint for leaks, check track tension, and ensure charge pump delivers rated pressure.
   
3. Clean or replace the case drain filter and test hydraulic fluid level and condition.
   
4. If external components check out, proceed to open-motor inspection: bearings, shafts, gears, and seals.
   

Brief Anecdote
A field operator faced a machine drifting to one side without logical cause. Pressure gauging failed to expose anomalies, yet forward drive remained strong. They discovered a worn internal motor valve that functioned better in forward motion but restricted reverse flow. Replacing that valve restored balanced drive performance in both directions.
Maintenance Recommendations

• Routinely monitor hydraulic pressure and case drain cleanliness.
  
• Rotate drive motors with refurbishments or replacements to distribute wear.
  
• Lubricate and inspect swivel joints regularly.
  
• Maintain proper track tension—both over- and under-tension damage motor performance.
  

Key Terminology Defined

• Final Drive Motor: The hydraulic motor driving track or wheel movement via a gear assembly.
  
• Swivel Joint: A hydraulic rotating coupling for continuous connection between static and rotating components.
  
• Case Drain Filter: Protects motor by filtering return oil; blockages can cause overpressure.
  
• Shock Valve: An internal valve that cushions sudden hydraulic pressure shifts.
  

Summary
A sluggish final drive motor in reverse—restored temporarily by slewing—points to a localized internal issue likely involving motor valves, not the control system. Pressure testing is the first step, followed by broader hydraulic system checks and, if necessary, motor disassembly. Addressing common wear points and maintaining clean hydraulic conditions ensures longer service life and consistent drive performance.