CAT 235 Swing Malfunction Insight

[MikePhua]

Excavator and Company Overview
The CAT 235 is a compact hydraulic excavator produced by Caterpillar Inc., a company with roots going back to the early 1900s. Caterpillar has long been a major force in heavy construction and mining machinery. The CAT 235 falls in the smaller-end of CAT’s line, typically used where maneuverability, lower operating weight, and moderate power are needed. These machines often operate in tight job sites, utility work, trenching, demolition, and landscaping. Key specs include: operating weight around 24,000-26,000 lbs, swing speed in the range of 10-12 rpm, and hydraulic systems designed for both power and precision.

Swing System Purpose and Components
In a hydraulic excavator, the swing refers to the rotation of the house (cab, boom, stick) relative to the undercarriage. The swing function is powered by:

• a swing motor (hydraulic motor) converting fluid pressure into rotational motion
  
• a reduction gear / swing gearbox which reduces motor output speed while increasing torque
  
• a swing bearing which supports the house load and allows rotation
  
• hydraulic control valves and pilot/servo systems managing operator inputs
  
• a swing brake or parking brake to hold the house stationary when swing is not active or when shut off
  

Symptoms of Swing Problems
When the swing mechanism malfunctions, common observable symptoms include:

• Swing motion sluggish or slow
  
• Swing stops entirely in one direction (e.g. will swing right but not left)
  
• Swing brake remains engaged even when swing is commanded
  
• Operator control (joystick) input does not produce swing or feels delayed
  

Possible Root Causes
Based on similar machines and known issues, the swing malfunction in a CAT 235 (or similar model) likely arises from one or more of the following:

• Hydraulic motor issues: Internal damage, worn seals, bad bearings causing low output, internal leakage reducing torque.
  
• Reduction gear or swing gearbox problems: Wear in gears, ring gear or planetary gear wear, or backlash too large impairing torque transfer.
  
• Swing brake or parking brake malfunctions: Brake not releasing properly, mechanical sticking, or hydraulic/pilot control failure.
  
• Hydraulic control circuit faults: Blocked flow paths, damaged valves, incorrect pressure settings, pilot line issues.
  
• Hydraulic fluid condition: Contamination, excessive temperature, low fluid levels, wrong viscosity affects performance.
  
• Mechanical binding or interference: Structural damage, bent frame, binding in swing bearing due to misalignment or worn bearing race.
  

Diagnostics and Inspection Steps
To isolate and fix swing problems, the following diagnostic procedure may help:

1. Check hydraulic fluid levels and condition: Ensure proper level, check for contamination or foaming, verify viscosity is correct per CAT spec.
   
2. Inspect swing motor
   • Check input and output ports for leaks.
     
   • Measure case-drain flow if specified (high flow may indicate internal leak).
     
   • Test pressure at motor inlet under load; low pressure suggests restriction or pump issue.
     
3. Assess swing gearbox / bearing
   • Inspect swing bearing for wear, broken bolts, misalignment.
     
   • Check for tooth damage on ring gear or planetary gears if accessible.
     
4. Examine swing brake / parking brake
   • Does the brake disengage when swing is commanded?
     
   • Is there hydraulic or pilot supply to release the brake?
     
   • Check solenoids (if used), control valves, and linkage.
     
5. Test control valves / pilot circuits
   • Verify joystick pilot pressure output.
     
   • Check swing control valve for proper function, spool movement, no sticking.
     
6. Operator input check
   • Ensure joystick and related linkages/sensors are working properly.
     
7. Inspect mechanical alignment
   • Ensure no unusual binding due to bent frames or misalignment in components.
     

Recommended Repairs and Solutions
Once diagnostics narrow down the issue, possible solutions include:

• Rebuild or replace the swing motor if internal leakage or damaged internal parts present.
  
• Replace or refurbish the reduction gear or swing gearbox if gear or bearing damage is detected.
  
• Service or replace brake components: clean or replace brake discs, pads, or hydraulic/pilot actuators.
  
• Clean or repair hydraulic control valves; ensure pilot circuits are free of debris and operate smoothly.
  
• Replace hydraulic fluid, filters, ensure correct viscosity, and keep fluid at proper operating temperature.
  
• Tighten or replace loose bolts on swing bearing; reassess alignment.
  

Case Story
A contractor in northern Georgia had a CAT 235 which would swing to the right but was locked when attempting to rotate left. The swing brake seemed engaged despite control input. They discovered that one of the swing control pilot hoses had been damaged, causing insufficient pilot pressure to release the brake in left swing. After replacing the hose and bleeding air from the pilot line, swing in both directions was restored. Also during inspection they found that the hydraulic fluid was overdue for change and quite dirty; after changing the fluid and filter they noted smoother swing motion and less overheating under heavy use.

Terminology Clarification

• Swing motor: The hydraulic motor that powers rotation of excavator house.
  
• Case drain: Returnflow of hydraulic fluid within the motor; excessive flow here can indicate internal leakage.
  
• Reduction gear / swing gearbox: Gears between motor and swing bearing, reduces speed while increasing torque.
  
• Parking brake / swing brake: Device to hold the house stationary. Can be hydraulic, mechanical, or a combination.
  
• Pilot pressure / pilot circuit: Lower-pressure hydraulic circuit used to operate control valves and actuators.
  

Preventive Maintenance Tips

• Regularly change hydraulic fluid and filters per manufacturer schedule; clean fluid means fewer unseen leaks or internal wear.
  
• Periodic inspection of swing motor seals, swing brake, bearing bolts, and valve operation.
  
• Keep swing brake adjustment and operation checked; even if not obviously faulty, corrosion or leakage can degrade performance.
  
• Protect hoses and pilot lines from damage; shield or reroute where exposed.
  
• Avoid overloading rotationally; quickly swinging heavy loads with high inertia stresses motor, gearbox, and brake heavily.
  

Summary
Swing failure or reduced swing performance in a CAT 235 excavator typically involve problems in the hydraulic motor, swing gearbox, brake, fluid condition, or control circuits. By systematically diagnosing motor function, hydraulic lines, brake release, and control pilot pressure, the underlying fault can usually be found. With proper maintenance, parts replacement, and ensuring alignment and fluid cleanliness, swing function can be reliably restored and preserved.