Diagnosing Track Misalignment on the Caterpillar D6D Dozer

[MikePhua]

The D6D and Its Undercarriage Design
The Caterpillar D6D was introduced in the 1970s as part of the company’s mid-size dozer lineup, offering a balance of power, maneuverability, and serviceability. With an operating weight around 30,000 lbs and powered by the reliable Cat 3306 engine, the D6D became a staple in construction, forestry, and land-clearing operations. Its undercarriage features a suspended track frame with equalizer bar articulation, designed to maintain ground contact and absorb terrain variations.
The track system includes a combination of single-flange and double-flange bottom rollers, a front idler, carrier rollers, and a sprocket-driven final drive. Proper alignment and tension are critical to keeping the track chain seated on the rollers and idlers during operation.
Symptoms of Track Displacement
Operators have reported that the track on one side of the D6D intermittently slips off the bottom rollers, especially during turns or when raising the machine with the blade and ripper. In some cases, the track realigns itself after rotating the chain a few turns, but the issue persists under load.
Key symptoms include:

• Side play in the front idler
  
• Track leaning outward at the front
  
• Uneven roller contact
  
• Track frame movement toward the outside
  
• History of wishbone failure and replacement
  

These signs suggest a combination of mechanical wear and improper component installation.
Front Idler Shimming and Frame Geometry
The front idler plays a crucial role in guiding the track chain and maintaining lateral stability. If the idler is not shimmed correctly, it can allow side-to-side movement, causing the track to walk off the rollers. Shimming involves inserting calibrated spacers between the idler mounting and the frame to eliminate excess play.
Improper shimming can result from:

• Incorrect installation of a replacement idler
  
• Missing or worn shims
  
• Bent or misaligned mounting brackets
  
• Uneven wear on the idler shaft or bushings
  

Before assuming a bent track frame, technicians should inspect the idler assembly and verify that all shims are present and properly torqued. A bent frame is rare and typically results from severe impact or structural fatigue.
Roller Configuration and Rail Wear
The D6D undercarriage uses alternating single-flange and double-flange bottom rollers to guide the track chain. Double-flange rollers help prevent lateral movement, especially on slopes or during side loading. If all rollers are single-flange, the track may be more prone to walking off.
Additionally, worn track rails can snake or twist under load, causing misalignment. Severely worn rails lose their ability to seat properly on the rollers and idlers, especially when the equalizer bar shifts or the machine articulates.
Recommendations include:

• Inspect rail height and wear pattern
  
• Measure roller flange thickness and spacing
  
• Replace worn rollers with correct flange configuration
  
• Check for shiny metal indicating abnormal movement
  

Wishbone and Equalizer Bar Integrity
The wishbone, or equalizer bar, connects the track frames to the main chassis and allows vertical movement for terrain adaptation. If the wishbone bushings are worn or the carrier on the dead axle is loose, the track frame may crowfoot—twisting outward and causing the track to misalign.
Key inspection points:

• Bushing wear at the wishbone pivot
  
• Carrier nut torque and condition
  
• Swing frame alignment
  
• Signs of crowfooting or lateral shift
  

A worn carrier bushing beneath the final drive plate can allow excessive movement, leading to track derailment at the front idler. Replacing the bushing and retorquing the retaining nut can restore proper geometry.
Anecdote from Oklahoma
An operator in Oklahoma noticed his D6D’s right track repeatedly slipping off the rollers. After replacing the front idler and all bottom rollers, the issue persisted. Upon closer inspection, he found that the wishbone had previously failed and been reinstalled without addressing the worn carrier bushing. Once the bushing was replaced and the idler properly shimmed, the track stayed aligned even under heavy ripping.
Preventive Measures and Long-Term Solutions
To prevent track misalignment:

• Shim the front idler to eliminate side play
  
• Use alternating flange rollers as per factory spec
  
• Replace worn rails and inspect for twist
  
• Check equalizer bar bushings and carrier integrity
  
• Monitor track tension and adjust regularly
  

Operators should also document undercarriage service history and inspect components after any major failure, such as wishbone collapse or idler replacement.
Conclusion
Track misalignment on the Caterpillar D6D is often caused by a combination of worn components, improper installation, and overlooked shimming. While a bent track frame is possible, it is rarely the root cause. By systematically inspecting the idler, rollers, rails, and equalizer bar, technicians can restore proper alignment and extend the life of the undercarriage. The D6D remains a workhorse in the field, and with careful attention to its track system, it can continue performing reliably for decades.