Flat Pad Tracks and Their Decline in Excavator Design

[MikePhua]

Origins and Use in Early Excavators
Flat pad tracks, often referred to as tumbler or crane-type tracks, were commonly used on crawler cranes, cable shovels, and early excavators from the mid-20th century. These tracks consisted of individual pads pinned directly to each other, without a continuous chain or bushing system. The pads featured a nub or tooth on the underside that engaged with a cog wheel—functionally similar to a sprocket—allowing the machine to propel itself forward.
Terminology annotation:

• Tumbler track: A track system where pads are pinned together without a chain, typically used on slow-moving machines.
  
• Cog wheel: A toothed wheel that engages with the pad nubs to drive the track.
  

This design was favored for machines that operated in stationary or near-stationary roles, such as crawler cranes lifting loads or trenchers working from stockpiles. The simplicity of the system made it easier to manufacture and maintain in low-mobility applications.
Limitations in Traction and Mobility
Compared to modern grouser-style tracks, flat pad systems offer significantly less traction, especially on slopes, wet terrain, or loose surfaces. The smooth surface of the pads lacks the aggressive ridges or cleats found on grousers, which are designed to bite into the ground and prevent slippage.
Operators noted that machines equipped with flat pads were prone to sliding on inclines and struggled to maneuver in mud or snow. In forestry and logging operations, some crews welded chunks of drill steel onto the pads to act as ice lugs, improving grip in frozen conditions.
Terminology annotation:

• Grouser: A raised ridge or cleat on a track pad that improves traction by penetrating the ground.
  
• Ice lug: A welded protrusion added to a track pad to enhance grip on icy surfaces.
  

Despite these adaptations, flat pad tracks remained best suited for level ground and minimal travel. Attempting to steer or climb with such systems often required external assistance, such as a dozer pulling the machine or careful alignment before loading onto trailers.
Wear Patterns and Maintenance Challenges
One of the most labor-intensive aspects of flat pad tracks is their maintenance. Over time, the pins connecting the pads wear into camshaft-like shapes, making removal difficult. The bores in the pads also elongate, leading to excessive play and misalignment. Unlike chain-type undercarriages, these systems lack recoil springs and hydraulic adjusters. Instead, large bolts are used to push the idlers or cog wheels outward, with shims inserted to hold the position.
Terminology annotation:

• Camshaft wear: A condition where cylindrical pins develop lobed profiles due to uneven wear, complicating removal and replacement.
  
• Shim: A thin spacer used to maintain alignment or preload in mechanical assemblies.
  

Bottom rollers in these systems often run directly on the pads, wearing grooves that can lead to stress fractures. Once cracks develop, the pads may fail under load, especially in machines with high drawbar pull.
Transition to Chain-Type Undercarriages
By the late 1970s, most excavator manufacturers had transitioned to chain-type undercarriages. These systems use track chains with bushings and pins, allowing smoother articulation and better load distribution. Chain-type tracks also support higher travel speeds and more aggressive terrain handling, making them ideal for modern excavation tasks.
Advantages of chain-type tracks:

• Improved traction and steering
  
• Easier maintenance with modular components
  
• Compatibility with recoil springs and hydraulic adjusters
  
• Greater durability under cyclic loading
  

Terminology annotation:

• Drawbar pull: The horizontal force a machine can exert to move itself or tow a load.
  
• Recoil spring: A spring mechanism that absorbs shock and maintains track tension during movement.
  

The shift was driven by the increasing mobility demands of excavators, which began to travel more frequently between job sites and perform dynamic digging operations.
Legacy and Niche Applications
Despite their decline in mainstream use, flat pad tracks still appear on large crawler cranes and legacy machines. In some cases, operators prefer them for their stability during lifting operations, where mobility is secondary. However, even in these roles, newer designs with high-walker undercarriages or rail-mounted pads offer better maneuverability without sacrificing load capacity.
In British Columbia, one operator still runs three crawler cranes on flat pads, noting that while the pins are difficult to remove when worn, new replacements restore function effectively. Attempts to bore out the pad holes often fail due to the hardened material, requiring alternative repair strategies like rebuilding roller paths or welding new dogs.
Terminology annotation:

• High-walker undercarriage: A track system with elevated sprockets and improved clearance, enhancing mobility and reducing wear.
  
• Dogs: Raised features on the pad surface that engage with rollers or drive components.
  

These machines are often kept in service due to their specialized roles and the high cost of replacement, but they require careful handling and maintenance to remain operational.
Conclusion
Flat pad tracks represent an earlier era of excavator and crane design, optimized for stability and simplicity rather than mobility. While they offer certain advantages in stationary applications, their limitations in traction, wear resistance, and maintenance have led to widespread adoption of chain-type undercarriages. For operators working with legacy equipment, understanding the nuances of flat pad systems—from pin wear to roller path rebuilding—is essential to keeping these machines productive. As technology continues to evolve, the flat pad track remains a reminder of the trade-offs between simplicity and performance in heavy equipment design.