Thunderbird TSY 255 Swing Bearing and the Mechanics of Logging Yarders

[MikePhua]

The TSY 255’s Role in Skyline Logging
The Thunderbird TSY 255 is a tower yarder designed for high-lead and skyline logging operations, particularly in steep terrain where ground-based equipment cannot reach. Manufactured by Thunderbird Logging Machines, a division of Washington Iron Works, the TSY 255 was built to handle large payloads with precision and durability. Its swing boom and tower configuration allow for lateral movement and cable control, making it ideal for hauling logs from inaccessible slopes to roadside landings.
Thunderbird machines were widely used across the Pacific Northwest, New Zealand, and British Columbia during the 1980s and 1990s. The TSY 255, in particular, became known for its robust frame, powerful winch system, and reliable swing mechanism. Though production numbers were limited compared to crawler dozers or excavators, the TSY 255 earned a loyal following among logging contractors for its ability to work in extreme conditions.
Understanding the Swing Bearing Assembly
The swing bearing is a critical component in the TSY 255’s boom rotation system. It allows the upper structure of the yarder to pivot horizontally, enabling the boom to swing left or right during cable operations. This movement is essential for positioning chokers, adjusting skyline tension, and managing log placement.
Terminology note:

• Swing Bearing: A large-diameter bearing that supports axial and radial loads while allowing rotational movement.
  
• Slew Ring: Another term for swing bearing, often used in crane and excavator applications.
  
• Raceway: The grooved track inside the bearing where balls or rollers circulate.
  
• Grease Port: An access point for injecting lubricant into the bearing raceway.
  

The TSY 255’s swing bearing is mounted between the base frame and the rotating upper structure. It must withstand dynamic loads from boom movement, cable tension, and log impact—all while maintaining smooth rotation and structural alignment.
Symptoms of Swing Bearing Wear or Failure
Operators may notice several signs when the swing bearing begins to degrade:

• Jerky or uneven boom rotation
  
• Audible grinding or popping during swing
  
• Excessive play or tilt in the upper structure
  
• Grease leakage or dry spots around the bearing
  
• Difficulty maintaining boom position under load
  

In one case, a yarder crew in Oregon reported that their TSY 255’s boom would drift during skyline tensioning. Inspection revealed that the swing bearing had developed flat spots in the raceway, causing resistance and misalignment. Replacing the bearing restored full swing control and reduced cable wear.
Another operator in British Columbia found that the bearing bolts had loosened over time due to vibration and thermal cycling. Retorquing the bolts and adding locking compound prevented further movement and improved boom stability.
Inspection and Maintenance Procedures
To assess and maintain the swing bearing:

• Visually inspect the bearing raceway for cracks, pitting, or corrosion
  
• Check bolt torque and look for signs of elongation or thread wear
  
• Grease the bearing regularly using high-pressure lithium-based lubricant
  
• Measure axial and radial play using dial indicators
  
• Listen for abnormal noises during swing operation
  

Recommended service intervals:

• Grease bearing every 100 hours or weekly in wet conditions
  
• Inspect bolt torque monthly or after heavy use
  
• Replace bearing seals every 2,000 hours or during overhaul
  
• Flush old grease annually and inspect for metal particles
  

One technician in Washington added a remote grease manifold to the swing bearing ports, allowing easy access from ground level. This reduced service time and ensured consistent lubrication.
Replacement and Retrofit Strategies
If the swing bearing must be replaced:

• Use OEM or precision aftermarket bearings with matching bolt patterns and load ratings
  
• Clean mating surfaces thoroughly and remove old sealant
  
• Align bearing raceway with boom pivot axis before torquing bolts
  
• Apply anti-seize compound to bolts and torque in a star pattern
  
• Test swing function under load before returning to service
  

Some operators retrofit their TSY 255s with upgraded swing systems, including larger bearings or hydraulic dampers to reduce shock loads. These modifications can extend service life and improve control during complex yarding operations.
Design Legacy and Industry Impact
The TSY 255’s swing bearing system reflects the broader evolution of logging yarders from cable-only rigs to semi-automated machines with hydraulic assist. Thunderbird’s engineering emphasized durability and field-repairability, allowing crews to maintain machines in remote locations with limited support.
Though Thunderbird Logging Machines ceased production decades ago, their equipment remains in use across North America. Restoration communities have emerged around models like the TSY 255, with owners fabricating replacement parts, digitizing manuals, and sharing field solutions.
Sales data from the 1990s suggests that fewer than 500 TSY 255 units were built, but many are still operational today—testament to their rugged design and the skill of the crews who maintain them.
Conclusion
The swing bearing in the Thunderbird TSY 255 is more than a mechanical pivot—it’s the heart of the machine’s directional control and operational flexibility. When properly maintained, it enables precise boom movement, safe log handling, and efficient yarding in the toughest terrain. Whether hauling timber from a steep slope or repositioning chokers in a dense forest, the swing bearing keeps the machine responsive, stable, and ready for the next pull.