Drive Sprocket Repair Strategies for Drott 40BLCW, Case 1280, and Case 1187 Excavators

[MikePhua]

Introduction: When Splines Strip and Options Narrow
The Drott 40BLCW and its mechanical siblings—the Case 1280 and Case 1187—share a common vulnerability: the final drive output shaft and its mating drive sprocket. Over time, especially under heavy load and poor lubrication, the splines on these components can strip, leaving the machine immobile and the operator facing a difficult repair. This article explores practical repair strategies, from field fixes to machine shop solutions, and offers guidance for sourcing parts, restoring torque transmission, and extending the life of these aging excavators.
Understanding the Final Drive Interface
The final drive system transmits power from the hydraulic motors to the track sprockets. In these models, the sprocket is mounted directly onto a splined output shaft, which is supported by pillow block bearings and bushings.
Key components:

• Output shaft (P/N S605277 or S616807)
  
• Drive sprocket (P/N S953545)
  
• Inner and outer pillow block bearings
  
• Spacer bushings and bearing caps
  
• Snap rings securing gear placement
  

Terminology clarification:
- Splines: Grooved ridges on the shaft and sprocket hub that interlock to transmit torque
- Pillow Block Bearing: A mounted bearing that supports a rotating shaft, often used in heavy equipment for alignment and load distribution
- Dowel Pins: Hardened steel pins used to lock components together, often drilled through mating parts
- Keyways: Machined slots that accept a key to prevent rotational slippage between shaft and hub
Field Repair Options: Creative but Risky
When replacement parts are unavailable or prohibitively expensive, operators often resort to creative repairs. These include:
- Drilling through the sprocket hub and shaft to insert hardened pins
- Welding nuts over the pins and securing them with set screws
- Using spring or dowel pins to lock the sprocket in place
- Machining keyways into the shaft and sprocket hub for mechanical engagement
- Welding the sprocket directly to the shaft
Each method has trade-offs:

• Pins offer a quick fix but may shear under heavy torque
  
• Keyways require precision machining and may weaken the hub
  
• Welding provides strength but complicates future disassembly and risks misalignment
  

Case Story: A Michigan Operator’s Multi-Pin Fix
In Michigan, an operator faced a stripped output shaft on his Drott 40BLCW. With no replacement sprocket available, he drilled two ⅝" holes through the sprocket hub and shaft, inserted hardened pins, and welded oversized nuts over the pins. Set screws were added to prevent pin walkout. The repair held through a full season of excavation work, though he noted increased vibration and eventual wear on the pillow block bearings.
Machine Shop Solutions: Precision and Longevity
For a more permanent fix, consider professional machining:

• Remove the final drive assembly and disassemble the shaft and sprocket
  
• Machine the sprocket bore smooth and weld the shaft to match
  
• Cut three equidistant keyways into the hub and shaft
  
• Install hardened keys and reassemble with proper torque specs
  
• Pressure test the final drive and inspect bearing preload
  

Advantages:

• Restores full torque transmission
  
• Allows future disassembly if keys are used
  
• Reduces risk of misalignment compared to welding alone
  

Challenges:

• Requires access to a skilled machinist
  
• Downtime may be longer than field fixes
  
• Cost may exceed the value of the machine if parts are scarce
  

Sourcing Replacement Parts: A Game of Patience
Finding a replacement sprocket or shaft for these older models can be difficult. Strategies include:

• Contacting legacy Case dealers or aftermarket suppliers
  
• Searching salvage yards with compatible machines
  
• Networking with crawler supply specialists
  
• Posting part numbers in equipment forums and classifieds
  
• Cross-referencing with similar models (Drott, Case, and early Link-Belt machines)
  

If a used sprocket is found:

• Inspect spline integrity and hub bore dimensions
  
• Check for cracks, wear, and previous weld repairs
  
• Verify compatibility with shaft diameter and bearing spacing
  

Disassembly Tips: Removing the Output Shaft
Some technicians claim the output shaft can be removed without full final drive disassembly. This involves:

• Removing the inner bearing cap
  
• Extracting the shaft through the opposite side
  
• Bypassing the need to remove the final drive gear
  

However, this method is debated. The snap ring securing the gear may be inaccessible without removing the bearing, and clearance may be insufficient for shaft extraction. Proceed only with detailed manual reference and visual inspection.
Welding Considerations: Where and How to Anchor
If welding is chosen:

• Avoid welding near the inner pillow block bearing to prevent heat damage
  
• Drill access holes through the sprocket to weld directly to the shaft splines
  
• Eliminate the outer spacer bushing if necessary to gain weld clearance
  
• Use low-hydrogen electrodes and preheat the shaft to reduce cracking risk
  
• Balance weld placement to minimize bending loads and vibration
  

Preventive Maintenance to Avoid Future Failures
To extend the life of the final drive:

• Maintain proper lubrication on splines and bearings
  
• Inspect sprocket alignment and torque regularly
  
• Avoid shock loads during track engagement
  
• Replace worn bushings and bearings before they cause misalignment
  
• Monitor for early signs of spline wear (metal shavings, vibration, track slippage)
  

Conclusion: Repairing with Ingenuity and Precision
The Drott 40BLCW, Case 1280, and Case 1187 excavators may be aging, but their mechanical simplicity allows for creative and effective repairs. Whether you choose pins, keys, or welds, the key is understanding the forces at play and respecting the tolerances of the final drive system. With a blend of field ingenuity and machine shop precision, even stripped splines can be reborn—and your excavator can return to the dirt with confidence.