Rebuilding the Drive System on Early Bobcat 863 Models with Eaton Motors

[MikePhua]

The Bobcat 863 and Its Hydraulic Drive Configuration
The Bobcat 863 skid steer loader was introduced in the mid-1990s as part of Bobcat’s transition into higher horsepower, hydrostatic-drive machines. With a rated operating capacity of 1,900 lbs and a turbocharged Kubota diesel engine, the 863 was designed for demanding applications in construction, agriculture, and landscaping. It featured dual hydraulic drive motors mounted to the rear frame, each powering a sprocket shaft that drove the track or wheel assemblies.
Early production units of the 863 were equipped with Eaton hydraulic motors, which interfaced with splined output shafts and mounted carriers that held the drive sprockets. These components were designed for high torque transfer but were vulnerable to wear and impact damage over time.
Terminology annotation:

• Hydrostatic drive: A propulsion system using hydraulic fluid to transfer power from the engine to the wheels or tracks.
  
• Carrier: A hub or flange that connects the motor output shaft to the sprocket or wheel assembly.
  

Failure of Output Shaft Splines and Carrier Damage
A recurring issue in early 863 models is the degradation of the Eaton motor output shaft splines. These splines transmit rotational force to the carrier, but under heavy load or poor lubrication, they can shear or strip. Once the splines fail, the motor spins freely without driving the wheels, rendering the machine immobile.
In one case, the bearing inside the carrier assembly pushed the retaining snap ring out of position, causing the internal knobs—designed to ride against the chain—to break off. This catastrophic failure compromised the entire drive system.
Terminology annotation:

• Spline: A series of ridges or teeth on a shaft that mesh with corresponding grooves in a mating part to transmit torque.
  
• Snap ring: A circular retaining ring used to hold components in place within a bore or groove.
  

Unfortunately, these carriers are now obsolete, and sourcing replacements from salvage yards has proven difficult. Many wreckers no longer stock early Eaton-equipped 863 parts due to low demand and high failure rates.
Exploring Rexroth Motor Compatibility
One potential solution is retrofitting the machine with Rexroth hydraulic motors, which were used in later Bobcat models and offer improved durability. However, compatibility is not guaranteed. Rexroth motors may have different mounting flanges, bolt patterns, and shaft dimensions, requiring custom adapters or frame modifications.
Recommended evaluation steps:

• Measure the bolt circle diameter and shaft spline count on both Eaton and Rexroth motors
  
• Compare carrier depth, bearing seat dimensions, and snap ring groove locations
  
• Consult Bobcat service bulletins for cross-reference part numbers and retrofit kits
  
• Consider machining custom carriers if Rexroth motors offer superior longevity
  

Terminology annotation:

• Bolt circle diameter: The diameter of the circle formed by the centers of mounting bolts, critical for flange compatibility.
  
• Retrofit: The process of upgrading or replacing components with newer alternatives not originally designed for the machine.
  

In some cases, operators have successfully adapted Rexroth motors by fabricating new carriers from hardened steel and using oversized bearings to accommodate shaft differences. However, this requires precision machining and may void any remaining warranty coverage.
Alternative Repair Strategies and Long-Term Viability
If sourcing OEM parts proves impossible, other strategies include:

• Rebuilding Eaton motors with new splined shafts, if available from hydraulic shops
  
• Welding and re-machining damaged splines (not recommended for high-load applications)
  
• Converting the drive system to a chain-and-sprocket configuration using external gearboxes
  
• Retiring the machine and salvaging usable components for other units
  

Terminology annotation:

• External gearbox: A mechanical transmission mounted outside the motor housing, used to adjust torque and speed.
  
• Salvage: The process of recovering usable parts from non-functional equipment.
  

While welding splines may seem cost-effective, it introduces heat distortion and weakens the shaft’s structural integrity. For machines used in grading, hauling, or demolition, this shortcut often leads to repeat failures.
Historical Context and Parts Obsolescence
Bobcat’s transition from Eaton to Rexroth motors in later 863 and 873 models was driven by reliability concerns and supplier consolidation. Eaton motors were known for their compact design but suffered from spline wear and seal leakage. Rexroth units, though heavier, offered better sealing and longer service intervals.
As Bobcat evolved into the S-series and T-series platforms, parts support for early 863 models dwindled. By 2010, many dealers had phased out inventory for pre-2000 machines, leaving owners reliant on salvage yards and independent rebuilders.
Terminology annotation:

• Seal leakage: Hydraulic fluid escaping from motor seals, often leading to pressure loss and contamination.
  
• Service interval: The recommended time or usage period between maintenance tasks.
  

In one anecdote, a contractor in Alberta rebuilt his 863 using a Rexroth motor sourced from a burned-out S185. After fabricating a custom carrier and adapting the mounting flange, the machine returned to service and ran for another 1,200 hours without issue.
Conclusion
Repairing the drive system on an early Bobcat 863 with Eaton motors presents a significant challenge due to spline failure and obsolete carriers. While retrofitting Rexroth motors offers a potential path forward, it requires careful measurement, fabrication, and risk assessment. For operators committed to preserving these machines, creative engineering and collaboration with hydraulic specialists can restore functionality and extend service life. As parts support fades, the 863 becomes a testament to mechanical ingenuity and the enduring value of field-proven equipment.