Gear Pump Failures and Retrofit Challenges in Skid Steers

[MikePhua]

The Role of Gear Pumps in Compact Equipment
Gear pumps are the heart of many hydraulic systems in compact machines like skid steers, loaders, and small excavators. Their simplicity, reliability, and cost-effectiveness make them ideal for high-pressure, low-flow applications. In a gear pump, two intermeshing gears rotate to create suction and discharge pressure, moving hydraulic fluid through the system. These pumps are typically mounted directly to the engine or transmission housing and are responsible for powering lift arms, steering, and auxiliary functions.
In older skid steer models—such as the Bobcat 843 series—gear pumps are often externally mounted and exposed to mechanical stress, vibration, and contamination. Over time, shafts can shear, seals degrade, and internal wear reduces efficiency. When a pump fails, replacement is not always straightforward, especially when aftermarket parts differ slightly in design.
Common Failure Modes and Terminology
- Sheared Shaft: A condition where the pump’s input shaft breaks due to torsional overload or misalignment. This renders the pump inoperable and may damage surrounding components.
- Face Plate: The mounting surface of the pump that interfaces with the machine’s hydraulic manifold or bracket. Variations in face plate design can affect compatibility.
- Mounting Hole: A drilled or cast hole in the pump’s face plate used for alignment, fluid passage, or securing the pump to the housing.
- Part Number Matching: The process of identifying replacement pumps based on manufacturer codes. Even with identical part numbers, physical differences may exist due to production changes or regional variants.
In one case, a technician removed a failed gear pump with a sheared shaft and discovered a drilled hole in the face plate—likely used for alignment or fluid routing. A replacement pump with the same part number lacked this hole, raising concerns about compatibility.
Can You Swap Face Plates Between Gear Pumps
Swapping face plates between gear pumps is technically possible but requires caution. The face plate is not merely a cosmetic component—it may contain fluid channels, pressure relief ports, or alignment features critical to system performance. If the replacement pump lacks these features, retrofitting the original face plate could restore functionality, but only if:

• Bolt patterns and gasket surfaces match precisely
  
• Internal gear alignment remains unaffected
  
• No fluid bypass or leakage occurs at the interface
  

Before attempting a swap, technicians should inspect both pumps for:

• Shaft diameter and spline count
  
• Gear tooth profile and pitch
  
• Mounting flange thickness
  
• Seal compatibility and pressure ratings
  

If the original face plate contains a drilled hole for fluid passage, omitting it could restrict flow or cause cavitation. In such cases, machining the new pump to match the original may be safer than swapping components.
Manufacturer Background and Pump Sourcing
The Bobcat 843 skid steer was produced in the 1980s and early 1990s by Melroe Company, which later became part of Ingersoll Rand and eventually Doosan. The 843 was known for its robust frame, mechanical simplicity, and ease of service. Gear pumps used in this model were often sourced from manufacturers like Cessna, Parker, or Permco—companies with long histories in hydraulic component design.
Cessna, for example, began producing hydraulic pumps in the 1950s and became a major supplier for agricultural and construction equipment. Their gear pumps are known for modular construction, allowing for field repairs and component swaps. However, production changes over decades mean that identical part numbers may not guarantee identical geometry.
Field Anecdotes and Practical Advice
One operator in Idaho reported that his pump shaft sheared during cold weather startup. The hydraulic oil had thickened, increasing resistance and torque on the shaft. After removing the pump, he noticed a drilled hole in the face plate—absent in the replacement unit. Rather than risk misalignment, he considered transferring the original face plate, but ultimately opted to machine the new pump to match the old geometry.
Another technician in Alberta shared that aftermarket pumps often omit minor features like alignment holes or pressure relief ports. He recommends always comparing old and new units side-by-side and consulting manufacturer drawings when available.
Recommendations for Gear Pump Replacement
To ensure successful gear pump replacement:

• Match part numbers but verify physical dimensions and features
  
• Inspect face plates for fluid passages, alignment holes, and bolt patterns
  
• Consider machining new pumps to match old geometry if swapping is risky
  
• Use high-quality hydraulic oil rated for cold starts to prevent shaft shear
  
• Torque mounting bolts evenly and check for gasket compression
  

If the pump is part of a closed-loop system, ensure that relief valves and filters are functioning properly. A failed pump may indicate upstream issues like clogged filters, stuck valves, or contaminated fluid.
Conclusion
Gear pump replacement in older skid steers like the Bobcat 843 requires more than just matching part numbers. Subtle differences in face plate design, shaft geometry, and fluid routing can affect performance and reliability. With careful inspection, machining, and attention to hydraulic principles, technicians can restore functionality and extend the life of these rugged machines. In the world of compact equipment, precision matters—even when the parts seem simple.