8 hours ago
The Bobcat S750 and Its Role in Compact Equipment
The Bobcat S750 skid-steer loader was introduced in the early 2010s as part of Bobcat’s M-Series, designed to offer improved visibility, cab comfort, and hydraulic performance. With a rated operating capacity of 3,200 lbs and a turbocharged 85 hp diesel engine, the S750 quickly became a favorite among contractors for grading, demolition, and material handling. Its vertical lift path and high-flow hydraulic options made it suitable for demanding attachments like cold planers and hydraulic breakers.
Bobcat, founded in North Dakota in the 1950s, has sold hundreds of thousands of skid steers globally. The S750, in particular, saw strong adoption in North America and Europe, especially in rental fleets and municipal operations.
Unusual Hydraulic Oil Loss and Chain Case Contamination
A recurring issue reported by operators involves hydraulic oil mysteriously disappearing without visible external leaks. In one case, a 2011 S750 with only 350 hours began losing oil gradually—about 5 gallons over 100 hours. No puddles or drips were found beneath the machine, and the hydraulic system appeared sealed.
Upon inspection, the oil was discovered inside the chain case—a sealed compartment housing the drive chains and sprockets. This is not a normal destination for hydraulic fluid, which should remain isolated within the reservoir, hoses, and valve bodies.
Terminology Clarification
Initial suspicion fell on the hydraulic motors, but further analysis pointed to the gearbox seals. These seals sit between the hydraulic motor and the chain case, preventing fluid from migrating into the drive compartment. When these seals fail, hydraulic oil can seep into the chain case unnoticed.
In this case, both sides of the machine were suspected to be leaking, though the rate was slow—roughly one quart every five hours. To isolate the faulty side, technicians suggested elevating the machine on stands, removing the chain case inspection cover, and running each side independently. This method allows visual confirmation of which motor is leaking based on oil accumulation.
Field Wisdom and Practical Advice
Experienced mechanics note that when one gearbox seal fails, the other is often close behind. This is especially true in machines exposed to vibration, temperature swings, or aggressive attachment use. In Indiana, a contractor running a hydraulic breaker on an S770 noticed similar symptoms. After replacing one seal, the other failed within 50 hours.
Another operator in Alberta used dye tracing fluid in the hydraulic system to confirm the leak path. The dye appeared in the chain case after several hours of operation, confirming seal failure without disassembling the motor.
Repair Strategy and Preventive Measures
Replacing gearbox seals requires removing the hydraulic motor and accessing the seal interface. While not overly complex, it demands precision and cleanliness to avoid contaminating the hydraulic system.
Recommended steps include:
Hydraulic oil leaking into the chain case of a Bobcat S750 is a subtle but serious issue that can compromise both drive performance and hydraulic integrity. While the root cause often lies in gearbox seal failure, early detection and methodical diagnostics can prevent costly repairs and downtime. With proper maintenance and field awareness, operators can keep their machines running smoothly—and ensure that hydraulic oil stays where it belongs.
The Bobcat S750 skid-steer loader was introduced in the early 2010s as part of Bobcat’s M-Series, designed to offer improved visibility, cab comfort, and hydraulic performance. With a rated operating capacity of 3,200 lbs and a turbocharged 85 hp diesel engine, the S750 quickly became a favorite among contractors for grading, demolition, and material handling. Its vertical lift path and high-flow hydraulic options made it suitable for demanding attachments like cold planers and hydraulic breakers.
Bobcat, founded in North Dakota in the 1950s, has sold hundreds of thousands of skid steers globally. The S750, in particular, saw strong adoption in North America and Europe, especially in rental fleets and municipal operations.
Unusual Hydraulic Oil Loss and Chain Case Contamination
A recurring issue reported by operators involves hydraulic oil mysteriously disappearing without visible external leaks. In one case, a 2011 S750 with only 350 hours began losing oil gradually—about 5 gallons over 100 hours. No puddles or drips were found beneath the machine, and the hydraulic system appeared sealed.
Upon inspection, the oil was discovered inside the chain case—a sealed compartment housing the drive chains and sprockets. This is not a normal destination for hydraulic fluid, which should remain isolated within the reservoir, hoses, and valve bodies.
Terminology Clarification
- Chain Case: A sealed compartment on skid steers that houses the drive chains connecting the hydraulic motors to the wheels.
- Hydraulic Motor: A device that converts hydraulic pressure into rotational motion to drive the wheels.
- Gearbox Seal: A seal that prevents hydraulic fluid from leaking between the motor and the chain case.
- Pilot Control: A low-pressure hydraulic system that actuates the main valves based on joystick input.
Initial suspicion fell on the hydraulic motors, but further analysis pointed to the gearbox seals. These seals sit between the hydraulic motor and the chain case, preventing fluid from migrating into the drive compartment. When these seals fail, hydraulic oil can seep into the chain case unnoticed.
In this case, both sides of the machine were suspected to be leaking, though the rate was slow—roughly one quart every five hours. To isolate the faulty side, technicians suggested elevating the machine on stands, removing the chain case inspection cover, and running each side independently. This method allows visual confirmation of which motor is leaking based on oil accumulation.
Field Wisdom and Practical Advice
Experienced mechanics note that when one gearbox seal fails, the other is often close behind. This is especially true in machines exposed to vibration, temperature swings, or aggressive attachment use. In Indiana, a contractor running a hydraulic breaker on an S770 noticed similar symptoms. After replacing one seal, the other failed within 50 hours.
Another operator in Alberta used dye tracing fluid in the hydraulic system to confirm the leak path. The dye appeared in the chain case after several hours of operation, confirming seal failure without disassembling the motor.
Repair Strategy and Preventive Measures
Replacing gearbox seals requires removing the hydraulic motor and accessing the seal interface. While not overly complex, it demands precision and cleanliness to avoid contaminating the hydraulic system.
Recommended steps include:
- Drain hydraulic fluid and clean the reservoir
- Remove the drive motor and inspect the seal surface for scoring
- Replace seals with OEM-grade components
- Flush the chain case and refill with appropriate gear oil
- Monitor fluid levels and chain case condition after reassembly
- Avoid overloading the machine with high-flow attachments
- Maintain proper hydraulic fluid levels and change intervals
- Inspect chain case regularly for signs of contamination
- Use infrared thermometers to monitor motor temperature during operation
- Log hydraulic fluid consumption and compare against usage hours
- Train operators to report subtle performance changes, such as sluggish travel or increased noise
- Stock seal kits for common models to reduce downtime
- Consider installing fluid sensors in the chain case for early leak detection
- Schedule seal inspections every 1,000 hours or during major service intervals
Hydraulic oil leaking into the chain case of a Bobcat S750 is a subtle but serious issue that can compromise both drive performance and hydraulic integrity. While the root cause often lies in gearbox seal failure, early detection and methodical diagnostics can prevent costly repairs and downtime. With proper maintenance and field awareness, operators can keep their machines running smoothly—and ensure that hydraulic oil stays where it belongs.
