10-22-2025, 06:49 PM
Bobcat 320 Mini Excavator Overview
The Bobcat 320 is a compact mini excavator introduced in the late 1990s, designed for tight-access digging, trenching, and light demolition. With an operating weight of approximately 3,600 pounds and a dig depth of around 7 feet, it is powered by a small diesel engine and features a swing boom, rubber tracks, and pilot-operated joystick controls. Its compact footprint and mechanical simplicity made it a popular choice for landscaping, utility work, and small-scale construction.
Bobcat Company, founded in 1947, has long been a leader in compact equipment. The 320 was one of its early mini excavator models, and while no longer in production, many units remain in use due to their reliability and ease of maintenance.
Swing Circle Noise Symptoms and Observations
After performing a full service—including oil and filter changes—a 1999 Bobcat 320 with approximately 3,500 hours began exhibiting a distinct rattling noise during swing operations. The noise occurs specifically when the upper structure is rotated to the right and the joystick is released gradually, allowing the swing valve to close slowly. If the joystick is released abruptly, the overpressure relief valve activates with a characteristic whistle, which is expected behavior.
The rattling noise is not present during rapid or aggressive swing stops, only during smooth deceleration. This suggests the issue is not mechanical backlash or gear damage, but rather a hydraulic or damping-related anomaly.
Terminology Notes
In Bavaria, a contractor operating a similar Bobcat 320 noticed a comparable noise after replacing the swing motor seals. The issue turned out to be a misaligned cushioning valve spring, which caused inconsistent damping during swing deceleration. After reseating the spring and flushing the swing circuit, the noise disappeared.
Recommendations for Technicians and Owners
Rattling noises during swing deceleration on a Bobcat 320 are often linked to hydraulic cushioning issues rather than mechanical failure. By focusing on the swing motor’s damping components and ensuring clean, properly pressurized fluid, operators can restore smooth swing performance. With over two decades of service, the 320 remains a capable machine when maintained with attention to detail.
The Bobcat 320 is a compact mini excavator introduced in the late 1990s, designed for tight-access digging, trenching, and light demolition. With an operating weight of approximately 3,600 pounds and a dig depth of around 7 feet, it is powered by a small diesel engine and features a swing boom, rubber tracks, and pilot-operated joystick controls. Its compact footprint and mechanical simplicity made it a popular choice for landscaping, utility work, and small-scale construction.
Bobcat Company, founded in 1947, has long been a leader in compact equipment. The 320 was one of its early mini excavator models, and while no longer in production, many units remain in use due to their reliability and ease of maintenance.
Swing Circle Noise Symptoms and Observations
After performing a full service—including oil and filter changes—a 1999 Bobcat 320 with approximately 3,500 hours began exhibiting a distinct rattling noise during swing operations. The noise occurs specifically when the upper structure is rotated to the right and the joystick is released gradually, allowing the swing valve to close slowly. If the joystick is released abruptly, the overpressure relief valve activates with a characteristic whistle, which is expected behavior.
The rattling noise is not present during rapid or aggressive swing stops, only during smooth deceleration. This suggests the issue is not mechanical backlash or gear damage, but rather a hydraulic or damping-related anomaly.
Terminology Notes
- Swing Circle: The slewing bearing and gear assembly that allows the upper structure to rotate on the undercarriage.
- Swing Motor: The hydraulic motor that drives the swing gear.
- Relief Valve: A pressure-limiting valve that protects hydraulic circuits from overpressure.
- Cushioning Circuit: A hydraulic feature that slows down movement at the end of stroke to reduce shock.
- Swing Motor Cushion Valve Wear: If the cushioning valve is worn or sticking, it may fail to dampen deceleration smoothly, causing a rattling or chattering noise as the swing slows.
- Hydraulic Bypass Flow Pulsation: When the joystick is released slowly, the valve may partially close, creating turbulent flow or pressure fluctuations that resonate through the swing gear.
- Loose Swing Gear Bolts or Bearing Wear: Although less likely given the noise only occurs during smooth stops, it’s worth checking for play in the swing bearing or loose bolts.
- Contaminated Hydraulic Oil: Even after a fluid change, residual debris or varnish in the swing circuit could affect valve performance.
In Bavaria, a contractor operating a similar Bobcat 320 noticed a comparable noise after replacing the swing motor seals. The issue turned out to be a misaligned cushioning valve spring, which caused inconsistent damping during swing deceleration. After reseating the spring and flushing the swing circuit, the noise disappeared.
Recommendations for Technicians and Owners
- Inspect Swing Motor Cushion Valve: Remove and clean the valve, checking for spring tension and spool movement.
- Check Hydraulic Pressure During Swing: Use a pressure gauge to monitor fluctuations during joystick release.
- Verify Swing Bearing Play: Lift the upper structure and check for axial or radial movement.
- Flush Hydraulic System Thoroughly: Especially if the machine sat idle before service.
- Use OEM Hydraulic Oil and Filters: Compatibility ensures proper valve function and lubrication.
Rattling noises during swing deceleration on a Bobcat 320 are often linked to hydraulic cushioning issues rather than mechanical failure. By focusing on the swing motor’s damping components and ensuring clean, properly pressurized fluid, operators can restore smooth swing performance. With over two decades of service, the 320 remains a capable machine when maintained with attention to detail.
