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The CAT 320BL and Its Hydraulic Swing System
The Caterpillar 320BL hydraulic excavator was introduced in the late 1990s as part of the B-series lineup, offering improved cab comfort, electronic control, and refined hydraulic performance. With an operating weight around 44,000 lbs and a 138 hp engine, the 320BL became a staple in earthmoving and demolition fleets. Caterpillar, founded in 1925, has sold hundreds of thousands of 320-series machines globally, and the B-series remains widely supported.
The swing motor on the 320BL is a hydraulic piston-type motor that rotates the upper structure. It relies on a cushion valve system, case drain routing, and replenishing pressure to prevent cavitation and overheating. When these systems fail or are misconfigured, catastrophic damage can occur—including slipper shoe failure, gear destruction, and complete lockup.
Terminology Notes
One 320BL experienced five consecutive swing motor failures. The most recent motor was rebuilt with new parts and tested by a CAT mechanic. Drive motors and swivel joints showed no issues. However, pilot pressure was initially set at 400 psi and later corrected to 650 psi. Case drain pressure spiked intermittently to 150 psi during multi-function operations (boom, dipper, bucket), though it remained around 50 psi during isolated movements.
The motor was rerouted to drain directly into the tank below fluid level to maintain wet conditions and reduce cavitation risk. After 50 hours of operation, the motor failed again—first showing swing drift, then grinding noises, followed by lockup.
Common Causes of Swing Motor Destruction
One operator on a CAT 322B destroyed two swing motors in two weeks due to a plugged case drain filter. He emphasized cutting open the old filter to inspect for debris and checking pressure with a tee’d gauge—not a blocked port.
A technician in Missouri explained that replenishing pressure is not the same as pilot pressure. Replenishing comes from the main pump’s return flow and is regulated by relief valves. If fluid bypasses the cooler due to cold temperature or open relief valves, pressure drops and cavitation occurs.
Another mechanic warned that thermoplastic case drain hoses can fail internally, causing hidden restrictions. He recommended replacing hoses with reinforced hydraulic lines and inspecting swivel seals for leakage.
Preventive Measures and Recommendations
Repeated swing motor failures on a CAT 320BL are often caused by high case drain pressure, incorrect hose routing, and insufficient replenishing pressure. Cavitation destroys internal components like slipper shoes, and residual debris can sabotage rebuilt units. By replacing filters, verifying pressure, and flushing the system thoroughly, operators can restore reliability and avoid costly downtime. The 320BL’s hydraulic system is robust—but only when its pressure pathways are clean, balanced, and correctly configured.
The Caterpillar 320BL hydraulic excavator was introduced in the late 1990s as part of the B-series lineup, offering improved cab comfort, electronic control, and refined hydraulic performance. With an operating weight around 44,000 lbs and a 138 hp engine, the 320BL became a staple in earthmoving and demolition fleets. Caterpillar, founded in 1925, has sold hundreds of thousands of 320-series machines globally, and the B-series remains widely supported.
The swing motor on the 320BL is a hydraulic piston-type motor that rotates the upper structure. It relies on a cushion valve system, case drain routing, and replenishing pressure to prevent cavitation and overheating. When these systems fail or are misconfigured, catastrophic damage can occur—including slipper shoe failure, gear destruction, and complete lockup.
Terminology Notes
- Slipper Shoes: Internal components in piston motors that ride against the swash plate, critical for smooth operation.
- Case Drain: A low-pressure return line that allows internal leakage and cooling fluid to exit the motor.
- Replenishing Pressure: The pressure required to keep the motor’s internal cavities filled with fluid, preventing cavitation.
- Pilot Pressure: Low-pressure hydraulic signal used to actuate valves and controls.
One 320BL experienced five consecutive swing motor failures. The most recent motor was rebuilt with new parts and tested by a CAT mechanic. Drive motors and swivel joints showed no issues. However, pilot pressure was initially set at 400 psi and later corrected to 650 psi. Case drain pressure spiked intermittently to 150 psi during multi-function operations (boom, dipper, bucket), though it remained around 50 psi during isolated movements.
The motor was rerouted to drain directly into the tank below fluid level to maintain wet conditions and reduce cavitation risk. After 50 hours of operation, the motor failed again—first showing swing drift, then grinding noises, followed by lockup.
Common Causes of Swing Motor Destruction
- High Case Drain Pressure: Should remain below 50 psi. Spikes to 150 psi indicate restriction or backpressure.
- Plugged Case Drain Filter: A clogged filter (e.g., CAT part 5I-7950) can cause pressure buildup and destroy slipper shoes.
- Incorrect Hose Identification: Misidentifying replenishing vs case drain lines leads to improper routing and pressure loss.
- Low Replenishing Pressure: Without sufficient pressure, cushion valves cavitate, leading to metal-on-metal contact.
- Residual Debris After Failure: Incomplete cleaning after motor failure can contaminate the new unit.
One operator on a CAT 322B destroyed two swing motors in two weeks due to a plugged case drain filter. He emphasized cutting open the old filter to inspect for debris and checking pressure with a tee’d gauge—not a blocked port.
A technician in Missouri explained that replenishing pressure is not the same as pilot pressure. Replenishing comes from the main pump’s return flow and is regulated by relief valves. If fluid bypasses the cooler due to cold temperature or open relief valves, pressure drops and cavitation occurs.
Another mechanic warned that thermoplastic case drain hoses can fail internally, causing hidden restrictions. He recommended replacing hoses with reinforced hydraulic lines and inspecting swivel seals for leakage.
Preventive Measures and Recommendations
- Replace Case Drain Filter During Every Motor Swap: Cut open the old filter to inspect for contamination.
- Verify Hose Routing: Use diagrams or port markings (e.g., “MU”) to confirm correct connections.
- Monitor Case Drain Pressure Under Load: Stall travel motors and observe pressure rise.
- Flush System Thoroughly After Failure: Remove all debris before installing a new motor.
- Use Tee Fittings for Pressure Testing: Avoid dead-end gauges that misread pressure.
Repeated swing motor failures on a CAT 320BL are often caused by high case drain pressure, incorrect hose routing, and insufficient replenishing pressure. Cavitation destroys internal components like slipper shoes, and residual debris can sabotage rebuilt units. By replacing filters, verifying pressure, and flushing the system thoroughly, operators can restore reliability and avoid costly downtime. The 320BL’s hydraulic system is robust—but only when its pressure pathways are clean, balanced, and correctly configured.
