An aggregate processing facility reported that a Doosan DL380-5 wheel loader took 8 seconds to raise an empty boom (normal is 4 seconds). Additionally, when turning the steering wheel, the wheel violently chattered and jumped, and a loud clanking noise was heard from the rear of the engine compartment.
The technician first checked the main hydraulic pressure at the boom cylinder, which only reached 120 bar (spec is 250 bar). The violent steering chatter usually indicates air in the system or a failing Orbitrol (steering control unit). He opened the hydraulic tank cap and found the oil was severely aerated (milky and full of foam). He traced the clanking noise to the tandem gear pump mounted on the transmission housing.
This loader uses a "Tandem Gear Pump"-a single input shaft driving two separate pump sections. The front section supplies the steering circuit, and the rear section supplies the boom and bucket implement circuit. He removed the pump assembly and disassembled the coupling between the two sections. The internal splined drive shaft connecting the front pump to the rear pump had completely sheared off.
The engine was driving the front (steering) pump, but the splined coupling that transferred torque to the rear (implement) pump was broken, meaning the rear pump was not turning. This explained the slow boom raise. However, the broken splines were jagged, and as the front pump turned, the jagged edges occasionally caught and spun the rear pump intermittently, creating the clanking noise. Furthermore, the broken spline allowed the drive shaft to wobble, destroying the pump's front shaft seal. The wobbling shaft drew air into the steering pump inlet, causing severe cavitation. The aerated oil was fed into the Orbitrol, causing the steering rotor to bind and release rapidly, resulting in the violent steering wheel chatter.
The repair involved replacing the entire tandem gear pump assembly (the internal spline wear was not field-repairable). The hydraulic tank was drained, and the system was flushed to remove the aerated, degraded oil. The Orbitrol was inspected; miraculously, the internal rotor was undamaged, but the O-rings had been shredded by the aerated oil and were replaced. Fresh ISO 68 hydraulic oil and a new return filter were installed. After bleeding the steering lines, the boom raised in 4 seconds, and the steering operated smoothly with no chatter.
