A brick distribution center faced a dangerous and frustrating issue with a Linde H25D internal combustion forklift. When the operator attempted to lift a full load of bricks, the mast would rise a few inches, stall completely, and then drop back down two inches before catching. Accompanying this lift failure was a loud, high-pitched whining noise emanating from the hydraulic compartment. The steering functioned normally, indicating the auxiliary pump circuit was intact.
The whining noise was a classic symptom of hydraulic pump cavitation-fluid starvation. The technician first checked the hydraulic oil level, which was correct, and inspected the tank breather cap, which was clear. A flow meter was connected to the pump outlet. When the hoist lever was deadheaded, the pump delivered a maximum of 8 GPM, a severe drop from the rated 22 GPM, and the pressure topped out at only 1,800 psi instead of the 3,000 psi system relief setting.
The Linde H25D utilizes a load-sensing, variable displacement axial piston pump. The pump relies on a compensator spool to sense system pressure and destroke the pump when the pressure reaches the set limit. The technician removed the compensator housing. Inside, the control spool was stuck in the fully destroked position. The spool was heavily varnished, and the centering spring was found broken into three pieces. With the spool stuck open, the pump was dumping its flow directly back to the tank internally, preventing pressure from building to lift the load.
However, the broken compensator spring did not explain the loud whining noise or the initial two-inch load drop. The suction line filter, located inside the hydraulic tank, was pulled for inspection. It was discovered that the inner core of the suction filter had collapsed inward. The filter element was so saturated with microscopic debris-mostly brass shavings from a previously failed tilt cylinder-that it had become a restriction. When the operator pulled the hoist lever, the pump tried to pull a high volume of fluid through the restricted filter, causing the fluid to vaporize (cavitate) in the suction line. The vapor bubbles would collapse violently when pressurized in the pump, creating the whining noise and causing momentary pressure drops that forced the mast to drop.
The repair required a complete hydraulic system flush to remove the suspended brass contamination. The collapsed suction filter was replaced with a high-capacity aftermarket unit, and the main return filter was also changed. The pump compensator was rebuilt with a new spool and centering spring. After bleeding the suction line to ensure the pump was primed, the forklift lifted a maximum capacity load smoothly and quietly, with system pressure holding rock-steady at 3,000 psi.
