A rock excavation contractor reported that a Caterpillar 320 excavator equipped with an H120 hydraulic hammer was striking rock weakly. The hammer fired at a much-reduced blow rate, then after about 10 minutes of operation, it stopped firing entirely and only produced a soft "chuffing" sound. The carrier excavator's boom, arm, and bucket functions were completely normal, indicating the main hydraulic system was healthy.
The technician first checked the carrier's hammer flow and pressure settings. The hammer flow was set correctly at 130 liters per minute, and the system pressure during hammer operation was 165 bar (spec is 160-170 bar), confirming the carrier was delivering proper hydraulic power. The fault was therefore isolated to the hammer itself.
He connected a nitrogen charging kit to the hammer's "Piston Return Chamber" (also called the backhead or nitrogen chamber). The nitrogen pressure gauge read only 4 bar (spec is 14-16 bar). The hammer's hydraulic piston is driven forward by hydraulic oil, but it is retracted by compressed nitrogen in the return chamber behind the piston. When the nitrogen charge is low, the piston cannot retract fully between strikes, dramatically reducing the blow energy and the blow rate. As the charge continues to leak down, the piston eventually fails to retract at all, and the hammer stops firing (the chuffing sound is oil flowing through the hammer without the piston cycling).
He attempted to recharge the chamber with nitrogen, but the pressure would not hold-it bled down within 30 seconds, indicating a leak. Upon disassembling the hammer's backhead, he found the "Piston Return Chamber Seal Kit" had failed: the O-ring sealing the backhead plug had extruded and cracked, and the check valve seal at the nitrogen charge port was also degraded. Additionally, the "Return-Line Filter" on the excavator's hammer return circuit was heavily clogged, causing backpressure that had further stressed the hammer's internal seals and accelerated the nitrogen leak.
The repair involved replacing the complete nitrogen chamber seal kit, recharging the piston return chamber to 15 bar with dry nitrogen, and replacing the clogged return-line filter element. After reassembly, the hammer fired at full blow rate and energy, breaking through the limestone formation at the expected production rate.
