An excavator operator complained that the machine was "lagging." When he pulled the joystick to curl the stick in, there was a distinct half-second pause before the cylinder moved. It made precision digging nearly impossible. The mechanic suspected a worn pump or sticky main valve spools.
We traced the pilot circuit. Pilot lines are small, low-pressure hoses that transmit the operator's signal to the main valve spools. Over time, the rubber in these hoses degrades due to heat and pressure cycling. The inner rubber liner can swell, effectively reducing the inside diameter of the hose.
When the joystick is moved, pilot fluid rushes into the hose. Because the inner diameter has swollen, the fluid velocity is restricted. It takes time to build enough pressure to overcome the centering spring of the main spool. That time delay is the "lag" the operator feels. We replaced the pilot hoses, and the machine responded instantly to the joystick inputs. On machines with more than 5,000 hours, replacing the pilot hose set is a low-cost maintenance item that can dramatically improve the "feel" of the machine.
