Excavator Structure, Components, and Working Principles
Excavator Structure, Components, and Working Principles
This domain tests how the machine is built and how power reaches the bucket. Know the undercarriage, upper structure, front attachment, hydraulic circuit, and the devices that hold loads and limit motion.
Core rules
- Undercarriage: track chain, idler, drive sprocket, track rollers, and tensioner. The sprocket drives; the idler guides.
- Upper structure: house, counterweight, swing bearing, cab, engine, hydraulic tank, and pumps.
- Front attachment: boom, stick (arm), bucket, pins, bushings, cylinders, and linkage.
- Hydraulic flow: engine drives main pumps; pilot pressure shifts control spools; main relief sets maximum pressure; load-holding valves stop drift; swing motor and brake control rotation.
- Counterweight balances the machine, but lift capacity still falls as load radius increases. Always use the load chart.
- Safety devices: ROPS/FOPS, seatbelt, travel alarm, horn, lockout lever, and boom drift reduction valve.
- Speed comes from flow; force comes from pressure and piston area.
Worked example
A boom cylinder has a 150 mm bore and is supplied at 25 MPa. Reasoning: area = π × (0.075 m)² = 0.0177 m²; force = 25,000,000 Pa × 0.0177 m² ≈ 442 kN. Flow does not set this force; it sets rod speed. If the cylinder is weak, check pressure at the cylinder, relief leakage, and internal bypass-not just pump flow.
Traps
- Confusing boom and stick when tracing linkage.
- Believing a heavier counterweight raises capacity at every radius.
- Ignoring pilot pressure when the main pump is fine.
- Treating a drifting cylinder as normal.