Excavator Systems and Components

Excavator Systems and Components

Chapter Introduction

This chapter covers all excavator hydraulic systems and components, as required by the Interprovincial Red Seal program. You must master not only the function of each component, but also the physical principles governing their operation, maintenance procedures, and applicable Canadian standards. This chapter is designed to be studied sequentially; each section builds on the previous one.

1. Excavator Structure and Chassis

1.1 Chassis Types

The excavator comes in two main chassis configurations: tracked and wheeled. The choice depends on soil conditions, required mobility, and available ground bearing capacity.

Trap to Avoid: Do not confuse ground pressure with total weight. Ground pressure is calculated by dividing the machine's weight by the track contact area. A 30-tonne tracked excavator can exert less ground pressure than a pickup truck.

1.2 Track Undercarriage

The undercarriage includes: the track frame, idlers, track rollers, carrier rollers, and the track chain with its links, bolts, and bushings.

Track tension is critical. Insufficient tension causes derailment; excessive tension accelerates bushing and pin wear. The standard inspection procedure:

Golden Rule: A properly tensioned track should be liftable 25 to 40 mm above the top carrier roller when suspended.

1.3 Counterweight and Stability

The rear counterweight is essential to machine stability. Its mass typically ranges from 2 to 8 tonnes depending on the model. Stability is governed by the moment of force principle (M = F × d). The stabilizing moment of the counterweight must always exceed the overturning moment created by the load at the end of the arm.

Practical Calculation: If an excavator has a 3,000 kg counterweight located 2.5 m behind the rotation axis, the stabilizing moment is:

M = 3,000 kg × 9.81 m/s² × 2.5 m = 73,575 N·m

If the bucket load is 1,500 kg at a 6 m reach, the overturning moment is:

M = 1,500 × 9.81 × 6 = 88,290 N·m

In this case, the machine is unstable. You must reduce the reach or the load. This simplified calculation ignores the self-weight of the boom and arm, which also contribute to stability (positively or negatively depending on position).

Applicable Standard: Excavator stability is covered by CSA B352.0 (Roll-over protective structures and falling object protective structures for construction machinery). Although this standard primarily addresses protective structures, it imposes static stability tests.

2. Main Hydraulic System

2.1 Fundamental Principles

The excavator's hydraulic system operates according to Pascal's principle: pressure applied to a confined fluid is transmitted undiminished in all directions. The fundamental relationship is:

P = F / A

where P is pressure (Pa), F is force (N), A is area (m²).

Flow rate (Q) is the volume of fluid displaced per unit of time (L/min). Speed of a cylinder is directly proportional to flow rate, while force is proportional to pressure.

2.2 Hydraulic Pumps

Modern excavators use variable-displacement axial piston pumps. These pumps adjust their displacement based on demand, optimizing fuel consumption and reducing heat generation.

Typical Working Pressure: Main circuits operate at 350 bar (5,000 psi) at peak. Pilot circuits operate at 30–50 bar.

2.3 Hydraulic Cylinders

Cylinders convert hydraulic energy into linear mechanical force. The force of a cylinder is calculated:

F = P × A_effective

For a double-acting cylinder, the effective area on the rod side is reduced by the rod cross-section:

A_effective = (π/4) × (D² − d²)

where D is the cylinder bore diameter and d is the rod diameter.

Example: Bucket cylinder with D = 120 mm, d = 60 mm, P = 300 bar (30,000 kPa):

A = (π/4) × (0.12² − 0.06²) = 0.785 × (0.0144 − 0.0036) = 0.00848 m²

F = 30,000,000 Pa × 0.00848 m² = 254,400 N ≈ 25.9 tonnes

Trap to Avoid: The pull force (retraction) is always lower than the push force (extension) due to the reduced area from the rod. Never confuse the two in lifting capacity calculations.

2.4 Control Valves and Directional Valves

The main control valve groups several sections, each controlling a cylinder or motor. The spools are moved by hydraulic pilot circuits or electrohydraulic systems.

Relief valves protect the system. The main relief valve is set to the system's maximum pressure (typically 350 bar). Secondary valves protect each individual circuit.

Anti-drop Function: The counterbalance valve prevents uncontrolled boom descent in the event of hose failure. It is mandatory on lifting circuits.

2.5 Hydraulic Fluid

Hydraulic fluid must meet manufacturer specifications. Typical viscosity grades are ISO VG 32, 46, or 68 depending on ambient temperature.

Contamination: Fluid cleanliness is critical. Abrasive particles cause premature wear of pumps and valves. The reference standard is ISO 4406 for particle counting. A healthy system should maintain a contamination level of 18/16/13 or better.

Sampling Procedure: Take the sample after the machine has reached operating temperature, at the filter return line, using a sterile kit. Have it analyzed in a laboratory at regular intervals (250–500 hours).

3. Boom and Arm Components

3.1 Attachment Geometry

The work attachment consists of three articulated elements: the boom, the stick/arm, and the bucket. The pivot points are equipped with bushings and pins that require regular lubrication.

Working angles are critical:

3.2 Kinematics and Reach

Maximum reach and maximum digging depth are determined by boom and arm lengths. Manufacturers provide load charts that indicate lifting capacity based on reach and height.

Reading a Load Chart: Capacities are shown in tonnes for each combination of reach (horizontal) and height (vertical). Values are given for the machine on tracks, stabilized, on firm, level ground.

Reduction Factors: When the machine is equipped with a long arm or long boom, capacity is reduced. A factor of 10–20 % is typical for each length extension.

3.3 Buckets and Attachments

Fill Factor: The actual excavated volume depends on the soil's swell factor. Compacted clay soil has a swell of 30–40%, meaning 1 m³ of in-situ soil becomes 1.3–1.4 m³ once excavated. The bucket fills with swollen soil, so bucket capacity must be chosen accordingly.

4. Swing System (Turntable)

4.1 Swing Bearing

The swing bearing allows the superstructure to rotate relative to the chassis. It consists of an inner ring and an outer ring with raceways for balls or rollers.

The bearing is lubricated through a central greasing system. The bearing backlash must be checked periodically. Excessive backlash indicates wear of the raceways or rolling elements.

Backlash Inspection Procedure:

4.2 Swing Motor and Gearbox

The swing motor is an axial piston hydraulic motor, often coupled to a planetary gearbox. Typical swing torque ranges from 50,000 to 150,000 N·m depending on machine size.

The anti-rebound valve on the swing circuit prevents jerking at the end of travel. The swing brake is normally a spring-applied disc brake, hydraulically released.

Trap to Avoid: The swing brake is not designed to stop a moving machine. It is only for holding the superstructure in a fixed position. The operator must decelerate gradually before engaging the brake.

5. Travel System

5.1 Travel Motors

Travel motors are axial piston hydraulic motors mounted in each track. They are equipped with spring-applied parking brakes and counterbalance valves to control descent on slopes.

Travel speed is controlled by the flow supplied to the motors. Most excavators offer two speed ranges:

5.2 Braking and Slope Safety

On slopes, braking is provided by the counterbalance valves which limit descent speed. The parking brake engages automatically when pilot pressure is cut off.

Safety Rule: When descending, the boom should be oriented downhill and the bucket lowered near the ground. When ascending, the boom should be oriented uphill. This rule maximizes stability and braking capacity.

6. Electrical System

6.1 Main Components

The excavator's electrical system includes:

6.2 Electrical Standards

The Canadian Electrical Code, Part I (C22.1-21) applies to building electrical installations, but excavators are covered by specific standards for mobile vehicles and machinery. CSA C22.2 No. 0 (General requirements) and CSA C22.2 No. 14 (Industrial control equipment) apply to industrial electrical components.

For mobile machinery, CSA M424.0 (Earth-moving machinery – General safety requirements) covers electrical and electronic aspects.

Rule 8-200 of the Canadian Electrical Code concerns wiring methods, but it applies primarily to buildings. For excavators, refer to CSA M424.0 and manufacturer specifications.

6.3 Electrical Fault Diagnosis

Systematic Diagnostic Procedure:

Trap to Avoid: Never disconnect the battery while the engine is running. The resulting voltage spike can destroy the ECM and electronic sensors.

7. Cooling and Lubrication Systems

7.1 Engine Cooling

The diesel engine is liquid-cooled. The circuit includes: the radiator, thermostat, water pump, and fan (often hydraulic or viscous).

Normal operating temperature is 80–95 °C. Overheating can be caused by:

7.2 Hydraulic Cooling

Hydraulic fluid is cooled by an oil cooler, typically mounted in front of the radiator. Normal oil temperature is 50–70 °C. Excessive temperature (>80 °C) causes fluid degradation and accelerated component wear.

7.3 Chassis Lubrication

Grease fittings (zerks) must be lubricated every 8 hours (daily) with lithium or molybdenum grease, per manufacturer specifications. Critical points are:

8. Preventive Maintenance and Inspections

8.1 Daily Inspection (before start-up)

8.2 Periodic Maintenance

8.3 Applicable Safety Standards

Excavators are covered by several Canadian standards:

Important Rule: Any machine modification (adding counterweight, changing boom, installing attachments) must be approved by the manufacturer and documented. Unapproved modifications can invalidate CSA compliance and create safety hazards.

9. Practical Calculations for the Operator

9.1 Hourly Production Calculation

Theoretical hourly production is calculated:

P = (V × N × E) / T

where V = bucket volume (m³), N = number of cycles per hour, E = efficiency (0.7–0.9), T = cycle time (hours).

Example: 1.5 m³ bucket, 30-second cycle, 0.8 efficiency:

N = 3,600 s / 30 s = 120 cycles/hour

P = 1.5 × 120 × 0.8 = 144 m³/h

This calculation must be adjusted for the soil's swell factor. For soil with 30% swell, the production in bank volume is:

P_bank = 144 / 1.3 = 110.8 m³/h

9.2 Breakout Force Calculation

Breakout force is the maximum force the bucket can exert at the teeth. It depends on hydraulic pressure and lever arm.

F = (P × A × L_cylinder) / L_arm

where L_cylinder is the perpendicular distance between the cylinder's line of action and the pivot point, and L_arm is the distance between the pivot point and the bucket teeth.

9.3 Common Unit Conversions

Trap to Avoid: Do not confuse imperial gallons with US gallons. The difference is 20%. Always use the metric system in your calculations, unless explicitly stated otherwise.

10. Traps to Avoid

11. Summary

Key Points to Memorize for the Exam:

This chapter covers all the theoretical and practical knowledge required to pass the "Systems and Components" section of the Red Seal exam. Review each section, redo the calculations yourself, and consult manufacturer manuals for model-specific values. Mastering these concepts will enable you not only to pass the exam, but also to operate an excavator safely and efficiently.