Undercarriage and Track Systems

Undercarriage and Track Systems

Introduction to the Undercarriage

The undercarriage is the assembly of components that supports the weight of the machine, transmits driving power to the ground, and ensures stability and mobility. For a heavy duty equipment technician (HDET), mastering the undercarriage is essential, as it represents approximately 15 to 20% of a track machine's maintenance costs over its service life.

Two configurations dominate the market: track systems (excavators, bulldozers, track loaders) and wheel systems (wheel loaders, off-highway trucks). This chapter focuses primarily on track systems but also covers critical components of wheel undercarriages where relevant.

Main Components of a Track Undercarriage

A typical undercarriage includes:

Undercarriage Geometry and Alignment

Component Alignment

Correct alignment is critical to undercarriage service life. Poor alignment can cause premature wear on track shoe edges, joint overheating, and loss of traction efficiency.

Alignment checkpoints:

Camber and Caster Angles

For wheeled machines, the following alignment angles are controlled:

Track Chains and Shoes

Types of Track Chains

Two main configurations exist:

Track Pitch and Calculations

Track pitch is the distance between the centers of two consecutive pins. Common pitches are 101.6 mm (4 inches), 152.4 mm (6 inches), 171.45 mm (6.75 inches), and 203.2 mm (8 inches).

Calculating the number of links:

Number of links = (Distance between sprocket/idler centers × 2) + (Sprocket diameter + Idler diameter) × π / Pitch

Example: For a machine with a center distance of 2500 mm, an 800 mm diameter sprocket, a 600 mm idler, and a 171.45 mm pitch:

Links = (2500 × 2) + (800 + 600) × π / 171.45

Links = 5000 + 1400 × 3.1416 / 171.45

Links = 5000 + 4398.24 / 171.45

Links = 9398.24 / 171.45 ≈ 54.8 → 55 links

Track Shoes: Width and Types

Track shoe width affects ground pressure and flotation:

Ground pressure = Machine weight / (Shoe width × Ground contact length)

Example: A 30,000 kg excavator with 600 mm wide shoes and a 3000 mm contact length:

Pressure = 30,000 × 9.81 / (0.6 × 3.0) = 294,300 / 1.8 = 163,500 Pa ≈ 163.5 kPa

Sprocket and Final Drive

Sprocket Tooth Profile

The sprocket transmits power to the chain. The teeth engage with the chain bushings. Sprocket wear is indicated by:

Rule of thumb: The sprocket should be replaced when tooth wear reaches approximately 15 to 20% of the original dimension. Measurement is done using a tooth depth gauge.

Final Drive

The final drive is the last reduction stage between the engine and the sprocket. It typically includes:

Typical reduction ratio: 30:1 to 60:1 for excavators.

Final drive oil checks:

Track Tension

Importance of Correct Tension

Incorrect tension is the most frequent cause of premature undercarriage wear.

Tension Adjustment Procedure

Typical sag values:

Measurement formula:

Sag = Vertical distance between the lowest point of the chain and the straight line between the two support points

Hydraulic Track Adjuster

The hydraulic adjuster uses a hydraulic cylinder with a spring cartridge. Grease is injected through a grease fitting to push the piston and tension the chain.

Greasing procedure:

Caution: Never loosen the adjuster relief valve without first relieving chain tension. The spring can eject the plug with force.

Track Rollers and Carrier Rollers

Track Rollers

Track rollers support the machine's weight and guide the chain. They are mounted on shafts with bearings and seals.

Roller types:

Roller wear:

Carrier Rollers

Carrier rollers support the upper portion of the chain and reduce whipping movement. They are less stressed than track rollers but must be inspected regularly.

Inspection points:

Idler and Track Adjuster

Idler

The idler guides the chain at the front (or rear, depending on the machine) and works with the adjuster to maintain tension.

Idler types:

Idler wear:

Track Adjuster

The track adjuster combines a hydraulic cylinder and a spring. The spring absorbs shocks and maintains minimum tension, while the cylinder allows fine adjustment.

Adjuster components:

Pressure release procedure:

Undercarriage Wear: Measurement and Evaluation

Measurement Methods

Wear evaluation is essential for planning replacements and avoiding costly breakdowns.

Track Chain Wear Calculation

Chain elongation:

Elongation (%) = (Measured length - Original length) / Original length × 100

Replacement criteria: The chain must be replaced when elongation exceeds 3% (some manufacturers recommend 2%).

Example: A new chain measures 1000 mm over 4 links. After 2000 hours, it measures 1030 mm.

Elongation = (1030 - 1000) / 1000 × 100 = 3%

The chain must be replaced.

Component Wear Relationships

Undercarriage component wear is interdependent. A worn chain accelerates sprocket wear and vice versa.

The "50-50" rule: If both the sprocket and chain are worn to 50% or more, it is generally more economical to replace both together. A new component with a worn component will wear prematurely.

Lubrication and Maintenance

Lubrication Points

The undercarriage requires regular lubrication to reduce wear and friction.

Oil Analysis

Oil analysis is an essential predictive maintenance tool:

Typical alert thresholds (final drive):

Safety and Procedures

Safety Rules

Track Shoe Replacement Procedure

Typical torque values:

Diagnostics and Troubleshooting

Common Symptoms and Causes

Abnormal Noises

Canadian Standards and Regulations

Applicable Standards

Although the undercarriage is not directly covered by electrical or gas standards, several Canadian standards apply to machinery and its maintenance:

Occupational Safety Requirements

Common Pitfalls to Avoid

Summary

The undercarriage is a complex system whose proper maintenance is essential to the longevity and performance of track machines. Key points to remember:

Mastery of these concepts will enable the candidate to diagnose problems effectively, plan maintenance, and perform repairs in accordance with industry standards.