Powertrain Systems (Transmissions, Drivelines, and Axles)

Transmission Systems (Gearboxes, Driveshafts, and Axles)

Chapter Introduction

This chapter covers all components of the power transmission system of heavy equipment, from the engine output to the wheel hubs. For the Red Seal exam, you must master not only the theory of the different types of gearboxes, but also diagnostic procedures, clearance measurements, gear ratios, and applicable safety standards. Mastery of this chapter typically represents 15 to 20% of the exam questions.

The Overall Function of the Transmission

The transmission's role is to:

The gear ratio (or reduction ratio) is the ratio between the input speed and the output speed. It is calculated as follows:

Ratio = Number of teeth on the driven gear ÷ Number of teeth on the driving gear

For a multiple gear train, the total ratio is the product of the individual ratios. For example, a transmission with a first gear ratio of 3.5:1 and an axle ratio of 4.88:1 gives a total ratio of 3.5 × 4.88 = 17.08:1.

Clutches

Single-Disc Friction Clutch

The friction clutch is the most common device on heavy equipment with manual transmissions. Its main components are:

The pedal free play must be checked regularly. Insufficient free play causes clutch slippage; excessive free play causes difficulty in engagement. The standard measurement is taken at the pedal: typical free play is 25 to 40 mm (1 to 1.5 inches) for a mechanical system.

Hydraulic Clutch (Torque Converter)

The torque converter is used in automatic transmissions and powershift transmissions. It consists of:

The converter multiplies torque at a ratio of up to 2.5:1 at full stall. The multiplication ratio gradually decreases until it reaches 1:1 when the turbine reaches approximately 90% of the pump speed. This point is called the coupling point.

Converter slippage is calculated as follows:

Slippage (%) = ((Pump speed − Turbine speed) ÷ Pump speed) × 100

Excessive slippage (greater than 5% at stabilized speed) indicates worn linings or an incorrect oil level.

Manual Transmissions

Constant-Mesh Gear Transmission

Constant-mesh gear transmissions use synchronizers to equalize gear speeds before engagement. The critical components are:

The synchronizer operates in three phases: (1) the friction cone equalizes speeds, (2) the blocker ring prevents premature engagement, (3) full tooth engagement occurs once speeds are synchronized.

Range-Type Transmission

Range-type transmissions combine a main transmission with a planetary gear reducer mounted at the output. The driver selects high or low range using an electropneumatic switch. Low range multiplies torque by a factor of approximately 2:1.

Splitter-Type Transmission

Splitter-type transmissions divide each main gear into two sub-gears. The total ratio is obtained by combining the main gear ratio and the splitter ratio. For example, an 8-speed transmission with a splitter gives 16 speeds.

Manual Transmission Diagnostic Procedures

Automatic Transmissions

Planetary Gear Automatic Transmission

The automatic transmission uses planetary gears combined with multi-disc clutches and band brakes. The main components are:

The planetary gear set offers three basic functions depending on the fixed element and the driving element:

Electronically Controlled Automatic Transmission

Modern transmissions use an electronic control module (ECM) that controls shift solenoids. The main sensors are:

Diagnostics for these systems are performed using an electronic scan tool. Diagnostic trouble codes (DTCs) are standardized according to SAE J1939 for heavy equipment.

Hydraulic Pressure Testing Procedures

To verify the hydraulic operation of an automatic transmission:

Low pressure in all ranges indicates a worn pump, a clogged filter, or a low oil level. Low pressure in a specific range indicates an internal leak in that range's circuit.

Powershift Transmissions

Powershift transmissions are widely used in construction equipment. They allow gear changes without power interruption thanks to hydraulically actuated multi-disc clutches.

The control system includes:

Modulation is essential to avoid shocks during gear changes. Progressive modulation valves allow gradual clutch engagement.

Modulation Pressure Adjustment

Modulation pressure must be adjusted according to manufacturer specifications. Pressure set too high causes harsh shifts; pressure set too low causes clutch slippage and disc overheating. The typical procedure:

Driveshafts

Components and Function

The driveshaft transmits power from the transmission to the differential. Its components are:

Angles and Alignment

The operating angle of universal joints must be equal and opposite at both ends of the shaft. An excessive angle (greater than 3°) causes vibrations. The maximum recommended angle is 7° for roller-type joints.

Alignment verification is performed with a precision bubble level or a digital inclinometer. Typical tolerances are:

Vibration Diagnostics

Driveshaft vibrations can have several causes:

The maximum play in a universal joint is typically 0.5 mm (0.020 inch) measured with a dial indicator. Beyond this, the joint must be replaced.

Universal Joint Replacement Procedure

Axles and Differentials

The Standard Differential

The differential allows the drive wheels to turn at different speeds when cornering. Its components:

The axle ratio is the ratio between the number of teeth on the ring gear and that of the drive pinion. For example, a ring gear with 41 teeth and a pinion with 9 teeth gives a ratio of 41 ÷ 9 = 4.56:1.

Limited-Slip Differential

The limited-slip differential (LSD) uses multi-disc clutches or a viscous system to limit the speed difference between the wheels. Common types:

Functional test of a limited-slip differential:

Differential Lock

The differential lock is used for difficult traction conditions. It can be actuated:

The lock should only be engaged on soft or slippery ground. Engaging it on hard ground can damage the gears.

Backlash Adjustment

The backlash between the pinion and the ring gear is critical. The measurement procedure:

Typical backlash is 0.15 to 0.25 mm (0.006 to 0.010 inch). Excessive backlash causes noise; insufficient backlash causes overheating and rapid wear.

Tooth Contact Pattern

The tooth contact pattern is checked by applying a marking compound (prussian blue) to the ring gear teeth and rotating the assembly. The correct pattern should be:

A pattern too close to the tooth tip indicates the pinion is too far from the ring gear. A pattern too close to the root indicates the pinion is too close.

Tandem Drive Axles

Tandem axles (two drive axles) are common on heavy trucks. They use a power divider (or transfer case) that splits power between the two axles.

The power divider can be:

The intermediate driveshaft (between the two axles) must be checked for alignment and play. Misalignment causes vibrations and premature wear of the universal joints.

Applicable Standards and Regulations

CSA Standards

The Canadian Electrical Code, Part I (CE Code) (CSA C22.1) applies to the electrical components of electronically controlled automatic transmissions. Rule 8-200 concerns the grounding requirements for electrical equipment.

The CSA B149.1 standard (Natural Gas and Propane Installation Code) applies to equipment operating on compressed natural gas (CNG) or propane, including associated transmission systems.

SAE Standards

The SAE J1939 standards (heavy-duty vehicle communication network) and SAE J1587 (diagnostics) are essential for electronic diagnostics of modern transmissions.

Canadian Vehicle Regulations

The Canada Motor Vehicle Safety Regulations (CMVSR) require that modifications to transmission systems comply with the original equipment manufacturer's specifications.

Safety Procedures

Before any intervention on a transmission system:

Summary

Common Pitfalls to Avoid