Overhauling a manual gearbox is a critical skill for automotive engineering technicians in Kenya, particularly given the diverse driving conditions and vehicle usage patterns encountered locally. Proper maintenance and repair of gearboxes ensure vehicle performance, fuel efficiency, and safety on Kenyan roads. Before commencing the overhaul process, organizing the work area and adhering to strict safety protocols are essential to protect technicians and maintain quality workmanship. This chapter focuses on these foundational aspects to prepare technicians for effective manual gearbox servicing.

3.2 Principles of transmission and gear system

A manual gearbox works by passing engine power through a train of shafts and gears, and by changing the gear ratio to match the load on the vehicle. Understanding how power flows through the transmission and how gear ratios multiply torque or speed is the foundation for every diagnosis, overhaul and assembly task that follows in this chapter. This topic explains the path of power from the engine to the wheels and shows how to calculate and apply gear ratios.

3.2.1 Power flow in transmission system

Power flow is the path that engine torque follows from the crankshaft to the drive wheels. In a rear-wheel-drive vehicle the power passes through the clutch, the gearbox, the propeller shaft, the final drive and the axle shafts, and each stage can change the torque and speed. Knowing the path helps a technician find which unit is at fault when the engine runs but the vehicle does not move. Example: a county government pickup revs freely in every gear but does not move, and tracing the power flow from the clutch outwards leads the technician to a worn clutch disc.

Power Flow Through the Gearbox

• Engine and flywheel: The crankshaft turns the flywheel, which carries the clutch cover and pressure plate.
• Clutch: With the pedal released, the clutch disc is clamped to the flywheel and carries torque to the input shaft.
• Input shaft and layshaft: The input shaft gear is always meshed with the large gear on the layshaft, so the layshaft cluster turns whenever the clutch is engaged.
• Selected gear: The synchronizer locks the chosen gear on the output shaft to the shaft, so power passes from the layshaft gear to that gear and out through the output shaft.
• Neutral: No gear is locked to the output shaft, so the layshaft turns but no power reaches the output shaft.
• Direct drive (top gear): The synchronizer locks the input shaft directly to the output shaft, and power passes straight through at a ratio of 1 : 1.
• Reverse: An idler gear is placed between layshaft and output shaft gears, which reverses the direction of rotation.

Power Flow Beyond the Gearbox

• Propeller shaft: It carries the gearbox output to the final drive through universal joints.
• Final drive and differential: The pinion drives the crown wheel, which gives a further reduction, turns the drive through 90 degrees and divides the torque between the wheels.
• Axle shafts and wheels: They carry the torque to the wheels, which push the vehicle along the road.
• Front-wheel drive: The gearbox and final drive are combined in a transaxle, and drive shafts with CV joints carry the power to the front wheels.
• Power losses: Friction in gears, bearings and oil churning lose some power as heat, so a well lubricated gearbox is more efficient.

3.2.2 Gear ratios

A gear ratio compares the speed or the number of teeth of two meshing gears. It shows how much torque is multiplied and how much speed is reduced. A low gear has a high ratio, giving strong pulling force at low speed, and a high gear has a low ratio, giving higher speed at lower engine revolutions. Example: a farm lorry pulling a loaded trailer out of a muddy field uses first gear for maximum torque, and changes to a higher gear on the tarmac to cruise at lower engine speed.

Calculating Gear Ratios

Formula: gear ratio = driven gear teeth ÷ driving gear teeth

Substitution: gear ratio = 45 ÷ 15

Working: 45 ÷ 15 = 3

Result: gear ratio = 3 : 1

For a pair of gears in two stages, the overall ratio is the product of the two stage ratios.

Formula: overall ratio = (layshaft teeth ÷ input gear teeth) × (output gear teeth ÷ layshaft gear teeth)

Substitution: overall ratio = (30 ÷ 15) × (36 ÷ 12)

Working: 30 ÷ 15 = 2, 36 ÷ 12 = 3, and 2 × 3 = 6

Result: overall ratio = 6 : 1

Effect of Gear Ratio on Torque and Speed

Formula: output torque = input torque × gear ratio

Substitution: output torque = 120 N·m × 3.8

Working: 120 × 3.8 = 456

Result: output torque = 456 N·m

Formula: output speed = input speed ÷ gear ratio

Substitution: output speed = 3,000 rpm ÷ 3.8

Working: 3,000 ÷ 3.8 = 789.5

Result: output speed ≈ 790 rpm

• Low gears (high ratio): They multiply torque for starting and climbing, but give low road speed.
• High gears (low ratio): They reduce engine speed at cruising speed, which saves fuel and reduces noise.
• Direct drive: In top gear with a ratio of 1 : 1, the input and output shafts turn at the same speed.
• Overdrive: A ratio below 1 : 1 makes the output shaft turn faster than the engine, for economical cruising.
• Reverse ratio: It is usually a little higher than first gear, and it reverses the direction because of the idler gear.

Overall Ratio Including the Final Drive

Formula: overall ratio = gearbox ratio × final drive ratio

Substitution: overall ratio = 3.8 × 4.1

Working: 3.8 × 4.1 = 15.58

Result: overall ratio = 15.58 : 1, so the engine turns 15.58 times for each turn of the wheels in first gear.

Typical Gear Ratios

3.2.3 Uses of Tools and Equipment

Cleaning brushes and solvents are essential to remove contaminants that can cause accelerated wear or corrosion. The workshop press is crucial for installing bearings without applying uneven force that could damage shafts or housings. Screwdrivers and gasket scrapers are basic yet vital for disassembly and preparation of mating surfaces for reassembly, guaranteeing a proper seal.

3.2.4 Maintenance of Tools and Equipment

Proper maintenance of tools extends their lifespan and preserves measurement accuracy, critical for quality assurance in gearbox servicing. Torque wrenches should be calibrated annually by certified service providers to ensure torque readings remain accurate. After use, all tools must be cleaned to remove grease and dirt that can cause rust or impair function.

Gear pullers and presses require lubrication of moving parts and inspection for wear or cracks before use. Dial indicators must be stored in protective cases and handled carefully to avoid damage to delicate internal mechanisms. Brushes should be cleaned of debris regularly, and cleaning solvents stored safely in labeled containers away from heat sources to prevent degradation or accidents.

3.2.5 Materials Used in Gearbox Overhaul

Materials used during gearbox overhaul include replacement gaskets, sealants, lubricants, and occasionally new bearings or gears. Gaskets and sealants must be compatible with gearbox oils and resistant to high temperatures and pressures. Lubricants such as gear oils conforming to the manufacturer’s specifications are essential for reducing friction and preventing wear. Replacement bearings and gears should meet or exceed OEM standards to ensure durability and performance.

Chapter Summary

This chapter covers the comprehensive process of overhauling a manual gear box unit, beginning with the organization of the work area to ensure safety, proper housekeeping, and effective waste disposal. It explains the fundamental principles of transmission and gear systems, including power flow and gear ratios, followed by an exploration of different types of gears such as spur, helical, and double helical gears. The construction and operation of both manual and automatic gearboxes are described to provide a clear understanding of their differences. The chapter addresses common gearbox problems through diagnosis and checks including slipping, grinding, hard shifting, noise, and fluid leaks. It details the tools, equipment, and materials needed, emphasizing their uses and maintenance. Procedures for dismantling manual and automatic gearboxes are outlined, along with servicing or replacing critical components like shafts, gears, synchronizers, bearings, seals, shift forks, and bushings. The assembly process focuses on correct alignment, gear clearance, fluid refilling, and adjustment for smooth operation. Finally, it covers gearbox mounting techniques, types of mounts, and necessary testing, concluding with the importance of documenting the gearbox condition and service history for future reference.