Unit 3 Transmission and Steering System Notes

Rajasthan Polytechnic (BTER) March 31, 2025

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Subject - AUTOMOBILE ENGINEERING ME 40041
Branch - Mechanical Engineering
Semester - 4th Semester

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UNIT-III: TRANSMISSION AND STEERING SYSTEM

In an automobile, the transmission system transfers power from the engine to the wheels, enabling smooth movement, while the steering system controls the direction of the vehicle. This unit covers clutches, gearboxes, propeller shafts, rear axles, and steering mechanisms.

3.1 General Arrangement and Principle of Friction Clutches

  • The clutch is a mechanical device used to connect and disconnect the engine from the transmission system smoothly.

  • It works on the principle of friction, where a frictional force is applied between two surfaces to transmit torque.

  • The clutch allows for smooth gear shifting, prevents jerks, and provides better control over the vehicle.

3.2 Types and Construction of Clutches

(a) Single Plate Clutch

  • Construction:

    • Consists of one friction plate mounted between the flywheel and the pressure plate.

    • The friction lining on both sides of the plate helps in engaging and disengaging the clutch smoothly.

  • Working:

    • When the clutch pedal is pressed, the friction plate is released, disconnecting the engine from the gearbox.

    • When the clutch pedal is released, the friction plate is pressed between the flywheel and pressure plate, allowing power transmission.

  • Used in: Light vehicles like cars and motorcycles.

(b) Multi-Plate Clutch

  • Construction:

    • Has multiple friction plates to increase the frictional surface area.

    • Plates are alternately arranged with metal plates to enhance power transmission.

  • Working:

    • Works similarly to a single plate clutch but can transmit more power due to multiple friction surfaces.

  • Used in: Racing cars, heavy vehicles, and motorcycles (where space is limited but high torque is required).

(c) Centrifugal Clutch

  • Construction:

    • Uses centrifugal force to engage the clutch automatically based on engine speed.

    • It consists of weighted shoes, springs, and a drum.

  • Working:

    • At low speeds, springs keep the shoes away from the drum, keeping the clutch disengaged.

    • As engine speed increases, centrifugal force pushes the shoes outward, making contact with the drum and engaging the clutch.

  • Used in: Scooters and automatic two-wheelers (where manual clutch operation is not required).

3.3 Necessity for Gear Ratios and Types of Gearboxes

(a) Necessity for Gear Ratios

  • The engine cannot provide the required torque and speed directly to the wheels for different driving conditions (starting, climbing, high-speed cruising).

  • A gearbox is used to vary torque and speed according to road conditions.

  • Lower gears provide more torque for starting and climbing, while higher gears offer higher speeds for cruising.

(b) Types of Gearboxes

(i) Sliding Mesh Gearbox

  • Construction:

    • Uses sliding gears to engage and disengage different gear ratios manually.

  • Working:

    • Gears slide to engage with the output shaft for different speeds.

    • Requires double-declutching to shift gears smoothly.

  • Used in: Older cars and commercial vehicles (not common in modern vehicles due to difficulty in shifting).

(ii) Constant Mesh Gearbox

  • Construction:

    • All gears are in constant mesh, but only one pair is engaged at a time using dog clutches.

  • Working:

    • A dog clutch locks the required gear to the main shaft without needing to slide gears.

  • Used in: Moderate-speed vehicles (offers smoother shifting than sliding mesh).

(iii) Synchromesh Gearbox

  • Construction:

    • Similar to a constant mesh gearbox but uses synchromesh devices for smooth engagement.

  • Working:

    • Synchronizers match gear speeds before engagement, ensuring smooth shifting without double-declutching.

  • Used in: Modern manual transmission cars (easier and smoother shifting).

(iv) Automatic Transmission (AT)

  • Uses hydraulic torque converters and planetary gears for automatic gear shifting.

  • No clutch pedal is required, and gear shifting happens based on vehicle speed and engine load.

  • Used in: Luxury cars, SUVs, and modern vehicles for effortless driving.

3.4 Propeller Shaft and Universal Joint

(a) Propeller Shaft

  • Purpose: Transfers power from the gearbox to the rear axle in rear-wheel-drive (RWD) vehicles.

  • Construction:

    • A long shaft with universal joints (U-joints) at both ends to allow flexibility.

  • Working:

    • As the suspension moves, the propeller shaft transmits power without breaking the connection between the gearbox and the rear axle.

(b) Universal Joint (U-Joint)

  • Allows the propeller shaft to flex and rotate even if the rear axle moves up and down.

  • Helps maintain smooth power transmission in varying driving conditions.

3.5 Types of Rear Axle

(a) Semi-Floating Axle

  • The wheel hub is directly mounted on the axle, which supports both weight and torque transmission.

  • Used in: Light vehicles (cars, small trucks).

(b) Full-Floating Axle

  • The axle only transmits torque, while the vehicle's weight is supported by the axle housing.

  • Used in: Heavy-duty trucks and off-road vehicles.

(c) Three-Quarter Floating Axle

  • A combination of semi-floating and full-floating axles, offering better strength.

  • Used in: Medium-duty vehicles.

3.6 Steering System: Working and Types

(a) Working of Steering System

  • The steering system allows the driver to control the direction of the vehicle by turning the front wheels.

  • It converts rotational motion of the steering wheel into the angular movement of the wheels.

  • Uses linkages and gears to ensure precise movement.

(b) Types of Steering Systems

(i) Manual Steering System

  • Mechanical linkages connect the steering wheel to the wheels.

  • Types:

    • Worm and Roller Steering – Used in trucks and older vehicles.

    • Rack and Pinion Steering – Used in modern cars (simpler and more efficient).

(ii) Power Steering System

  • Uses hydraulic or electric power assistance to reduce steering effort.

  • Types:

    • Hydraulic Power Steering (HPS) – Uses hydraulic fluid to assist steering.

    • Electric Power Steering (EPS) – Uses an electric motor for assistance, offering better efficiency.

    • Electro-Hydraulic Power Steering (EHPS) – A combination of both systems.

Conclusion

  • The transmission system is crucial for power transmission and speed control.

  • Clutches (single plate, multi-plate, centrifugal) allow smooth power engagement.

  • Gearboxes (sliding mesh, constant mesh, synchromesh) vary speed and torque.

  • The propeller shaft and U-joints enable flexibility in power transmission.

  • Rear axles (semi-floating, full-floating) support weight and torque.

  • The steering system (manual or power-assisted) provides control and stability.

Rajasthan Polytechnic (BTER)

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