Manual transmissions are rare in new consumer cars, but millions of units still exist in older cars, commercial, and racing applications. For this explanation, we’ll focus on a front-wheel-drive arrangement with a constant mesh type design where gear sets are always in contact with each other.
The Clutch
The clutch assembly is securely attached to the flywheel, which is bolted to the engine. It’s always spinning if the car is running. The components inside this assembly allow power flow to be disconnected from the transmission input shaft.
- Connection to Engine Power: This is accomplished with a sliding friction disc, sometimes called a clutch plate, sandwiched between the flywheel surface and a movable pressure disc. The clutch plate assembly is splined to the transmission input shaft but can slide back and forth.
- Pressure Plate: The pressure disc gradually squeezes everything together as the driver releases the clutch pedal for a smooth connection to engine power. To ensure smooth power delivery, the clutch plate has a built-in damping system that absorbs vibrations or other potentially harmful anomalies in the flow of power.
Pressure Plate Movement
The pressure plate is connected to a special diaphragm spring. This strong spring naturally presses the pressure plate to the flywheel.
- Diaphragm Spring: The outer cover has hooks that hold the diaphragm spring securely in place, acting as a fulcrum. There are also supporting leaf springs attached to the pressure plate and cover.
- Clutch Fork: The clutch fork sits at the center of the diaphragm spring. As the clutch fork pivots, it presses against a release or throwout bearing, depressing the diaphragm spring’s inner prongs, which in turn lifts its outer edge, and the connected pressure plate.
Hydraulic Actuator
The clutch fork acts as a lever with a fulcrum point on the transmission case interior surface. It’s driven by a hydraulic actuator whose lines lead back to the clutch pedal with its own hydraulic actuator and fluid reservoir.
The Gears
Each speed has a gear set so that input speed can be different from output speed. The speed gears in this constant mesh transmission are always linked and have diagonal or helical cut teeth for quieter operation.
- Reverse Gear: The reverse gear set has straight teeth, making a familiar gear whining sound when in reverse. Engine power flows through the main shaft to the countershaft, to the differential assembly, and out through front axles.
Synchronizing Gears
To make gear shifting possible, one gear in each set floats freely on its shaft, riding on a roller bearing. The other gear in each set is either connected with splines or directly machined into its respective shaft.
- Synchronizer Hub: Between each gear set, there’s a synchronizer hub that’s splined to and rotates with the shaft. A sliding shift sleeve, driven by this hub, can be moved back and forth by a selector fork. Forks are connected to sliding rods held by the external casing.
- Blocker Ring: During gear selection, the fork moves the sleeve towards the desired gear. A blocker ring sits between the sleeve and the gear, ensuring everything spins at the same speed for synchronization. The hub has slots for three synchronizer keys that spin the blocker ring, allowing it to adjust on the fly as the sleeve teeth approach.
- Final Synchronization: The shift sleeve’s internal teeth push against keys, pressing the blocker ring against the gear. Gradually, the gear begins to spin with the blocker ring, and once enough pressing force builds, the keys move out of the way, allowing the sleeve to lock into its final synchronized position with gear locking teeth.
Switching Gears
To switch gears, the clutch is pressed in, pressure is relieved, and the sleeve can slide into synchronization with an adjacent floating gear.
- Selector Forks and Rods: For a six-speed transmission, there are three selector forks and rods. The first and second floating gears operate from the output shaft, but this doesn’t fundamentally change functionality.
- Shift Change Assembly: Forks and rods move back and forth as directed by the shift change assembly, which can swing side to side to select a specific rod and fork.
Reverse Gear
The reverse gear has its own rod and selector fork. The transmission must come to a complete stop to shift into reverse as there are no synchronization components. Adding a third idler gear reverses the final output.
Neutral Position
When in the neutral position with no gear selected and the clutch engaged, the transmission input shaft spins, but no power flows out of the transmission.
Oil System
Oil rests at the bottom of the case and is splashed up onto gears for lubrication. There’s no oil pump or filter.
Conclusion
Understanding how a manual transmission and clutch work involves knowing the clutch assembly, gear sets, synchronization process, and how power is transferred through the system. This knowledge is crucial for anyone interested in automotive mechanics.
FAQs
Q1: What is a constant mesh transmission?
A1: A constant mesh transmission is a type where all gear sets are always in contact with each other, ensuring smoother gear shifts.
Q2: How does the clutch fork work?
A2: The clutch fork pivots, pressing against a release bearing, which in turn depresses the diaphragm spring’s inner prongs, lifting the pressure plate.
Q3: What is the purpose of the synchronizer hub?
A3: The synchronizer hub helps synchronize the rotating speed of gears during gear shifts, allowing for smooth transitions between gears.
Q4: Why does the reverse gear make a whining sound?
A4: The reverse gear has straight teeth, which produce a familiar gear whining sound when engaged.
Q5: How is oil used in a manual transmission?
A5: Oil lubricates the gears by resting at the bottom of the case and being splashed onto the gears as they spin. There’s no oil pump or filter.
