Manual Transmissions

Purpose • To change the torque going

to the drive wheels

• Needed to start vehicle from a stand still

Basic Theory

• Manually shifted

• Clutch disc rotates the input shaft

• Output shaft powers wheels

Transaxle • A differential and transmission

in the same housing

• Common in four wheel drive vehicles

• Manual or automatic transmissions

Parts • Input shaft

• Gears

• Synchronizers

• Shift fork

Parts• Gear shift lever

• Output shaft

• Transmission case

Input Shaft

• Driven by clutch• Turns gears inside transmission

Transmission gears

• Provides a means of changing output torque and speed

Synchronizers • Devices that help gears

mesh into desired arrangement

Shift fork

• Unit for moving gears in and out of arrangement

Gear shift lever• A lever that provides the

driver with the ability to change the gear arrangement

Output Shaft • Shaft that transfers power from

the transmission to the wheels

• Transfers power to drive shaft in a rear wheel drive vehicle

Transmission Case

• Housing that encases transmission shafts, gears, and lubricant

Gear Ratio• Definition: the number of

rotations a drive gear has to turn before the driven gear turns once

Determining Gear Ratio

• Found by dividing the number of teeth on the driven gear by the number of teeth on the drive wheel

Gear Reduction

• A smaller gear drives a larger gear

• Increases torque and decreases the speed

Over Drive Gear

• A large gear drives a smaller gear

• Torque is decreased and speed is increased

Gear types

• There are two types of gears found in manual transmissions

–Spur gears

–Helical gears

Spur Gears

• Teeth cut parallel to center shaft • Not used for drive gears • Used for gears that have to slide

in and mesh with other gears • Noisy

Helical Gears

• Teeth cut out of parallel to drive shaft

• Used for constant drive components

• More efficient

• Quiet

Gear back lash

• Small clearance between meshing gears

• Allows lubricant to flow between high friction gears

• Allows gears to expand during operation

Lubricant • Splash lubricant

• Gears fling lubricant around transmission

• Gear oil: 80W to 90W (always check owner’s manual)

Transmission shafts

• Input shaft

• Counter shaft

• Reverse idler shaft

• Output shaft

Input shaft (AKA clutch shaft)

• Transfers power from clutch to the counter shaft

• Splined on clutch side and a fixed gear on the transmission side

• Whenever clutch is turning the input shaft is turning

Counter shaft (AKA cluster gear shaft)

• Holds counter shaft gears into mesh with the input shaft gear

• Counter shaft does not turn

Reverse idler shaft • Small shaft that supports idler

gear • Reverse idler gear meshes

with gears on both the input shaft and the counter shaft

Output shaft (AKA Main shaft)

• Holds output gears and synchronizers

• Extends out of the transmission to power the drive wheels

Shift forks

• Sits around synchronizer sleeves

• Transfers movement from the gear shifter linkage to the synchronizers

Problems

• Grinding sounds when shifting – Worn gear change linkage – Clutch grabbing – Synchronizer gears grinding going into mesh

Problems

• Transmission noise

–Roaring, rumbling, whirling sounds •Low lubricant, metal contaminants

Problems

• Difficulty shifting

–Problem with the linkage for the shifter mechanism

–Clutch problems

Problems

• Jump out of gear –Worn clutch pilot bearing

causes vibration and wobbling of the transmission input shaft. The vibration and wobbling shakes the synchronizers out of gear

Problems

• Locked in gear

–Shifter assembly problems –Broken drive gear teeth, bits stuck in gears