Manual transmission is also referred to as stick shift transmission because you need to use the transmission stick every time you change the gears. To perform the gear shift, the transmission system must first be disengaged from the engine. After the target gear is selected, the transmission and engine are engaged with each other again to perform the power transmission. Figure 5 shows the components of the manual transmission gearbox, and Figure 6 shows how the clutch works. From Figure 5, the gear selector fork can be moved forward and backward to move the collar to engage it to the target gears (blue). The gears (blue) are driven by the engine, while the collar (purple) connects to the wheels. To engage the collar to the gear, the power from the engine must be temporarily removed so the gear will lose the driving force. Therefore, the gear and the collar can be engaged without causing grinding due to different spin speeds. However, the collar and gear still have different spinning speeds due to the gear ratio change when shifting. For the modern car, a mechanism called a synchronizer is added to the collar to synchronize the spinning speed between collar and gears to make the gears mesh smoothly. The synchronizer is made of frictional materials. When the collar tries to mesh with the gear, the synchronizer will touch the gear first and use friction force to drive the gear to spin at the same speed as the collar. This will ensure that the collar is meshed into the gear very smoothly without grinding (see Figure 7). For some cars without synchronizers, a driving skill called "double clutching" must be performed during gear shifting to avoid gear grinding.
For a standard 5-speed manual transmission system, three different gear selector forks are used. Figure 8 and Figure 9 show the side-view and top-view of the respective positions of three forks. This "H" shift pattern enables the driver to select fivc different gear ratios and a reverse gear.
The clutch, used to temporarily disconnect power from the engine, is shown in Figure 6. When the thrust pad is pushed, it lifts the pressure pad so the driven pad (clutch plate) is disengaged from the flywheel. The flywheel, which is connected to the gears of the transmission system, will stop spinning. On the other hand, when the thrust pad is released, the spring pushes the driven pad back to engage the flywheel and drives it spinning. The whole procedure is shown on the following Shockwave file: http://static.howstuffworks .com/flash/clutch-fig5.swf.
The driven pad is made of frictional materials like brake pads, which use friction to drive the flywheel spinning. Like the brake system, the pad will become thinner and thinner, and eventually it will need to be replaced. The life of the driven pad really depends on the driver's skill and how carefully the clutch is maintained. Like brake pads, if signs of wear occur, a proper adjustment is needed. Regular adjustment is the best way to keep the clutch and brakes in good shape.
With the progress of technology, a new mechanism was invented called a self-adjusting clutch, which uses hydraulic pressure, instead of the traditional school linkage, to push the thrust pad . This idea came from the hydraulic brake, which was first invented in 1927 by Magura located in Bad Urach, Germany and used for BMW motorcycles. The company also invented the first "adjust-on-the-fly" clutch in 1968 . Regular adjustment will no longer be necessary, but transmission oil must still be frequently changed to enhance the transmission's life. Almost all of the manual transmission vehicles built later than 10 years ago were equipped with a self-adjusting clutch, which is said to last longer than 120,000 miles without breaking down.
Basically, the gear is shifted by depressing the clutch and releasing
the gas, shifting the gear, and releasing the clutch and stepping
on the gas. For some very old cars, the transmission collars aren't
equipped with synchronizers. For some
heavy-duty trucks, the synchronizers are removed in order to carry
more heavy products. For some rally race cars, to reduce gear shift
time, the synchronizers are removed so the driver can quickly move
the stick into the target gear. To avoid gear grinding, a technique
called "double clutching" is always used. To perform double clutching,
instead of shifting to the target gear directly, it is necessary
to shift to neutral first, release the clutch, step on the gas when
down-shifting or release the gas when up-shifting so the gear can
reach the same spin speed as the collar, depress the clutch again
and release the gas, shift to the target gear, and finally release
the clutch and step on the gas .
This link of a video clip from youtube , shows how to do double
clutching when driving a 1960 GMC bus:
Most modern manual transmission cars have been equipped with synchronizers so double clutching is no longer necessary. However, you still can use this technique when driving to enhance the synchronizers and clutch pad's lives.