All cars and road vehicles have transmissions. They are even mentioned in some new car advertisements, making certain ones a selling point for cars. But how do they work, and how many different kinds are there?
First off lets begin with the basics: A transmission in mechanical terms essentially transmits power from the engine to the wheels so that a car can move. The different gears or speeds determine when a car uses a specific amount of pulling power (torque).
The Manual Gearbox. It was the first mechanical transmission used in automobiles. It is relatively simple in design and function. There is a plate, clutch and gears that mesh. These gears are linked to the engine and transmit the power to the drive wheels. Manual transmissions require operation by the driver of individual gears, also known as stick shift. The clutch is what stops the gears from moving so that another gear can be selected without grinding them together and damaging the transmission. That is why you have to press the clutch in while shifting or else you hear a grinding sound, and risk damaging the transmission. The gears of a transmission: These gears essentially transfer the kinetic energy of the pistons within the engine to themselves, and then from the gears to the drive shaft. The energy travels along the drive shaft to the differential, where they are transmitted to the drive wheels. The reverse gear works exactly like the other gears, it just spins in the opposite direction to cause the kinetic energy that is transferred along the drive train to spin that same opposite direction, which turns the wheels in the opposite direction.
To understand how the clutch works, one must understand what role friction plays. Friction is a force that can stop motion. When a car is in gear and moving, the gears within the transmission rotate quickly. If you try to change gears while in gear and moving without stopping them from spinning, the grooves (called teeth) on the gears will grind together, causing lots of damage to the transmission. So the role that friction plays is in stopping the gears from spinning when you change gears. The clutch is made of two main pieces; the pressure plate and the clutch disc. The disc is coated in a high friction material, which is what aids in stopping the gear spin while changing gears. When you press the pedal in, the pressure plate applies pressure to the high-friction disc, and that disc presses against the shaft, which stops the gears from spinning. The pressure plate has coil springs within it that apply pressure to the clutch disc and prevent it from slipping. This is what allows the gears to switch without grinding together and causing bad damage. In order to keep these gears in track with each other, modern manual transmissions have a gear synchronizer for each gear. The two parts of the transmission that engage the gear are the gear collar and the fork. The fork is connected to the shifter, and it moves the collar in the direction that you move the shifter, which changes gears, respectively. Many years ago, cars used non-synchronous transmissions and a trick that all drivers had to learn to use was "double-clutching". In order to manually synchronize the dog teeth on the collar with the holes on the selected gear, drivers had to engage and disengage (let out and press in, pushing in the clutch disengages the engine from the transmission, letting out the clutch links the transmission to the engine and they spin together) the clutch, then rev the engine a bit to match the engine speed required to mesh the collar with the gear teeth. After that, you would press the clutch in again and that would lock the collar in the selected gear, then simply shift. That is quite a bit of trouble to change gears, imagine driving around town in traffic and having to do this each time!
Basically, what the synchros on each gear do is ensure that the output shaft and the gear itself are spinning at the same speed so that they can mesh without grinding. Ideally, you want them spinning at the same RPM (revolutions per minute) so that when they make contact, the gear and the collar do not grind at all and the gear is engaged. The synchro does this by locking the selected gear to the output shaft so that the power is able to transfer through the gear, out the shaft and to the drive wheels. This is why it is important to press the clutch in all the way when shifting a manual transmission because it minimizes friction and prevents grinding damage. Also, you have to be careful when letting the clutch out when shifting, if you do it too fast then that can strain the entire drive train. Another part of the system is the throwout bearing. It is the automotive equivalent to a thrust bearing and it is what pushes against the clutch system to disengage the gear spin in the transmission.
Diagram of what a manual transmission looks like inside
Six speed manual shifter
Clutch with pressure plate and disc
Set of pedals on a manual transmission car. The third pedal from right to
left is the clutch
The Automatic Gearbox. This is used by most cars in the United States today. It is much more complex in design because it is automated instead of operated manually. In an automatic transmission, there is a clutch between each gear that engages when gears change, this basically automates what pushing the clutch in and shifting does. The gears are automatically engaged, disengaged and synchronized to allow for shifting of gears and transmitting power to the drive wheels without manual operation. Modern cars use a shift computer that is linked to the engine for optimal shift times and fuel economy. Because of its convenience, it is preferred by many drivers on the road today, but it is more expensive to manufacture and more complex.
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