An Introduction to Manual Transmission - By Priti Gadgil June 21, 2009 If you drive a stick-shift car, then you may have several questions floating in your head: •
How does the funny "H" pattern that I am moving this shift knob through have any relation to the gears inside the transmission? What is moving inside the transmission when I move the shifter?
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When I mess up and hear that horrible grinding sound, what is actually grinding?
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What would happen if I were to accidentally shift into reverse while I am speeding down the freeway? Would the entire transmission explode?
Mercedes C class 6-speed Manual transmission In this article, we'll answer all of these questions and more as we explore the interior of a manual transmission.
Need for transmission Cars need transmissions because of the physics of the internal combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per minute (though this varies, and is typically less for diesel engines), while the car's wheels rotate between 0 rpm and around 1800 rpm. Any engine has a redline -- a maximum rpm value above which the engine cannot go without exploding. Furthermore, the engine provides its highest torque outputs approximately in the middle of its range, while often the greatest torque is required when the vehicle is moving from rest or traveling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits, is required. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so the engine can stay below the redline and near the rpm band of its best performance.
The object of the gearbox is to enable the engine’s turning effect and its rotational speed output to be adjusted by choosing a range of under and overdrive gear ratios so that the vehicle responds to the driver’s requirements within limits of various load conditions. Manual transmissions dominate the car market outside of North America. They are cheaper, lighter, usually give better performance, and fuel efficiency (although the latest sophisticated automatic transmissions may yield results slightly closer to the ones yielded by manual transmissions), and it is customary for new drivers to learn, and be tested, on a car with a manual gear change. In Japan, Philippines, Germany, the Netherlands, Austria, the UK, Ireland, Sweden, France, Australia, Finland, Lithuania and Israel, a test pass using an automatic car does not entitle the driver to use a manual car on the public road; a test with a manual car is required. Manual transmissions are much more common than automatic transmissions in Asia, Africa, South America and Europe.
The transmission is connected to the engine through the clutch. The input shaft of the transmission therefore turns at the same rpm as the engine.
A 5-speed transmission applies 1 of 5 different gear ratios to the input shaft to produce a different rpm value at the output shaft. Some typical gear ratios :
=> lower gear, higher gear ratio, lower speed, higher torque output.
Understanding a standard transmission system
A simple 2-speed transmission system •
The green shaft comes from the engine through the clutch. The green shaft and green gear are connected as a single unit. The green shaft and gear turn at the same rpm as the engine.
Clutch fundamentals Clutches are designed to engage and disengage the transmission system from the engine. When you push in the clutch pedal, the engine and the transmission are disconnected so the engine can run even if the car is standing still. When you release the clutch pedal, the engine and the green shaft are directly connected to one another. The gradual increase in transfer of engine torque to the transmission should be smooth. •
The red shaft and gears are called the layshaft. These are also connected as a single piece, so all of the gears on the layshaft and the layshaft itself spin as one unit. The green shaft and the red shaft are directly connected through their meshed gears so that if the green shaft is spinning, so is the red shaft. In this way, the layshaft receives its power directly from the engine whenever the clutch is engaged.
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The yellow shaft is a splined shaft that connects directly to the drive shaft through the differential to the drive wheels of the car. If the wheels are spinning, the yellow shaft is spinning.
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The purpose of the collar is to connect one of the two blue gears to the yellow drive shaft. The collar is connected, through the splines, directly to the yellow shaft and spins with the yellow shaft. However, the collar can slide left or right along the yellow shaft to engage either of the blue gears. Teeth on the collar, called dog teeth, fit into holes on the sides of the blue gears to engage them.
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The blue gears ride on bearings, so they spin on the yellow shaft. If the engine is off but the car is coasting, the yellow shaft can turn inside the blue gears while the blue gears and the layshaft are motionless.
First gear
Neutral gear
First Gear: collar engages the blue gear on the right. The green shaft from the engine turns the layshaft, which turns the blue gear on the right. This gear transmits its energy through the collar to drive the yellow drive shaft. Meanwhile, the blue gear on the left is turning, but it is freewheeling on its bearing so it has no effect on the yellow shaft. Neutral: collar is between the two gears. Both of the blue gears freewheel on the yellow shaft at the different rates controlled by their ratios to the layshaft. Therefore, when you make a mistake while shifting and hear a horrible grinding sound, you are not hearing the sound of gear teeth mis-meshing (since all gear teeth are fully meshed at all times). The grinding is the sound of the dog teeth trying unsuccessfully to engage the holes in the side of a blue gear.
Standard 5-speed manual transmission
There are 3 forks controlled by 3 rods that are engaged by the shift lever. The shift lever has a rotation point in the middle. When you push the knob forward to engage first gear, you are actually pulling the rod and fork for first gear back. As you move the shifter left and right you are engaging different forks (and therefore different collars). Moving the knob forward and backward moves the collar to engage one of the gears. A top view of the shift rods is as shown:
Reverse Gear: is handled by a small idler gear (purple). At all times, the blue reverse gear in this diagram is turning in a direction opposite to all of the other blue gears. Therefore, it would be impossible to throw the transmission into reverse while the car is moving forward -- the dog-teeth would never engage. However, they will make a lot of noise!
Double clutching Double-clutching was common in older cars and is still common in some modern race cars. In double-clutching, you first push the clutch pedal in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you can move the collar into neutral. Then you release the clutch pedal and rev the engine to the "right speed." The right speed is the rpm value at which the engine should be running in the next gear. The idea is to get the blue gear of the next gear and the collar rotating at the same speed so that the dog teeth can engage. Then you push the clutch pedal in again and lock the collar into the new gear. At every gear change you have to press and release the clutch twice, hence the name "double-clutching" or Unsynchronized transmission.
Synchronizers: Manual transmissions in modern passenger cars use synchronizers to eliminate the need for double-clutching. A synchro's purpose is to allow the collar and the gear to make frictional contact before the dog-teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to engage, like this:
The cone on the blue gear fits into the cone-shaped area in the collar, and friction between the cone and the collar synchronize the collar and the gear. The outer portion of the collar then slides so that the dog-teeth can engage the gear. Every manufacturer implements transmissions and synchros in different ways, but this is the general idea.
Gear selection using a floor-mounted shifter In most modern passenger cars, gears are selected through a lever attached to the floor of the automobile—this selector is often called a gearstick, gear lever, gear selector, or simply shifter. Moving this lever forward, backward, left, and right allows the driver to select any given gear. In this configuration, the gear lever must be pushed laterally before it is pushed longitudinally.
5-speed shift stick of a 1999 Mazda Protege. Most front-engined, rear-wheel drive cars have a transmission that sits between the driver and the front passenger seat. Floor-mounted shifters are often connected directly to the transmission. Front-wheel drive and rear-engined cars often require a mechanical linkage to connect the shifter to the transmission.
1) The most common five-speed layout found in the USA and the UK:
2) This five-speed layout, found on many race cars and some older model passenger cars, is commonly referred to as a "dog-leg first" or "racing" pattern, because of the "up and over" 1-2 shift.
3) Another five-speed shift pattern (common on many European cars) is this:
This style of pattern (including depressing the gear lever) is common on BMWs, Opels, most Volkswagens (though some have reverse towards second gear,) older Volvo 240s and some Renault models (12, 9, 19, 5, Mégane, Twingo and Clio). 4) A typical pattern for the more modern six-speed transmission is shown here:
Gear selection using a column-mounted shifter Some cars have a gear lever mounted on the steering column of the car. It was common in the past but is no longer common today. However, many automatic transmissions still use this placement. Column shifters are mechanically similar to floor shifters, although shifting occurs in a vertical plane instead of a horizontal one. Column shifters also generally involve additional linkages to connect the shifter with the transmission. Also, the pattern is not "intuitive," as the shifter has to be moved backward and upward into R to make the car go backward.
1) A 3-speed column shifter, nicknamed "Three on the Tree" (alternatively, "Three in the Tree"), began appearing in America in the late 1930s and became common during the 1940s and 50's. Its layout is as shown below:
2) First gear in a 3-speed is often called "low," while third is usually called "high." There is, of course, no overdrive. Later European and Japanese models began to have 4-speed column shifter and some of these made their way to the USA. Its layout is shown here:
Historically, 4-speed floor shifters were sometimes referred to as "Four on the Floor", when steering column mounted shifters were more common.
Clutch and torque transfer
Pedal setup on a 2007 Subaru Legacy.
From left to right, the dead pedal, clutch pedal, brake, and accelerator. When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is transferred from the engine to the transmission (and by extension to the drive wheels). In this uncoupled state it is possible to select gears or to stop the car without stopping the engine.
When the clutch pedal is fully released, the clutch is fully engaged, and practically all of the engine's torque is transferred. In this coupled state, the clutch acts as rigid coupling, and power is transmitted to the wheels with minimal practical waste heat. Clutch slip Between these extremes of engagement and disengagement the clutch slips to varying degrees. When the clutch slips it still transmits torque despite the difference in speeds between the engine crankshaft and the transmission input. Because this torque is transmitted by means of friction rather than direct mechanical contact, considerable power is wasted as heat (which is dissipated by the clutch).
Properly applied, slip allows the vehicle to be started from a standstill, and when it is already moving, allows the engine rotation to gradually adjust to a newly selected gear ratio. Because of the heat that a slipping clutch generates, slip should not be maintained beyond necessity; skilled drivers rarely allow a clutch to slip for more large fraction of a second or so.
Note: Automatic transmissions also use a coupling device; however, a clutch is not present. In these kinds of vehicles, the torque converter is used to separate the engine and transmission.