# How a manual transmission works

All road vehicles powered by internal combustion engines have a transmission as part of the powertrain. The simplest type of transmissions is the manual transmission. It’s called “manual” because the driver has both roles of decision making (when to perform a gearshift) and actuation (actual gearshift process).

The traction characteristics of an internal combustion engine makes it impossible to propel a vehicle without a transmission. The torque and speed output of the internal combustion engine are either too low or too high to match the dynamic requirements of a vehicle. Thus, the role of a transmission is to:

• adapt the torque output of the engine function of the road load
• make possible the backwards movement of the vehicle, for the same direction of rotation of the engine
• allow engine disconnection from the rest of the powertrain

What is the difference between a transmission and a gearbox ?

Usually a transmission consists of a gearbox plus a differential. The gearbox contains all the gear assemblies, shafts, synchronizers, rails, etc. The gearbox can be regarded as the transmission without the differential.

For front-wheel drive (FWD) vehicles, the powertrain (engine + gearbox + differential) is completely contained on the front axle. Thus, for this type of vehicles, when we refer to the transmission, we consider that it contains both the gearbox and the differential.

Image: Vehicle powertrain for a front-wheel drive arrangement – kinematic view

For rear-wheel drive (RWD) vehicle, the powertrain is split between the front and rear axles. The front axle contains usually the engine and gearbox, while the rear axle contains the differential. Thus, for this type of vehicles, transmission or gearbox has the same meaning.

The transmission is mounted after the coupling device (clutch, torque converter), takes the clutch torque and speed as input and converts and distributes them to the wheels through the half shafts.

Types and main components of a manual transmission

Every manual transmission consists of input and output shafts, several permanent-mesh gears and an actuation mechanism. Depending on the number of ratio stages used to make up the gears, the transmissions are classified as:

• single-stage transmissions
• two-stage transmissions
• multi-stage transmissions

In a single-stage transmission, a gear ratio is formed with only one pair of gears. Also, there are only two shafts in the transmission: an input shaft and an output shaft. This type of transmissions are primarily used in front-wheel drive vehicles.

 Image: Getrag 5MTT170 5-speed single-stage manual transmission – components Credit: Getrag Image: 5-speed single-stage manual transmission – kinematic view

A particular feature of this type of transmission is that there is no direct drive gear (ratio = 1.00). This is because all gear ratios are formed by a pair of permanent-mesh gears. There is an equivalent gear for the direct drive gear, with the gear ratio close to 1.00 (e.g. 0.98 or 1.02).

Two-stage transmissions are used for standard powertrain configuration (engine on front axle with rear-wheel drive). Most of these transmissions have an input shaft, a counter shaft and an output shaft. There are also configurations with only two shafts (input and output).

Image: ZF S6-37 6-speed two-stage manual transmission – components
Credit: ZF

In the case of a two-stage transmission, the input shaft and the output shaft have a coaxial arrangement (their axis is common), while on single-stage transmissions the axis of the input and output shafts are different, with an offset between them.

Both single-stage and two-stage transmissions have the input shaft connected to the clutch.

All the forward gear assemblies have synchonizers for engagement. The purpose of the synchronizer is to align the input shaft speed to the output shaft speed when a gearshift is performed.

Image: Two-stage manual transmission

Two-stage transmissions have a constant gear which mechanically links the input shaft to the counter shaft. Thus, every gear ratio is made up with two permanently-meshed gear assemblies, the constant gear plus the gear assembly for the specific gear. Because of this arrangement, two-stage transmissions have slightly less overall efficiency.

The direct drive gear (4th gear in the image above) is the gear which connects the input shaft directly to the output shaft, without going through a gear mesh. Thus the gear ratio for a direct drive gear is 1.00 (no conversion of speed or torque).

Image: Manual transmission gearshift animation (click on image for animation play)

In every transmission, except for the reverse gear, all the forward gears are permanently meshed. For the example above, all the gears on the counter shaft are fixed (they rotate together), and all the gears on the output shaft are free (they rotate independently of the output shaft).

The synchronizers are fixed on the output shaft. When engaging a gear, the synchonizer will make the connection between input/counter shaft and output shaft.

 Image: 5-speed manual transmission power flow – 1st gear engaged Image: 5-speed manual transmission power flow – 2nd gear engaged

The reverse gear contains an extra gear in order to change the direction of rotation of the output shaft. The reverse gear doesn’t have a synchronizer since the reverse gear is engaged after the vehicle has come to a complete stop.

Image: Engagement of the Reverse gear for a manual transmission

All the gearshifts in a manual transmission are performed with torque interruption. Before a gearshift, the clutch is opened and there is no more engine torque transmitted to the input shaft. After the gearshift is complete, the clutch is closed back in order to allow the flow of the engine power (torque and speed).

In the case of a manual transmission, the gearshift can be:

• upshift: the gear number is incremented (e.g. from 1st gear to 2nd gear)
• downshift: the gear number is decremented (e.g from 3rd gear to 2nd gear)

Modern manual transmissions have 5, 6 or even 7 forward gears and 1 reverse gear. Each gear is characterized by a gear ratio.

Multi-stage transmissions are using more than two permanently meshed gears assemblies for a gear ratio formation. They are primarily used in commercial vehicle applications.

How the engine, speed, torque and power is modified by the transmission ?

The core element of a manual transmission is the meshed gear assembly. It consists of two toothed wheels (gears) meshed together. The gear that is connected to the input/counter shaft is the input gear, the gear connected to the synchronizer is the output gear. Every gear has a fixed gear ratio.

Image: Gear ratio calculation

The gear ratio (i) is the ratio between the number of teeth of the output gear (zout) and the number of teeth of the input gear (zin). For the example above the gear ratio is:

$i = \frac{z_{out}}{z_{in}} = \frac{24}{16} = 1.5$

For a given speed of the input gear (nin = 4500 rpm) and a gear ratio (i = 1.5), the speed of the output gear (nout) will be:

$n_{out} = \frac{n_{in}}{i} = \frac{4500}{1.5} = 3000 \text{ rpm}$

For a given torque of the input gear (Tin = 200 Nm) and a gear ratio (i = 1.5), the torque of the output gear (Tout) will be:

$T_{out} = T_{in} \cdot i = 200 \cdot 1.5 = 300 \text{ Nm}$

We can see that, for a gear ratio higher than 1.00, the output speed is reduced while the output torque is amplified.

What happens to the power, does it change? To find the answer to this question we need to calculate the power at the input gear and the power at the output gear, with the equation:

$P \text{ [W]} = T \text{ [Nm]} \cdot \frac{\pi}{30} \cdot n \text{ [rpm]}$

For our input data above, we’ll get:

$\begin{equation*} \begin{split} P_{in} &= T_{in} \cdot \frac{\pi}{30} \cdot n_{in} &= 200 \cdot \frac{\pi}{30} \cdot 4500 &= 94248 \text{ W}\\ P_{out} &= T_{out} \cdot \frac{\pi}{30} \cdot n_{out} &= 300 \cdot \frac{\pi}{30} \cdot 3000 &= 94248 \text{ W} \end{split} \end{equation*}$

As we can see a gear ratio doesn’t transform the power also, but only the torque and speed, keeping the power constant. In reality there is a small power drop, at the output gear, due to gear mesh efficiency. For one gear mesh assembly, the efficiency is around 0.98 – 0.99. In this case the output power will be:

$P_{out} = P_{in} \cdot \eta_{gear} = 94248 \cdot 0.98 = 92363.04 \text{ W}$

Example of real-world manual transmission: TREMEC TR-6070

Source: http://www.tremec.com

The TREMEC TR-6070 seven speed manual transmission was designed specifically for premier North American sports cars and integrates an awe inspiring shift technology. The TR-6070 is based on the well respected TR-6060 six speed transmission. A triple overdrive gear was added to improve fuel economy and lower emissions. Incorporated in the TR-6070 is a Gear Absolute Position (GAP) sensor. The technology provides a signal from the transmission to the engine controller, inferring the real time position of the shift selector. With this information, the engine RPM can be controlled to match the next gear selection which enhances driveability.

Image: TREMEC TR-6070 7-speed manual transmission
Credit: Tremec

Design features of the TR-6070 synchronizers include a combination of double-cone and triple-cone rings, utilizing a hybrid solution on all forward gears. The hybrid rings are a combination of carbon and sintered bronze cones providing higher capacity and shift performance. Linear bearings lower the friction of the shift rail movements, making the shifter feel naturally lighter and more direct.

TR-6070 Features at a Glance:

• Rear wheel drive, seven-speed manual overdrive transmission
• Triple overdrive for improved fuel economy and lower emissions
• Gear ratio spread of up to 6.33
• Triple- and double-cone synchronizers
• Advanced and asymmetric clutch teeth in second and third speed gears
• Two-piece gear design for high torque capacity
• Low mass, hollow shaft design available
• Sensors include:
• Temperature
• Speed
• Gear position

TREMEC TR-6070 Transmission Specifications:

 Type Rear wheel drive, seven-speed manual overdrive transmission Maximum gross vehicle weight (reference) [kg/lb] 2400 / 5291 Case Die-cast aluminum alloy Center distance [mm] 85 Overall length [mm] 782 Clutch housing Integrated Synchronizer type Double and triple cone; hybrid friction material Lubricant type Dexron III ATF Lubricant capacity (approximate) [L / pt] 3.5 / 7.4 Transmission weight [kg / lb] 65.2 / 143.75 Power take off No Available Gear Ratios Alternative ratios available upon request; may result in different maximum input torque Gear A B C 1 2.97 2.66 2.29 2 2.07 1.78 1.61 3 1.43 1.30 1.21 4 1.00 1.00 1.00 5 0.71 0.74 0.82 6 0.57 0.50 0.68 7 0.48 0.42 0.45 R 2.85 2.53 2.70 Input Torque [Nm / lb-ft] 625 / 460 740 / 545 860 / 635

Manual transmissions have relatively simple mechanics, do not require maintenance, are robust and with very good overall efficiency. Understanding how a manual transmission works is critical in order to advance to more complex topics as automatic or double-clutch transmissions.

For any questions or observations regarding this tutorial please use the comment form below.

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