Editing Manual transmission (section)

==Internals==
[[File:ZF-16S181-transmission-hous.jpg|thumb|16-speed (2×4×2) [[ZF Friedrichshafen|ZF]] 16S181 – opened transmission housing ('''''2×4'''''×2)]]
[[File:ZF-16S181-range.jpg|thumb|16S181 — opened [[Epicyclic gearing|planetary]] range housing (2×4'''''×2''''')]]

===Shafts===
A manual transmission has several shafts with various gears and other components attached to them. Most modern passenger cars use 'constant-mesh' transmissions consisting of three shafts: an ''input shaft'', a ''countershaft'' (also called a [[layshaft]]) and an ''output shaft''.<ref>{{cite book |last1=Bosch |first1=Robert |title=Automotive Handbook |date=2004 |publisher=Bentley Publishers |isbn=978-0-8376-0333-9 |page=741 |edition=6th |url=https://books.google.com/books?id=zsRHPwAACAAJ |access-date=10 March 2020 |language=en}}</ref>

The ''input shaft'' is connected to the engine and spins at engine speed whenever the clutch is engaged.<ref>{{cite web |title=How Manual Transmissions Work |url=https://auto.howstuffworks.com/transmission2.htm |website=www.howstuffworks.com |access-date=13 March 2020 |language=en |date=1 April 2000}}</ref> The ''countershaft'' has gears of various sizes, which are permanently meshed with the corresponding gear on the input shaft.<ref>{{cite web |title=Basic Anatomy – How The Manual Transmission Works |url=https://www.gotodobbs.com/blog/how-manual-transmission-works/ |website=www.gotodobbs.com |access-date=13 March 2020 |date=29 April 2017}}</ref> The gears on the ''output shaft'' are also permanently meshed with a corresponding gear on the countershaft; however, the output shaft gears are able to rotate independently of the output shaft itself (through the use of bearings located between the gears and the shaft).<ref>{{cite web |title=How It Works: Manual transmissions |url=https://driving.ca/auto-news/news/how-it-works-manual-transmissions |website=www.driving.ca |access-date=13 March 2020 |language=en |date=12 September 2018}}</ref> Through the use of [[Shaft collar|collars]] (operated using the ''shift rods''), the speed of the output shaft becomes temporarily locked to the speed of the selected gear.<ref>{{cite web |title=Manual Transmission Basics |url=https://www.edmunds.com/car-technology/manual-transmission-basics.html |website=www.edmunds.com |access-date=13 March 2020 |language=en-us |date=25 April 2001}}</ref> Some transmission designs—such as in the Volvo 850 and S70—have two countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's ring gear. This allows for a narrower transmission since the length of each countershaft is halved compared with one that contains four gears and two shifters.

The fixed and free gears can be mounted on either the input or output shaft or both. For example, a five-speed transmission might have the first-to-second selectors on the countershaft, but the third-to-fourth selector and the fifth selector on the main shaft. This means that when the vehicle is stopped and idling in neutral with the clutch engaged and the input shaft spinning, the third-, fourth-, and fifth-gear pairs do not rotate.

When neutral is selected, none of the gears on the output shaft are locked to the shaft, allowing the input and output shafts to rotate independently. For reverse gear, an idler gear is used to reverse the direction in which the output shaft rotates. In many transmissions, the input and output shafts can be directly locked together (bypassing the countershaft) to create a 1:1 gear ratio which is referred to as ''direct-drive''.

In a transmission for [[longitudinal engine|longitudinal engined]] vehicles (e.g. most rear-wheel-drive cars), it is common for the input shaft and output shaft to be located on the same axis, since this reduces the [[torsion (mechanics)|torsional forces]] to which the transmission casing must withstand. The assembly consisting of both the input and output shafts is referred to as the ''main shaft'' (although sometimes this term refers to just the input shaft or output shaft). Independent rotation of the input and output shafts is made possible by one shaft being located inside the hollow bore of the other shaft, with a bearing located between the two shafts.

In a transmission for [[transverse engine]]d vehicles (e.g., front-wheel-drive cars), there are usually only two shafts: input and countershaft (sometimes called input and output). The input shaft runs the whole length of the gearbox, and there is no separate input pinion. These transmissions also have an integral [[Differential (mechanical device)|differential]] unit, which is connected via a pinion gear at the end of the counter/output shaft.

<gallery caption="Gear selection in a constant-mesh transmission" mode="packed">
Image:Manual transmission clutch First gear.PNG|First gear (blue, to back)
Image:Manual transmission clutch Second gear.PNG|Second gear (blue, to front)
Image:Manual transmission clutch Third gear.PNG|Third gear (purple, to back)
Image:Manual transmission clutch Fourth gear.PNG|Fourth gear (purple, to front)
</gallery>
<gallery mode="packed">
Image:Manual transmission clutch Reverse gear.PNG|Reverse (green engaged)
Image:Manual transmission clutch Neutral.PNG|Neutral (all disengaged)
</gallery>

===Dog clutch===
In a modern constant-mesh manual transmission, the gear teeth are permanently in contact with each other, and [[dog clutch|dog clutches]] (sometimes called ''dog teeth'') are used to select the gear ratio for the transmission. When the dog clutches for all gears are disengaged (i.e. when the transmission is in neutral), all of the gears are able to spin freely around the output shaft. When the driver selects a gear, the dog clutch for that gear is engaged (via the gear selector rods), locking the transmission's output shaft to a particular gear set. This means the output shaft rotates at the same speed as the selected gear, thus determining the gear ratio of the transmission.<ref>{{cite web |title=How Manual Transmissions Work |url=https://auto.howstuffworks.com/transmission4.htm |website=www.howstuffworks.com |access-date=7 June 2020 |language=en |date=1 April 2000}}</ref>

The dog clutch is a sliding selector mechanism that sits around the output shaft. It has teeth to fit into the [[Spline (mechanical)|splines]] on the shaft, forcing that shaft to rotate at the same speed as the gear hub. However, the clutch can move back and forth on the shaft, to either engage or disengage the splines. This movement is controlled by a selector fork that is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the selector. The selector is typically symmetric: it slides between two gears and has a synchromesh and teeth on each side in order to lock either gear to the shaft. Unlike some other types of clutches (such as the foot-operated clutch of a manual-transmission car), a dog clutch provides non-slip coupling and is not suited to intentional slipping.

==={{Anchor|Synchronized transmission}}Synchromesh===<!-- This subsection is linked from non-synchronous transmission, don't remove anchor! -->
[[File:Sincronizzatore.jpg|thumb|Synchronizer rings]]
In order to provide smooth gearshifts without requiring the driver to [[double-clutching (technique)|manually match the engine revs]] for each gearshift, most modern passenger car transmissions use 'synchromesh' (also called 'synchronizer rings') on the forward gears. These devices automatically match the speed of the input shaft with that of the gear being selected, thus removing the need for the driver to use techniques such as [[double-clutching (technique)|double-clutching]]. The synchromesh transmission was invented in 1919 by [[Earl Avery Thompson]] and first used on production cars by Cadillac in 1928.<ref>{{cite web|url=https://hooniverse.com/synchromesh-gear-box-hows-that-work/|title=Synchromesh Gear Box- How's That Work?|date=29 April 2014|work=hooniverse.com|access-date=23 September 2019}}</ref>

The need for synchromesh in a constant-mesh transmission is that the dog clutches require the input shaft speed to match that of the gear being selected; otherwise, the dog teeth will fail to engage and a loud grinding sound will be heard as they clatter together. Therefore, to speed up or slow down the input shaft as required, cone-shaped brass synchronizer rings are attached to each gear. When the driver moves the gearshift lever towards the next gear, these synchronizer rings press on the cone-shaped sleeve on the dog collar so that the friction forces can reduce the difference in rotational speeds.<ref>{{cite web|url=http://www.howstuffworks.com/transmission3.htm|title=Synchronizers; graphic illustration of how they work|website=Howstuffworks.com|access-date=2007-07-18|date=April 2000}}</ref> Once these speeds are equalized, the dog clutch can engage, and thus the new gear is now in use. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed. Many transmissions do not include synchromesh on the reverse gear (see [[#Reverse gear|Reverse gear]] section below).{{fact|date=February 2023}}

The synchromesh system must also prevent the collar from bridging the locking rings while the speeds are still being synchronized. This is achieved through 'blocker rings' (also called 'baulk rings'). The synchro ring rotates slightly because of the frictional torque from the cone clutch. In this position, the dog clutch is prevented from engaging. Once the speeds are synchronized, friction on the blocker ring is relieved and the blocker ring twists slightly, bringing into alignment certain grooves or notches that allow the dog clutch to fall into the engagement.{{fact|date=February 2023}}

Common metals for synchronizer rings are [[brass]] and [[steel]], and are produced either by [[forging]] or sheet metal shaping. The latter involves stamping the piece out of a sheet metal strip and then [[machining]] to obtain the exact shape required. The rings are sometimes coated with anti-wear linings (also called 'friction linings') made from [[molybdenum]], [[iron]], [[bronze]] or [[carbon]] (with the latter usually reserved for high-performance transmissions due to their high cost).<ref>{{cite web |author=Diehl Metall Stiftung & Co. KG |url=http://www.diehl.com/en/diehl-metall/products/synchronizer-rings.html |title=Synchronizer Rings: Diehl Metall |website=Diehl.com |access-date=2017-03-06 |archive-date=7 March 2017 |archive-url=https://web.archive.org/web/20170307203812/http://www.diehl.com/en/diehl-metall/products/synchronizer-rings.html |url-status=dead }}</ref>

Mechanical wear of the synchronizer rings and sleeves can cause the synchromesh system to become ineffective over time. These rings and sleeves have to overcome the momentum of the entire input shaft and clutch disk during each gearshift (and also the momentum and power of the engine, if the driver attempts a gearshift without fully disengaging the clutch). Larger differences in speed between the input shaft and the gear require higher friction forces from the synchromesh components, potentially increasing their wear rate.{{fact|date=February 2023}}

=== Reverse gear ===
Even in modern transmissions where all of the forward gears are in a constant-mesh configuration, often the reverse gear uses the older ''sliding-mesh'' ("crash box") configuration. This means that moving the gearshift lever into reverse results in gears moving to mesh together. Another unique aspect of the reverse gear is that it consists of two gears—an [[idler gear]] on the countershaft and another gear on the output shaft—and both of these are directly fixed to the shaft (i.e. they are always rotating at the same speed as the shaft). These gears are usually [[spur gear]]s with straight-cut teeth which—unlike the helical teeth used for forward gear—results in a whining sound as the vehicle moves in reverse.

When reverse gear is selected, the idler gear is physically moved to mesh with the corresponding gears on the input and output shafts. To avoid grinding as the gears begin to the mesh, they need to be stationary. Since the input shaft is often still spinning due to momentum (even after the car has stopped), a mechanism is needed to stop the input shaft, such as using the synchronizer rings for fifth gear. However, some vehicles do employ a synchromesh system for the reverse gear, thus preventing possible crunching if reverse gear is selected while the input shaft is still spinning.<ref>{{cite web|url=http://www.alfisti.net/311.2.html?&L=1 |title=Buyers Guide Alfa Romeo Spider & GTV 916 |publisher=Alfisti.net |access-date=2010-10-16}}</ref>

Most transmissions include a lockout mechanism to prevent reverse gear from being accidentally selected while the car is moving forwards. This can take the form of a collar underneath the gear knob which needs to be lifted or requiring extra force to push the gearshift lever into the plane of reverse gear.

=== Non-synchronous transmission ===
{{Main|Non-synchronous transmission}}
[[File:Crash gearbox 3gears and reverse.gif|thumb|right|3-speed non-synchronous "crash" gearbox; used in automobiles pre-1950s and semi-trailer trucks]]
Another design of transmission that is used in older [[car]]s, [[Semi-truck|trucks]], and [[tractor]]s, is a non-synchronous transmission (also known as a crash gearbox). Non-synchronous transmissions use a sliding-mesh (or constant-mesh, in later years) design and have the nickname "crash" because the difficulty in changing gears can lead to gear shifts accompanied by crashing/crunching noises.
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