Gear box

A gear box is a machine element making it possible to modify the transmission ratio between the number of revolutions and the couple of a driving tree and an output shaft. A given number of report/ratio is realizable.

Its most frequent case of use is the transmission couple of a thermal engine to the Roue S motor coaches of a Véhicule. It is also used in different multiples contexts such as the Machine-outil, farming machines…

Need for a gear box

The gear box is the element which makes it possible to adapt the engine torque to the resistive torque of the stop to the displacement of the vehicle, under all the conditions of rolling (on the dish, in the rises, the descents, the turns…).

Motor vehicles thermal

The car is today in quasi exclusiveness driven by a Thermal engine. These engines have operating features (maximum couple and power) which do not allow their direct coupling on the wheels of the terrestrial vehicles:

Their output varies much with the mode (in particular at the very low speeds), and their starting is delicate.
They turn only in one direction.
the driving couple is rather weak (600 N.m corresponds to a large engine) ; moreover it is very inconstant in the beach of use of these engines.

These general characteristics impose for the use of a vehicle terrestre :

the possibility of dissociating the engine of the wheels of the vehicle, for speeds close to the arrêt : it is partly the role of the Embrayage. But that supposes that this one can remain in disconnected position.
the possibility of reversing the direction of rotation on the level them wheels to ensure walk behind.
increase in the couple available on the wheel (the driving axle), the gears reducer being the most common solution.

Moreover, usual terms: short Stop, Speed limit, Deceleration, Relief, imposes different Allure S of operation. A vehicle must thus have a transmission system of power proposant :

a null report/ratio (called Not dead).
a report/ratio of Step back.
One or more ranges of reports/ratios staged for the Walk before.

It is thus about a whole of Réducteur S laid out in the same case.

The denomination " box of vitesses" is partly erroneous since, actually, its role is to adapt the couple. This name is related to the first machines Industrie lles which received the driving energy of a common driving shaft, animated by a Water mill or a Steam engine turning at constant speed. Systems of pinions allowed the choice a speed of operation. The Anglo-Saxon denomination is thus more juste : gearbox (literally: box with Gears ), however, all the gear boxes are not built on the principle of toothed gearing.

Alternatives to the gear box

The reappearing technology of the electric vehicles announces the abandonment of this transmission system. Indeed the electrical motors are increasingly powerful (important couple and unequalled compactness, output without equal). They can henceforth be directly established in the wheel, which eliminates all the elements from transmission (differential, Cardan joints…).

The transmission Hydrostatique for a long time adopted on the vehicles of agricultural Chantier and S allows a continuous variation speed with a natural distribution of the couples the unit of the wheels with exceptional Rendement S. On the other hand this technology is more cumbersome (tank and Refroidissement of Huile), but also particularly simple and reliable to implement. It was the subject of tests only on vehicles of small size and is used only on Engin S with slow displacement.

Classification of the gear boxes

For the vehicles with Driving thermal, one will be able to distinguish the boxes according to three independent technical functions:

the technology of the reducers: by Gears S traditional, Epicyclic gear, Belt…
the control device: manual, semi-automatic, automatic, sequential…
synchronization: authorizing the change of report/ratio moving or to the stop only.
Finally the orientation which has of influence only on the geometry of the connections with the driving shaft and the differential. Architecture is different according to whether the engine is established transversely (axis of rotation of the Vilebrequin Parallèle with the axis of rotation of the wheels) or Longitudinal ement (orthogonal axes ) and, that the couple is transmitted to the nose gear wheels, back, or with the four wheels.

To each Combinaison corresponds a different model of gear box.

Manual parallel gears box

Description

The manual box called “to pinions always in catch” is used nowadays; it is distinguished on this point from the boxes from machine tools which have disconnected gears.

This type of box is generally made up of two tree S carrying of the pinions:

the tree of entry (or primary education) related to the driving shaft via the Clutch, carries the fixed primary education pinions. There are as many pinions as of reports/ratios of box.
the output shaft (or secondary) carrying insane pinions (gearing respectively with a pinion of the tree of entry), the systems of first shaft and the synchronizers.
the third tree intervenes only for step back. It contains a pinion which can slide and intercalate themselves between a pinion of the tree of entry and another of the tree of sortie ; thus, one has gears moreover between the two trees (either two inversions of direction of rotation instead of one), from where reversed walk. It is the only case where the gears are not always in catch.
the output shaft is related to the conical couple of the differential, integrated into the Carter of the box (for the vehicles with traction front) or deferred on the back bridge (for the vehicles with propulsion).
the change of report/ratio is done by handling of slides actuating direct-drive dog clutches and synchronizers thanks to the Fourchette S of orders temporarily related to the gear shift lever.

Operation

Selection of a report/ratio

The principle of this type of box rests on the choice of several couples of pinions (called Engrenage S) offering different transmission ratios. Each gears consist of a pinion of entry interdependent of the input shaft, and of another pinion of exit in Liaison pivot with the secondary tree. A report/ratio is engaged when one of the pinions of exit becomes interdependent of the secondary tree. During this time the other pinions turn freely. It is said that they are insane.

After disengaging, to make a pinion insane interdependent of its tree, it should initially be synchronized with its tree, i.e. to cancel the number of relative revolutions, then to block it in rotation. The operation is ensured by a synchronizer (synchro) and a direct-drive dog clutch assembled on grooves, therefore in Liaison slide with the tree, and ordered in translation by one of the forks. The coupling between the direct-drive dog clutch and the corresponding pinion is carried out by means of teeth, which can be frontal (crenels) or peripherals (grooves). The shapes complementary to the two elements ensure a transmission of the power by obstacle. These are the teeth which grognent when one magnifying glass the operation, and not the teeth of gears.

The maintenance of the engaged report/ratio is done using a system of locking of the direct-drive dog clutches on the output shaft, but also of the slides of fork (pulled by the gear shift lever) on the casing of box.

Gears

In this type of box, one generally adopts gears with helicoid teeth . They are quieter because the teeth in catch are more numerous (report/ratio of control improved); they thus undergo each one less load than the pinions with right teeth. Unfortunately, they cause a thrust, which imposes the use of bearing S adapted and a reinforcement of the stage S. It should be noted that step back is obtained with pinions with right teeth, which explains the so characteristic noise. This same noise is identifiable on the old cars. This choice is imposed by the principle even step back since a pinion is moved axially to connect the two trees (additional gears for the inversion).

Direct catch

On certain boxes, when the two trees are on line, it is possible to couple them directly: for the direct catch, one of the sleeves returns the pinion of exit of the interdependent tree of entry to the secondary tree. The insane pinions thus have 2 different teeth: one, peripheral, which is used to transmit the movement of the input shaft to the secondary and the other, right or peripheral, for the passage speeds. Thus, the peripheral teeth of the pinions are not damaged at the time of the passage speeds because the shocks during engaging are supported by the whole of the teeth of the synchro. For step back, the tertiary axis is moved by a fork and the pinion which it door comes to place between the two pinions of the couple corresponding to reverse the direction of rotation.

Robotized box

They are manual boxes standards to which one grafted a automated system, electrotechnical often associated with the Hydraulique, which is behave:

In automatic mode: like an automatic box changing the reports/ratios at the most convenient time;
Is in semi-automatic mode, assisting the driver by leaving it to him to require, (using buttons, pallets, or of a lever), the passage of the reports/ratios, but which acts only when the conditions ad hoc are met.

One finds this system in various manufacturers, under name Sensodrive at Citroen, Quickshift at Renault, Selespeed at Alfa Romeo, F1 at Ferrari, etc

Sequential box

Description

It is a derived version from the manual gear box, which takes again the same mechanical principle of pinions and direct-drive dog clutches, but with a configuration which implies that, the driver cannot choose a report/ratio randomly, but only the report/ratio immediately higher or lower than that in service, which constitutes a sequence in the passage speeds. It is the case of the boxes of Motocyclette S as well as many automobile of races.

Operation

Except obligation: Regulation of automobile race or Manufacturing costs, henceforth the passage speeds is not done mechanically any more by the operation of the driver of a clutch lever and pedal, but by means of a automatism electrohydraulic Mécanique or which ensures only the passage speeds. Electronic piloting of the injection and the butterfly motorized makes it possible to put the engine and the pinions at the number of revolutions wanted during the passage speeds, so that the position of the Accélérateur does not have any more an influence throughout change of report/ratio.

Direct-drive dog clutch box

Description

Used in competition (rally in particular) and on the motor bikes, this box has the advantage of being able to do without Embrayage and to have a better output, at the price of a wear which can be fast and of a slapping at the time of the passages speed.

Operation

A manual box, synchronized , has pinions and rings ( synchros ) which make it possible to grant the number of revolutions of the input shaft with that of the secondary tree , before engaging the pinions. A direct-drive dog clutch box does not have these synchronizers, speed passes from a blow and, sometimes violently, if the trees do not have close number of revolutions. These boxes often have gears with right teeth which support best the jolts, but which make definitely more noise, on the other hand they have a better output, because the lateral thrust of the pinion S with helical teeth is absent.

Automatic box

Description

The automatic transmission is a system able to determine in an autonomous way the best possible report/ratio according to information external with the system such as the couple and speed engine, the depression of the accelerator pedal, the speed of the vehicle, the operating process of limps (D, S, handbook), the resistive torque of the vehicle (gone up, descent) and other more complex functions which depend on the technological level of the automatic gear box. With identical of a robotized gear box, it is an electrohydraulic system controlled by an electronic computer electronic computer which manages the passages speeds. On the other hand the transfer of power is continuous in an automatic gear box, which is not the case for the other types of box.

Several approaches for the ordering of the gear box are today on the marché :

the selector of order " PRNDL" is characterized by the fact that each position defines a specific operating process. One can thus find for example the positions suivantes :

P : Parking brake ( Park ): the box is blocked by the interlocking of a bolt in a revolving element of the stage of exit.
R : Step back ( Reverse ).
N : Not dead ( Neutral ).
D : Normal control with the totality of the reports/ratios ( Drive ).
3 and/or 2 : Control is:

with the only reports/ratios of 1st, 2nd or 3rd, which lets the engine take turns, adapted to control in mountain.
with the only report/ratio of 2nd (even with starting) to facilitate the departure on snow or glaze.

1 or L : Control in 1st or (Low), used for example in the strong descents.

the selector " steptronic" is in general on the more recent models. It keeps the positions P, R, NR and D of the " PRNDL" but generally a position sport offers. This position makes it possible to obtain a more aggressive selection of the reports/ratios at the time of a control on sinuous road for example. Moreover, when the selector is in position D or S, a manual mode making it possible the driver to directly choose the report/ratio is available. In parallel of this system, each manufacturer can choose to add buttons or levers on the wheel to select the report/ratio.

the common point between these two operating process is that there exists a mechanical bond between the lever and the gear box. Several manufacturers now offer a system approaching the system " steptronic" but using an electric drive for all the operations. The choice of the mode is done according to the current location of limps and the direction in which the driver moves the lever. For example, if the driver is in position " Drive" and that it pushes of a notch the selector, limps it automic will order the position " Neutre" ; On the other hand if the driver pushes the selector of two notches, limps it orders the position " Reverse". To pass in position " Park" it is generally necessary to press on a button. This type of order is interesting because the manufacturer can place the lever where he wants in the cockpit of the vehicle but request also a higher level of security. Indeed if electronics were to have a problem, the driver can more not have the means of being put in a sure position (" Neutre").

Operations

Seen outside, this box is laid out identically with the manual box between the engine and the Différentiel - bridge. However, the management of the shiftings of speed requires adaptations technologiques :

Upstream, the Embrayage is replaced by a hydraulic Torque converter. Its role is to transmit the input torque while authorizing slips (equivalent of inevitable ice-skating at the time of the passage of report/ratio), it also serves to separate the engine from the remainder as the chain like a clutch.
the most widespread constitution is that of epicyclic gears in cascade. While solidarizing by the means of Clutch S or Brake S, certain parts of these trains, one obtains different reports/ratios. Selection of a report/ratio depending then only on one combination of order of the pistons of order. So there is no first shaft to control.
a stage, or epicyclic gear is made up various components, planet gears, satellites, carries satellite, which freely turn the ones compared to the others (2 mobilities per stage).
a hydraulic pump high pressure, integrated in the box, trained directly by the engine, provides energy for the Actuateur S Hydraulique S necessary to the clutches and with the brakes, it also lubricates under pressure the various elements.
the control device takes into account the intention of the driver (state of the brake and accelerator pedals) the mode of the engine, the speed of the vehicle, as well as the transmitted couple. The best report/ratio is then applied. A distributer (equivalent of the chart with holes) then sends the pressure towards the elements concerned.

the first automatic generations of boxes are controlled by mechanical, electromechanical systems (box Cotal) or hydraulics. A cable connecting the box to the accelerator, brings under control the box of the load of the engine. The variable pressure produced by the pump gives information proportional to the driving mode. These parameters make it possible the control system to start the passage of the reports/ratios according to the needs.
the generations of current boxes are controlled by electronic calculator S, which, associated with many sensors, allow an increasingly intelligent management of the box.

Note importante : the control of the oil level of an automatic box is done driving turning, the oil pump " remplissant" the Torque converter. This implies that an automatic box has a gauge.

Staging of a gear box

The ideal box would be that proposing the positive ratio for each situation. One cannot however multiply the reports/ratios ad infinitum. The range of reports/ratios suggested must thus be most universal possible. Limited a long time to 3 speeds, the automobile box at gradually taken 4 then 5 even 6 speeds on the top-of-the-range vehicles. The Camion S have, for a long time, of the stagings of transmission on more than 10 reports/ratios. The motor cycles have some, usually, 6 (one saw models of series with 7 reports/ratios, and more in competition).

However certain vehicles (Daf in the Years 1960) had a variator proposing a continuous range of reports/ratios.

Nowadays, one finds new vehicles of tourism based on the same principle of the Transmission with Continuous Variation (CVT). Those have the advantage of permanently adapting the mode of the engine and the couple applied to correspond as well as possible at the equation economy/the speed imposed by the driver. In addition, the variation of the report/ratio applied gradually being done, no jolt is perceptible, contrary to certain robotized automatic boxes and seldom sequential. The stress is thus laid on comfort and the economy. Currently, these automatic boxes equip in particular the Nissan Micra n-CVT (93~2002) and the Honda Jazz CVT in Europe.

Transmission ratio

The total transmission ratio (box (S) + bridge) is a relationship between speeds of the engine and the wheels, but also (with the output near) between the engine torque and the couple available to the wheel.

$R_ {I} = \ frac {N_ \ mathrm {wheel}} {driving N_ \ mathrm {}} = \ eta. \ frac {driving C_ \ mathrm {}} {C_ \ mathrm {wheel}} = R_ \ mathrm {Bi}. R_ \ mathrm {bridge}$

The engines proposing a couple rather moderate and functioning correctly with high modes (thousands of turns/minute), it is thus advisable to reduce speed (hundreds of turns/minute) what will cause to increase the couple with the wheel, i.e. effort of Propulsion (in the broad sense) of the vehicle.

The series of reports/ratios available on a box is optimized by taking of account the characteristics of the engine:

couple, power and output (specific consumption) maximum, and modes associated with the engine. The majority of the engines have an operating feature according to the diagram opposite;

but also characteristics of the vehicle:

Mass of the car (PTRA)
Size of the Wheel S, report/ratio of the Differential .
desired Maximum speed
Cx coefficient of drag.

Report/ratio of first

The report/ratio of first is used only to tear off the vehicle of its position of stop. Its determination rests on the consideration of an unfavourable case of starting, namely the vehicle in maximum loading and a coast with 15 %. One thus determines by a mechanical study Statique, the couple necessary to the wheel.

Step back thus is often proposed with the same transmission ratio as the first (even a little shorter).

Intermediate reports/ratios

Once the moving traffic, to reach cruising speed, or maximum speed, acceleration imposes a force of permanent propulsion. The engine must thus give the maximum of couple. For that, at the time of the passage of the higher report/ratio (for example 1st worms 2nd), the engine leaving the mode with maximum power must fall down at least on the mode with maximum couple. It results an ideal staging from it from the transmission ratios according to a geometrical Suite whose reason is the relationship between the modes with power and stall torque.

Higher report/ratio

The penetration in the air of the car and frictions in the whole of the transmission offer a resistance to the advance which evolves/moves with speed. The consumption thus increases with speed. There thus exists a value of power equivalent to the maximum power delivered by the engine. This mode of operation makes it possible to fix the higher report/ratio of the gear box. Of course this report/ratio does not enter systematically the continuation defined previously. The staging is then corrected.

The ecological considerations led the manufacturers to reduce the consumption of the vehicles. Today the last report/ratio is often given for a mode of cruising with minimal specific consumption (consumption by minimal tower, is a maximum output). This point of operation of the engine is sufficient to maintain the car at established speed.

Some particular cases

the cross-country vehicles are often equipped with one second gear box called box with reduction or transfer, most of the time not synchronized, therefore handled to the stop. The reduction offering more force to advance for crossings. The same device allows the farm tractors to lay out of more than force for traction of the plowshares and other plow.

Bugatti Royales, whose Cut Napoleon of Ettore Bugatti, are been driven by an engine with 8 cylinders and 15 000 cm ³, with a couple such, that the only report/ratio of 2nd is enough to lead them of 0 until not very reasonable speeds, 3rd taking over only to go to tickle 200 km/h.

the 2CV Citroen proposed since 1948 a box with 4 reports/ratios whereas the Traction, queen of the road then had about it only 3. Also, to remain in the spirit of announced modesty, 4th will be called S (overgeared) not to be regarded as a speed. The staging of this box is curiously according to a arithmetic Suite what generates enormous holes between 1st, 2nd and 3rd. Finally its constitution is rather particular with 4 trees two to two coaxial and of the reductions obtained with 1 or 3 successive gears.

the Ford T was in its time the car produced in the greatest number of specimens. It was characterized however by its particular gear box which was worth a driving license to him special. Resting on the architecture of an automatic box with epicyclic gears, the reports/ratios were selected via 1 lever and 2 pedals actuating the straps of orders. It often happened that the first remainder engaged (system stuck in winter); then, as in films of Laurel and Hardy, many users were made crush during the crank-starting; a parade being to raise beforehand a driving wheel using the jack, the Differential then making turn the wheel in the vacuum.

See too

Thermal engine
Gears
Epicyclic gear
Tiptronic

External bonds

very complete Study on the gear boxes
complete Article with very a good part on synchronization
Study of gear boxes

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