﻿ Continuously variable transmission

# Continuously variable transmission

A continuously variable transmission is a uniformly transmitting gear unit, characterized by a steplessly adjustable translation. It often comes in machine tools and vehicles. To refer to such transmission abbreviations are often taken from English:

• CVT stands for Continuously Variable Transmission, which is a continuously variable transmission with a certain ratio range
• IVT stands for Infinitely Variable Transmission, a continuously variable transmission, in which a direction can be set to infinity slow. Practically, this means that the drive can turn the transmission input and output of the speed 0 is present without a clutch must be opened. Because the ratio of input rotational speed / output rotational speed, in this case infinity ( ∞) is the symbol ∞ is given as a limit of the control range.
• Variator

Continuously variable transmissions are mostly mechanical gears. Other continuously variable transmission are fluid transmission, in which the torque transmission is hydraulic.

• 3.1 Mechanical continuously variable transmission
• 3.2 Hydrostatic continuously variable transmission
• 3.3 Electric continuously variable transmission

## Basics

In continuously variable transmissions, the ratio of the rotational speeds of the driving and driven shafts, the translation may be continuously changed in a certain range. This can also standstill ( IVT) or include the reversal of rotation of a shaft. The ideal design of such a stationary infinitely adjustable gear is still one unresolved engineering challenge dar.

The torque transmission in a continuously variable mechanical transmission is frictionally engaged in the rule. Exception is the positive quasi-continuous chain of transmission in which a slat chain in constant engagement with two radially grooved conical disks. The grooves toward the center of the conical discs continuously narrower. The chain teeth formed by the thin lamellae are also close, but this can only be done almost continuously in small increments, that is.

In frictional torque transmission is the coupling of components whose geometries determine the transmission efficiency according to the lever principle, strong.

A gear change is achieved by changing this component geometries, but the same can not be done without changing the geometry of the location of the coupling components.

Developed for vehicle continuously variable transmission are realized primarily through friction couplings, which are characterized by so-called slip. For this slip, there are two main causes:

• Due to the principle there with all designs, a pressure area over which the mechanical power is transmitted, for example, the contact surface of the belt in the DAF VARIOmatic. Geometrically, the contact surfaces move against each other, because they have transverse to the direction of slightly different speeds. When VARIOmatic there is only one position ( translation ) in the transmission operates without slip. Here, the extension of all the tapered surfaces intersect at a point. In all other positions of the conical disks created Bohrschlupf or pivoting friction, which can be reduced by a narrow area of ​​contact of the belt.
• The elastic elongation expansion slip occurs (see also belt drives ) on the contact surface, which depends on the elasticity / rigidity of the friction partner, and the transferred forces.

The use of friction continuously variable transmissions is limited by several factors:

• Maximum possible Hertzian pressure between the coupling contact surfaces to prevent pitting or pitting;
• Permissible temperature rise due to the frictional losses;
• Direct energy use, for example, to compress conical disks.

In addition to mechanical continuously variable transmissions, there are other solutions:

• Hydrostatic transmission: The power is transmitted through moving a pressurized fluid between a hydraulic pump and a hydraulic motor. The mechanical is thus replaced by a hydraulic coupling.
• Hydrodynamic torque converter: the power is transmitted through a flowing movement of a fluid at low pressure. Hydrodynamic torque converter are usually completely self-regulating, the translation may not come from outside or only partially be influenced. In the application as a continuously variable transmission, they are unsuitable for most uses.
• Electric transmission: Power is transmitted as a controllable electric power between a generator and an electric motor.

## Description

With stepless mechanical transmissions, the force via traction, transferred either wide belts, slat chains or thrust belts. There are also Wälzkörpergetriebe (cone ring gear and Extroid CVT).

The central element of the power transmission with a traction mechanism gear is the variator. It consists of two axially displaceable conical disk pairs and the current between the traction means.

The endless consists of either

• A push belt, which runs between the two conical disk pairs,
• A wide V-belt running between two smooth conical disk pairs (see also: belt drive )
• A link chain that runs between two smooth conical disk pairs (see also: Multitronic ) or
• A chain with laterally displaceable laminations that runs between two toothed conical disk pairs (see also: chain transmission )

Main difference between the versions is:

• The push belt transmits in most operating conditions, the torque due to shearing forces, that is, although it is a flexible band, it is not taken for power transmission, but pressed.
• The belt works positively and has slip. For this, the operating noise of the belt is lower and he can run without lubrication. The maximum transmissible torque is relatively low.
• The link chain is working strong. The maximum transmissible torque is up to 400 Nm. The ratio change is possible with CVT transmissions in automobiles in the ratio of 1:7 to 1:8.
• The slat chain operates positively and thus slip.

### Function

The one cone pulley pair is connected to the drive and the other to the output. One diagonally opposite half of the cone pulley pairs being fixed on the shafts, while the other disc on the shaft is axially guided, can be moved controlled.

When the conical disks are far away from each other, then the traction element describes a small radius on the cone surface. Are the conical disks, however close together, then the traction mechanism follows a large radius on the cone surface.

The conical disk pairs thereby move in opposite directions, so that for the same Zugmittellänge center distance and the traction mechanism remains biased.

A small pull-means radius in the drive cone pulley pair corresponds to a small passage of a conventional transmission, a large radius of a large gear.

Are the conical disks far apart and the conical disks are pressed together, then the traction element radius and thus the ratio increased from small to big. Since this process is continuous, the ratio also varies continuously.

### Infinitely Variable Transmission ( IVT)

Certain variable transmission, such as the toroidal, hydraulic transmission, or the combination of one or more planetary gear with a continuously variable transmission can cover an "infinite " field of translation. The vehicle comes in Translation "infinite" in the state, with the engine running, without additional separating clutch from.

An example of this make modern continuously variable transmission in tractors represent about the Vario transmission of the tractor manufacturer Fendt. A planetary gear mechanism serving as a power switch, and divides the power of the driving motor to the driving wheels and the infinitely variable displacement hydraulic pump. The hydrostatic part of the transmission energy of the hydraulic fluid is also fed to two hydraulic motors, and is continuously variable to the drive wheels again available. The transmission ratio is controlled by the hydrostatic branch. Through the power split has yet another advantage: the power portion of the continuously variable ( hydrostatic ) power branch can be kept low in most operating points, whereby its poorer efficiency does not fall so much weight.

The transmission (Power Split Device) of the hybrid vehicle Toyota Prius is similarly constructed, the variable part consists here of an electrical transmission (generator and motor ). The system is marketed under the name of Toyota 's Hybrid Synergy Drive.

### Special shapes

#### NuVinci transmission

His basic principle was described by Leonardo da Vinci. The name NuVinci (new, eng. Newly ) is derived from it. It is a planetary gear, in which the force is concluded by bullets. By tilting the balls in relation to the lateral disks arranged the gear ratio can be varied continuously. Bicycle gear hub of the company Fallbrook operate according to this principle.

#### X - CONI - gear

The so-called X - CONI - gear sit on the function principle of the bevel ring gear, but can transmit more torque. You are coaxial in the rule and the function, power-split run and well suited for rear-wheel drive vehicles. The basic principle is a concentric arrangement of almost all components in the transmission. The power flows to the starting element (clutch, torque converter, etc.) to the planet carrier. One part goes to the output shaft with sun, the other part to a ring gear with internal and external teeth. The ring gear drives the oriented than outer planets cone. They are inclined by half the cone angle, and thereby form a "cylinder", where the " center drive" the transmission gear can be moved in translation on the output shaft of a displacement unit. The time required for the toroidal transmission pressure force generated by the cones themselves. In order to transfer high torque, can also be a separation between traction area and lubrication region realize.

The remarks as QuattroCONI (4 cones) or HexaCONI (6 cones) are useful, but a different number from two upward cone is conceivable, for example, as a modular design for various services.

When the highest torque transmission is required, eg for commercial vehicles, the execution of Duo -X CONI offers, in a second set of downstream cone.

## Applications

### Mechanical variable transmission

( " PIV " is a brand name ) widely used in mechanical engineering, especially in machine tools, before the widespread introduction of electronically controllable motors the continuously variable transmission with slat chain was.

Also in the vehicle continuously variable transmission were used. They became known to the general public for the first time as VARIOmatic in vehicles of DAF, which could forward and reverse drive at the same speed. The French Moped Makers Motobécane already began in the late 1940s with the development of a continuously variable transmission and manufactured in the 1950s, the resulting type Mobymatic. Here, the centrifugal clutch is driven by the rear wheels. A power circuit only came into existence from a speed of about 7 km / h at which the vehicle had to be previously accelerated by muscle power. From 1954 to 1957 DKW produced the scooter DKW hobby with belt type continuously variable transmission ( system Uher ) in an edition of 45 303 units. In contrast to Mobymatic - type, the driving speed was determined solely by the engine speed, which then caused me problems with driving on slopes with to the system Uher. Since the 1980s mopeds ( scooter / moped ) in large series of Honda (eg moped Camino ) and the Italian manufacturer Piaggio were made ​​(eg in the hexagon ) with a belt type continuously variable transmission and a centrifugal clutch. In 1987, the car manufacturer Subaru Justy the small car with an electronically controlled, continuously variable transmission ECVT mentioned on the market. In the same year Ford introduced the variable CTX automatic transmission for the second series of the Fiesta. The Fiat Group is using this gear in the automatic versions of the Fiat Panda I ( 141A ), Uno, Punto I, Tipo and Lancia Y10 Lancia Y and under the name Selecta or Selectronic.

Today, continuously variable transmission, see for example in drives of scooters and increasing again in cars up to 191 kW ( 260 hp ) (eg Audi Multitronic ), for chain applications and up to 350 Nm for steel thrust applications. An electronic system controls the continuously variable transmission so that the engine is always - is at the optimal speed range - for the selected mode of operation. This means depending on driving conditions, the engine ( usually called "Economy", while accelerating the motor in the speed range of the maximum torque remains ) in a speed range with the lowest possible fuel consumption and the highest possible performance (for example to overtake, usually called the "Power" runs in the speed range of the maximum power output ). During acceleration, the engine can deliver its maximum power at a constant speed, while the speed increase is compensated by the continuously variable transmission. Initially this has the irritating effect that, for a constant engine noise, the speed increases continuously ( " rubber band effect").

A variant of the CVT as a friction angle with varying roles, the 1999 presented ( Extroid Nissan CVT ) transmission. It is because of an oversized hydraulic and an unfavorable efficiency in small cars still too heavy and too expensive. However, the technical potential of rolling friction and therefore extremely quiet, comfortable stepless power transmission is apparent. A modern variant with rolling friction transmission is the cone ring transmission.

The Mercedes -Benz A- and B-Class, as well as the Dodge Caliber was equipped with a continuously variable automatic transmission with virtual transitions, which can be switched by hand also optional. Even the Honda Jazz ( from 1,4 l, up to 09 /2008), the Mini and the Ford Focus ( just 1.6 liters of diesel, and 1.8 L petrol engine and only until February 2007) are optionally available with a continuously variable transmission. Also in the Toyota iQ, Toyota Verso, Toyota Verso- S, Toyota Avensis (from 2009) (from 2009) and Toyota RAV4 is an optional continuously variable transmission ( Multidrive -S) available. With Subaru is for some models a "Linea Tronic " called continuously variable transmission available ( 2.0i and 2.5i petrol ).

### Hydrostatic continuously variable transmission

In Construction hydrostatic continuously variable transmission are common. On tractors purely hydrostatic transmission, however, could hardly prevail because of the moderate efficiency. Because of the power split, only a small proportion of power is usually transmitted hydrostatically and the rest mechanically, thereby improving the overall efficiency of the transmission with respect to a purely hydrostatic solution. Other manufacturers later brought similar solutions on the market. Today, all the leading manufacturers offer continuously variable transmission tractor, with standard tractors a clear trend for hydro-mechanical continuously variable transmission is observed.

### Electric continuously variable transmission

For diesel locomotives, submarines and ships an electric transmission (generator motor) is often used. Because of its weight and moderate efficiency, this solution could not prevail in cars. Similar to a hydrostatic continuously variable transmission, however, the transmission efficiency can also be significantly improved if a power-split design is used, such as the Toyota Prius, Yaris Hybrid and Auris Hybrid and Lexus vehicles RX400h, GS450h, LS600h and CT200h.

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