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transmission,in automobiles, system of parts connecting the engine to the wheels. Suitable torque, or turning force, is generated by the engine only within a narrow range of engine speeds, i.e., rates at which the crankshaft is turning. However, the wheels must turn with suitable torque over a wide range of speeds. While its speed is held roughly constant, the engine turns an input shaft on the transmission whose output shaft can be adjusted to turn the wheels at an appropriate speed.
The simplest transmissions are manual transmissions, and consist of a system of interlocking gearwheels. These wheels are arranged so that by operating a lever the driver can choose one of several ratios of speed between the input shaft and the output shaft. These ratios are called gears, first gear being the arrangement that gives the lowest output speed, second gear the next lowest, and so forth. To allow smooth shifting from one gear to another, a clutch is provided to disengage the engine from the transmission. The commonly used dry single disk clutch has a steel disk with a friction lining that is sandwiched between a flywheel on the engine shaft and a pressure plate on the transmission input shaft. When the driver takes his foot off the clutch pedal, springs squeeze the friction disk into the space between the flywheel and the pressure plate, enabling the engine shaft to turn the transmission.
For many cars and for normal driving conditions a transmission with three forward gears and one reverse gear is sufficient. In cars having small engines transmissions with four or five forward speeds are used; racing cars often have as many as six forward speeds. A synchromesh transmission is a manual transmission in which all forward gear wheels are held in mesh at all times. Used on most American cars with a manual transmission, it allows the driver to shift gears more smoothly and makes the car run more quietly.
The automatic transmission, introduced in 1939, switches to the optimum gear without driver intervention except for starting and going into reverse. The type of automatic transmission used on current American cars usually consists of a fluid device called a torque converter and a set of planetary gears. The torque converter transmits the engine's power to the transmission using hydraulic fluid to make the connection. For more efficient operation at high speeds, a clutch plate is applied to create a direct mechanical connection between the transmission and the engine.
The introduction of microprocessor-controlled electronic sensors has enhanced the performance of automatic transmissions still more. Data about engine speed, exhaust pressure, and other performance characteristics are sent to a processor that controls the changing of gears and the clutch plate in the torque converter via electrical switches, or solenoids. New approaches to transmission design combine the best features of manual and automatic transmissions to provide more efficient ways of channeling engine power to the wheels. A manumatic transmission is an automatic transmission with an added manual-shift mode; typically, a floor-mounted shifter offers an alternative selector path supplemented by buttons mounted on the steering wheel. A continuously variable transmission (CVT) uses a belt that connects two variable-diameter pulleys to provide an unlimited number of ratio changes and uninterrupted power to the wheels; CVT transmissions offer better fuel efficiency than conventional automatic transmissions, which change the transmission ratio by shifting gears. A sequential manual gearbox (SMG), developed for Formula One cars, uses computer-controlled actuators to operate the clutch and change gears when prompted by the driver; both manual and automatic modes are possible, and there is no clutch pedal. The dual clutch transmission (DCT), also called the direct shift gearbox (DSG), substitutes dual clutches for the conventional single-sided clutch to transfer power from the engine through two parallel paths; the gearbox features two sets of gears, identical to those in conventional manual transmissions—one set being the odd gears (1st, 3rd, 5th) and the other the even gears (2nd, 4th, 6th)—the gears must be shifted in sequence, and power to the wheels is never interrupted.
a mechanism for stepped variation of the gear ratio (the speed of rotation or feed rate). A transmission consists of interchangeable gear drives installed in a separate housing or in a common housing with other mechanisms. The transmission used to change the speed of the main cutting motion of metal-cutting machines is also called a gearbox. The gearbox designed for changing the feed in metal-cutting machines, which has certain special kinematic features, is called a feed box.
Transmissions are widely used to transmit power to the driving wheels of automobiles and other transportation vehicles powered by internal-combustion engines, which cannot develop high torque and power at low speeds of rotation and also do not permit a change in the direction of shaft rotation (reversing). The use of a transmission makes possible the generation of sufficient torque at the wheels, as well as providing reversing and efficient engine performance at various speeds.
The gear ratios u of transmissions usually correspond to a geometric progression (u1,u2 = ϕu1, u3 = ϕ2u1,…), which provides the same relative increase in u upon shifting from one speed to the next. In the USSR the values of u are standardized: 1.06, 1.12, 1.26, 1.41, 1.58, and 2.00. A stepped geometric progression with a different ϕ for certain sections of the control range, or an arithmetic progression (for example, in a feed box), is sometimes used.
The construction of a transmission depends on its purpose, the method of changing gears and the specifications of the machine or tool: transmitted power, speed, number of speeds (up to 48), and control range. For overdrive transmissions, a gear ratio u of not less than 1:2 is usually used; for underdrive transmissions, the ratio is not more than 4, and the number of transmissions between two shafts is not more than 6–8.
According to the method of changing gears, a distinction is made among transmissions with sliding gear clusters (with dog and toothed clutches), transmissions that have toothed clutches with synchronizing devices, transmissions with friction clutches and brakes, and transmissions with overrunning clutches. Gear boxes with sliding clusters permit shifting only after stopping or at a low idle speed; they are of simple and compact construction and are widely used in metal-cutting machines with a large number of speeds. At low values of ϕ the transmission may be simplified in design and made shorter by means of gear shaving, which allows one gear to mesh with different gears on other shafts. Transmissions that have gear clutches with synchronizing devices make it possible to first put in contact small friction surfaces, which equalize the angular speeds of the shaft and engaged gear, and then to engage the clutch coupling. Such transmissions provide smooth shifting at idle and are used mainly in motor vehicles. Transmissions with friction clutches and brakes permit shifting while running under load, and the use of clutches and brakes with electromagnetic, hydraulic, or pneumatic control provides remote shifting and the possibility of automation. Because of their complex construction and large size, such transmissions are used for a low number of speeds; brake shifting is used in planetary transmissions. Transmissions with overrunning clutches shift gears and change the direction of rotation of the drive shaft while the direction of rotation of the driven shaft remains unchanged; such transmissions are rarely used.
The changing of gears in transmissions is accomplished by individual control mechanisms, in which each gear cluster or clutch coupling is shifted by a separate lever; by centralized control mechanisms (of consecutive and selective engagement and with preliminary selection, or preselective), in which all gear cluster or clutch couplings are shifted by a common lever; by electrical and other remote-control devices; and by automatic control devices that shift speeds in relation to changing operating conditions.
The disadvantages of transmissions in comparison with mechanical variometers are stepped changing of the gear ratio and less convenient control. Their advantages—a rigid kinematic connection (strict uniformity of the gear ratios between the driving and driven shafts), high reliability and durability, and compactness and simplicity of construction—have brought about their widespread use in modern machines.
REFERENCEMashinostroenie: Entsiklopedicheskii spravochnik, vol. 11. Moscow, 1948.
N. IA. NIBERG
in optics, the passage of optical radiation through a medium, occurring without a change in the composition or in the relative intensities of the monochromatic component frequencies of the radiation. Transmission is classified as (1) direct, if there is no scattering of light in the medium (or if such scattering is negligibly small), (2) diffuse, if scattering in the medium is the determining factor and refraction within the medium and direct transmission are of no marked significance, or (3) mixed, if the transmission is partly direct and partly diffuse. A uniformly diffuse transmission is a special case of diffuse transmission in which the spatial distribution of diffuse radiation is such that the luminance is the same in all directions.
a device for the transfer of mechanical energy from a motor to the moving parts of machines, such as lathes and mills. Rotation is usually transmitted from the driveshaft to the machine by a drive belt or countershaft.
In modern technology, the term “transmission” is understood to mean an entire set of devices, from the engine shaft to the working parts of the machine in which it is installed. Thus, in an automobile or tractor, the power transmission, clutch, Cardan mechanism, and differential are components of the mechanical transmission. Hydromechanical, hydraulic, and electromechanical transmissions are also used in diesel locomotives, ships, trucks, and tractors. Hydromechanical transmissions consist of a torque converter and a mechanical gear box, hydraulic transmissions have a hydraulic pump and hydraulic motors, and electromechanical transmissions have a generator and electric motors.