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In mechanics, a transmission is the gear and/or hydraulic system that transmits mechanical power from a prime mover (which can be an engine or electric motor), to some form of useful output device. Transmissions are used for the following basic reasons:
- To reduce the often unsuitable high speed and low torque of the prime mover output shaft to a more useable lower speed with higher torque.
- To provide a mechanical advantage (i.e increase in torque) to allow higher forces to be generated.
- To change the physical direction in which power is transmitted.
In its original usage, a transmission was a mechanical assembly, usually with gears, for changing the nature of motion supplied by some sort of power source to make it more suitable for utilization equipment. Early transmissions included right-angle drives and other gearing in windmills, horse powered devices, and steam engines, mainly in support of pumping, milling, and hoisting applications.
Most commonly today, the transmission is a component of the automobile. Two main types are in use today: the automatic transmission, and the manual, synchronized transmission. Earlier cars and trucks used an unsynchronized transmission.
Transmissions are also used in agricultural, industrial, construction, and mining equipment. In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the hydrostatic drive and Ward-Leonard controls .
The simplest transmissions, sometimes called gearboxes, provide gear reduction (or, more rarely, an increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft. These are often used on PTO powered agricultural equipment, since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as with rotary mowers), or horizontally extending from one side of the implement to another (as with manure spreaders , flail mowers , and forage wagons ). More complex equipment, such as silage choppers and snowblowers, has drives with outputs in more than one direction.
Regardless of where they are used, these simple transmissions all share an important feature: the gear ratio cannot be changed during use. It is fixed at the time the transmission is constructed.
An automotive transmission is an intermediary device for transmitting the rotary energy of the car's engine at a suitable rotation speed, to the differential and from there to the driving wheels.
The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Engines typically operate over a range of 600 to about 6000 revolutions per minute (though this varies from design to design), while the car's wheels rotate between 0 rpm and around 2500 rpm. Furthermore, the engine provides its highest torque outputs approximately in the middle of its range, while often the greatest torque is required when the vehicle is moving from rest or travelling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits, is required. Transmissions perform this transformation.
Most transmissions and gear boxes used in automotive and truck applications are contained in a cast iron case, though sometimes aluminum is used for lower weight. There are three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the output shaft towards the rear axle. The shaft is suspended by the main bearings, and is split towards the input end. At the point of the split, the pilot bearing holds the shafts together. The gears and clutches ride on the mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
The countershaft is generally below the mainshaft and turns in the opposite direction, driven by a bevel gear on the input shaft.
The earliest automotive transmissions were entirely mechanical unsynchronized gearing systems. They could be shifted, with multiple gear ratios available to the operator, and even had reverse. But the gears were engaged by sliding mechanisms or simple clutches, which required a skilled operator who could use timing and careful throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged.
When upshifting, double declutching was sometimes used. However, many transmissions were easier to shift from one gear to another without the use of the clutch at all. The clutch, in these cases, was only used for starting and stopping.
Synchronized manual transmissions began to appear in the 1930s, and incorporated synchronizers that were capable of bringing the countershaft up to speed, and aligning the gears in phase, prior to engagement. By the 1960s most passenger automobiles with manual transmissions were synchronized.
Many modern cars have an automatic transmission, which uses hydraulics as well as mechanical gearing, and partially automates the gear selection process. Recent developments have included electronically controlled "clutchless-manuals" which technically resemble traditional manual transmissions but from a driver perspective work like an automatic, and continuously variable transmissions which rather than providing a limited set of "ratios", allow the relationship between the speed of the engine and the speed of the wheels to be varied continuously.
Hydrostatic transmissions transmit all power with hydraulics, there is no mechanical coupling of the input and output. One half of the transmission is a variable displacement piston pump and the other half is a hydraulic motor. A movable swash plate controls the piston stroke to change the pump's displacement.
They are used in the drive train of some types of heavy equipment and applications requiring continuously variable control. Their disadvantages are high cost and sensitivity to contamination.
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