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A Transmission specifies, generally, the gears that transmit mechanical power from an engine via a driveshaft to an axle. Alternatively, Transmission is the act of transmitting an electrical messages (and the associated phenonomena of radiant energy that passes through mediums). In medicine, it refers to incidents inwhich infectious diseases are transmitted.

Table of contents
1 Mechanical
2 Electrical


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, horsepowers, 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.

Simple transmissions

The simplest transmissions, sometimes called gearcases, 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.

Unsynchronized transmission

The earliest automotive transmissions were unsynchronized. 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 clutching was sometimes used. However, many transmssions 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.

Most transmissions 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 coutershaft, 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.

Synchornized transmission

Synchronized 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.

Automatic transmission

[ to be written ]


In telecommunication, the term transmission has the following meanings:

1. The dispatching, for reception elsewhere, of a signal, message, or other form of information.

2. The propagation of a signal, message, or other form of information by any means, such as by telegraph, telephone, radio, television, or facsimile via any medium, such as wire, coaxial cable, microwave, optical fiber, or radio frequency.

3. In communications systems, a series of data units, such as blocks, messages, or frames.

4. The transfer of electrical power from one location to another via conductors.

Source: from Federal Standard 1037C and from MIL-STD-188