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At one time, all large trucks used transmissions derived from farm tractors. This made sense decades ago, when those driving heavy haulers often had agricultural roots and had plenty of practice operating manual transmissions. Today, the transport industry has come to embrace those from all walks of life, which has had a number of surprising benefits.
Manual Transmissions
There is a mistaken assumption that a heavy truck transmission is simply an oversized version of those found in cars. While the operating principle is the same, heavy truck transmissions often make do without the synchronizers that ease shifting in cars. These slider-gears fit between the gear sets in a transmission and allow the driver to engage a gear without perfectly matching RPMs. These transmissions require a great deal of practice to use correctly. Manual truck transmissions also have a separate, pneumatically-controlled, two-speed transmission built in, which controls the high and low range. On most transmissions, the driver will start out in low range and shift through all of the transmission gears before engaging the high range. On others, the range control must be used between gear-shifts. The shift pattern on these transmissions goes something like: first gear (low), first gear (high), second gear (low), second gear (high), etc.
Automated Manuals
An automated manual (mistakenly called an "automatic" by many drivers) is internally identical to a standard manual transmission, but uses a computer-controlled series of servos to eliminate the need for manual shifting. The simplest way to understand how an automated manual works is to imagine that there is a robot sitting in the passenger seat shifting for you. Because they always shift at the right RPM and never grind gears, automated manuals have a number of advantages over a standard manual. Increased ease of use, longer transmission life, better fuel economy and increased acceleration are all to be expected from an automated transmission.
Planetary-Gear Automatic
This is the transmission that most people think of when they hear the word "automatic" and is the type found in most cars. Planetary-Gear Automatics (PGAs) are not generally used in heavy-vehicle applications. This is due primarily to the fact that PGAs rely on a series of hydraulically controlled clutches to transfer power; clutches that are prone to slippage and do not offer the positive engagement of a standard transmission. This is vitally important for trucks because they rely on engine back-pressure to safely descend hills. If the transmissions internal clutches begin to slip while a truck is going downhill, it may begin to accelerate uncontrollably with the force of gravity. This dangerous condition is called a "runaway" and is responsible for dozens (if not hundreds) of deaths a year.
