In 1842, the reversing lever was invented and patented as the Walschaerts valve gear in Belgium. Industrial marketing has since then coined spectacular names for various vehicle parts. Changing from the Locomobile, a 1906 race-car to what is now called the automobile, advertisers used design wording from the engineering departments to give new ideas a desirable appeal for sales promotions. From 1932, synchronizer mechanisms began to appear in automotive transmissions. The split off of automotive transmission types that has prevailed in engineering designs uses three major categories: automatic, manual, and non-synchronous. Some of the differences are improvements, including the continuously variable transmission installed in hybrid vehicles that are powered partly by an internal combustion engine, and partly by an electric motor. The concepts of transmission continue to employ methods for transferring the most conceivably efficient use of power.
All automatic transmissions have synchronizing mechanisms. Most manual transmissions also have synchronizers. But, there are still other types of transmissions used mostly in commercial applications that are non-synchronous.
Most people are familiar with the park, reverse, neutral, and drive positions of their automatic transmission. These are fully synchronous transmissions. Their gear ratios and hydro-pneumatic systems are designed to change gears based on engine performance and other velocity indicators, delivering torque to drive wheels.
Some people drive a standard or manual transmission. They learn that when they dump or pop the clutch, that the vehicle will lurch or lunge as the clutch plates engage. But even these transmissions have synchronizing mechanisms (called cone & collar synchronizers) that are designed to keep gear dog-teeth from being broken off.
Non-synchronous transmissions are designed to depend upon an operator experienced in changing gears. These types of transmissions are known to heavy equipment operators as non-synchronous transmissions. The operators must understand how to shift these transmissions into and out of gear. Many learn how to do this in certifying schools.
Heavy equipment for industrial, military, or farm use have different torque issues. They have unique stress from massive horsepower that makes converter faces shear. For the reasons of engineering a dependable, longer-life piece of equipment, these machines often use non-synchronous transmissions.
Any transmission that requires the operator to manually synchronize engine crank-shaft revolutions (RPM) with drive-shaft revolutions is non-synchronous.
They are engineered with the understanding that a trained operator will be shifting gears in a known coordination of timing. Commercial vehicle operators use a double-clutching technique that is taught in driver's trade schools. With payloads of cargo ranging in commercial freight of 80,000 lbs (40 tons) or more, some heavy haulers have over 24 gears that an operator will shift through before reaching a top cruising speed of 70 mph. Many low-low (creeper) gears are used in farm equipment to plow, till, or harvest. Also see Engineering vehicle. An inexperienced operator would suddenly find a piece of heavy equipment stuck in gear under full power, or even worse unable to shift into gear a runaway vehicle in neutral headed down a steep slope, unless he understood the synchronizing skill, and torque issues in non-synchronous transmissions. Many mountain roads require heavy equipment operators to remain in gear and not shift while passing down a steep grade. For more details about steep grade operation see either jake brake, or engine brake. Many other circumstances face operators of non-synchronous transmissions. Safety and operator skills need to be learned before operating any of these types of vehicles.
Operators of 18-wheelers, farm equipment, tractors & other heavy equipment learn to float the transmission in & out of gear, beginning with dis-engaging the clutch by pressing the clutch pedal only part way, enough to pull the transmission out of gear, re-engaging the clutch in neutral (between gears by letting the clutch pedal all the way back out) to let the engine revolutions deccelerate enough for the idle sprockets to shift, and free gear shafts to slow their revolutions per minute (RPM), then dis-engage the clutch again (by pressing the clutch pedal only part way to the floor) a 2nd time, and float the higher gear into engaging the drive coupling & fly wheel and engaging the clutch plates. Professional operators of heavy equipment take extensive safety training before ever learning how to double-clutch. Once an operator is familiar with range, range selector, rpm, velocity, and torque of heavy equipment like an 18-wheeler, they can begin to anticipate when to shift gears. Operators become familiar with ranges of gears. They also learn not to leave their foot on the clutch while driving, because these types of transmissions use the clutch for several very different purposes. The depth the clutch is depressed to the floor will determine what the clutch will be doing as a synchronizing function.
Unlike any other type of transmission, non-synchronous transmissions often have a mechanism for slowing down, or stopping an idle gear. In commercial motor vehicles, this mechanism is called the clutch brake, and is used by depressing the clutch all the way to the floor. This is useful in 18-wheelers that have just started their diesel engines, and are releasing parking locks, and engaging the transmission from a stop. The clutch brake not only slows or stops the idle gear axis, but can also prevent shifting into gear until the clutch is lifted a few inches off the floor. In order to shift into gear, the clutch must be half way off the floor, otherwise the clutch brake will prevent the transmission from being shifted into or out of gear.