Non-synchronous transmission is a form of transmission system based on gears that do not use synchronizing mechanisms. They are found commonly in in various types of commercial, military and agricultural vehicles. The operation of these vehicles is sometimes regulated by local, regional or national laws. Some countries require the operator to obtain a specialist Commercial Driver's License. The non-synchronous transmission type requires an understanding of gear range, torque, engine power, range selector, multi-functional clutch and shifter functions. Some vehicles may use a combination of transmissions for different mechanisms. An example would be a PTO, or power-take-off.
Non-synchronous Transmission of the Titanic seen as a visual example
The movie Titanic starring Leonardo DiCaprio & Kate Winslet has a scene of the engine room of the ship just before it struck an iceberg. The captain orders full reverse of the port propeller. Why does the engineer hesitate? Because he has to wait for the timing of the gear teeth to line up, in order to throw the gear lever, otherwise the lever would just be thrown back out at him. Non-synchronous transmissions won't go in gear until the operator times it. Also see mechanical engineering.
Drive A Standard, or Automatic?
All automatic transmissions have synchronizing mechanisms. Most manual transmissions also have some gear synchronizing mechanisms. 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, especially sports car & motorcycle enthusiasts. They learn that when they pop the clutch, that the vehicle will bolt as the clutch plates engage. But even these transmissions have synchronizing mechanisms that are designed to keep gear teeth from being broken off. These are referred to in the article transmission (mechanics) as unsynchronous. Don't confuse unsynchronous with non-synchronous. They're not the same.
Different than synchronous & unsynchronous, non-synchronous transmissions were designed without synchronizing mechanisms. They depend upon the operator to be experienced in changing gears. These types of transmissions are known to the world of 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 mega-machines often use non-synchronous transmissions.
Any transmission that requires the operator to manually synchronize engine cam-shaft revolutions (RPM) with drive-shaft revolutions is non-synchronous. Inexperienced operators grind gears, unable to engage, or disengage the transmission.
How does a non-synchronous transmission synchronize?
The operator uses a double-clutching technique that is taught in many ATA & PTDI associated truck driving schools, including the American Trucker Association, & Professional Truck Driver's Association school at the Houston Community College N.E. campus in northeast Houston, TX (USA). 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 they understood the synchronizing skill, and torque issues in non-synchronous transmissions (not to be confused with manual, or automatic). 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 visit Jacobs Vehicle Systems, or see diesel engine jake brake. Many other circumstances face operators of non-synchronous transmissions. Safety & operator skills have to be learned before operating any of these types of vehicles. There are many recommended colleges and schools offering training courses in this area.
Double Clutching
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(let the clutch pedal all the way out & take your foot off of it). If you've never done this, don't try this at home. 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.
The Clutch Brake
Unlike any other type of transmission, the non-synchronous transmission has a mechanism for slowing down, or stopping an idle gear. 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 it from engaging the fly wheel. In order to shift into gear, the clutch must be half way off the floor, otherwise the clutch brake will prevent a non-synchronous transmission from being shifted into or out of gear. Experienced operators are familiar with the use of the clutch brake. Many inexperienced operators don't know what it does. This can cause a clutch brake to burn out, and is one of the maintenance issues a good operator will check for before operating a commercial vehicle.
See Also
References
- Maxitorqueincludes graphic displays of non-synchronous transmissions
- Core Transmissions-for display only: this is not an endorsement
- Jacobs Vehicle Systems-for explaination only. What is a jake brake?
- ATA - American Trucking Association- not a global reference
- Professional Truck Driver's Institute- pertains to U.S. only
- Federal Motor Carrier Safety AdministrationNew Hampshire Dept. of Motor Vehicles 2005 Commercial Driver's License Manual, sec. 13.1.11 Section 13 page 13-3 says Double clutch if vehicle is equipped with non-synchronized transmission. (warning - this file is a complete manual in Adobe Acrobat format with a file size of over 10 Megabytes).