AgriEngineers.com:
Types of Tractor Transmission Systems
Manual Transmission
A manual transmission is a type of transmission used in automotive applications. Manual transmissions often feature a driver-operated clutch and a movable gear selector, although some do not. If you have a manual transmission, you have to shift the gears yourself, usually with a stick located on your console and the clutch pedal. Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Manual transmissions are generally available with four to six forward gears and one reverse gear, although manual transmissions have been built with as few as 2 and as many as 7 gears. Some manuals are referred to by the number of forward gears they offer (e.g., 5-speed) as a way of distinguishing between automatic or other available manual transmissions.
Manual transmissions are of these types:
Simple unsynchronized systems:
In this systems, gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth. It required skills of timing and careful throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise the teeth would refuse to mesh.
It is the plain old standard shift transmission. The two most popular styles of manual shift transmissions are the sliding gear, and the collar. shift. In the sliding gear style, the gears are splined to the main shaft, and gear selection is made by actually moving the gears, via shift forks, into the appropriate location.
For the collar shift style, the gears are built up into a stack. The gears do not slide back and forth. These gears are not splined to the main shaft but are free to rotate when not engaged. There is a shift collar in between each gear pair, i.e. 1st & 2nd or 3rd & 4th. This collar is splined to the main shaft, and is the movable component when a speed change is called for.
Manual shift transmissions, while not as user-friendly as some of the other types, tend to have cast iron durability. It has no synchronizers thus one have to stop and clutch for each gear change. One can稚 shift on the go without grinding gears. Its usually have 6-8 forward gears and 1-2 reverses.
Synchronized systems:
In the system, the gearbox is of constant mesh type, in which all gears are always in mesh but only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, the others being allowed to rotate freely; thus greatly reducing the skill required to shift gears. These systems that will automatically "mesh" while changing gears. Basically the same as the manual shift except it has synchronizers and can be shifted on the go. The clutch should always be used for starts, stops as well as shifting. It usually has 6-8 forward gears and 1-2 reverses.
Let's suppose our transmission is synchronized between 3rd and 4th gears. We'll start out in 3rd gear, and then shift into 4th. As we shift, the first occurrence in the chain of events is that we move the transmission out of 3rd gear, and into neutral. As we continue moving the shift lever towards 4th gear, a brass cone applies friction to 4th gear, increasing or decreasing it's speed to match that of the rotating collar. Once the speeds have equalized, the gears still may not be lined up with each other, so there are little triangular shaped teeth around the outer circumference of the brass cone, which serve to ever so slightly rotate the shift collar teeth and the gear teeth into perfect alignment. This whole process occurs rapidly, usually allowing a straight-through shift, directly out of one gear and into the next. Synchro transmissions range from simple, where only a single pair of gears are synchronized, on up to full synchronization of all speeds, including forward and reverse
Shuttle-Shift or Synchro-Shuttle:
This is a standard transmission that may have as many gears in reverse as forward. There is a lever on the left of the steering that performs the forward to reverse gear change. Most Shuttle Shifts have some synchronized gears or they may be fully synchronized. i.e. there is synchronization between forward and reverse. The clutch must be used to start, stop and change directions. Usually has 9, 12 or 16 gears in each direction. May have a gear as low as ｼ MPH on the 16 speed models.
Shuttle-Shift is a real benefit to have on a tractor that will be used for front loader work. Still a reliable transmission, but a little more subject to failures because of the extra bearings, synchro rings, etc. that are in use.
Power-Shuttle:
With this Standard transmission, you can change forward to reverse directions and also start and stop without using the clutch. Most if not all Power Shuttle transmissions have fully synchronized gears. Usually has 9, 12 or 16 gears in each direction. May have a gear as low as ｼ MPH on the 16 speed models. (JD calls this a Power Reverser)
Power-Shift:
This standard transmission has all of the benefits of the power shuttle as well as being able to shift between gears without the clutch. It can be started, stopped, changed from forward to reverse and the gears shifted without the clutch. Power shift gives you the ability to select several different speeds while on the go without having to use the clutch pedal. When everything is working properly, they are really nifty to use. Beware of poor maintenance habits, though. Neglecting to change the transmission oil and filters, as recommended, will result in this transmission biting a big chunk of money out of your wallet.
It usually has 9, 12 or 16 gears in each direction. It may have a gear as low as ｼ MPH on the 16 speed models. (Massey Ferguson痴 DynaQPS is a powershift and Kubota痴 GST is essentially a powershift.)
Automatic Transmission
An automatic transmission is an automobile gearbox that can change gear ratios automatically. If you have an automatic transmission, the mechanism changes without any help from you, thus freeing the driver from having to shift gears manually. This is accomplished through a system that works by oil pressure. Each shift of the gears is controlled by a shift valve; the gears shift change depending on speed, the road, and load conditions. Automatic transmissions feature clutch packs to select gear ratio. Transmissions that employ clutch packs but allow the driver to manually select the current gear are called semi-automatic transmissions. A 5-speed automatic transmission is referred to as a 5-speed automatic
Hydrostatic Transmission (HST):
Hydrostatic transmissions transmit all power with hydraulics i.e. with the power of oil.. One half of the transmission is a variable displacement pump and the other half is a hydraulic motor. A movable swash plate controls the piston stroke to change the pump's displacement. A hydrostatic transmission works as being a variable-displacement hydraulic pump, driving a fixed-displacement hydraulic motor.
The greatest advantage of a hydrostatic transmission is the ability to infinitely vary the ground speed and quickly change directions. Another advantage is reliability. This transmission is self-protecting from operator abuse. Also, on foot pedal controlled transmissions, there is a built in safety factor in that you need only lift your foot from the pedal, to bring the tractor to a controlled stop. Their disadvantages are high cost, sensitivity to contamination and a slight loss of power at the PTO shaft. You must also remember to apply the parking brake should you park the tractor on a slope.
Hydrostatic is, by far, the best choice for turf mowing applications or for any tasks that require constant speed and direction changes within a small area. They are used in the drive train of riding lawnmowers and lawn tractors and applications requiring continuously variable control.
A hydraulic automatic transmission consists of the following parts:
Fluid coupling or Torque converter:
A hydraulic device connecting the engine and the transmission. It takes the place of a mechanical clutch, allowing the engine to remain running at rest without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.
Planetary gearset:
A compound planetary set whose bands and clutches are actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.
Valve body:
A hydraulic control center that receives pressurized fluid from a main pump operated by the fluid coupling/torque converter. The pressure coming from this pump is regulated used to run a network of spring-loaded valves, check balls and servo pistons. The valves use the pump pressure and the pressure from a centrifugal governor on the output side (as well as hydraulic signals from the range selector valves and the throttle valve or modulator) to control which ratio is selected on the gearset; as the car and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. The hydraulic pressure controlled by these valves drives the various clutch and brake band actuators, thereby controlling the operation of the planetary gearset to select the optimum gear ratio for the current operating conditions. However, in many modern automatic transmissions, the valves are controlled by electro-mechanical servos which are controlled by the Engine Management System or a separate transmission controller.
Continuously variable Transmission (CVT):
A different type of automatic transmission is the continuously variable transmission or CVT, which can smoothly alter its gear ratio by varying the diameter of a pair of belt or chain-linked pulleys, wheels or cones. Some continuously variable transmissions use a hydrostatic drive consisting of a variable displacement pump and a hydraulic motor to transmit power without gears. CVT designs are usually as fuel efficient as manual transmissions in city driving, but early designs lose efficiency as engine speed increases.
A slightly different approach to CVT is the concept of toroidal CVT or IVT (from infinitely variable transmission). These concepts provide zero and reverse gear ratios.
