Types of Transmission System
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’t shift on the go
without grinding gears. Its usually have 6-8 forward gears and 1-2 reverses.
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 ¼ 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.
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 ¼ MPH on the 16 speed models. (JD calls this a Power Reverser)
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 ¼ MPH on the 16 speed models. (Massey Ferguson’s DynaQPS is a powershift
and Kubota’s 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:
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.
A compound planetary set whose bands and clutches are actuated by
hydraulic servos controlled by the valve body, providing two or more gear
ratios.
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:
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.
