>> How Things Work >> How Manual Transmissions Work
drive a stick-shift car, then you may have several questions
floating in your head:
does the funny "H" pattern that I am moving this
shift knob through have any relation to the gears inside
the transmission? What is moving inside the transmission
when I move the shifter?
I mess up and hear that horrible grinding sound, what is
would happen if I were to accidentally shift into reverse
while I am speeding down the freeway? Would the entire transmission
article, we'll answer all of these questions and more as we
explore the interior of a manual transmission.
transmissions because of the physics of the gasoline engine.
First, any engine has a redline -- a maximum rpm value above
which the engine cannot go without exploding. Second, if you
have read How Horsepower Works, then you know that engines
have narrow rpm ranges where horsepower and torque are at
their maximum. For example, an engine might produce its maximum
horsepower at 5,500 rpm. The transmission allows the gear
ratio between the engine and the drive wheels to change as
the car speeds up and slows down. You shift gears so the engine
can stay below the redline and near the rpm band of its best
the transmission would be so flexible in its ratios that the
engine could always run at its single, best-performance rpm
value. That is the idea behind the continuously variable transmission
has a nearly infinite range of gear ratios. In the past, CVTs
could not compete with four-speed and five-speed transmissions
in terms of cost, size and reliability, so you didn't see
them in production automobiles. These days, improvements in
design have made CVTs more common. The Toyota Prius is a hybrid
car that uses a CVT.
is connected to the engine through the clutch. The input shaft
of the transmission therefore turns at the same rpm as the
transmission applies one of five different gear ratios to
the input shaft to produce a different rpm value at the output
shaft. Here are some typical gear ratios:
at Transmission Output Shaft
with Engine at 3,000 rpm
Very Simple Transmission
the basic idea behind a standard transmission, the diagram
below shows a very simple two-speed transmission in neutral:
Let's look at each of the parts in this diagram to understand
how they fit together:
green shaft comes from the engine through the clutch. The
green shaft and green gear are connected as a single unit.
(The clutch is a device that lets you connect and disconnect
the engine and the transmission. When you push in the clutch
pedal, the engine and the transmission are disconnected
so the engine can run even if the car is standing still.
When you release the clutch pedal, the engine and the green
shaft are directly connected to one another. The green shaft
and gear turn at the same rpm as the engine.)
red shaft and gears are called the layshaft. These are also
connected as a single piece, so all of the gears on the
layshaft and the layshaft itself spin as one unit. The green
shaft and the red shaft are directly connected through their
meshed gears so that if the green shaft is spinning, so
is the red shaft. In this way, the layshaft receives its
power directly from the engine whenever the clutch is engaged.
yellow shaft is a splined shaft that connects directly to
the drive shaft through the differential to the drive wheels
of the car. If the wheels are spinning, the yellow shaft
blue gears ride on bearings, so they spin on the yellow
shaft. If the engine is off but the car is coasting, the
yellow shaft can turn inside the blue gears while the blue
gears and the layshaft are motionless.
purpose of the collar is to connect one of the two blue
gears to the yellow drive shaft. The collar is connected,
through the splines, directly to the yellow shaft and spins
with the yellow shaft. However, the collar can slide left
or right along the yellow shaft to engage either of the
blue gears. Teeth on the collar, called dog teeth, fit into
holes on the sides of the blue gears to engage them.
see what happens when you shift into first gear.
below shows how, when shifted into first gear, the collar
engages the blue gear on the right:
this picture, the green shaft from the engine turns the layshaft,
which turns the blue gear on the right. This gear transmits
its energy through the collar to drive the yellow drive shaft.
Meanwhile, the blue gear on the left is turning, but it is
freewheeling on its bearing so it has no effect on the yellow
collar is between the two gears (as shown in the first figure),
the transmission is in neutral. Both of the blue gears freewheel
on the yellow shaft at the different rates controlled by their
ratios to the layshaft.
From this discussion, you can answer several questions:
you make a mistake while shifting and hear a horrible grinding
sound, you are not hearing the sound of gear teeth mis-meshing.
As you can see in these diagrams, all gear teeth are all
fully meshed at all times. The grinding is the sound of
the dog teeth trying unsuccessfully to engage the holes
in the side of a blue gear.
transmission shown here does not have "synchros"
(discussed later in the article), so if you were using this
transmission you would have to double-clutch it. Double-clutching
was common in older cars and is still common in some modern
race cars. In double-clutching, you first push the clutch
pedal in once to disengage the engine from the transmission.
This takes the pressure off the dog teeth so you can move
the collar into neutral. Then you release the clutch pedal
and rev the engine to the "right speed." The right
speed is the rpm value at which the engine should be running
in the next gear. The idea is to get the blue gear of the
next gear and the collar rotating at the same speed so that
the dog teeth can engage. Then you push the clutch pedal
in again and lock the collar into the new gear. At every
gear change you have to press and release the clutch twice,
hence the name "double-clutching."
can also see how a small linear motion in the gear shift
knob allows you to change gears. The gear shift knob moves
a rod connected to the fork. The fork slides the collar
on the yellow shaft to engage one of two gears.
next section, we'll take a look at a real transmission.
five-speed manual transmission is fairly standard on cars today.
Internally, it looks something like this:
are three forks controlled by three rods that are engaged
by the shift lever. Looking at the shift rods from the top,
they look like this in reverse, first and second gear:
mind that the shift lever has a rotation point in the middle.
When you push the knob forward to engage first gear, you are
actually pulling the rod and fork for first gear back.
see that as you move the shifter left and right you are engaging
different forks (and therefore different collars). Moving
the knob forward and backward moves the collar to engage one
of the gears.
gear is handled by a small idler gear (purple). At all times,
the blue reverse gear in this diagram is turning in a direction
opposite to all of the other blue gears. Therefore, it would
be impossible to throw the transmission into reverse while
the car is moving forward -- the dog teeth would never engage.
However, they will make a lot of noise!
transmissions in modern passenger cars use synchronizers to
eliminate the need for double-clutching. A synchro's purpose
is to allow the collar and the gear to make frictional contact
before the dog teeth make contact. This lets the collar and
the gear synchronize their speeds before the teeth need to
engage, like this:
on the blue gear fits into the cone-shaped area in the collar,
and friction between the cone and the collar synchronize the
collar and the gear. The outer portion of the collar then
slides so that the dog teeth can engage the gear.
manufacturer implements transmissions and synchros in different
ways, but this is the general idea.
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