transmission OEM identifies the model number with an identification
plate riveted to the side of the transmission. Each OEM uses
minor variations in its model-numbering system. As you can see
from the tables shown below, there are some similarities and
also some differences:
Typical Single Countershaft 5-Speed Transmission
manual transmissions use a countershaft to obtain gear selection
options. Shown below is a diagram of how a single countershaft
transmission functions. The input shaft which connects to the
clutch, is labeled IS. The output shaft which connects to the
driveshaft is labeled OS.
input shaft is attached to the top left red gear, which turns
all the time that the clutch is engaged. This drives the lower
left red gear at all times. The lower red gears are all attached
to the counter shaft and are called the cluster. Any time that
the clutch is engaged, the entire cluster spins.
the lower right hand red gear. This is two gears, one stacked
on top of the other gear which is a part of the cluster. That
top red gear is called a reversing gear. The reversing gear
has its own pivot shaft which is not shown in this drawing.
The reversing gear simply spins on its shaft and reverses the
direction of the 1-R blue gear when it slides above the reversing
bright white arrows in the diagram show the direction of gear
rotation and power flow. The faint white arrows represent no
power flow, just gears spinning freely.
drawing above shows the transmission in neutral. In this neutral
position, none of the blue gears are engaged, so the black gears
just spin freely on the output shaft (driven by the lower red
gears), and the output shaft does not transmit any power.
the 5-speed shift pattern to the left. There are three fore
and aft throws on the shifter. Each throw moves a separate shift
fork, which moves a blue gear. When the shifter is moved to
the left throw, then forward selects reverse gear, and a shift
fork moves the 1-R blue gear to the right. Moving the shifter
backwards selects first gear and the shift fork moves the 1-R
blue gear to the left. Likewise, the middle shifter throw moves
the 3-2 blue gear, and the right shifter throw moves the 5-4
blue gear. By selecting a gear with the shifter, the selected
blue gear is moved in the appropriate direction.
drawing below shows the transmission in first gear. The 1-R
blue gear is slid to the left and now provides first gear power
to the output shaft.
drawing below shows the transmission in second gear. The 3-2
blue gear is slid to the right and now couples second gear power
from the largest black gear to the output shaft.
drawing below shows the transmission in third gear. The 3-2
blue gear is slid to the left and now couples third gear power
from the middle black gear to the output shaft.
drawing below shows the transmission in fourth gear. The 5-4
blue gear is slid to the right and now couples fourth gear power
from the smallest black gear to the output shaft.
drawing below shows the transmission in fifth gear. The 5-4
blue gear is slid to the left and now couples fifth gear power
from the input shaft red gear directly to the output shaft.
drawing below shows the transmission in reverse gear. The 1-R
blue gear is slid to the right and now couples power from the
red reversing gear to the output shaft, forcing the output shaft
to turn in reverse. Notice the gear ratio difference between
first and reverse red gears. The reversing gear is about 1/3
the diameter of the red cluster gear used for first gear. Since
the gear diameter ratio or the gear teeth ratio both determine
the gear ratio, the reverse gear will drive the 1-R gear (and
the output shaft) about 1/3 the speed that first gear would
drive the output shaft.
Countershaft Transmission Types
single counter shaft transmission places high forces upon the
gear bearings and upon the gear teeth. For increased transmission
life, twin counter shaft and triple counter shaft transmissions
were developed. The most popular twin counter shaft transmission
is the RoadRanger. The drawing below shows an end view of the
three countershaft types. The twin counter shaft divides the
forces between two countershaft gear clusters, and the triple
countershaft divides the forces between three countershaft gear
that the twin countershaft has twice as many cluster gear teeth
meshed, and the triple countershaft has three times as many
cluster gear teeth meshed. The more teeth that are meshed, the
longer the teeth wear. The input shaft bearing loading of the
twin is lighter because the input gear is balanced between both
countershafts and is not trying to move sideways. The input
shaft bearing loading of the triple countershaft transmission
is even less because it is balanced on all three sides by countershafts.
It is not trying to move up or down either. The triple countershaft
transmission is primarily manufactured by Mack Trucks.
Countershaft Transmission Timing Requirements
countershaft transmissions have a unique requirement for timing
while being assembled. If you look at the drawing below and
right, you will notice that gear A is floating around the
output shaft. This design was incorporated to reduce unused
gear drag on the output shaft. The teeth from the surrounding
countershaft gears, hold gear A in proper alignment around the
output shaft. Gear A floats on the countershaft gear teeth.
transmission would have five of these floating gears. Because
each of these five floating gears has a different number of
teeth, the associated gears on the countershafts have different
phasing positions. These differing phases are required to float
each gear centered about the output shaft properly. If the twin
countershafts are out of time (one or more teeth out of position),
one or more of these floater gears will operate slightly out
of position with the output shaft. The misalignment will usually
allow all the gears to be selected, but that transmission is
now doomed for failure.
a floater gear is slightly out of position, it will wear on
gear B as shown in the diagram to the left. Gear B is what couples
power from gear A to the output shaft when that gear is selected.
The out of alignment floater will cause much friction, which
will breakdown the lubricant properties, and transmission failure
will occur within about 200 miles of use after untimed transmission
assembly. Always refer to the manufacturer's timing requirements
during assembly of multiple countershaft transmissions.
we have only talked about the main 5-speed section of the transmission
case. When more shift gears are required, then an auxiliary
gear case is bolted on behind the primary 5-speed case. These
auxiliary gear cases usually employ 2, 3, or 4-speed gears.
The drawing to the left shows a 3-speed auxiliary gear case,
which is shown selected in the low gear range.
shaft of the 5-speed is replaced with a shaft which splines
to the input shaft of the aux unit. The input shaft of the aux
unit is now connected directly to the output shaft of the primary
5-speed gear case. The output shaft of the aux unit is now splined
to the driveshaft.
the requirement of a second shift handle for the aux unit, air
pistons inside the aux unit are controlled by air valves on
the primary gear case shifter handle. The driver flips a low
switch to set the low aux gear range as shown above. While the
low gear is selected, the high and mid gear air piston is locked
out in the middle position and can not be selected.
the driver switches out of the low range, then the other switch
on the shift handle allows shifting between mid and high gears.
diagram to the right shows the mid gear selection for this aux
unit. While in the high or mid gears, the low gear air piston
is locked out and can not be selected.
trucks have the 2-speed aux unit which does not have the low
gear range. They just have the mid and high gears. These 2-speed
aux units are commonly called splitters.
these diagrams only show single countershaft units, the same
principles apply for twin and triple countershaft aux units.
If the primary 5-speed is multiple countershaft, then the aux
unit is also multiple countershaft.
next drawing to the left shows the aux unit in high gear. Notice
that this is just like the 5-speed in that the input shaft is
coupled directly to the output shaft. No power flows through
units have an overdrive output. Instead of making the top gear
direct, they simply gear the output shaft to turn faster than
the input shaft. In this situation, there is usually a direct
and over gear choice. This once again permits splitting of the
5-speed gears to simulate 10 speeds, but it now offers overdrive
thing you can always count upon in the good ole USA, is a lot
of choices. You have probably heard of 5-speeds, 10-speeds,
13-speeds, 15-speeds, and 18-speeds. With the new E-engines
(electronic diesels), performance and fuel efficiency has forced
the engineers to keep the engine RPMs lower while using more
torque output. This combination reduces the need for many forward
gears, so fewer speed transmissions are in the trucker's future.
In fact, this leads us into the next transmission type which
is called the automated top gear transmissions. Below is an
exploded view of a typical twin countershaft transmission with
a 3-speed auxiliary unit.
Top Gear Transmissions
heavy duty highway rigs run on smaller percent grade hill climbs,
have high torque producing computer controlled engines, and
get by with 10-speed transmissions operating in cruise control
mode most of the time. In fact, most linehaul drivers spend
93% of their time in either 9th or 10th gear.
wide use of electronics on both engines and transmissions today,
automatic top two shifters have come into play. The cruise control
computer, engine computer and transmission computer all share
information and manage the auto shift of 9th and 10th gears.
earlier how the auxiliary shifter was air operated to preclude
a second shift handle. The top-2 transmissions simply allow
the engine computer to operate the air pistons which perform
the shifts between 9 and 10. The transmission computer reports
the input shaft RPM, and the engine computer measures the engine
RPMs. The cruise control decides when a shift is required. When
a shift is required, the engine computer breaks torque on the
transmission input shaft (adjusts engine throttle to remove
the power applied to the transmission), orders a transmission
deselect of the current gear, matches engine RPM to the new
gear input shaft RPM, and then selects the new gear, and then
reapplies torque (adds power back). No clutch action is required
because of the computer engine RPM gear matching. This entire
process takes place when the cruise control computer determines
that a change is necessary.
in the automated shift mode, a couple of requirements must be
met. The driver shifts through the first 8 gears, clutching
in a normal manner. When the vehicle speed is above 40 MPH and
the engine speed is 1400 RPMs or greater, and the driver shifts
from 8th gear to the A position, and the cruise control is active,
then the engine computers take control of the top two shifts
and supports the cruise control requirements.
that can occur is called transmission hunting. For a given road
speed, throttle position, and engine load, the system will shift
back and forth between 9 and 10. When this occurs, just change
your road speed a little bit and the hunting will stop.
also have a hold switch which allows the driver to override
the auto-shift, and when this hold switch is activated, the
transmission remains in the current gear.
of the system are identified when the system fails 3 attempts
to shift within a 9 second time period. Failures result in trouble
codes which are particular to the truck engine. Each engine
has its own trouble codes for transmission failures. Once the
failure mode has been detected, the transmission resorts to
manual mode and will not return to auto-shift mode until after
the truck has been stopped and the ignition key has been turned
off for at least 10 seconds.
the specific transmission and engine manufacturer documentation
for specific fault codes and troubleshooting tips. We hope to
publish some of the fault codes as soon as they become available
Oil Pumps & Oil Coolers
operate under high torque at speeds below 30 miles per hour,
such as during a steep grade climb. The high torque conditions
generate considerable friction which can drive the oil temperature
above 250 degrees. Oil temperatures above 250 degrees can breakdown
the lubricating properties of the oil and result in shortened
transmission life. Synthetics oils reduce these negative effects.
transmission models, the gear, bushing and bearing lubrication
is obtained by oil thrown off of the countershaft cluster gears
spinning in the oil. When this transmission operates at an angle
steeper than 12 degrees, poor lubrication can occur with serious
component damage resulting.
countershaft transmissions use an oil pump and oil cooler to
overcome the above problems. The oil pump can circulate the
transmission oil through an external oil cooler, and the oil
pump can ensure proper lubrication during steep grade climbs.
can be mounted inside the transmission or they can be mounted
on the power takeoff mount of the transmission. When used with
an oil cooler, the combination can reduce the transmission oil
temperature by 50 degrees. This greatly reduces the possibility
of the high torque condition taking the oil temperature above
that dangerous 250 degrees. The oil coolers can be water cooled
or air cooled. Oil coolers are recommended for engines above
350 HP, and are required for engines above 399 HP.
lube oil brands are readily available for use in truck manual-gearshift
transmissions. Transmission oils specified by an OEM are categorized
by the American Petroleum Institute (API) and the Society of
Automotive Engineers (SAE) classifies the viscosity of lubricants.
A "W" appearing after the oil viscosity grade (15W-40)
indicates that the product has met requirements for winter use.
SAE viscosity grade requirements can vary between SAE 30 and
SAE 90 based upon the ambient temperature of the transmission
oil. The following table lists the oil recommendations for the
heavy duty Rockwell twin-countershaft transmissions. These same
lub oil specifications are adopted by Eaton/Fuller, Spicer,
Mack, Volvo, GMC, and ZF.
|Heavy Duty Engine Oil
|Above 10 deg F
|MIL-L-2104B, C, or D
or API-SF, -SG, -CD or -CE
|Above 10 deg F
Below 10 deg F
|Mineral gear oil with rust and
oxidation inhibitor (API-GL-1)*
|Above 10 deg F
Below 10 deg F
|Synthetic oil, Rockwell spec. 0-81*
|*Multiweight and EP gear oils are not recommended.
Do not mix oils in the transmission.
lubricants have such great advantages that they are used in
all jet aircraft and space vehicles. These same advantages are
the basis for use by many transmission manufacturers. The CD50
classification is widely accepted by Eaton/Fuller, Spicer, and
Rockwell products. Synthetic oils have been chosen as lubricants
of choice for transmissions and differentials. Some OEMs fill
this equipment with these oils at the factory. Some OEMs also
offer a longer warranty if the proper synthetic lubricant is
used instead of mineral oils.
people have heard that synthetics are better than petroleum
based lubricants but they don't know why. Well, here's why.
Because synthetic oil is composed of molecules that are uniform
in weight and shape, its heat of vaporization is much higher
(more than 300°C compared to conventional oil which evaporates
at temperatures as low as 175°C). This means that you will burn
less oil and have less sludge, which is the result of evaporated
slipperiness is another positive attribute of synthetics. The
uniform length of synthetic oil polymers allows them to more
easily slide over one another. Mineral-based oil has a film
strength of about 500 psi, synthetics are closer to 3000 psi.
This means that the oil is less likely to be pushed out from
between two metal surfaces where a lot of pressure exists, like
bearings and gears. The advantages of synthetics are so great
that they out weigh the initial high cost. A
synthetic lubricant will sell for 2 to 6 times the price of
petroleum based oil. The synthetic lubricants resist oxidation
and will not create varnish and sludge at high temperatures.
This feature is important in the newer truck designs where the
transmissions are smaller (can dissipate less heat), where aerodynamics
reduces airflow over the transmission, and where the high torque
E-engines increase the loading and torsional forces on transmission
weather operation, the synthetic lubricant provides better low
temperature fluidity to improve startup protection. Synthetics
also posess improved wear protection and oil film strength,
and extended drain intervals due to the oil's resistance to
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