a Clutch Master
is a necessary evil to get your rig on down the road. Most drivers,
after many miles of experience, seem to learn the art of start-out
clutching on their own (and I'm not talking about holding on to
your sweet heart), and do a fairly good job of it. Unfortunately,
other truck drivers never seem to
get it right, and this results in frustration for the driver and
premature failure of clutches and drive-line parts. Even if you
are pretty good at it today, you can be better at it after reading
clutching is really quite simple to explain, but not as simple
to master. First you stop the transmission gears from spinning
by pressing the clutch pedal all the way down. Trucks with a transmission
brake, will brake the spinning gears with the pedal all the way
down. Now select a gear which is appropriate for the situation
you have to do is provide little if any throttle, gradually increase
the transfer of engine power to the transmission (using the clutch),
until the rig starts moving and the clutch is fully engaged. Once
the clutch fully engages, continue to increase engine throttle
until the first shift point is reached. See our topic shifting
gears for after the rig is moving.
We have all
seen that tractor with the front end bouncing up and down while
the frustrated driver attempts to get the rig moving. We have
all heard that clutch chatter and truck shudder when the clutch
has become grabby from improper use. So lets take a look at the
forces which create these problems.
Major Clutch Components
above shows a heavy duty dual disc spicer clutch assembly. The
top left corner component is the pressure plate, and the top right
component is the spacer disc which has tabs that are driven by
the pressure plate so that it becomes a second pressure plate
gripping surface between both discs. The pressure plate has heavy
springs which press the outer most clutch disc against the spacer
disc, which presses the inner most disc against the flywheel generating
double grip when the clutch is engaged. The bottom two components
are the twin clutch discs. There are eight facing pads riveted
onto each of these clutch discs (four on each side).
Little Red Wagon and Mass
..... way back when you were about eight years old, you probably
loaded some pretty heavy loads onto your wagon and then tried
to pull it around. This is when you first experienced the effects
of mass at rest and mass in motion. Isaaic Newton determined that
a mass at rest tends to stay at rest until acted upon by an external
force. Your wagon wouldn't move until you pulled on it. And even
when you pulled on it, there was a slight delay before it started
moving. The heavier the wagon was, the more delay before it would
also discovered that a mass in motion tends to stay in motion
until acted upon by some outside force. When you stopped pulling
on your heavily loaded wagon, it would take time to coast to a
stop. Sometimes you wouldn't count on this extended coasting,
and your wagon would ram into something or somebody (ouch). Welcome
to the world of mass.
the Clutch and Transmission Do
has a pretty big job to do and it needs your help to pull it off.
Your choice of transmission gear selection has a profound impact
upon your clutching success. Look at your transmission as a torque
multiplier for start-ups. Torque is a measure of twisting force.
To get your 70,000+ pound rig to move, requires:
action on the drive axles and tires
comes from the drive shaft(s)
comes from the transmission output shaft
comes from the selected gearing
comes from the transmission input shaft
comes from the clutch disc
comes from the clutch pressure plate and flywheel
comes from the engine.
transmission is a torque multiplier, your engine has no problem
providing adequate torque in the lowest transmission gears to
get your rig moving. In fact, it is typical for drivers to give
the engine to much throttle for the gear selected, resulting in
to much torque, which makes the clutch's job much harder as we
shall see in the next two paragraphs. So the driver's first
task is to select an appropriate gear, and use just enough throttle
to get the rig moving as the clutch fully engages.
problem the driver must overcome is allowing sufficient time for
the loaded transmission input shaft RPMs to catch up with the
engine RPMs while the rig first starts moving. During this catch
up time, the clutch is slipping. While the clutch is slipping,
it is generating heat from the slippage friction. The longer this
engagement takes, the more heat the clutch has to dissipate. Heat
is the death of a clutch, so if the driver uses excessive engine
throttle, then excessive heat is generated, and the clutch wears
out much quicker. The over throttle takes longer for the transmission
input shaft to catch up with the engine RPMs.
the driver can use excessive throttle and engage the clutch to
quickly. This combination is even worse than the previous problem.
In this situation, two spring forces come into play. The first
spring in the torsional springs located inside the clutch disc(s).
The second spring is the tractors suspension system. Both spring
systems can absorb momentary excessive forces, but then they have
to release this force. This absorption and release of spring energy
is what makes the tractor shake and bunny hop.
Spring Forces Explained
disc has three parts, the facing assembly, the torsional springs,
and the splined hub assembly. If you look at the spicer clutch
assembly in the picture above, you will find two disc assemblies.
The disc facings is what the clutch pressure plate grabs onto
to make the facing rotate. There are springs connecting the facing
assembly to the splined hub assembly. The splined hub assembly
the transmission input shaft and and transfers torque to the transmission
input shaft. The facing assembly as it is grabbed by the pressure
plate during clutch engagement, sees the engine torque. The radial
springs transfer energy from the facing assembly to the splined
hub assembly which transfers energy to the transmission input
are present to absorb some of the torque mismatch as the clutch
is engaged. No driver is perfect at clutch action, so the clutch
disc usually fully engages while the engine RPMs and transmission
input shaft RPMs are still slightly mis-matched. These clutch
springs absorb this slight mismatch and prevent any sudden stresses
on the drive-line.
the driver has done his job right, the clutch fully engages just
as the engine RPMs match the transmission input shaft RPMs and
the springs absorb the excess force for any slight RPM mis-match
If the clutch
fully engages before the engine RPM matches the transmission input
shaft RPM, then these clutch springs are fully compressed and
at that point they don't exist. Now the sudden and premature clutch
engagement results in a sudden twisting strain on both the engines
crankshaft and the truck's drive-line. Repeated abuse of these
springs weakens them, and they have less protective qualities
for future mismatches, which accelerates the next problem.
source of clutch chatter is from roughened surfaces of the clutch
discs, pressure plate, and flywheel. Repeated RPM mis-match during
full clutch engagement, roughens these surfaces and makes the
surfaces (and clutch) grabby. Once the clutch is grabby, the torsional
springs get excessive workout until they are to weak to assist
is smoothing out the mismatch, and the problem gets worse.
all these problems are a direct result of repeated premature full
engagement of the clutch while the engines RPMs are not matched
to the transmission input shaft RPMs.
Suspension Forces Explained
Take a close
look at the height of your fifth-wheel pin above the center of
your drive wheels. It will probably be about 20 inches. How much
force can you exert using a 20 inch crow-bar? Quite a bit more
than with your bare hands! Where is most of your rigs weight?
On the tractor or on the loaded trailer? Of course, most of the
weight is on the loaded trailer.
As your drive
wheels try to move forward, all of that mass of the loaded trailer
resists this motion. Since this drag is 20 inches above the drive
wheels, this drag has a crow-bar effect on the tractor chassis,
and tends to lift the front of the tractor chassis.
engage the clutch to quickly, this trailer crow-bar effect is
amplified and jerks up the front of the tractor more than usual.
Once that slight mass delay of the trailer catching up with the
tractor is past, then the front of the tractor comes back down
because the crow-bar effect has lessened.
Now we have
another spring effect. As the front of the tractor falls back
to its normal position, the mass of the front of the tractor forces
the front to drop lower than normal, which now levers the trailer
to move slightly faster than the tractor. When the front tractor
springs rebound from this excess compression force, the front
of the tractor rises again and reduces the pull on the trailer
so it now gets slightly behind the tractor speed. This entire
bouncing process repeats over and over until the excess forces
eventually die out and the trailer and tractor are both moving
at the exact same speed.
suspension force to recognize is the twisting of the drive axle
upon its suspension. When the drive wheels turn clockwise, then
the drive axle exerts the opposite force and twists counter clockwise.
This twisting action along with front end nose bounce, delays
the trailer drag response and adds to the crow-bar forces described
above. Excesive power in a low gear increases the axle twist and
increases the truck surge.
all these problems are a direct result of applying excesive engine
throttle for the gear selected.
If you don't
understand something that was explained above, then reread that
section until you understand. Once you understand all these forces
at play, you will also understand how to improve your start-up
clutching, and you will understand the importance of the following
- Dont select
a gear that is to low. This will multiply the engine torque
is required and makes it harder to feather the clutch into full
causing front end bounce.
over throttle the engine because that makes clutch engagement
much harder and genertes excessive clutch heat.
- If the
truck surges during start up, then you are using to much throttle
are allowing insufficient time for the clutch to fully engage.
the clutch fully engages, then apply more power to run out that
- If the
truck surges after the clutch is fully engaged, then you are
applying to much
power for that gear, and the truck drive axle and suspension
is twisting back and forth
and causing the surging effect. Use less throttle when you feel
Do's and Dont's Which Affect Clutch Life
contains information that is provided on the Eaton/Fuller web
site concerning clutch do's and don'ts. Excessive slipping is
the major cause of clutch failure. Extreme operating temperatures
can cause the clutch to fail because the heat generated between
the flywheel, driven discs, intermediate plate, and pressure plate
is high enough to cause the metal to flow and the friction material
to be destroyed.
adjusted or slipping clutch will produce sufficient heat to rapidly
burn up. There are many factors which have significant impact
on clutch life:
1 . Starting
the Vehicle in the Proper Gear. The correct gear will allow
you to start the vehicle with your foot off the throttle.
2. Gear Shifting Techniques. Many drivers upshift into
the next gear or even skip-shift into a higher gear before the
vehicle has reached the proper speed. This type of shifting is
almost as damaging as starting off in a gear that is too high,
since the engine speed and vehicle speeds are too far apart, requiring
the clutch to absorb the speed difference as heat.
3. Excessive Vehicle Overload or Overloading the Clutch.
Clutches are designed and recommended for specific vehicle applications
and loads. These limitations should not be exceeded. Excessive
or extreme overloading is not only injurious to the clutch but
to the entire vehicle powertrain as well. If the total gear reduction
in the powertrain is not sufficient to handle excessive overloads,
the clutch will suffer, since it is forced to pick up the load
at a higher speed differential.
4. Do Not Ride the Clutch Pedal. Riding the clutch is very
destructive to the clutch since a partial clutch engagement permits
slippage and excessive heat. Riding the clutch pedal will also
put a constant thrust load on the release bearing, which can thin
out the lubricant and also cause excessive wear on the pads. Release
bearing failures can often be attributed to this type of operation.
5. Do Not Slip the Clutch to Hold the Vehicle on an Incline.
This procedure uses the clutch to do the job normally expected
of the wheel brakes. A slipping clutch accumulates heat faster
than it can be dissipated, resulting in early failures.
6. Do Not Coast with the Clutch Released and Transmission in
Gear. This procedure can cause high driven disc RPM through
multiplication of ratios from the final drive and transmission.
It can result in “throwing” the facing off the clutch discs. Driven
disc speeds of over 10,000 RPM have been encountered in such simple
procedures as coasting tractors down an unloading ramp. While
an ample safety factor is provided for normal operation, the burst
strength of the facing is limited.
7. Do Not Engage the Clutch while Coasting. This procedure
can result in tremendous shock loads and possible damage to the
clutch, as well as the entire drivetrain.
8. Reporting Erratic Clutch Operation Promptly. Drivers
should report erratic clutch operation as soon as possible, to
give the maintenance personnel a chance to make the necessary
inspection, internal clutch adjustment, linkage adjustment and
lubrications, thereby avoiding possible clutch failures and breakdowns
while on the road. The importance of free pedal travel (sometimes
referred to as a pedal lash) should be brought to the driver’s
attention as well as the mechanic. This item should be included
and commented on daily in the driver’s report, since clutch free
pedal is the maintenance personnel’s guide to the condition of
the clutch and the release mechanism.
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