Proper analysis of failed clutch components
is critical for evaluation of manufacturing process control, product
life expectancy and warranty analysis. Rebuilt products which
are application and installer sensitive are easily blamed for
failures which may actually be the result of an installation error,
poor maintenance procedures, or an improper application identification.
In the evaluation of failed heavy duty clutches,
the majority of clutch failures or complaints can be traced to
(in no particular order):
- Improper installation or adjustment;
- Improper application;
- Operator abuse.
The clutch is a mechanical device designed
to transmit torque from the engine to the input shaft of the transmission.
The basic operating principle behind the operation of the clutch
is friction. We recognize friction as heat. The major cause of
clutch failure and premature wear can be summarized in two words
– "excessive heat."
Clutches are designed to absorb and dissipate
more heat than would be encountered during normal vehicle operation,
without resulting in damage or breakdown of the friction surfaces.
Clutch installations are designed to last many thousands of miles
under normal operating temperatures, and if properly used and
maintained, will give satisfactory service.
Under proper plate load, heat and wear are
nonexistent when the clutch is fully engaged. However, during
the moment of engagement, when the clutch is picking up the load,
it generates considerable heat. Also, an improperly adjusted or
slipping clutch will rapidly multiply the generation of heat,
allowing the clutch to deteriorate and self-destruct.
There are several troubleshooting diagnostic
and service procedures that can be used to find out what caused
a clutch to become inoperative, or a failure complaint to occur.
Poor clutch release is the most common complaint regarding the
operation of the clutch. Poor release is a symptom which has many
possible causes; these are similar whatever class of truck you
may be working with.
Depending on the history of the vehicle and
the timing of the complaint, poor release could be caused by a
number of possible sources. These sources include: drive pin misalignment,
warped driven discs, a worn transmission drive gear, improper
internal adjustment, damaged release bearing or damaged linkage
components, incorrectly set separator pins, facing contamination,
or an inoperative clutch brake.
On the 14" cast Easy-Pedal/Angle-Spring
clutch, proper drive pin alignment is essential for the clutch
to perform properly. Each of the six drive pins is designed to
transmit an equal amount of torque from the engine. These drive
pins need to be equally spaced, 90° from the flywheel surface,
and parallel to each other. If any pin is misaligned, then the
clutch will suffer from poor release. The more the pins are cocked,
the greater the intermediate plate hangs up, resulting in poor
clutch release problems.
A poor release complaint only when the clutch
is warm indicates a clearance problem with one or more of the
pins-to-slot clearance. As the intermediate plate absorbs heat,
it expands, closing the clearance tolerance. At a point of intermediate
plate expansion, it pinches the drive pin and hinges, or pivots
instead of sliding as designed. The intermediate plate will have
an elliptical bluish or black mark under the offending pin location.
If cocked drive pins are suspected as the cause
of poor release, the transmission/clutch assembly must be removed
and the pins must be individually checked for squareness using
a machinist square. If a square is not available, squareness can
be checked by first removing the flywheel and setting it flat
on a work bench.
Second, replace the front/flywheel side driven
disc down against the flywheel surface, then slide the intermediate
plate over the drive pins. While the plate is rotated and held
firmly in place against the drive pins, use a .006" feeler
gauge on the back side of the drive pins to check for the required
Acceptable clearance for a new clutch is a
minimum of .006". Check the back side of each drive pin.
If any of the gaps are less than .006", then that drive pin
must be straightened to become parallel to the rest of the pins.
Straightening can be accomplished by loosening the set screws
(except on Mack) which hold the drive pins securely in place.
Then align the pin to be square to the flywheel, and tighten the
two set screws. Mack uses a drive pin with a threaded shank and
nut assembly to secure the pin in place.
Do not enlarge the slots on the intermediate
plate to allow the pins to slide. This causes unequal loading
of the drive pins, flexing of the intermediate plate, a change
in balance, and voids the warranty on the clutch.
When a clutch is replaced, the flywheel should
be cleaned and verified to be within specifications. If the flywheel
is not clean and within specifications, poor clutch engagement
and release will result. On a 14" cast clutch, a verified
surface is essential to assure that the drive pins have a common
reference surface so that drive pins can be properly aligned.
Another poor clutch release problem can be
attributed to a driven disc that is either warped or distorted.
The discs usually are warped or distorted by poor handling practices
or bad installation methods. When replacing a clutch, a technician
should never force the transmission drive gear into the disc hubs,
or leave the transmission hanging unsupported in the clutch. This
forcing and hanging will cause distorted discs that will lead
to poor release.
A worn or distorted transmission input shaft
is another frequent cause of poor clutch release. Worn splines
will not allow the discs to slide freely. Excessive wear can be
caused by an engine’s torsional vibrations. Torsional vibrations
generated by the engine can virtually fracture the driven disc
hub. These torsional vibrations damaging the clutch hubs can be
reduced by using the properly specified dampened driven discs
for the application. If the transmission input shaft splines are
worn, the input shaft must be replaced.
All dampened discs are not created equal. If
an eight-spring disc is removed, care should be taken when replacing
that disc to replace it with an eight-spring disc which has the
same dampening capacity. Replacing with an eight-spring disc,
where the springs have less dampening capacity, will cause the
springs to over compress and fail.
Replacing with a disc, where the springs also
have a lower rate of spring dampening, in certain vehicles, will
cause the clutch hub to fracture and fail. This is because the
dampener has not altered the damaging torsional vibrations occurring
at a critical resonance point near peak engine torque. Replacing
with a disc that has more dampening capacity than necessary will
also result in a fractured disc hub. In this case, the springs
will not dampen the torsional, but will transfer the torsional
to the input shaft of the transmission similar to a rigid disc
or one in which rubber encased springs are used.
Clutch driven discs are "tuned" to
accept the torsional characteristics of certain engines and those
engines only. Generally, disc hub fracture can be attributed to
four factors: under spec’ed, over spec’ed, shock load, or misalignment.
The first three will produce a symmetrical failure, while the
last will produce an irregular break.
Another common clutch failure is the result
of improper adjustment. Internal and external clutch adjustments
are many times misunderstood. When the clutch release yoke contacts
the cover, improper adjustment to the linkage can be suspected.
This improper adjustment will also produce a pattern on the back
of the pressure plate where only one set of "witness"
marks can be seen. It will also cause the bearing retainer to
contact the rear driven disc.
To assure optimum performance of a 14"
or 15-1/2" Spicer Easy-Pedal/Cast Angle-Spring pull-type
clutch, a technician will want to set the clutch bearing release
travel to between 1/2" to 9/16". Proper release travel
assures that the release bearing is capable of releasing far enough,
allowing the two driven discs to spin freely, avoiding clutch
This release measurement can be checked by
removing the inspection cover at the bottom of the clutch bell
housing, and checking the measurement between the release bearing
housing and the clutch brake. If release travel measurements are
not within 1/2" to 9/16", a technician should now make
an internal adjustment to the clutch.
To make an internal clutch adjustment, a technician
must first release the clutch by depressing the clutch pedal to
the end of pedal travel and remove the lock strap on the adjusting
ring, or use the Kwik-Adjuster. The next step is to rotate the
clutch adjusting ring until 1/2" to 9/16" release travel
is achieved. If the release travel is greater than 9/16",
a technician will rotate the adjusting ring clockwise, which moves
the release bearing toward the transmission.
If the travel is less than 1/2", a technician
will rotate the adjustment ring counter clockwise, which moves
the release bearing toward the engine. Once the internal clutch
setting is made, reinstall the lock strap, or re-engage the Kwik-Adjuster.
Once release travel adjustment is made, the
next step in properly adjusting a clutch is to set clutch free
travel. Clutch free travel adjustment is a clearance found between
the release yoke and release bearing wear pads. It should be set
at 1/8". A technician can set free travel to the 1/8"
dimension by adjusting the external linkage.
It is important that technicians consult OEM
linkage adjustment procedures for required free pedal dimension
and description of linkage adjustment. Clutch free travel dimensions
regulate how much free pedal is obtained in the cab. In most applications,
1/8" of free travel should correlate to a free pedal of approximately
1-1/2" to 2" in the cab. When recommended free pedal
is no longer present, the clutch must be properly readjusted.
After release travel and free travel settings
have been adjusted, clutch brake squeeze setting can be performed.
Proper clutch brake squeeze adjustment assures that the input
shaft will stop rotating when the vehicle is stationary. The clutch
brake is only used when shifting into first or reverse gears from
neutral. Proper clutch brake squeeze adjustment should occur approximately
1/2" to 1" from the end of pedal stroke.
To check clutch brake adjustment, a technician
should insert a .010" feeler gauge between the release bearing
and clutch brake. Depress the clutch pedal and squeeze the gauge.
Let the pedal up slowly. Stop when the gauge can be pulled out.
The pedal should be 1/2" to 1" from the end of the pedal
Once internal and external adjustments have
been made, it is important that a technician remembers to reinstall
the transmission inspection cover, so that road grit does not
get into the clutch assembly. Such contamination can lead to adjusting
ring freeze-up. Following these easy installation steps will help
assure maximum life from a clutch installation.
Release bearing failure
Another common clutch failure is caused by
release bearing failure. A release bearing failure is most generally
attributed to lack of periodic lubrication. Another reason a release
bearing might fail would be a failure by the driver to fully release
a clutch, or by riding the clutch pedal. When a driver rides the
clutch pedal, a continual thrust load on the bearing will create
higher temperatures and consequential loss of lubricant. Failure
to use NLGI #2 lubricant as recommended can also cause loss of
lubricant even under normal operating conditions. An impending
release bearing failure is accompanied by noise.
Bent, altered or worn clutch external linkages
can cause clutch failures. Any alteration to linkage after a clutch
is properly internally and externally adjusted may cause the clutch
to not release correctly. When a clutch is not set up properly,
a tremendous amount of heat can build up. This is what destroys
Proper driving technique will allow for optimum
clutch life, however, not all driving techniques are proper. Typical
driver misuse which leads to excessive heat and eventual clutch
failure include: starting the vehicle rolling in the improper
gear, overloading the clutch, riding the clutch pedal and holding
the vehicle on an incline with a slipping clutch.
An operator could cause catastrophic failure
when coasting down a hill with the clutch released and the transmission
in gear. This procedure will lead to higher driven disc rpm through
a multiplication of ratios from the final drive and transmission.
It can result in "throwing" the facings off the clutch
Driven disc speeds of more than 10,000 rpm
have been encountered in such simple procedures as coasting tractors
backwards into a below grade loading dock. While an ample safety
factor is provided for normal operation, the burst strength of
the facing is limited.
Understanding how the clutch operates in relationship
to the rest of the vehicle system is important to the proper evaluation
of failed clutch products. People sell and service what they know
and understand. Since terms like coefficient of friction, plate
load, torque and thermal dynamics, as well as their effects on
each other, are not known or understood by most, is it any wonder
why clutch failures are such a sensitive issue?
Dave Dennis is a technical instructor at
Dana Corporation’s Dana University.