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Extending Hd Clutch Life: Proper Analysis Is Critical For Evaluation Of Manufacturing Process Control

By Dave Dennis

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 clearance.

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 drag.

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 stroke.

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 clutches.

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 discs.

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.