Become a Clutch Master

Clutch operation 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 this article.

Start-up 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 at hand.

Now, all 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

The picture 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).

Your Little Red Wagon and Mass

Back when ..... 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 start moving.

Isaaic Newton 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.

What the Clutch and Transmission Do

Your clutch 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:

  • torque action on the drive axles and tires
  • which comes from the drive shaft(s)
  • which comes from the transmission output shaft
  • which comes from the selected gearing
  • which comes from the transmission input shaft
  • which comes from the clutch disc
  • which comes from the clutch pressure plate and flywheel
  • which comes from the engine.

Because your 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.

The second 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.

In contrast, 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.

Clutch Spring Forces Explained

The clutch 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 slides over 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 shaft.

These springs 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.

So, when 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 error.

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.

The primary 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.

Remember, 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.

Truck 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.

When you 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.

Another truck 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.

Remember, all these problems are a direct result of applying excesive engine throttle for the gear selected.

Clutching Conclusions

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 clutching rules:

  • Dont select a gear that is to low. This will multiply the engine torque beyond what
    is required and makes it harder to feather the clutch into full engagement without
    causing front end bounce.
  • Don't 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 and/or you
    are allowing insufficient time for the clutch to fully engage.
  • After the clutch fully engages, then apply more power to run out that shift point.
  • 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 this happening.

Factory Do's and Dont's Which Affect Clutch Life

This section 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.

An improperly 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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