Clutches: Middle Management

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As transmission technology and engine electronics change at a blazing pace, the clutch is still doing what it’s always done-linking the engine’s spinning flywheel to the gears of the transmission. The difference is that the clutch is becoming more invisible to the driver: electronic controls, small servo motors, and even the engine brake combine to make gear changes through automated “float shifting,” or else the clutch is engaged or disengaged by those little servos. In a typical semi-automated mechanical box, the driver may use his clutch foot only to start the truck off and then to stop it again.

Before long, a much wider range of fully automated transmissions will use torque converters, in which case the traditional clutch-“hidden” or not-will be a thing of the past for many fleets. Whether your manual gearbox is automated or not, you still have to spec a clutch. The good part is that today’s clutches are more capable than they’ve ever been-they’re both easier to use and easier to maintain, if they need maintenance at all.


A clutch has three key components: the driven disc (heavy trucks have two), which is splined to the transmission’s input shaft and houses the friction material that mates to the flywheel when the clutch is engaged; the pressure plate, which applies the force required to engage the clutch, by spring power; and the release bearing, which regulates movement of the mechanical linkage to engage or disengage the clutch.

Other essentials include the centre plate, which may be connected either to the flywheel or clutch cover, depending on design. It separates the driven discs and increases clutch capacity by adding more surface area. There’s also the clutch brake, a device on the input shaft of non-synchronized transmissions that stops gears from rotating to allow faster and grind-free engagement of first or low gear while the truck is stationary. It’s activated by pressing the clutch pedal all the way to the floor.

There are two clutch types: angle-spring and diaphragm-spring, referring to the way in which pressure is applied to force the driven discs against the flywheel and keep them there. The angle-spring type, most common in North America, uses six or more powerful coil springs set at an angle to the transmission input shaft. Its competitor (dominant in Europe) uses a metal plate, which acts as a diaphragm-style spring, flexing fore and aft to engage or disengage the clutch.

There are two choices in friction material: ceramic or organic. The former dominates in a very big way (90%-plus of all heavy-duty clutches) because it wears some 75% better than organic. Metal is often incorporated into the ceramic material, in which case it’s called “cerametallic.”

Organic friction material provides much easier, much smoother clutch engagement, however. The downside of shorter life won’t be a factor for drivers who float-shift-without using the clutch at all. In that event, you may reasonably decide that the softer action comes without a cost.

There are also two clutch sizes: 14-inch and 15.5-inch, though the latter has come to dominate in class-8 trucks.

You will also see reference made to “pull-type” or “push-type” clutches, referring to how actuation occurs, but the difference is of little consequence.

Finally, do you need some sort of clutch assist? Accomplished by either air or hydraulic means, this eases the burden on the driver’s left leg. But improvements to angle-spring clutches have brought increasingly easier pedal action, and diaphragm-spring types are inherently easier at the pedal.


These days the clutch likely does more than grab the spinning flywheel to send power rearwards. Some clutch models can also be called upon to act as something of a shock absorber when used behind a typical modern engine with its combination of high torque and low rpm. In fact, they’ve become standard equipment on almost all heavy trucks.

The need for that shock absorber role arose a few years back when it was learned that powertrains can literally self-destruct because of torsional vibrations. In simple terms, such vibrations occur when the natural resonance of the drivetrain is “excited” by the engine’s own resonance at certain frequencies. This amplifies the drivetrain’s dynamic torques and motions by as much as three times the torque-capacity rating of components like gear teeth and axle shafts.

One transmission engineer explains it this way: “The particular resonance that is of most concern is the one where the transmission, propshaft and axle act as a single mass, rotating back and forth, sprung between the clutch and axle shafts. This resonance occurs at a frequency of 40 times per second, so if the engine or propshaft excite it at that frequency, the dynamic pulsations are amplified significantly, often to destructive levels.”

Failures of the clutch-plate hub, springs, or even the transmission input shaft and driveshaft universal joints can all be attributed to operating a truck at or close to these highly damaging natural frequency points for extended periods.

Testing showed that many factors had an effect on the engine speed at which this happens-anywhere from 500 to 1700 rpm, depending on the truck and its configuration-and also that a “softer” clutch could lower the speed at which torque is amplified in the drivetrain.

That brought the recommendation that “soft clutch” models, with damping abilities by way of coaxial coil springs in the driven disc, be used universally with heavy-duty engines. The alternative is called a “rigid” disc; it’s almost never standard equipment.

The soft-spring and damping characteristics protect driveline components from destructive torsional vibrations because they allow more “twist” between the engine’s flywheel and the transmission input shaft.

They literally cushion the blows rendered by the engine’s firing pulses and send the destructive resonances to lower operating speeds where they can do less damage.

The cushioning effect is created by the springs that couple the clutch’s driven discs to the input-shaft spline. In softer clutches, these springs are more compliant and they offer more travel, thus becoming better shock absorbers.

All clutch makers offer a soft model, but only in the 15.5-inch size. Note that spec’ing a soft-damped clutch assures warranty coverage for torsion vibration-related failures that would not otherwise be covered.


Obviously, the clutch can’t be spec’d in isolation. As part of the drivetrain, it’s subject to the same general considerations, the most important one being engine torque.

You’ll have some choices when ordering a new truck, but by and large you can depend on the truck maker to cover the basic needs-assuming the truck’s intended use is communicated to them. It’s at replacement time, especially if you’re the vehicle’s second owner, when the most care must be taken.

In either case, spec’ing components to the intended vocation is the key, and this is not a moment to go for low price. There isn’t a bad clutch out there, but there are certainly clutches that won’t stand up to your line of work, that don’t take into consideration your maintenance habits and capabilities, that don’t account for the nature of your drivers’-or your own-tastes.

In order to avoid a mismatch of components to the vocation, make sure that the dealer salesperson fully understands your specific application. And if there’s a chance that it may change, spec to the toughest case. Second owners doing a clutch replacement should be especially careful to stipulate the vehicle’s intended use. Given that trucks in their second lives often do different-and frequently more demanding-work, don’t assume that a clutch is a clutch is a clutch.

What’s at risk in either case, new truck or old, is the potential for early failure, but there are also warranty considerations.

“Second owners should take the time to register a warranty application and verify specs and the vehicle’s previous usage before assuming there is warranty coverage,” notes one clutch maker. As for torque considerations, a larger clutch is better for high-torque engines, because a bigger clutch has more clutch-plate surface area, providing less slippage and less wear.

The old adage is never more true: spec to match the engine’s output-and to your vocational needs.


SIDEBAR: Self-Adjusting Clutches

Clutch life and performance are related to preventive maintenance and the number of regular adjustments it gets.

The self-adjusting clutch now represents about 40% of production for the leading clutch maker and has been growing in popularity year by year since its launch in 1996. It eliminates the need for periodic wear adjustments.

One such clutch uses an interesting adjusting mechanism that’s referred to as “continuous wear compensation.” It uses opposing sets of spring-loaded, tapered ramps. As wear occurs, one set of ramps slides up the other in an amount equal to the wear. There are also wear sensors located in four quadrants around the clutch.

Whatever the mechanism, the self-adjusting clutch should last considerably longer than ordinary types. The market leader claims a 60% increase, with a 50% hike in flywheel life as well.

The self-adjusting clutch costs more up front, not surprisingly, but there’s a reasonably good payback. With most operators adjusting a two-plate clutch 13 times or more over its life, and assuming a cost of about $35 per adjustment, the total saving would be at least $455, says one manufacturer. And that’s not counting the cost of clutch failures due to poor adjustment.

Eaton believes it has gone one better: the so-called “No-Maintenance Clutch”(above). Available as a 14-inch medium-duty model or a 15.5-inch heavy-duty type, the company says its Solo clutch never needs lubrication. It eliminates that need mainly by way of a sealed release bearing using high-temperature grease and seals. Other features, for heavy-duty models, include a transmission input-shaft sleeve to protect both the shaft and the release sleeve bushings.

Meritor Automotive, the other main clutch maker, also offers a reduced-maintenance model (the AutoJust) by way of an optional sealed release bearing with an extended lube interval of 100,000 miles.

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Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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