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The clutch: “Middleman” of your truck’s powertrain

BUCKHORN, Ont. - Just as the thrust of many new technologies has been to remove the "middleman," technological innovation in transmission and engine electronics may also one day do away with the tradi...



BUCKHORN, Ont. – Just as the thrust of many new technologies has been to remove the “middleman,” technological innovation in transmission and engine electronics may also one day do away with the traditional clutch – the “middleman” linking the engine’s flywheel to the transmission gearing.

But Canadian truck owners seem to have a particular affinity for manual gearboxes, and automatic transmissions, although growing in popularity, still don’t dominate the truck market on either side of the border. That means the clutch will remain an important spec for the foreseeable future.

The more you understand about clutches, the more likely you are to make the best spec for your operation. Although the clutch can not be spec’d in isolation (it must be matched to the engine’s output and you can depend on the truck manufacturer to address most of your needs), it’s at replacement time that your knowledge will help you make a “clutch” decision.

The clutch connects and disconnects the engine from the rest of the power train as the driver applies it. It includes a middle plate, or clutch disc, which is fastened by splines to a shaft connected to the transmission. The disc (there are two in heavy trucks) fits into the grooves on the shaft so the plate and shaft turn together. The plate can slide backward and forward on the shaft. The clutch disc is the “driven” member of the assembly.

The clutch system also includes two “driving” members. A strong spring forces the two driving members towards each other. This squeezes them against the middle plate until they all turn as one unit. The clutch is engaged when that happens, and disengaged when they are apart.

The first driving member is the flywheel and it has a smooth surface where it squeezes the driven plate. The other driving member is the pressure plate, which applies the necessary force to engage the clutch.

A series of coil springs, or sometimes one large flat spring, acts between the clutch cover and the pressure plate, pushing or pulling the pressure plate toward the flywheel and squeezing the clutch disc between the cover and the plate in the process. The springs exert a constant force to engage the clutch and usually they are strong enough to keep the clutch from slipping. The series of coil springs approach, called an angle spring, is what’s most dominant in North America and employs six or more coil springs to do the job.

There are several things to consider when spec’ing a clutch. Application and engine torque top the list, as the clutch must be able to transmit and control torque from the engine. The higher the capacity to control torque, the larger the torque needed. There are two clutch sizes to choose from: 14-inch and 15.5-inch. The latter is the dominant spec with Class 8 trucks. A bigger clutch has more plate surface, providing less slippage and less wear.

Two types of clutch friction material are in use: organic and ceramic, and each has its own unique performance characteristics.

Organic facings will provide drivers with smoother clutch engagements but they also tend to wear at a faster rate than ceramic facings because they allow more slippage. The ceramic option is harder and more durable. Its downside is that it grabs hold immediately: transferring power abruptly.

For heavy-truck applications and fleets where multiple drivers may drive the same truck, ceramic facings will be the better spec. For medium-duty applications, heavyweight jobs where frequent clutching is not an issue, organic facings are the smarter spec.

One issue you may encounter in evaluating the right clutch choice for your operation is torsional vibration and how the choice of clutch can or can’t address the problem. Torsional vibration results from several factors, such as improper air suspension ride height and driveline angle set-up, but chief among them is the capability of high-efficiency diesel engines to run at lower rpms. ArvinMeritor’s engineers explain that “engine firing frequencies at these lower operating speeds are closer to the drivetrain’s natural frequency for torsional resonance, which means there will be a natural tendency for excessive vibration throughout the drivetrain.” If the truck’s cab isolation is good enough, the driver may not actually be aware of torsional vibration, but this hammering of the drivetrain can occur as many as 50 times per second and its damaging results eventually become apparent. All drivetrain components exposed to this vibration face increased wear, fretting and fastener loosening. According to ArvinMeritor, some of the more frequent failures caused by torsional vibration include broken clutch damper springs and hubs, damage to transmission input shafts, spline wear and universal joint failures.

The industry’s solution to the problem is soft-damped clutches. These units, offered by all manufacturers, have hubs with coaxial springs and secondary shock-absorbing devices for the severe vibrations and torque loads that pass through the engine flywheel.

Your final decision involves the choice between a new or a remanufactured clutch. Obviously, a new clutch should deliver longer service life and less downtime. But a remanufactured clutch can be an economical alternative, which offers a longer service life than a rebuilt clutch. n


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