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

To that end, the more you understand about clutches, the more likely you are to make the best spec for your operation. While the clutch is obviously not something that can be spec’ed 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, excuse the pun, a “clutch” decision.

Essentially, 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 (grooves or slots) to a shaft connected to the transmission. The disc (there are actually two in heavy trucks) fits into the grooves on the shaft so that 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 said to be 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 larger clutch is better for high-torque engines, because a bigger clutch has more clutch plate surface area, 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 wear at a faster rate than ceramic facings because they allow more slippage. The ceramic option is harder and more durable. Downside is that it grabs hold immediately when the clutch is engaged and so transfers engine torque to the transmission input shaft a tad abruptly.

For heavy-truck applications and fleets where the same truck may be driven by multiple drivers, ceramic facings will be the better spec. For medium-duty applications, heavyweight jobs where frequent clutching is not an issue, and operations where smoother clutch engagement is a priority, organic facings are the smarter spec.

One issue you may run up against in evaluating the right clutch choice for your operation is torsional vibration and how the choice of clutch can address the problem. Torsional vibration results from several factors, chief among them the capability of high-efficiency diesel engines to run at lower rpms. As ArvinMeritor’s engineers explain, “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.” All drivetrain components exposed to this vibration are subject to increased wear, fretting and fastener loosening. The industry’s solution is soft-damped clutches. These clutches have hubs with coaxial springs and secondary friction devices designed to act as shock absorbers for the severe vibrations and torque loads that pass through the engine flywheel.

In doing so they reduce wear and tear on the other clutch components, the transmission and the rest of the drivetrain. In comparison, the alternative, rigid discs, are solid and provide virtually no damping of vibration.

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 and does offer longer service life compared to rebuilt.