Today's low-revving, high efficiency engines have exceptionally high torque output that subjects clutches, transmissions and other drive train components to extremes of stress, heat and wear. To combat these conditions and their impact on clutch life and performance, manufacturers have developed improved components and materials that extend clutch service life and help reduce maintenance costs and downtime.
The key to taking advantage of these improvements, however, it is understanding the performance characteristics of these components and how to properly maintain the total clutch system for maximum service life. We've developed several "rules of thumb" that may be useful in this regard. First, however, some background on friction materials might be helpful.
There are two types of clutch frictions materials in use – organic and sintered ceramic. Each material has a unique performance characteristics. Organic facings wear at a faster rate than ceramic because they allow more slippage for smother clutch engagement.
Ceramic material, being harder, lasts longer but it grabs hold immediately when the clutch is engaged and abruptly transfers engine torque to the transmission input shaft.
Because of their toughness and resistance to wear, ceramics are used primarily in heavy-duty, extreme-clutch-use applications where clutch forces are high. Ceramic discs are normally specified for engines of 350 hp or above and fleet truck applications with multiple drivers.
Organic facings, on the other hand, are recommended for medium-weight applications and heavyweight jobs where there isn't frequent clutching. Organics are also find favor where smoother engagements are desired.
Dampened Discs vs. Rigid Discs
Both organic and ceramic-faced discs are available in either dampened-disc or rigid-disc versions. The dampened discs typically have hubs that are fitted with coaxial springs and secondary friction devices. These absorb and dampen the severe vibrations and torque loads passed through from the engine flywheel. Dampened discs make life easier on the other clutch components. The transmission and the rest of the drive train.
Rigid discs simply are solid discs. They have virtually no vibration or stress dampening capability. A major drawback of rigid discs is that they transfer torque vibration loads directly through their hubs to the transmission input shaft splines and other drive train parts, thereby causing excessive wear.
In designing modern clutches and transmissions, however, engineers are striving to eliminate drive train shocks and stresses and minimize heat build-up – a goal that will eliminate rigid discs.
Although engineers are continually trying to develop new materials to extend service life, reducing clutch wear is a tough challenge. Consider the fact that a clutch with approximately 135-sq. in of surface area under clamping loads of 2800 to 3600 lbs. Converts up to 1550 – ft – lbs of engine torque to move 40 to 70 ton trucks from a dead stop and it's no wonder clutch materials wear out! With proper adjustment, wear can be minimized.
Wear doesn't occur only when engaging the clutch to accelerate from a stop. It results from other factors as well, such as when a driver rides the clutch pedal, or when clutch components have worked their way out of adjustment causing slipping wear.