An upcoming TMC Recommended Practice offers spec'ing tips for new, replacement, and reman clutches.

Clutch life in a heavy-duty truck depends on four main factors:

* The powertrain spec (engine, clutch, transmission, axle, tires);

* Control of drivetrain torsionals;

* Driver technique (starting gear selection, engagement technique, driving habits); and,

* Clutch maintenance (adjustment, lubrication, protection against rust and contamination).

At its recent Summer Meeting in Tucson, The Maintenance Council (TMC) of the American Trucking Assns. neared completion of a Recommended Practice (RP) on clutch spec'ing and maintenance. The proposed RP will delve into each of these factors, and it is intended to supplement, not replace, vehicle and clutch manufacturers' published recommendations.

After reviewing the effects of transmission selection and startability ("Powertrain startability has the most significant effect on clutch life. . . . It is important that startability be determined in the normal startup gear, which is not necessarily the lowest ratio gear. . . Great startability in low gear is of no use if that gear is not used for normal starting."), the RP looks at spec'ing both new and replacement clutches.

New clutches. Historically, the primary basis for clutch selection has been engine torque and flywheel style, with some modifications for duty cycle. Today, the selection process is more complex, as the RP points out, because of "system" concerns driven by the interaction of all the drivetrain components.

Engine peak torque is the first item to consider in selecting a new clutch. Clutch manufacturers use a clutch torque-capacity calculation (combined with a safety factor) to establish their application charts. Clutch torque capacity is influenced by plate load, size, type of friction material, and number of driven discs.

Although clutch dampers are not part of clutch torque-capacity calculations, the RP notes, dampers can be critical to performance and life.

Soft-rate dampers are recommended -- sometimes required -- by clutch manufacturers, and several truck builders make them standard.

What about the "More torque capacity is better" philosophy? Increasing plate load to meet higher engine torque or standardizing at a higher plate load will result in higher bearing load, and this, the RP says, means the driver must exert more pedal effort. For example, for a two-plate clutch, a 400-lb. increase in plate load results in a 12% increase in torque capacity, but pedal effort will be at least another 6 lb.

The "I've used this model/size before and it will work now" selection approach should not be automatic. Slight engine upgrades or other drivetrain component changes might cause a problem. TMC recommends a driveline analysis of possible component interaction whenever such changes are made.

Replacement clutches. Replacements should be "like-for-like," but a rigid-type damper should never be used. Organic facing should not be substituted for cerametallic; slippage and/or premature failure may result.

A soft-damped clutch (damped driven disc with a low spring rate) should not be replaced by a non-soft-damped unit. Potentially damaging torsional vibrations may result. And a replacement clutch must have the same (or higher) clamp load as the original to avoid slippage.

If, however, engine power or torque rating, vehicle configuration, or vocation has changed, TMC recommends following its guidelines for new clutch spec'ing or consulting the clutch supplier.

Remanufactured clutches. The RP points out that "There are good reman clutches and there are bad reman clutches." For best performance, a user must be certain that OEM-grade linings and genuine OEM replacement parts are used, and that no components are added that weren't in the original. TMC cautions that reman damped discs using rubber-encased springs have damper-stiffness characteristics similar to those of rigid discs and are not effective replacements. And rigid discs are not recommended and should never be used as replacements.