Bobby Baker is proud to describe himself as a former truck driver, and he draws on that experience when discussing factors that affect clutch life. “It all comes down to that left foot,” says the technical director of Setco Automotive, which produces LIPE clutches. “You have some drivers called ‘cowboys’ and some drivers who know what they’re doing.”
It would be difficult to overlook the impact of that left foot, especially if a driver pits the power of a truck against the resistance of something like an ungreased fifth wheel. “The weakest part of the driveline is not the yoke. It’s not the driveshaft. It’s the clutch,” Baker continues. “The clutch is basically the failsafe so you don’t ‘puke’ a driveline or bust a gear.”
Beyond the issues that surround outright abuse, there is no overlooking the fact that clutches will require some level of maintenance support over time, and factors ranging from component choices to lubrication intervals will all play a role in the life of any design.
Mileage is actually a poor indicator when trying to determine the potential life of the clutch, notes Andreas Gebauer, senior design engineer with ZF’s Commercial Vehicle Group. The same facing material that lasts 200,000 miles in a linehaul truck could wear away within 50,000 miles under the pressures of a severe-duty application.
ASSESSING SERVICE
When it comes to ongoing maintenance, a series of detailed inspections will offer the best indication of service needs.
The initial adjustment of a new clutch begins by setting the proper gap between the rear face of a release bearing and the front face of the clutch brake that has been pushed into contact with the transmission. That gap should measure
530 thousandths of an inch (thous), plus or minus 30 thous, Gebauer explains.
While the existing Recommended Practice from the Technology & Maintenance Council (RP 638) specifically calls for a half inch, he is hoping that will be refined during a scheduled review of the document. Many widely distributed service manuals have already made the shift.
It is an important measurement. While some technicians believe they can improve the level of contact by reducing the amount of clearance, the decision can affect the pressure plate’s ability to retract, warns Vince Fuleki, product support manager for Roadranger Field Marketing. Drivers at the controls of such a clutch will begin to complain about the trouble they have getting into gear, and that could lead to an unnecessary replacement.
Meanwhile, one of the best ways to determine the need for an adjustment has traditionally involved using a tape measure to identify free pedal travel. If the amount of free play is within one inch of clutch pedal movement at the top of the clutch pedal stroke, an internal adjustment is needed. The locking mechanism may vary by manufacturer, but the clutch adjusting ring will have to be turned to compensate for the internal wear, re-establishing the original settings that existed when the clutch was first installed.
