AutoJust™ Clutch

Ceramic, twin disc, manual clutches, that are common in the North American heavy duty truck market, require frequent adjustments in order to function properly and perform according to their specifications. Wear on disc facings and friction surfaces of the flywheel, intermediate and pressure plates causes changes in the thickness of these components, resulting in a change of the release bearing position. As wear increases, the driver experiences a decrease and finally a complete loss of free play in the clutch pedal. The clutch is then out of adjustment. By manually adjusting the clutch, its efficiency can be restored, but this maintenance activity incurs labor costs and causes downtime losses. A self-adjusting clutch is a clutch containing a mechanical, automatic control system that measures instantaneous wear and then compensates the release bearing position. In this way the clutch is always in proper adjustment. The Meritor Clutch Company (MCC) has introduced a pull type, self-adjusting clutch called the AutoJust™. This paper describes and discusses the self-adjustment concept used in the MCC design. Emphasis is put on a functional analysis of the control system. Both over- and under-adjustment conditions are evaluated. Static and dynamic models of clutch engagements and disengagements are presented. This modeling is used to demonstrate how the basic parameters of the self-adjusting mechanism were derived. Special consideration is devoted to the selection of pre-load in the sensing device and its relationship to dynamic parameters within the drivetrain. Furthermore, the initial (factory) adjustment is discussed in association with tolerance stack ups within the clutch. AutoJust™ clutches have been extensively tested by MCC in many accelerated in-truck tests. The accelerated wear testing method is explained in the paper and results from these tests are presented. In conclusion, the evaluation of the AutoJust™ design is presented and future trends in the clutch industry are discussed.