Some Aspects of Parasitic Drag In Dry Friction Clutches 2001-01-2802
Clutch drag is caused by friction between clutch parts and adjacent components having different rotational speeds when the clutch is released. The root cause of drag is the presence of normal reactions between friction surfaces when the clutch is disengaged. In most cases, especially in multiple disc clutches, these reactions cannot be eliminated but they can and should be minimized by proper design.
The major difficulty in dealing with drag problems is associated with the relatively small magnitudes of drag torque in comparison to useful torques carried by the drivetrain. From the history of drag problems, especially in the automotive industry, it appears obvious that the problem of dragging clutches has no single solution. There are many potential sources of friction forces contributing to drag torque. In addition, contributions from particular sources are very small and not necessarily consistent. Many external disturbances, and some independent variables in drag problems, are strongly stochastic in their nature. Their conclusive measurement under industrial conditions seems to be very problematic. On one hand, there is a problem with accuracy of measurement and on the other hand, in industrial conditions, all significant items involved in the drag phenomenon cannot be tested or observed. Therefore, engineers have to resort to sampling. Any sampling in drag problems has proven itself to be impractical. Drag tests are usually non-repeatable. In this situation the authors try to deal with some practical questions: what is a practical way of dealing with dragging clutches and what is a practical way of making design improvements?
This paper includes some design recommendations to reduce drag and a strategy for dealing with drag cases in industrial conditions. It also presents a specific design problem for the spline interface. Drag, introduced by friction forces between the hub and input shaft splines, is investigated for various design parameters of the hub. The determination of threshold conditions for self-locking on the spline is included.