Intermittent change-over control from two-wheel drive to four-wheel drive

Abstract

In a vehicle with a permanently active rear-axle drive train and with a front-axle drive train which can be connected via an electrohydraulically controllable clutch and a power take-off gear, and which is connected each time for a specific period of a few seconds when the drive slip in the rear-axle drive train exceeds a predetermined threshold value. A torque direction sensor responds to pulling and pushing stresses in the front-axle drive train and which generates electrical output signals characteristic of such stresses. These output signals are used with the output signals from a forward-reverse motion sensor in a logic circuit, in such a way that the clutch is released when admissibly high stresses occur in the front-axle drive train within the connection period, and the clutch is engaged again when these stresses have diminished. The torque direction sensor comprises a rotary-slide reversing valve, the piston of which is connected fixedly in terms of rotation to the cardan shaft of the connectable drive train, on the one hand, and to the output gear wheel of the power take-off gear via a torsion rod, on the other hand. For alternative directions of the torque in the connectable drive train, this valve assumes alternative through-flow positions, in each of which a dynamic pressure in one of two flow branches is produced. The monitoring of the pressures in the two flow branches by a pressure/voltage converter device generates the torque direction signals necessary for logic circuit.