1. Shifting means for a two-speed transmission (2) which is shiftable under load and driven by a hydromotor (1), particularly for mobile excavators and dredgers and similar construction machines, comprising two transmission stages, which are adapted to be connected to and disconnected by means of hydraulically operable friction clutches (6, 12), an oil pump (10), which is driven in constant dependence on the speed of the hydrometer (1) and serves to supply hydraulic oil and lubricating oil to the transmission, and a regulating valve (25) for maintaining the required oil pressure, characterized by the following features : a) The friction clutch (12) for the first-speed mechanism consists of an energy storage spring clutch which is biased in an engaging sense ; b) the supply of hydraulic oil for engaging the friction clutch (6) for the second-speed mechanism is effected by means of a single line in conjunction with a simultaneous supply of hydraulic coil for disengaging the friction clutch (12) for the first-speed mechanism by means of an externally operable shift valve (30; 77) ; c) a spring-loaded disconnecting valve (27; 80) is provided, which prevents an engagement of the friction clutch (6) for the second-speed mechanism when the oil pressure is too low and the first-speed mechanism is operative and which effects a shift to the first-speed mechanism when the oil pressure is too low and the second-speed mechanism is operative.
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