1. Field of the Invention
The present invention relates to a change-speed control construction, and more particularly to an improvement of a change-speed construction including a hydraulic clutch for a propelling transmission system and a hydraulic control mechanism operable to disengage the hydraulic clutch to start a change-speed operation or to engage the hydraulic clutch to complete the change-speed operation.
2. Description of the Prior Art
A typical prior construction for controlling a hydraulic clutch for a change-speed operation is known from e.g. a Japanese patent laid open under No. 62-52247. A hydraulic control mechanism of this construction includes a plurality of logic valves disposed in oil passages extending to the hydraulic clutch, the logic valves being controlled by pilot pressures, and valve units provided for hydraulic actuators corresponding to the logic valves and also for a change-speed manual control device, respectively, such that the hydraulic clutch is engaged or disengaged in accordance with opening or closing actions of the logic valves associated with the valve units.
In the above construction, each of the actuators comprises a hydraulic cylinder type actuator whereas the pilot pressure controlling valve unit associated therewith is constructed by forming predetermined lands, oil passages or the like between a piston and cylinder of the actuator.
Further, the above conventional construction uses the further pilot pressure controlling valve unit for the manual control change-speed device, which valve unit is of a spool type for example.
In short, the conventional construction requires numerous logic valves which number corresponds to the number of shifting units and the associated valve units each of which must be slided for an amount corresponding to a shifting amount needed for a desired change-speed operation. The first drawback of this construction is the difficulty of manufacturing such valve units as mentioned immediately above which require a considerable manufacturing precision. The second drawback is the numerous number of components which is disadvantageous in terms of manufacturing costs and efficiency. In these respects, the conventional construction has much room for improvement.
Taking the above state of the art into consideration, the primary object of the present invention is to provide an improved change-speed construction through maximum simplification and rationalization of its hydraulic control mechanism to engage or disengage the hydraulic clutch for a change-speed operation.