A modulating valve capable of shortening the operating time of the clutch and also preventing the generation of an impulsive torque in the clutch-transmitted power by discontinuously changing the rate of variation of pressure used for pressing together the frictional members of the friction clutch. In order that the force, acting upon the valve body of the pressure regulating valve, will vary discontinuously relative to time, a plurality of independently slidable pistons are provided so that a discontinuously varying force will be generated by the strokes of these pistons. Thus, according to this arrangement, pressure, varying with the time in a manner that is not readily achievable with conventional systems, can be used for pressing together the frictional members, thereby realizing an operation with reduced working time and which is free of impulsive torque.
A switch-over valve 6 is provided in a pressure circuit 31 between a hydraulic pump 2a and a clutch 7, and a pressure limiting valve 5 is provided between a reflux circuit 9 for the valve 6 and the pressure circuit 31. The clutch actuating hydraulic circuit includes another hydraulic pump 2b, and a switch-over valve 8 for switching over a discharged fluid of the hydraulic pump 2b. A downstream side of the switch-over valve 8 is connected to the pressure circuit 31 and to the reflux circuit 9. Consequently, when the clutch is engaged or disengaged, the discharged fluid of the hydraulic pump 2b is combined with the discharged fluid of the hydraulic pump 2a so as to increase the flow rate to the clutch 7.
A multifunction hydraulic flow metering valve and a circuit therefore comprising a main valve, a bleed flow orifice, and a servo valve acting with the main valve provides for proportional speed control for lowering loads and as an anti-cavitation check valve. A pilot valve in the circuit acting with the main valve provides the circuit with a pressure limiting relief valve.
A pressure holding valve for pressure-fluid-actuated leveling systems in vehicles, having at least one seat valve with a valve opening, a valve seat and a valve member in which the valve member is opened by a control piston acted upon by pressure fluid. In order to achieve an absolutely pressure-tight seat valve with a long service life, an elastomer seal which seals off from the valve member is disposed on the valve seat, and pressure equalizing means are provided such that upon an impingement of pressure on the control piston in an opening direction of the seat valve, pressure build up upstream of the valve member is such that only a slight pressure difference arises at the seal at the onset of opening. In addition, the valve member is embodied such that upon displacement in a closing direction of the seat valve, the valve member substantially closes the valve opening in advance of the valve member becoming seated upon the seal.
A hydraulic pressure control device for use, for example, in an automatic transmission, in which a line pressure to be supplied from a hydraulic pressure source to a hydraulic pressure servo-device is controlled in three stages by means of a main spool which is displaced in a bleed-off valve of the line pressure by a subsidiary spool under a varying hydraulic control pressure. The subsidiary spool has at least one land forming a piston slidingly fitted in a movable sleeve having a closed end, the movable sleeve also forming a piston by being in turn slidingly fitted for limited movement in a stationary sleeve having a closed end. A first space between the outer wall of the closed end of the movable sleeve and the inner wall of the closed end of the stationary sleeve, and a second space within the piston chamber of the movable sleeve have ports for the introduction of hydraulic control pressure. A spring is confined between the subsidiary spool and the movable sleeve with an initial load for urging the movable sleeve away from the main spool and minimizing the first space. The initial load on the spring and the diameter of the spaces are so determined that an increasing hydraulic control pressure displaces only the spools until the product of the control pressure and the difference in the areas of the cross sections of the spaces exceeds the initial load on the spring. Further increase of the control pressure displaces both the movable sleeve and the subsidiary spool to the limit of movement of the movable sleeve, the movement of the spools being a function of the strain on the spring. Further increase of the control pressure acts to move only the subsidiary spool and thereby the main spool. The control device preferably includes a second land on the subsidiary spool operating in an additional space of larger diameter in the movable sleeve and an annular space between the movable sleeve and the stationary sleeve. Both the additional space and the annular space are accessed to a second control pressure for additional displacement of the subsidiary spool and thereby of the main spool.
The present invention provides an apparatus for controlling a hydraulic pressure usable for a clutch wherein the apparatus is employed for a transmission mounted on a construction machine or the like in such a manner that it is provided separately for a plurality of clutches and it is controlled individually. The apparatus includes a first valve for allowing a large amount of hydraulic oil to flow in the clutch and a second valve for electronically controlling a clutch pressure so that the termination of filling and the clutch pressure are detected by a simple structural arrangement on the basis of a displacement of a spool for the first valve and a separately disposed piston. In addition, appearing of an overshoot pressure on the termination of filling is prevented by providing a difference in pressure receiving areas on the spool for the first valve.
