Valving using reciprocating spools to control the flow of fluids is well known in the hydraulics art. For instance, spool valves, arranged radially, are used as part of hydraulic pump/motor apparatus (e.g., see U.S. Pat. No. 5,513,553 entitled "Hydraulic Machine with Gear-Mounted Swash-Plate"). In most such known valving, each spool reciprocates axially within a cylinder formed in the valve body. Most commonly, each cylinder is provided with a pair of ports defining first and second fluid passages, and the spool has a pair of port-blocking portions separated by a stem so that, when the spool is moved axially to a first position, the first fluid passage is blocked while fluids are permitted to move past the stem and through the second fluid passage. Likewise, when the spool is moved axially to a second position, the second fluid passage is blocked while fluids are permitted to move past the stem and through the first fluid passage.
Traditionally in such valving, one end of the spool portion of the valve acts as a cam follower that rides on a revolving cam surface, and each spool is spring biased toward the cam surface so rotation of the cam controls the successive and continuous axial movement of the respective spools in each valve set. However, it is known that the response time and general operation of such spring-biased spool systems are often affected by dirt and counter-pressure problems. Also, it is well known that the individual spools of such known valving often rotate (albeit, very slowly) about their central axes when being operated within their respective cylinders. Therefore, the narrowed stem section of each spool has preferably been designed with a cylindrical shape (see FIGS. 3 and 4) so that, should such spool rotation occur, changes in the orientation of its stem section do not result in any change in the shape of the fluid passageway formed about the cylindrical stem section when the valve is opened.
Valve design is of particular importance when the valving is used to control the flow of hydraulic fluids under high speed and high pressure conditions, e.g., in automotive pump/motors which are capable of developing high horsepower and must be able to achieve speeds as high as 4000 rpm and to withstand pressures as high as 4000 p.s.i. Consistent fluid flow under such conditions is critical.
The invention disclosed herein is primarily directed to such critical fluid flow. Valving according to the invention overcomes the response time problems of spring-biased valving and not only assures consistency of valve timing but also significantly increases the efficiency of fluid flow past the stem portion of each spool.