This invention relates to hydrostatic steering systems for motor vehicles, especially vehicles of the articulated frame type. Cross reference is hereby made to prior copending application Ser. No. 79,838 filed Sept. 28, 1979, now U.S. Pat. No. 4,258,752 and Ser. No. 084,183, filed Oct. 12, 1979, now U.S. Pat. No. 4,295,805 owned in common with the present application by the same assignee.
The present invention is associated with a hydrostatic steering system of the type that includes a hand operated metering gear pump having an internally toothed stator enmeshed with the external teeth on a rotatable piston element operatively connected to the inner rotatable valve element of a steering control valve. The control valve also includes an outer rotatable valve element mounted in a valve body housing in concentric relationship to the inner valve element. The inner rotatable valve element is rotatable in opposite directions by the vehicle steering wheel to control the flow of fluid between a high pressure source and expanding and contracting fluid displacing pockets formed between the meshing teeth of the metering gear pump. A plurality of valve flow passages conducting fluid to and from the gear pump are controlled by two series of relatively movable apertures equal in number to twice the number of teeth associated with the rotatable piston element of the gear pump. Such a steering system is disclosed, for example, in German Pat. No. 2,253,575. In this type of steering system, the rotatable piston of the hand metering gear pump has four teeth while the stator has five teeth. Operation of such metering gear pump produces relatively large fluid pulsations causing a jerking movement in the operation of fluid steering motor connected to the control valve. Heretofore, these pulsations were reduced by increasing the number of teeth associated with the metering gear pump. Thus, the metering gear pumps according to current power steering practice are provided with rotatable pistons having six teeth and a stator having seven teeth. Attempts to further increase the number of teeth in the hand metering gear pump have resulted in an intolerable increase in production costs.
Where a stator having seven teeth is utilized, the valve housing is provided with seven passages to establish fluid communication between the tooth gaps of the stator and a first series of apertures extending through the cylindrical bearing surface of the housing journaling the outer rotatable valve element having a second series of apertures therein cooperating with the first series of apertures to control flow to and from the gear pump pockets. According to prior steering systems, the number of apertures in the second series were twice the number of teeth of the rotatable piston in order to accommodate steering in both directions. Where six teeth are utilized, twelve apertures spaced by 30.degree. from each other were provided in the outer rotatable valve element. The inner valve element through which the steering direction is reversed, must have the same angular spacing for the apertures therein. Accordingly, the control apertures in the neutral position of the control valve for one of the valve elements are disposed centrally between the control apertures in the other of the valve elements. Thus, the maximum adjusting angle for the inner rotary slide valve element relative to the outer valve element was limited to 15.degree. in both directions.
Where the steering system is to be used for self-propelled, heavy vehicles such as articulated frame vehicles, an adjusting angle for the control valve larger than 15.degree. is required in order to reduce the sensitivity of the vehicle to rotational motion of the hand steering wheel. Otherwise, during rapid travel of the vehicle, the large masses of the articulated parts of the vehicle would oscillate about their hinged connection and impede smooth steering of the vehicle.
It is therefore an important object of the present invention to provide a hydrostatic steering system with greater flexibility in selecting the number of gear teeth provided for the hand metering gear pump in order to enable design of the steering system in accordance with criteria such as the quantity of fluid displaced, installational space, production costs, and fluid stream pulsation amplitude. An additional object of the present invention in accordance with the foregoing object is to provide the control valve associated with a hydrostatic steering system with cooperating series of control apertures angularly spaced by an amount suitable for the desired purpose and, in particular, for installations where angular spacing between the control apertures must be made larger than 15.degree. in both directions even though a rotatable piston with seven or more external teeth is utilized.