Abstract:
A hydraulic power steering system which includes a tank and a power steering pump which takes in a pressure medium from an intake area which is decoupled from the prevailing atmospheric pressure on the side of the tank wall and delivers it under pressure to the power steering system. A valve device builds up a pre-pressure in the intake area and prevents a negative pressure and a positive pressure from building up on the interior of the oil tank.

Description:
The present invention relates to a hydraulic power steering system having a tank and a power steering pump which draws in a pressure medium from a suction area and delivers it under pressure into the power steering system. 
     BACKGROUND 
     Power steering systems of this kind are generally known. One problem with these systems is that cavitation occurs at the power steering pump because of insufficient suction pressure, which limits the speed and capacity of the power steering pump. Alternatively, expensive injector designs must be used with such power steering pumps in order to achieve higher suction pressures. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a hydraulic power steering system which will overcome these disadvantages. 
     The present invention provides a hydraulic power steering system having a tank and a power steering pump which draws in a pressure medium from a suction area and delivers it under pressure into the power steering system, the suction area of the power steering system being hermetically isolated from the atmosphere and having a valve device for building up a bias pressure in the suction area. In a preferred power steering system, the valve device includes a check valve for drawing in air when there is negative pressure in the suction area, and further includes a pressure-limiting valve, or counterbalance valve, for releasing air when there is excessive pressure in the suction area. 
     In accordance with an embodiment of the present invention, the present invention provides that the suction pressure of the pump may be improved when the low-pressure or suction area of a steering system is isolated from the atmosphere using a hermetic tank or a hermetically sealed suction area in conjunction with an auxiliary device which controls the pressure level required in the suction area. In steering systems commonly used today, the level of the pressure medium in the reservoir or tank constantly changes due to temperature changes during operation. An air vent may ensure that there is always a balancing pressure with respect to the environment, irrespective of the level in the reservoir. If the air vent is replaced with a self-priming pressure-limiting valve, the oil level changes, in conjunction with this valve, which can be used to selectively control the pressure inside the reservoir, and thus, the suction pressure of the pump. In this process, the liquid column in the tank functions as a piston pump. 
     In an embodiment of a power steering system according to the present invention, the opening pressure of the check valve may be smaller, possibly significantly smaller, than the opening pressure of the pressure-limiting valve. This may have the advantage that at low temperatures, a slight decrease in pressure in the reservoir may be sufficient to cause the check valve to open, and to cause air to be drawn from the environment into the reservoir until the pressure is nearly equalized to atmospheric pressure. Therefore, the pressure difference across the check valve should be as small as possible. When the temperatures increase during operation, a positive pressure may be generated in the system due to the rise of the oil level in the tank and because of this additional volume of air. When the pressure exceeds a defined value, air can be readily released through the pressure-limiting valve. Thus, the pressure in the suction area can be automatically adjusted without additional aids, and the maximum suction pressure of the power steering pump is determined by the opening pressure of this pressure-limiting valve. Since these effects occur automatically as the system warms up after start up, it may not be absolutely necessary to pre-adjust the system when opening the reservoir or during initial filling. 
     A power steering system is preferred in which the two functional valve units may be arranged in a common valve housing to form a single valve device or valve assembly. This has the advantage that the two functional valve units, for example, the valve assembly needed for the above-described functions, can be disposed as a single unit in the suction area, where this unit can, for example, replace a filler cap of a tank or reservoir. Thus, existing systems can be readily converted to pressure bias in the suction area. It may be also possible to retrofit existing reservoirs. 
     An embodiment of a power steering system according to the present invention provides the valve seat sleeve of the check valve and the closure member of the check valve, especially a ball, together may also form a pressure-limiting valve piston. Also preferred may be a power steering system in which the pressure-limiting function may be accomplished by the joint movement of the valve seat sleeve and the pressure-limiting valve closure member against an opening pin which pushes the closure member or ball away from the seat, thus opening the check valve. Thus, the check valve may be opened once against its closing spring by negative pressure in the suction area, and a second time by a pin when there is excessive pressure in the suction area. Consequently, the check valve may have two modes of operation. 
     A preferred power steering system design of the present invention provides the opening pin which may be mounted on a sleeve having passage openings. This has the advantage that the opening pin may be reliably guided by the sleeve, while nevertheless allowing passage of air from the atmosphere into the tank through the passage openings within the valve housing. 
     Another preferred design of the present invention provides a power steering system in which the closure member, or ball, of the check valve may be pressed against the seat by a first, weak spring. The weak spring may ensure that the check valve already opens when the pressure in the suction area falls slightly below atmospheric pressure, and that it thereby may ensure the equalization of pressure and the increase in air volume in the reservoir. 
     A further preferred design of the present invention provides a power steering system in which the valve seat sleeve may be pressed by a second, stronger spring against a stop in the valve housing, for example, away from the opening pin. This has the advantage that, due to the stronger spring in conjunction with the piston area that provides the pressure-limiting function, a correspondingly high bias pressure or charge pressure can be adjusted for the suction area of the power steering pump. 
     Another preferred design of the present invention provides a power steering system in which the valve housing contains a valve seat sleeve, a ball check valve, a first spring, a second, stronger spring, an opening pin with a sleeve having passage openings, and an opening to the atmosphere. This has the advantage that this one valve assembly structurally combines two valve functions, namely that of a check valve and that of a pressure-limiting valve, which allows it to be used in any desired location in the suction area of the power steering system, or to replace any venting devices located there, so as to convert the suction area at atmospheric pressure to a suction area that is at a bias pressure. 
     Another preferred design of the present invention provides a power steering system in which the valve housing may be disposed in or on the tank or on the tank cap. 
     In accordance with certain embodiments of the present invention, a power steering system is provided having the oil column in the tank acting as a piston for biasing and drawing in the air in the suction area or tank. The level in the tank constantly changes due to temperature changes of the oil and of the power steering system during operation. This change in level can be used for drawing in air and compressing the residual air in the oil tank, using the oil column in the tank as a piston. 
     Another preferred design provides a power steering system in which the suction pressure of the power steering pump may be increased by the bias of the air in the tank or suction area. Advantageously, this may allow avoidance of cavitation at high speeds and high delivery rates, which may eliminate the need for expensive injector designs. 
     Another preferred design provides a power steering system in which the valve device can be integrated into an existing filler cap of the tank or suction area; for example, in which the valve device can replace the existing venting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, with reference to the FIGURE. 
       The FIGURE shows a cross-sectional view of a valve device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A valve device  3  according to the present invention is disposed in a wall  1  of an oil reservoir or tank  40  for a pump  42 , the wall possibly forming part of a reservoir cap  48 . Side  5  of the reservoir wall is at atmospheric pressure, while side  7  of reservoir wall  1  faces a suction area  44  of the power steering system, and therefore constitutes the inner side of the oil reservoir  40 . The oil reservoir  40  itself and the suction area  44  of the power steering system are hermetically sealed off from the ambient pressure. Valve device  3  has a valve housing  9 . A valve seat sleeve  11  is slidably supported in valve housing  9 . Valve seat sleeve  11  includes a conical valve seat  13 , which serves as a valve seat for a closure member  15  in the form of a ball. Ball  15  and valve seat  13  are shown in an open position to better illustrate the component parts. However, in reality, ball  15  would be pressed against valve seat  13  by a first, relatively weak spring  17 . A second, significantly stronger spring  19  presses valve seat sleeve  11  against a stop  21  of valve housing  9 . Spring  17  rests against a support disk  23 , which has an opening  25  to the interior of the tank  40 . An opening pin  27  is mounted on a support sleeve  29  in the upper portion of valve housing  9 , said support sleeve having passage openings  31  and resting, with a collar  33 , against second spring  19  within valve housing  9 . Valve housing  9  further has an opening  35  to the atmosphere. 
     The following is a description of the operation of the valve device of the present invention. If, due to low oil temperatures or low oil levels in the reservoir  40 , the pressure within the reservoir, for example, in area  7 , drops below a preset level, which can be adjusted by the closing pressure of the check valve, which is determined by ball  15  and spring  17 , check valve  15  opens, as illustrated in the figure, and air is drawn into the reservoir  40  from the environment, for example, from area  5 , through opening  35  and openings  31 , past opening pin  27  and check valve piston  15 , and through opening  25  until the pressure is nearly equalized to atmospheric pressure. The pressure will be slightly below atmospheric, which is due to the pressure difference determined by spring  17  and piston area  15  of the check valve. 
     When the temperature of the pressure medium increases during operation, a positive pressure is generated within the hydraulic system in the suction area  44  due to the rise of the pressure medium level and the resulting compression of this additional volume of air in addition to the already existing volume of air, and by the heating and expansion of the air. When the pressure exceeds a maximum pressure against second spring  19 , which is now determined by the sum of the piston area of check valve  15  and the piston area of valve seat sleeve  11 , then valve seat sleeve  11  is moved upward until ball  15  moves against opening pin  27 , as a result of which ball  15  is pushed away from its seat  13 . Thus, no further increase in pressure can occur in the suction area  44 ; the pressure remains at the level determined by spring  19  and the two interacting effective pressure areas of the so-called pressure-limiting valve. Thus, the maximum pressure in the suction area  44 , for example, the bias pressure, can be automatically adjusted without additional aids. 
     An oil column  46  in the tank  44  may act as a piston for biasing and drawing in the air in the suction area  44  or tank  40 . The level in the tank  40  constantly changes due to temperature changes of the oil and of the power steering system during operation. This change in level can be used for drawing in air and compressing the residual air in the oil tank  40 , using the oil column  46  in the tank  40  as a piston. 
     If valve device  3  is integrated directly into the filler cap of an oil reservoir, existing systems can be readily converted into biased systems. It is possible to retrofit existing reservoirs. Possibly, the steering gear and the power steering pump may have to be rated for higher suction pressures or idle pressures. In any case, it is an advantage of the present invention that the biased suction pressure enables cavitation-free operation of power steering pumps at significantly higher speeds and delivery rates. 
     List of Reference Numerals
       1  wall of an oil reservoir     3  valve device     5  reservoir wall side at atmospheric pressure     7  reservoir wall side in the suction area     9  valve housing     11  valve seat sleeve     13  conical valve seat     15  closure member (ball)     17  first, weak spring     19  second, stronger spring     21  stop of valve housing  9       23  support disk     25  opening to the interior of the tank     27  opening pin     29  support sleeve     31  passage openings of support sleeve  29       33  collar of support sleeve  29       35  opening of valve housing  9  to the atmosphere