Patent Publication Number: US-2004040485-A1

Title: Power assist marine steering system

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates to marine steering systems and, more particularly, relates to a power assist steering system for a boat or other watercraft. Specifically, the invention relates to a steering system that incorporates an operator controlled helm and a separate hydraulic steering cylinder that is controlled by the helm in a master/slave fashion to steer the watercraft.  
       [0003] 2. Discussion of the Related Art  
       [0004] In a conventional marine steering system, a watercraft such as a boat is steered by pivoting a rudder and/or outboard motor on the stem of the watercraft about a vertical steering axis upon steering actuation by an operator stationed at the helm. One typical steering system for a boat having a hull-mounted motor comprises a steering cable extending between the steering helm and the motor so that steering at the helm actuates the cable to pivot the motor about the steering axis. The cable typically comprises a push-pull cable having a reciprocatable inner core slidable in a protective, flexible outer sheath or housing. One end of the cable is connected to the steering helm, and the other end is connected to a tiller arm coupled to the motor or rudder. When the wheel is turned at the helm, the cable is actuated by a push-pull movement of the inner core, thereby pivoting the tiller arm. These systems work reasonably well on small boats, but the steering forces required for pivoting the tiller arm increase progressively with system size to the point that many larger boats can be steered manually only with great difficulty, if at all.  
       [0005] In order to reduce the forces required to steer a watercraft, it is well-known with marine outboard drives, particularly those employing large displacements, to employ a hydraulic power steering assist system for assisting the operator in steering the boat. The typical hydraulic power steering assist system includes a hydraulic cylinder that is connected to a tiller arm or other steered mechanism and that is energized in response to operator control to actuate the steered mechanism. Specifically, a helm-responsive controller is coupled to a hydraulic cylinder assembly that, in turn, is coupled to the steered mechanism, either directly or via an intervening push-pull cable. When the steering wheel is turned one way or the other, hydraulic fluid is pumped from the steering helm to one end or the other of the cylinder assembly to pivot the motor one way or the other.  
       [0006] A power steering assist system that is generally of the type described above is described in U.S. Pat. No. 5,603,279 (the &#39;279 patent). The system described in the &#39;279 patent comprises a hydraulic cylinder-piston assembly and a helm. The cylinder-piston assembly has a reciprocally mounted piston and first and second chambers in the cylinder on opposite sides of the piston. The steering cylinder has a balanced piston. In fact, as with most systems of this general type, a rod extends through both ends of the steering cylinder making for a longer assy. The helm includes two separate cylinder assemblies that are divided into four separate internal chambers by a stepped flanged piston. One of the cylinder assemblies forms a master cylinder that is actuated directly by a control valve assembly under power supplied from the pressure source. The portion of the piston in this part of the assembly is stepped so as to form an unbalanced cylinder in the helm. The second cylinder assembly comprises a slave cylinder divided into third and fourth chambers by an annular flange on an extension of the piston. The third and fourth chambers are coupled to respective chambers of a steering cylinder. The control valve assembly is actuatable to regulate the flow of hydraulic fluid into and out of the second chamber to drive the piston and, thereby, vary the volumes of the third and fourth chambers and driving the steering piston one way or the other within the steering cylinder to effect a steering operation. The actuator of the valve assembly comprises a rotatable valve body that has first and second valves mounted in it. A rotatable input member (e.g., a steering shaft or extension thereof), actuable upon steering at the helm, is operably connected to the valve actuator. Thus, steering at the helm actuates the valve actuator to regulate the flow of pressurized hydraulic fluid through the cylinder, thereby driving the piston in one direction or the other depending upon the steering direction.  
       [0007] The system disclosed in the &#39;279 patent, while effective, exhibits several drawbacks and disadvantages. For instance, because its helm has four chambers and, in effect, two pistons, it requires a great many seals. The helm is also relatively large (both axially and radially). In fact, it is so large that it must be formed from a casting rather than machined components. It is therefore difficult to mount on the back of the dashes of many smaller boats. Several of the hydraulic fittings on the helm also are necessarily located on the periphery of the helm rather than on the rear end, rendering it difficult to access those fittings after the helm is installed behind the dash.  
       [0008] In addition, the rotary valve employed by the &#39;279 patent is relatively expensive to manufacture and difficult to assemble.  
       [0009] Moreover, in the system disclosed in the &#39;279 patent, only part of the system (namely, the first and second chambers of the helm) is pressurized directly by the pressure source. The remainder of the system (namely, the third and fourth chambers of the helm and both chambers of the steering cylinder) is pressurized indirectly via translation of the slave portion of the piston. Air in the lines of that portion of the system can lead to noticeable “looseness” or play of the cylinders.  
       [0010] The need therefore has arisen to provide a power assist marine steering system that is relatively simple in construction and easy to assemble.  
       [0011] The need further exists to provide a power assist marine steering system including a helm that is relatively compact so as to be easily mountable to the dash and accessible from behind the dash of a boat.  
       SUMMARY OF THE INVENTION  
       [0012] In accordance with a first aspect of the invention, a power steering assist system for a watercraft comprises a hydraulically actuated, unbalanced steering cylinder assembly, a pressure source, and helm that is spaced from the steering cylinder assembly. The steering cylinder assembly is configured for connection to a steered mechanism of the watercraft. It includes a steering cylinder, a steering piston that is mounted in the steering cylinder to define first and second chambers on opposite sides thereof, and a rod that is affixed to the steering piston, wherein either the rod or the steering cylinder is movable relative to the other and is configured for connection to the steered mechanism. Fluid pressures in the first and second chambers act on first and second different effective areas of the steering piston. The helm includes a helm cylinder having a slave chamber fluidically coupled to the second chamber in the steering cylinder, a high pressure port fluidically coupled to the outlet of the pressure source and to the first chamber in the steering cylinder, and a return port fluidically coupled to a vent. The helm additionally includes a helm piston that is slidably mounted in the helm cylinder so as to form the slave chamber and a control chamber on opposite sides thereof, and a control valve assembly that is movable between at least first and second positions to alternatively couple the control chamber to the high pressure and return ports.  
       [0013] Preferably, the control valve assembly is movable into a third, neutral position in which the control chamber is isolated from both the high pressure and return ports. In this case, the helm further comprises an operator-manipulatable steering mechanism. The control valve assembly comprises first and second two-way/two-position valves that are configured to be actuated by the steering mechanism such that 1) both the first and second valves remain closed when the steering mechanism remains stationary, 2) movement of the steering mechanism in a first direction opens the first valve while leaving the second valve closed, and 3) movement of the steering mechanism in a second direction opens the second valve while leaving the first valve closed. The control valve assembly comprises a valve body that houses the first and second valves and a valve actuator that is linearly translatable between first, second, and third positions thereof, the valve body having a first passage formed therein that couples the high pressure port to the control chamber and a second passage formed therein that couples the return port to the control chamber, and wherein the first and second valves are located in the first and second passages, respectively. The resultant system is simple and compact. It is also pressurized directly by a single source. It therefore does not exhibit the looseness experienced by some other systems.  
       [0014] In order to facilitate mounting of the helm to the dash of the watercraft, the helm has only three ports (namely, a slave port that is fluidically connected to the second chamber in the steering cylinder, the high pressure port, and the return port), and all three ports are all located on a rear axial end of the helm cylinder. The helm cylinder also is very compact.  
       [0015] In accordance with other aspects of the invention, an improved helm cylinder and an improved power assist steering method are also provided.  
       [0016] These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:  
     [0018]FIG. 1 is a schematic top plan view of a boat incorporating a power steering assist system constructed in accordance with a preferred embodiment of the present invention;  
     [0019]FIG. 2 is somewhat schematic perspective view of the power steering assist system of FIG. 1;  
     [0020]FIG. 3 is an elevation view of a portion of a dash of the boat of FIG. 1, showing a steering wheel and a helm of the power steering assist system mounted on the dash;  
     [0021]FIG. 4 is a hydraulic circuit schematic of the power steering assist system;  
     [0022]FIG. 5 is a side sectional elevation view of the power steering assist system, illustrating the system in a first operational state thereof;  
     [0023]FIG. 6 is a detail sectional elevation view, illustrating a valve assembly of the helm of the power assist steering system in a first operational state thereof;  
     [0024]FIG. 7 corresponds to FIG. 6 and illustrates the system in a second operational state thereof;  
     [0025]FIG. 8 is a side sectional elevation view of the detail “V” in FIGS. 5; and  
     [0026]FIG. 9 corresponds to FIG. 5 and illustrates the system in a second operational state thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0027] Turning now to the drawings and initially to FIG. 1, a boat  12  incorporates a power steering assist system  10  (hereafter simply “power steering system”) constructed in accordance with a preferred embodiment of the present invention. The boat  12  includes a hull  14  having a bow  16  and a stern  18 , an outboard motor  20  mounted on the stern  18 , and a cowling or dash  22  extending laterally across the hull  14  near the bow  16 . As is conventional, the motor  20  is mounted on the boat  12  by a pivoting mount assembly (not shown) that permits the motor  20  to be pivoted about a vertical axis to cause a rudder formed on or by the motor  20  to steer the boat  12 . The motor  20  could alternatively be a non-pivoting inboard or outboard motor, and boat  12  could be steered by one or more rudders movable separately from the motor  20 .  
     [0028] Referring now to FIGS.  1 - 2 , the steering system  10  for the boat  12  includes a tiller arm  24  coupled to the motor  20  and forming the boat&#39;s steered mechanism, a helm  26  including a steering wheel  28  serving as the boat&#39;s steering mechanism, a pressure source  30 , and a steering cylinder assembly  32 . The present embodiment contains no mechanical linkage connecting the helm  26  to the steering cylinder assembly  32 . Both assemblies  26  and  32  are pressurized by a single power source. The helm  26  is mounted through the dash  22  and is actuated by the steering wheel  28 . The steering cylinder assembly  32  is actuated by the helm  26  to move the tiller arm  24  and pivot the motor  20  on its mount under power supplied by the pressure source  30 . In order to minimize the size and weight of the components that are mounted behind the dash  22 , the steering cylinder assembly  32  is located remote from the helm  26 , possibly adjacent the motor  20  as illustrated or on the motor, so as to be connectable directly to the tiller arm  24 . Alternatively, the steering cylinder assembly  32  could be mounted at some other location on the boat  12  and connected to the tiller arm  24  by a push-pull cable or the like. The helm  26  is connected to the pressure source  30  by a high pressure line  34  and a return line  36 . It is also connected to the steering cylinder assembly  32  by the high pressure line  34  and a slave line  38 .  
     [0029] The fluid pressure source  30  could comprise any structure or assembly capable of generating hydraulic pressure and of transmitting it to the helm  26  and the steering cylinder assembly  32 . It also can be located virtually anywhere on the boat  12 . In the illustrated embodiment, the fluid pressure source  30  includes a pump  40  and a reservoir  42 , best seen in the assembly illustrated in FIG. 2. The pump  40  has an inlet connected to an outlet of the reservoir  42  and has an outlet  44  connected to or, as in the illustrated embodiment, forming the pressurized outlet of the pump assembly  30 . An accumulator (not shown) could be provided between the pump outlet  44  and the helm  26 , if desired. The reservoir  42  has an inlet  46  connected to or, as in the illustrated embodiment, forming the unpressurized inlet of the pressure source  30 .  
     [0030] Referring to FIGS. 2, 4,  5 , and  6  the steering cylinder assembly  32  comprises a hydraulically actuated, unbalanced steering cylinder assembly operatively coupled to the helm  26 , the pump outlet  44 , and the tiller arm  24 . “Unbalanced” as used herein means that the cylinder assembly&#39;s piston has different effective surface areas on opposite sides thereof such that equal fluid pressures on both sides of the piston generate an intensification effect on the side of the piston having a greater effective surface area and drive the piston to move towards the side of the cylinder facing the side of the piston having a smaller effective surface area. The steering cylinder assembly  32  includes a steering cylinder  50 , a steering piston  52  mounted in the steering cylinder to form first and second chambers  54 ,  56  on opposite sides of the steering piston  52 , and a rod  57  connected to the steering piston  52 . First and second ports  58 ,  60  open into the first and second chambers  54  and  56  for connection to the high pressure line  34  and the slave line  38 , respectively. The steering cylinder  50  of this embodiment is stationary and is mounted on the stem  18  of the hull  14  by a suitable bracket  62 . The rod  57  extends axially through a rod end  64  of the steering cylinder  50  (disposed opposite a cylinder end  66 ) and terminates at a free end that is coupled to the tiller arm  24 . The unbalanced condition of the assembly  32  therefore is created by virtue of the attachment of the rod  57  to the steering piston  52  and the consequent reduction in piston surface area exposed to fluid pressure in the first chamber  54 . Alternatively, the rod  57  could extend completely through the steering cylinder  50  and could be affixed to a stationary support, in which case the steering cylinder  50  would be coupled to the tiller arm  24  and would reciprocate relative to the stationary piston  52 . In this case, the unbalanced condition of the assembly  32  would be achieved by other measures, e.g., by making one end of the steering rod  57  diametrically smaller than the other.  
     [0031] Referring to FIG. 3, the helm  26  is mounted through the dash  22 . It includes the steering wheel  28 , a steering shaft  68  extending forwardly from the dash  22 , and a helm cylinder  70  located behind the dash  22 . The helm cylinder  70  is relatively compact, having a body  72  and a cap  74  screwed onto the front end of the body  72 . The back end of the cap  74  is mounted on the front surface of the dash  22  by bolts  76 . The body  72  is cylindrical, having a front axial end  78 , a rear axial end  80 , and an outer radial periphery  82 . It is very narrow, having a diameter of no more than 4 inches and preferably no more than about 3 inches. The body  72  also is relatively short, having a total length of no more than about 6″ to 7″. The entire helm cylinder  70 , including the body  72  and the cap  74 , is no longer than 11″ to 12″. Mounting behind the dash  22  is facilitated by the fact that the helm cylinder  70  has only a limited number of fittings (three in the preferred embodiment), and all of those fittings extend from the relatively easily-accessible rear axial end  80  of the helm cylinder  70 . The helm  26  therefore is considerably smaller than the helm disclosed in the &#39;279 patent and easier to mount to the dash. It is also considerably lighter, weighing 6 to 7 pounds less than the commercial version of the helm disclosed in the &#39;279 patent. The helm cylinder also need not be formed from a casting.  
     [0032] The hydraulic circuitry contained within the pressure source  30 , the helm  26 , and the steering cylinder assembly  32  will now be described with reference to FIG. 4. The helm cylinder  70  has a high pressure inlet port  84  connected to the high pressure line  34 , a slave port  86  connected to the slave line  38 , and a return port  88  connected to the return line  36 . Located within the helm cylinder  70  are a control valve assembly  90 , a helm piston  92 , a relief valve  94 , and check valve  96  and  201 . The helm piston  92  is slidably disposed in the helm cylinder  70  to form a slave chamber  98  and a control chamber  100  on opposite sides thereof. The slave chamber  98  is in constant fluid communication with the second chamber  56  in the steering cylinder  50  via the slave line  38 . The control chamber  100  is in constant fluid communication with the control valve assembly  90  which, in turn, is coupled to the pressure source outlet  44  and inlet  46  by the high pressure line  34  and the return line  36 , respectively. Check valve  200  is located in high pressure line  34  and prevents backflow into the pump  40 .  
     [0033] The control valve assembly  90  includes first and second normally, closed two-way/two-position valves. Still referring to FIG. 4, the first valve is a supply valve  102  having an inlet port  104  coupled to the high pressure inlet port  84  and having an output port  106  coupled to the control chamber  100 . The second valve is a vent valve  108  having an inlet port  110  coupled to the control chamber  100  and an outlet port  112  connected to the return port  88  via the valves  94  and  96 . Both valves  102  and  108  are coupled to a common actuator (preferably the steering shaft  68 ), such that movement of the actuator in a first direction opens one of the valves  102  or  108  while leaving the other valve closed, and movement of the actuator in a second direction opens the other valve  108  or  102  while leaving the one valve closed.  
     [0034] It can thus be seen that the first chamber  54  of the steering cylinder  50  will always be at a pressure P1 that is the same pressure as the pump outlet pressure. The slave chamber  98 , control chamber  100  of the helm cylinder  70  and the second chamber  56  of the steering cylinder  50  will all be at a second pressure P2 when no load is applied to the rod  57 . The pressure P2 will, depending upon the operational state of the valve assembly and the direction of load applied to rod  57 , vary from a low of essentially 0 psi relative to the atmosphere to a high of P1 (typically on the order of 1000 psi). Due to this arrangement, pressurized fluid flow into the control chamber  100  from the supply valve  102  drives the helm piston  92  to the left as seen in FIG. 4 to create a pressure differential across the steering piston  52  (generated by the unbalanced nature of the steering piston) and drive the steering piston  52  and rod  57  to the right as seen in FIG. 4. Conversely, venting of the control chamber  100  upon opening of the vent valve  108  causes the helm piston  92  to move to the right as seen in FIG. 4, leading to the fluid flow into the slave chamber  98  from the second chamber  56  of the steering cylinder  50  and creating a reverse pressure differential that drives the steering piston  52  to the left as seen in FIG. 4.  
     [0035] The relief valve  94  is locatable either internally of the helm cylinder  70  as illustrated in FIG. 4 or externally of the helm cylinder. Valve  94  is operable to normally permit unrestricted flow from the control chamber  100  upon opening of the vent valve  108  and to restrict the flow of fluid from the control chamber to a preset pressure if the pump  40  is not operational and, accordingly, the inlet port  84  is not pressurized. The relief valve  94  is normally held open by pilot pressure from the high pressure line  34  or another constantly pressurized portion of the system. The relief valve  94  closes in the absence of that pilot pressure to prevent fluid flow to the return port  88  unless the fluid pressure upstream of the relief valve  94  is above a check pressure (typically 300 psi).  
     [0036] Turning now to FIG. 5, the physical structure of the helm assembly incorporating the hydraulics of FIG. 4 can be seen to include the steering shaft  68 , the control valve assembly  90 , the helm piston  92 , the relief valve  94 , and the check valve  96 . The steering shaft  68  is rotatably borne in the helm cylinder  70  by thrust bearings  114  that permits rotation of the steering shaft  68  relative to the helm cylinder  70  but that prevents relative axial movement therebetween. The inner end the steering shaft  68  is threaded for cooperation with a valve actuator  120  of the control valve assembly  90 . A number of threaded shafts could be used for this purpose. A particularly preferred shaft is a so-called “acme screw” having a high pitch that effects a relatively large stroke of the actuator with relatively small rotation of the shaft. The control valve assembly  90  includes the valve actuator  120  and a valve body  122  coaxially surrounding the valve actuator  120 . The valve body  122  and helm piston  92  are coaxially located within the helm cylinder  70  and bolted to one another in an end-to-end relationship so as to move as a unit within the helm cylinder  70 . The valve body  122  also houses the supply and vent valves  102  and  108  and cooperates with the valve actuator  120  to selectively open and close the valves  102  and  108  upon steering shaft rotation.  
     [0037] Still referring to FIG. 5, the helm piston  92  is mounted in the helm cylinder  70  so as to form the control and slave chambers  100  and  98  on opposite sides of it. The two chambers  98  and  100  are sealed from one another by a single O-ring  124  mounted in a groove in the outer periphery of the helm piston  92 . The helm piston  92  also has a deep counterbore  126  formed in its front end to accommodate movement of the helm piston  92  over the steering shaft  68  as seen in FIG. 7. The counterbore  126  terminates in an extension  128  on the rear end of the helm piston  92 . The extension  128  bottoms out in a counterbore  130  in the rear end  70  of the helm cylinder  70  when the helm piston  92  assumes its right-most position within the helm cylinder. The slave port  86  opens into the counterbore  130 . Supply and return bores  132  and  134  are formed axially through the helm piston  92  on opposite sides of the counterbore  130 . Supply and return tubes  136  and  138  extend partway into the bores  132  and  134  from the high pressure and return ports  84  and  88 , respectively. The tubes  136  and  138  are sealed against the respective bores  132  and  134  by respective seals  140 ,  142  to permit fluid to flow into and out of the helm piston  92  from the rear end  80  of the helm cylinder  70  while permitting relative axial movement between the helm piston  92  and the tubes  136  and  138 .  
     [0038] Referring to FIGS. 5 and 6, the valve body  122  includes a tubular element disposed in the control chamber  100  in a non-fluid tight manner so that the control chamber  100  surrounds both ends of the valve body  122 . The valve body  122  has a central axial through bore  150  and axial supply vent passages  152  and  154  on opposite sides of the bore  150 . The bore  150  is counterbored at both axial ends  156  and  158  to receive rings  170  and  172  of the valve actuator  120  as detailed below. The supply passage  152  opens into the control chamber  100  at the front end  156  of the valve body  122  and is connected to the mating bore  132  in the helm piston  92  at the rear end  158 . The vent passage  154  similarly opens into the control chamber  100  at the valve body front end  156  and opens into the vent passage  134  in the helm piston  92  at the valve body rear end  158 . The supply valve  102  seats towards the front end of the supply passage  152 . It includes a ball-valve element  160  and a return spring  162  that biases the ball-valve element  160  toward the front end  156  of the valve body  122 . Conversely, the vent valve  108  seats towards the rear end of the vent passage  154 . It includes a ball-valve element  164  and a return spring  166  that biases the ball-valve element  164  toward the rear end  158  of the valve body  122 .  
     [0039] Still referring to FIGS. 5 and 6, the valve actuator  120  includes a nut  168  that is mounted on the threaded end of the steering shaft  68  and that is coaxially surrounded by the valve body  122 . Hence, rotation of the shaft  68  in one direction or the other drives the valve actuator  120  to move linearly either towards or away from the rear end  80  of the helm cylinder  70 . First and second rings  170  and  172  are mounted on opposite ends of the actuator  120  within the counterbored ends  156  and  158  of the valve body  122 . The first ring  170  is press-fit against a notch on the front end of the valve actuator  120 , or, alternatively, formed integrally with the valve actuator. The second ring  172  is clamped in a notch in the rear end by a nut  174  or otherwise affixed to the rear end. The rings  170  - and  172  are spaced from one another by a distance L1 that is greater than the length L2 of the counterbored portion of the valve body  122 , thereby forming a clearance at each end of the valve body  122  (having a maximum length of ((L1−L2)/2) that permits limited movement of the valve actuator  120  relative to the valve body  122  before one of the rings  170  or  172  contacts the associated counterbored end  156  or  158  in the valve body. Each ring  170 ,  172  has a tab  176 ,  178  that receives an actuator pin  180 ,  182  extending toward a respective passage  152 ,  154  in the valve body  122 . Accordingly, when the valve actuator  120  moves relative to the valve body  122  in a first direction, the pin  180  engages the ball-valve element  160  to open the supply valve  102  (compare FIG. 5 to FIG. 7). Conversely, when the valve actuator  120  moves relative to the valve body  122  in a second direction, the pin  182  engages the ball-valve element  164  to open the vent valve  108  (compare FIG. 5 to FIG. 6). Both pins  180  and  182  are mounted on axially movable adjusters  184 ,  186  to permit selected setting of the valve clearance.  
     [0040] Referring to FIG. 8, the relief valve  94  is configured to permit continued manual steering of the system  10  in the event of failure of the pressure source  30 . The relief valve  94  comprises a pilot valve mounted in a passage  190  that extends perpendicularly to the return passage  134  and that terminates in the supply passage  132 . A ball  192 , located in the passage  134 , is biased towards its closed position by a spring  194 . A plunger  196  is located on the other side of the ball  192  and extends into the supply passage  132 . Fluid pressure in the supply passage  132  normally forces the plunger  196  downwardly to hold the valve  94  open and to permit unhindered fluid flow through to the outlet port  88  from the return passage  134 . In the event of pump failure, the plunger  196  will no longer be forced downwardly by the supply pressure, at which point valve opening will be opposed by the return force of the spring  194 . Flow through the return passage  134  will continue only for so long as the fluid pressure in the return passage  134  imposes an opening force on the ball  192  that exceeds the closing force imposed by the spring  194 , thereby assuring at least minimal fluid pressure in the control chamber  100  and permitting manual steering of the system as detailed below.  
     [0041] Still referring to FIG. 8, the check valve  96  is located in parallel with the relief valve  94 . Valve  96  permits fluid to be drawn into the return passage  134  from the reservoir  42  if the control chamber  100  requires make-up fluid. It comprises a conventional ball-valve element  198  biased to its closed position by a relatively weak return spring  200 .  
     [0042] The operation of the power assist steering system  10  will now be described, with the assumption that the components are in the positions illustrated in FIG. 5 and the steering wheel  28  and steering shaft  68  are stationary. The valve actuator  120  is balanced in the valve body  122  at this time, and both the supply and vent valves  102  and  108  are closed to block flow into or out of the control chamber  100 . The pressures across both the helm piston  92  and the steering piston  52  are therefore balanced, and the helm piston  92  and steering piston  52  both remain stationary. Initial rotation of the steering shaft  68  in either direction drives the actuator  120  to move axially relative to the valve body  122  until one of the actuator pins opens the associated valve. Hence, clockwise shaft rotation drives the actuator  120  towards the front end  78  of the helm cylinder  70  and opens the vent valve  108  as illustrated in FIG. 6, thereby permitting pressurized fluid to flow out of the control chamber  100  through the vent valve  108  and the vent passage  134 . Continued clockwise rotation of the shaft  68  will cause the actuator  120 , valve body  122 , and helm piston  92  to move axially as a unit within the helm cylinder  70  from the position illustrated in FIG. 5 toward the position illustrated in FIG. 9. This movement permits fluid to flow from the second chamber  56  of the steering cylinder  50  into the slave chamber  98  of the helm cylinder  70  via the slave conduit  38 , to drive the steering piston  52  to the left as viewed in the drawings under the assistance of the pressure differential across the steering piston  52 . When steering shaft rotation ceases, steering piston  52 , the helm piston  92 , and the valve body  122  continues to move to the left relative to the valve actuator  120 , but only until the vent valve  108  closes and the valve body  122  rebalances on the valve actuator  120 . The helm piston  92  and steering cylinder piston  52  will thereafter remain in those positions until the steering shaft  68  is once again rotated.  
     [0043] Counterclockwise rotation of the steering shaft  68  drives the valve actuator  120  to the right relative to the valve body  122  to open the supply valve  102  and couple the control chamber  100  to the supply passage  132 . Subsequent movement of the helm piston  92  forces fluid into the second chamber  56  of the steering cylinder  50  from the slave chamber  98  and the slave conduit  38 , thereby forcing the steering piston  52  to the right as seen in the drawings. This motion is assisted by the increasing fluid pressure in the control chamber  100 . When shaft rotation ceases, the valve body  122  and helm piston  92  will continue to move to the right until the supply valve  102  closes and the valve body  122  rebalances on the valve actuator  120 . The helm piston  92  and the steering cylinder piston  52  move at different rates. The rate is determined by the ratio of the area of the piston faces. It should again be noted that the total volumes of chambers  56  and  98  are equal and that the total stroke of the helm piston  92  results in the total stroke of the steering cylinder piston  52 .  
     [0044] If the pump  40  fails, the system can be operated manually. Specifically, when the steering shaft  68  is turned counterclockwise, the actuator  120  unseats ball  160  and contacts the valve body  122 . At this time the force exerted by operator input causes the actuator  120  to push the valve body  122  and helm piston  92  to the right. Fluid is forced out of the slave chamber  98  into chamber  56  of the steering cylinder  50 , moving the steering cylinder piston  52  to the right. Fluid from chamber  54  of the steering cylinder  50  is forced out into the control chamber  100  past the unseated ball  160 . Because the volume of the control chamber  100  is larger than the volume of the steering cylinder chamber  54 , a negative pressure is created in control chamber  100 . This negative pressure will lift check balls  198  and  201  from their seats, and fluid will be drawn from the reservoir  42  into control chamber  100 . Check valve  200  prevents fluid from returning to the power source  30 .  
     [0045] When the steering shaft  68  is turned clockwise, the valve actuator  120  unseats ball  190  and contacts the other side of the valve body  90 . At this time the force exerted by operator input causes the actuator  120  to pull the valve body  122  and the helm piston  92  to the left. At this time, there is a decrease in pressure in chamber  56  of the steering cylinder  50  and the slave chamber  98  of the helm. There is also an increase in pressure in control chamber  100  of the helm. This increase in fluid pressure forces ball  160  off its seat, and fluid flows from control chamber  100  into chamber  54  of the steering cylinder  50 . Because the volume of the control chamber  100  is larger than the volume of chamber  54  of the steering cylinder  50 , the excess fluid in control chamber  100  must flow past ball  164 , through passage  134  and past pressure relief valve  94 , which is closed. The fluid pressure in control chamber  100  has to reach a predetermined level before the excess fluid can flow past the relief valve  94  back to the reservoir  42 . This pressure is the back-up pressure used to move the steering cylinder piston  52  to the left.  
     [0046] Many changes and modifications could be made to the invention without departing from the spirit thereof. Some of these changes are discussed above. Other changes will become apparent from the appended claims.