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RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/243,508 filed Sep. 17, 2009, which is hereby incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to hydraulic drive systems for spreaders used to distribute a road surface treatment material, such as sand or salt, across a road surface. 
     BACKGROUND 
     Hydraulic drive systems have been used to drive a feed auger and spinner of a spreader typically carried by a vehicle for spreading a road surface treatment material, such as sand or salt, across a road being traversed by the vehicle. The feed auger delivers the road surface treatment material from a supply thereof, such as a hopper, to the spinner which distributes the material across a road surface. 
     Many of these systems are designed as self-contained units that can be mounted in the bed of a pickup dump truck. Pressurized hydraulic fluid is supplied by a hydraulic pump typically driven by a gasoline engine. Cab-mounted, manually operated hydraulic flow control valves have been used to adjust the rotary speeds of the material conveyor (auger) and spinner fan (spinner). 
     Electrically-operated hydraulic valves have been used for spreading applications. These systems have used a parallel type flow configuration for supplying hydraulic fluid to the hydraulic motors that drive the auger and spinner. These systems require relatively large pumps to supply adequate flow to the auger and spinner motors. 
     U.S. Patent Application Publication No. 2005/0204587 discloses a microprocessor-controlled hydraulic system for snow-ice removal trucks that uses digital hydraulic valving control responsive to the instantaneous speed of the truck. According to this document, a binary form of digital valving removes a requirement for vulnerable feedback lines and associated sensors. As disclosed, the hydraulic motors for driving the auger and spinner are serially connected. The valves are either in an open position or a closed position depending on the desired amount of hydraulic flow. When no hydraulic pressure is to be supplied to the auger and/or spinner motors, i.e. when the digital control valves are all closed, hydraulic flow is routed back to reservoir by a bypass valve. When the bypass valve is open, pressure cannot buildup at the inlets to the auger and spinner motors. 
     SUMMARY OF INVENTION 
     The present invention provides a hydraulic drive system for a spreader used to distribute a road surface treatment material, such as sand or salt, across a road surface. The hydraulic drive system enables the use of pressure-compensated proportional control valves in series relationship with the auger and spinner motors, and thus eliminates the need for digital valving equipped with a bypass valve that prevents pressure buildup in the system when hydraulic fluid is not being supplied to the auger motor and/or spinner motor. 
     A preferred embodiment of the invention is characterized by the use of a lower cost and smaller displacement hydraulic pump that enables installation in the engine compartment of a vehicle so it can be driven by the engine&#39;s fan belt. This eliminates the need for and cost of a gasoline-powered auxiliary engine, as well as the associated noise, pollution, and maintenance requirements. The hydraulic pump need only be sized to provide hydraulic flow satisfying the larger of the flow requirements for the auger and spinner motors, rather than the sum of the requirements as in the case of parallel systems. 
     More particularly, a hydraulic system for operating the feed auger and spinner of a spreader includes first and second fluid motors for driving the feed auger and spinner. The fluid motors are connected in series with one another and first and second solenoid-operated pressure-compensated proportional control valves each including a pressure compensating spool. The first valve has an inlet configured to receive pressurized fluid from a source thereof such as an engine compartment mounted hydraulic pump, a regulated flow outlet connected to the inlet of the first fluid motor, and a bypass flow outlet, whereby operation of the valve controls the volume of flow of pressurized fluid supplied to the inlet of the first fluid motor via the regulated flow outlet with the balance of flow bypassing the first fluid motor. The second valve has an inlet connected to the outlet of the first fluid motor and the bypass flow outlet of the first valve, a regulated flow outlet connected to the inlet of the second fluid motor, and a bypass flow outlet, whereby operation of the second valve controls the volume of flow of pressurized fluid supplied to the inlet of the second fluid motor via the regulated flow outlet of the second valve with the balance of flow bypassing the second fluid motor. The system additionally includes first and second pressure-relieving restricted flow passages respectively connected between the regulated flow outlets and bypass flow outlets of the first and second valves for preventing pressure buildup at the regulated flow outlets of the first and second valves, thereby to assure proper operation of the compensator spools of the first and second valves. 
     In a preferred embodiment, the first and second restricted flow passages include an orifice of no greater than about 0.020 inch in diameter. In another preferred embodiment, the first and second restricted flow passages each is sized to accommodate the leakage flow through the respective valve that exceeds the leakage flow through the respective fluid motor. In another preferred embodiment, the first and second restricted flow passages are sized to provide a leakage flow of about 0.12 gallons per minute. 
     In another preferred embodiment the restricted flow passage includes an orifice and a filter upstream of the orifice to prevent clogging of the orifice. 
     The system may further include the pump that provides pressurized fluid to the inlet of the first valve. The pump may be configured for mounting in the engine compartment of the snow-ice control vehicle and for being driven by an engine-driven fan belt in the compartment. 
     The invention also provides a snow-ice vehicle including the hydraulic drive system as well as the engine, wherein the engine has an engine-driven belt, and the pump is driven by the belt. 
     In still another preferred embodiment, the system is employed in combination with the auger and spinner that are connected to the first and second motors, respectively. 
     The invention also provides a corresponding method of operating the hydraulic drive system. 
     The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a snow-ice control vehicle including a hydraulic drive system for operating a feed auger and spinner in the snow-ice control vehicle in accordance with the invention; 
         FIG. 2  is a cross-sectional view of an exemplary manifold assembly according to the invention; and 
         FIG. 3  is a cross-sectional view of a valve in the hydraulic drive system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings in detail, and initially to  FIG. 1 , a snow-ice control vehicle  10  includes a hydraulic system  12  for operating a feed auger  14  and spinner  16  of a spreader carried by the snow-ice control vehicle. The hydraulic system can be installed in various snow-ice control vehicles, such as a pickup dump truck, and allows pressure-compensated proportional control valves in series relationship to be operated from the cab of the vehicle to provide independent control of the feed auger  14  and spinner  16 . Accordingly, the system can be used in combination with the snow-ice control vehicle  10 , having an engine  18 , wherein the engine  18  has an engine-driven belt  20  that in conjunction with pulleys  22  and  24 , couples the engine  18  to a pump  26 , thereby allowing the pump  26  to be driven by the belt  20 . The pump  26  may be configured for mounting in an engine compartment of the snow-ice control vehicle  10  to allow the pump  26  to be driven off the engine  18 . 
     The pump  26  supplies pressurized fluid to a solenoid-operated pressure-compensated proportional control valve  28  when driven by the engine  10 . Pumps suitable for use as the pump  26  are conventionally available and often sold as kits with engine compartment mounting hardware. As is typical, the outlet of the pump may be provided with a pressure relief valve  78  to prevent damage to the system. 
     The solenoid-operated pressure-compensated proportional control valve  28 , herein also referred to as an auger control valve, has an inlet  30 , a regulated flow outlet  32 , and a bypass flow outlet  34 . The auger control valve  28  also includes an orifice spool  35  biased by a spool spring  36  toward its closed position, i.e. its position blocking flow from the inlet  30  to regulated flow outlet  32 . As described further below, the valve has a compensator for compensating for pressure variations whereby the position of the orifice spool is a function of current applied to a solenoid  28  of the auger control valve. As energization of the solenoid  38  is increased, the orifice spool  35  will move a corresponding amount permitting flow of hydraulic fluid from the inlet  30  to the regulated flow outlet  32 , while the balance of flow exits through the bypass flow outlet  34 . 
     Accordingly, the inlet  30  receives pressurized fluid from the pump  26  (or other source), and the regulated flow outlet  32  controllably delivers the pressurized fluid to a fluid motor  40 . The fluid motor  40 , herein also referred to as the auger motor, includes a fluid inlet  42  connected to the regulated flow outlet  32  and a fluid outlet  44  connected to a solenoid-operated pressure-compensated proportional control valve  46 , herein also referred to as a spinner control valve. Operation of the auger control valve  28  controls the volume of flow of pressurized fluid supplied to the inlet  42  of the auger motor  40  via the regulated flow outlet  32  with the balance of flow bypassing the auger motor  40  via the bypass flow outlet  34 . 
     Upon receiving the pressurized fluid supplied by the regulated flow outlet  32 , the auger motor  40  is configured to drive the feed auger  14  (or other device). When the fluid exits the auger motor  40  via the fluid outlet  44 , it is combined with the fluid from the bypass flow outlet  34  to provide essentially all the fluid flow from the pump  26  to the spinner control valve  46 . If an operator wishes to bypass the auger  14  (or other device), the pressurized fluid may be supplied to the spinner control valve  46  solely from the bypass flow outlet  34 . 
     The spinner control valve  46  operates in manner similar to that described above in respect to the auger control valve  28  for controllably driving the spinner  16 . The spinner control valve  46  has an inlet  48 , a regulated flow outlet  50 , and a bypass flow outlet  52 . The spinner control valve  46  also includes an orifice spool  53  biased by a spool spring  54  toward its closed position, i.e. its position blocking flow from the inlet  48  to regulated flow outlet  50 . The valve has a compensator for compensating for pressure variations whereby the position of the orifice spool is a function of current applied to a solenoid  56  of the auger control valve. As energization of the solenoid  56  is increased, the orifice spool  53  will move a corresponding amount permitting flow of hydraulic fluid from the inlet  48  to the regulated flow outlet  50 , while the balance of flow exits through the bypass flow outlet  52 . 
     Accordingly, the inlet  48  of the spinner control valve  46 , which is connected to the outlet  44  of the auger motor  40  and the bypass flow outlet  34  of the auger control valve  28 , receives the pressurized fluid from the outlet  44  and the bypass flow outlet  34 , and the regulated flow outlet  50  controllably delivers the pressurized fluid to a fluid motor  58 . The fluid motor  58 , herein also referred to as the spinner motor, includes a fluid inlet  60  connected to the regulated flow outlet  50  and a fluid outlet  62  connected to a reservoir  64 . 
     Operation of the spinner control valve  46  controls the volume of flow of pressurized fluid supplied to the inlet  60  of the spinner motor  58  via the regulated flow outlet  50  with the balance of flow bypassing the spinner motor  58  via the bypass flow outlet  52 . Upon receiving the pressurized fluid supplied by the regulated flow outlet  50 , the spinner motor  58  is configured to drive the spinner  16  (or other device). Fluid exiting the spinner motor  58  via the fluid outlet  62  is directed back to the reservoir  64  as is the fluid exiting the bypass flow outlet  52 . If an operator wishes to bypass the spinner  16 , the pressurized fluid may be returned to the reservoir  64  solely from the bypass flow outlet  52 . 
     Pressure compensated valves, even when de-energized, have some leakage flow through the regulated flow outlets. The inventor recognized that this can result in pressure buildup at the inlet of the respective motor, particularly when the auger/spinner motor has low leakage. This is particularly a problem with new fluid motors when tolerances are very tight. As a result of this pressure buildup, the pressure compensated valves may not work properly and can result in pressure buildup at the outlet of the pump. To prevent pressure buildup at the regulated flow outlets of the valves, thereby assuring proper operation of the compensators, pressure relieving restricted flow passages  66  and  72  are connected, respectively, between the regulated flow outlets  32  and  50  and bypass flow outlets  34  and  52 . The restricted flow passages  66  and  72  can each be sized to accommodate the difference between leakage flow through the regulated flow outlets  32  and  50  of the respective valves  28  and  46  and the respective fluid motor  40  and  58  when the valves  28  and  46  are de-energized and flow is directed to the bypass outlet. 
     The restricted flow passages  66  and  72  may include respective orifices  68  and  74  to restrict flow from the regulated flow outlets  32  and  50  to the bypass flow outlets  32  and  50 . In one embodiment, the restricted flow passages  66  and  72  can have an orifice having a diameter no greater than about 0.020 inch and provide a leakage flow of about 0.12 gallons per minute. Additionally, the restricted flow passages  66  and  72  can include respective filters  70  and  76  upstream of the orifices  68  and  74  to help prevent clogging of the orifices  68  and  74 . 
     The restricted flow passages  66  and  72  may be provided in a respective manifold block  80 ,  82  in which the valves are installed, as depicted by the broken lines in  FIG. 1 . The manifold blocks may be different or they may be the same. In the illustrated embodiment, the manifold blocks are the same. An exemplary manifold assembly  100 , including the valves and orifices is illustrated in  FIG. 2 . 
     Turning now to  FIGS. 2 and 3 , the solenoid-operated pressure-compensated proportional control valve is indicated at  102  and is of a cartridge type threaded into the manifold block indicated generally as  104 . The valve  102  may be of any suitable type, such as a valve available from Parker Hannifin Corporation under part number DFA125C31SN. As seen, the valve  102  has a valve body  106  having a central bore housing the valve components. The valve body  106  is threaded into the manifold  104  to secure the valve  102  in the manifold block  104 . The valve  102  includes an inlet  110 , a regulated flow outlet  112 , and a bypass flow outlet  114  that are coupled to an inlet  111 , a regulated flow outlet  113  and a bypass flow outlet  115  of the manifold  104 , respectively. The valve  102  also includes a pressure compensating spool  108  that is biased by a spool spring  116 , the compensating spool compensating for pressure variations in the valve. A sense port  123 , which is connected by a sense line (as illustrated in  FIG. 1 ) to the outlet  112  of the valve, allows the compensating spool  108  to sense the pressure on both ends of the spool  108  to compensate for the pressure variations in the valve. 
     The valve  102  additionally includes an orifice spool  117 , which is biased by spool springs  119  and  121  toward its closed position, i.e. its position blocking flow from the inlet  110  to regulated flow outlet  112 . A radial flow path  128  is provided between the inlet  110  and regulated flow outlet  112  that opens during axial movement of the orifice spool  117  to allow the fluid to flow from the inlet  110  to the regulated flow outlet  112 . 
     A solenoid  118  is provided including a solenoid plunger  120  that is configured to be axially movable under the magnetic influence of a solenoid coil  122  toward and away from the orifice spool  117 . The solenoid plunger  120  is coupled to a rod  124  and guided in a pole piece  126 , thereby allowing the plunger  120  to move the orifice spool  117  a corresponding amount and permit fluid flow from the inlet  110  to the regulated flow outlet  112  when the solenoid  118  is energized. The position of the orifice spool  117  is a function of current applied to the solenoid  118  by a control device. The solenoid  118  is coupled to the control device by a coupling device  130 , the control device preferably being located in the vehicle cab. The control device includes suitable controls that may be operated by the vehicle operator to vary the speed of the auger and spinner by varying the current supply to the auger control valve and spinner control valve. This may be implemented by a suitable microprocessor controller. 
     The manifold block  104  also has a flow passage  132  connected between the regulated flow outlet  113  and bypass flow outlet  115 . The passage has disposed therein an orifice  134  and a filter  136  upstream the orifice, as described above. The restricted flow passage  132  can be sized to accommodate the difference between leakage flow through the valve  102  and the respective fluid motor when the valve is de-energized. To mount the manifold assembly  100  in different positions in the snow-ice vehicle  10 , the assembly  100  includes mounting holes  138 . 
     Although the auger is shown upstream of the spinner, it should be appreciated that the positions may be reversed. 
     Principles of the invention can be applied to other applications and thus, it should be appreciated that devices other than the auger or spinner may be driven by the fluid motors. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

Summary:
A hydraulic drive system for a spreader used to distribute a road surface treatment material, such as sand or salt, across a road surface. The hydraulic drive system enables the use of pressure-compensated proportional control valves in series relationship with the auger and spinner motors, by the provision of small restricted flow passages across the outlets of the valves.