Abstract:
A relief valve in a hydraulic system in a machine selectively sends pressurized oil to both a brake cooling apparatus and a cylinder used to hoist a load. A relief valve lowers pressure spikes in a brake cooling line by dumping fluid in a brake cooling line to tank responsive to a pressure spikes at the head end of the cylinder during hoisting or other transitions in the hydraulic system. Reducing the pressure spikes in the brake cooling line improves reliability of the brake cooling apparatus by protecting seals from undue stresses.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a hydraulic system and more specifically to reducing pressure spikes in a brake oil cooling system. 
     BACKGROUND 
     Oil or other hydraulic fluid used to operate hydraulic systems, such as a truck bed hoist cylinder of a machine such as an off-road truck may also be used to cool the brakes of the machine. The oil may be circulated to one or more brake coolers by the main hydraulic pump except when the main hoist cylinder is activated to raise the truck payload bed. Upon demand, the oil flowing through the brake cooler may be redirected to the main hoist cylinder to raise the truck payload bed. This change in flow from the brake cooler to the main hoist cylinder and other changes in the flow path of oil or operation of the machine may cause a spike in fluid pressure, also known as an fluid hammer, in the brake cooler hydraulic circuit. This spike in fluid pressure has been linked to premature failure in seals in the brake cooler or related components. 
     WO 2013/112109 A1 (the “109 publication) discloses an unloading valve that uses two pilot orifices to prevent spool oscillations due to sudden pressure fluctuations. The 109 publication fails to teach an unloading valve that relieves pressure from a brake cooling line responsive to pressure changes in the cylinder end of a hoist hydraulic line. 
     SUMMARY 
     In one aspect, a hydraulic system includes a hoist cylinder having a rod end and a head end and a brake cooler having a brake cooler port configured to receive dispelled fluid from the rod end in response to pressurized fluid being directed to the head end to extend the hoist cylinder. The hydraulic system may also include an unloading relief valve in fluid communication with the rod end and the brake cooler port, the unloading relief valve being movable between a biased first position and a second position, in response to fluid being dispelled from the rod end to the brake cooler port and pressurized fluid being directed to the head end. The unloading relief valve is movable to the second position based on a signal pressure of the head end to allow the dispelled flow from the rod end to flow to a tank. 
     In another aspect, a hydraulic system in a machine having a hoist cylinder with a head end and a rod end includes a hydraulic valve configured in a biased first position and a second position that connects a first port and a second port, the hydraulic valve including a first signal pressure port and a second signal pressure port, wherein the hydraulic valve is urged to the second position by a predetermined pressure on either the first or second signal pressure ports. The hydraulic system may also include a first hydraulic line having a first end coupled to a brake cooling circuit and further coupled to both the first port and the first signal pressure port, the first hydraulic line having a second end selectively coupled via a hoist valve to one of a pump or a hoist cylinder rod end hydraulic line. The hydraulic system may also include a second hydraulic line coupled between a hoist cylinder head end hydraulic line and the second signal pressure port and a third hydraulic line coupled between the second port and a tank. 
     In yet another aspect, a method of operating a hydraulic system includes, responsive to a hoist valve in a raise position, providing pressurized fluid to a head end of a hoist cylinder and dispelling fluid from a rod end of a hoist cylinder to a port of the brake cooler, and moving an unloading relief valve to a position in response to a pressure of a head end of a hoist cylinder being above a threshold pressure to allow the dispelled fluid from the rod end to flow to a tank. 
     These and other benefits will become apparent from the specification, the drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an off-road machine; 
         FIG. 2  is a diagram of a simplified hydraulic system suitable for use in the machine of  FIG. 1 ; 
         FIG. 3  is an alternate diagram of the simplified hydraulic system of  FIG. 2  in a first state; 
         FIG. 4A  is an alternate diagram of the simplified hydraulic system of  FIG. 2  in a second state with an unloading valve closed; 
         FIG. 4B  is an alternate diagram of the simplified hydraulic system of  FIG. 4A  with the unloading valve open; 
         FIG. 5A  is an alternate diagram of the simplified hydraulic system of  FIG. 2  in a third state with an unloading valve closed; 
         FIG. 5B  is an alternate diagram of the simplified hydraulic system of  FIG. 5A  with the unloading valve open; 
         FIG. 6  is a diagram of another embodiment of a hydraulic system suitable for use in the machine of  FIG. 1 ; and 
         FIG. 7  is a flowchart of a method of operating a hydraulic system. 
     
    
    
     DESCRIPTION 
       FIG. 1  illustrates a machine  10  having a payload bed  12  that can be lifted by a hoist cylinder  14 . In some applications there may be more than one hoist cylinder  14 . The machine  10  may be used in a variety of applications including mining, road construction, construction site preparation, etc. More details about the machine  10  and in particular a hydraulic system associated with the machine  10  are discussed in more detail below. 
       FIG. 2  is a diagram of a hydraulic system  21  suitable for use in the machine  10 . The illustrated hydraulic system  21  shows only component elements that are relevant to the current disclosure for the sake of simplicity and leaves out many elements commonly found in hydraulic systems of such machines including, but not limited to, additional pumps, safety relief valves, filters, counterbalance valves, additional cylinders, and auxiliary hydraulic units. 
     The hydraulic system  21  of  FIG. 2  may include a pump  102  to provide pressurized fluid to the system, a hoist valve  106 , also known as a hoist spool valve, to selectively direct pressurized flow from the pump  102  to hydraulic functions, such as the hoist cylinder  14 . The hoist cylinder  14  has a rod end  16  and a head end  18  capable of receiving pressurized flow from the pump  102  through the hoist valve  106 . The hoist valve  106  moves in a known fashion between positions to accommodate hydraulic connections between the elements of the hydraulic system  21 . In an embodiment, the hoist valve  106  may include pilot pressure controls (not depicted) on each end of a spool that move the spool back and forth using pressurized fluid controlled by a joystick or other operator interface. In some embodiments, the spool may be moved by an electrohydraulic valve using electrical signals from a joystick control. 
     A tank  120  can be provided in the system  21  to receive flow from hydraulic functions, such as the hoist cylinder  14 . In one example, a port  138  leading to the head end  18  is provided and coupled to the hoist valve  106  via a hydraulic line  116 . Also, a port  136  leading to the rod end  16  is provided and coupled to the hoist valve  106  via a hydraulic line  114 . 
     The hydraulic system  21  may also include a brake cooler  20  connected to the hoist valve  106  via a brake cooling line  118  at port  140 . A port  141  may connect the brake cooler  20  to the tank  120 . In an embodiment, the pump  102  may supply hydraulic fluid, or oil, to the brake cooler  20  when the hoist cylinder  14  is not in use. When the pump  102  is in use, for example, when the hoist cylinder  14  is actively lifting the payload bed  12  using fluid directed into the head end  18 , fluid being dispelled from the rod end  16  may be routed to the brake cooler  20  to extend the time during which brake cooling can take place. The dispelled fluid may be routed to the brake cooler  20  through the hoist valve  106 . Discharge from the brake cooler  20  may carried to the tank  120  via a tank line  122 . A relief valve  124  may be provided with the system  21 . The relief valve  124  can be disposed in the brake cooling line  118  between the hoist valve  106  and the tank  120 . The relief valve  124  can offer protection to the brake cooler  20  should the brake cooling line  118  experience a high pressure. The relief valve  124  may also be used to limit pressure spikes created on the brake cooling line  118  under circumstances discussed in more detail below. 
       FIGS. 3-5  show different operating states of the hydraulic system  21 .  FIG. 3  shows additional detail of the hoist valve  106  illustrating various exemplary positions including a lower position  108 , a hold position  110 , and a raise position  112 . The pump  102  may be coupled to the hoist valve  106  at more than one location as depicted by lines  104   a  and  104   b . The line  114  may connect the hoist valve  106  to the rod end port  136  and the line  116  may connect the hoist valve  106  to the head end port  138 . 
     As illustrated in  FIG. 3 , the hoist valve  106  is set in the hold position  110  so that both the rod end hydraulic line  114  and the head end line  116  are disconnected from the pump in order to maintain the hoist cylinder  14  at its current position. While in the hold position  110 , the hoist valve  106  connects the pump  102  to the brake cooling port  140  to provide cooling to the brake cooler  20  ( FIG. 2 ). 
       FIG. 3  also illustrates additional detail about the unloading relief valve  124 . The valve  124  includes an inlet port fluidly coupled to the line  118  via line  119 , and outlet port coupled to line  122 . Also, the valve  124  is biased in the closed position by a spring located on one side of the valve  124 . When moved to the open position, an internal passage couples the inlet and outlet ports to allow variable flow (0-100%) based on the position of the valve  124 . A signal pressure line  130  is coupled to the inlet port to communicate the pressure of the line  118  to the side of the valve opposite the spring. A line  125  is provided to communicate the pressure of the line  116  to the outlet port and the line  122 . A second signal line  132  is coupled to the line  125  to communicate the pressure of the line  116  to the side of the valve opposite the spring. A check valve  126  can be provided to the line  125 . In an embodiment, another check valve  127  disposed in the valve  124  may allow flow only in the direction of the tank and prevent pressurized fluid in the tank line  122  from passing into the line  118 . 
     An orifice  128  can be provided to line  125 . The orifice  128  can restrict flow through the check valve  126  in a known manner so that the signal line  132  is able to activate the unloading relief valve  124  at a given pressure or flow rate. The unloading relief valve  124  may be set to respond to a first threshold pressure applied at the signal line  130  and a second threshold pressure applied to the additional signal line  132 . In some embodiments, the first and second threshold pressures may be the same. 
       FIG. 4A  illustrates the hydraulic system  21  with the hoist valve  106  set to the lower position  108  so that the head end port  138  is connected to the tank  120 , the pump  102  is connected to the rod end port  136  as well as the brake cooling port  140 . Because lowering the empty payload bed  12  is substantially easier than raising the payload bed  12  when loaded, the pump  102  is generally capable of supplying fluid to both the rod end  16  as well as the brake cooler  20 . 
       FIG. 4B  illustrates the hydraulic system  21  shown in  FIG. 4A  with the unloading relief valve  124  in the open position so that fluid in the line  118  is shunted to tank  120 . This state of the unloading relief valve  124  may be in response to a high enough pressure in the signal line  130 , corresponding to a high pressure in the brake cooling line  118 , to urge the valve  124  into the open position. This state of the unloading relief valve  124  may also be in response to an impulse or fluid hammer in the head end line  116  that causes sufficient pressure in the signal line  132  to urge the valve  124  open. 
     In an embodiment, the functions of the unloading relief valve  124  may be separated into two valves, one that only functions as a relief for high pressure in the brake cooling line  118 , and another valve that unloads the brake cooling line in response to high pressure on the head end line  116 . 
       FIGS. 5A and 5B  illustrates the hydraulic system  21  with the hoist valve  106  set to the raise position  112 . In this state, the pump  102  is connected to the head end port  138  and the rod end port  136  is connected to the brake cooling port  140  via line  115   a  as well as the tank  120  via line  115   b . In some embodiments the rod end port  136  is only connected to the brake cooling portion  140  via the hoist valve  106  and line  115   a  where line  115   b  is not present. In either embodiment, the oil dispelled from the rod end  16  may be used to provide additional brake cooling during payload bed  12  hoist operations even though the pump  102  is not connected to the brake cooling line  118 . As illustrated in  FIG. 5A , there may be times, either at when initially connected or during some portions of the hoist cycle such as when a load shifts, the unloading relief valve  124  may be in the closed position. In most cases, after pressure builds in the head end line  116 , the unloading relief valve  124  will be open during the hoist cycle. 
       FIG. 5B  illustrates the hydraulic system  21  shown in  FIG. 5A  with the unloading relief valve  124  in the open position so that fluid in the line  118  is shunted to tank  120 . This state of the unloading relief valve  124  may be in response to a high enough pressure in the signal line  132 , corresponding to a high pressure in the head end line  116  to urge the valve  124  into the open position. 
     As illustrated in  FIGS. 3-5 , when pressure in the brake cooling line  118  exceeds a threshold pressure of the unloading relief valve  124 , pressure in line  130  urges the unloading relief valve  124  from a first biased position to a second, open, position. The valve  124  in the open position can allow oil to flow from brake cooling line  118  through the valve  124  to the tank  120 , preventing damage to the brake cooler  20  resulting from sustained high pump pressure or the like. 
     Beyond that, several points in the payload bed  12  unloading cycle may cause transient high pressure spikes to occur in the hydraulic system  21  that cause a hydraulic hammer condition (referred to herein as a fluid hammer) sometimes associated with a rise in head end line  116  pressure before flow starts in the brake cooling line  118 . For example, transient pressure spikes in the head end line  116  may occur at the initiation of a payload bed  12  raise, at some point during a payload bed  12  raise such as when a load slides from the payload bed  12 , when the payload bed  12  reaches maximum height, at some point during the payload bed  12  lower phase such as when a weight transfer of the payload bed  12  itself compresses the hoist cylinder  14 , or when the payload bed  12  fully drops to its corresponding support on the machine frame. 
     Even though the brake cooling line  118  may not be directly connected to either the pump  102  or the head end hydraulic line  116  when some of these transients occur, the resulting fluid hammer may be transmitted via the rod end hydraulic line  114  and/or the hoist valve  106 . Because the signal line  130  that activates the unloading relief valve  124  for over pressure conditions is essentially in a parallel relationship with the line  119  and the brake cooler  20 , the unloading relief valve  124  may not be activated by an fluid hammer in the signal line  130  in time to prevent or mitigate the fluid hammer from reaching the brake cooler  20 . This pressure spike or fluid hammer may cause premature wear on the seals of the brake cooler  20 . 
     Without regard to how the fluid hammer is transmitted to the brake cooling line  118 , the presence of a pressure spike on the head end line  116  can indicate the subsequent occurrence of the fluid hammer in the brake cooling line  118 . The signal line  132 , being responsive to pressure spikes on the head end line  116  therefore allows the unloading relief valve  124  to activate and connect the brake cooling line  118  to the tank line  122  in time to shunt an fluid hammer associated with such a pressure spike before it reaches the brake cooler  20 . 
     While it may be desirable to have the unloading relief valve  124  respond to a pressure spike at the signal line  132  and open as fast as possible, the sudden closing of the unloading relief valve  124  may generate a transient pressure spike of its own. So, the unloading relief valve  124  may be dampened so that the valve  124  will open quickly and close slowly. In one embodiment, the pressure in line  132  that causes the valve  124  to open may only be bled off through orifice  128 , so that high pressure in the head end line  116  causes the valve  124  to open quickly, but when the pressure in the head end line  116  is reduced, the valve  124  will close slowly as fluid is discharged through the orifice  128  to tank. 
       FIG. 6  illustrates another embodiment of a hydraulic system  100  similar in many respects to the hydraulic system  21  that may also be suitable for preventing or limiting fluid hammer damage to a brake cooler  20 . In this embodiment, a controller  150  in communication with a first pressure sensor  152  and a second pressure sensor  154  may control an electrohydraulic valve  156  to provide protection for over pressure situations as well as pressure-spike related fluid hammers. In this embodiment, high pressure on the brake cooling line  118  may be sensed by the sensor  152  and the electrohydraulic valve  156  may be opened in order to connect the brake cooling line  118  to the tank  120 . Similarly, the sensor  154  may be sensitive to pressure spikes occurring on the head end hydraulic line  116  so that the controller  150  may cause the electrohydraulic valve  156  to open prior to an associated fluid hammer reaching the brake cooling port  140  and subsequently the brake cooler  20 . 
     INDUSTRIAL APPLICABILITY 
     The use of an unloading relief valve  124  configured to operate in response to pressure in a hoist cylinder head end hydraulic line  116  protects a brake cooler  20  from pressure spikes that can damage seals and cause a machine  10 , such as an off-road large truck, costly downtime. 
       FIG. 7  is a method  240  of operating a hydraulic system  21 ,  100 . The method  240  helps reduce pressure spikes or fluid hammers from propagating through the hydraulic system  21 ,  100  to a brake cooler  20  and therefore protects seals in the brake cooler circuit. 
     At a block  242 , a machine  10 , such as an off-road truck using in mining or construction operations, may have a hydraulic system  21 . Responsive to a hoist valve  106  being in the raised position, dispelling fluid from the rod end  16  to a port  140  of the brake cooler  20 . 
     At a block  244 , moving an unloading relief valve  124  to a position in response to a pressure at the head end  18  of a hoist cylinder or a head end hydraulic line  116  being above a threshold pressure to allow rod end fluid flow to a tank  120 . Setting the unloading relief valve  124  to the second position may include providing a signal line  132  from the head end  18  or the head end hydraulic line  116 , to a signal pressure port of the unloading relief valve  124 , wherein the signal line  132  connection includes a check valve  126  that prevents reverse flow into the head end  18  or its associated hydraulic line  116 . 
     At a block  246 , the unloading relief valve  124  may also be set to the second position responsive to a pressure in the brake cooling line  118  above a second threshold pressure. This protects the brake cooler  20  from high pressure in the brake cooling line  118  unrelated to pressure spikes in the head end line  116 . In various embodiments, the pressures at signal pressure lines  130  and  132  required to move the unloading relief valve  124  from the biased first position to the second position may be the same or may be different. 
     At a block  248 , optionally movement of the unloading relief valve  124  may be dampened when moving from the second position back to the biased first position. This may reduce the introduction of additional pressure spikes or fluid hammers caused solely by a chance in flow in the brake cooling line  118 . At a block  250 , movement of the unloading relief valve  124  from the biased first position to the second position may have no damping so the unloading relief valve  124  may open as quickly as possible to intercept an fluid hammer propagating through the hydraulic system  21 . 
     The use of an unloading relief valve  124  configured to respond to sources of pressure spikes in relatively remote hydraulic circuits provides increased reliability for any seals and other wear components that are otherwise subject to these pressure spikes. While applicable in the above disclosed embodiment of a brake cooling environment, use for fans, motors, other coolers, transmissions, etc. are equally applicable. 
     In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.