Hydraulic system for extremely cold environments
BACKGROUND OF THE DISCLOSURE 
     In the use of vehicles in extremely cold environments, as for example a crawler tractor in Alaska during the winter, an operator sometimes finds that the hydraulic fluid increases in viscosity to a magnitude sufficient to make the hydraulic system slow to respond. One of the principal locations at which the hydraulic fluid becomes excessively viscous is in the relatively long conduits which provide the fluid pathway between the control valve and the hydraulic cylinders which actuate the clutch and brakes of the crawler tractor. During extended operation without clutch actuation, the fluid in these long conduits can cool to a value at which the pump pressure is slow to move the viscous fluid in the conduit upon actuation of the control valve. 
     This invention solves the problem of hydraulic line &#34;freeze up&#34; during inactive periods of the associated work element by providing means for substantially continuously passing hydraulic fluid between the control unit and the hydraulic cylinder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a portion of a hydraulic system of a vehicle having the apparatus of this invention; 
     FIG. 2 is a diagrammatic, enlarged view of a portion of the hydraulic system showing the control valve and associated hydraulic conduits; and 
     FIG. 3 is a diagrammatic view of a portion of the apparatus of FIG. 2 with the spool of the control valve positioned at another location. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a vehicle hydraulic system 10 is provided for controllably passing hydraulic fluid to a hydraulically actuated cylinder 12 of a work element (not shown), for example a vehicle clutch or brake. The hydraulic system 10, as is known in the art, has a fluid reservoir 14, a pump 16, and a control valve 18 connected between the pump 16 and cylinder 12 and positioned a relatively great distance from the cylinder 12. 
     A conduit 20 is connected to an intake of the pump 16 for passing fluid from the reservoir 14 to the pump 16. A first means 22, for example a conduit, is provided for passing fluid from the pump 16 to the control valve 18, a second means 24, for example another conduit, is provided for passing fluid from the control valve 18 to the hydraulic cylinder 12, and a third means 26, for example another conduit, is provided for returning hydraulic fluid from the control valve 18 to the reservoir 14. 
     Referring to FIG. 2, the control valve 18 has a chamber 28 and first, second, and third ports 30,32,34 in fluid communication with the chamber 28. Each of the ports 30,32,34 is connected to a respective one of the first, second, and third means 22,24,26 for passing hydraulic fluid, as set forth above. 
     A spool 36 is slidably positioned within the housing chamber 28 and is movable in response to actuating control means 38, such as a brake or clutch pedal. The spool 36 is movable between a first position (FIG. 2) and a second position (FIG. 3). At the first position, a fluid pathway is open from the pump 16, through the control valve 18, and to the reservoir 14. At the second position, a fluid pathway is open from the pump 16, through the control valve 18, and to the hydraulic cylinder 12, and is of a magnitude sufficient for actuating said cylinder 12 by fluid passing through the pathway to the cylinder at a preselected rate. 
     In this invention, a fourth port 40 is opened into the control valve chamber 28 at a first end portion 42 of the valve 18. A circulating conduit 44 has one end connected to the fourth port 40 and the other end connected to the cylinder 12 or the second means 24 at a location immediately adjacent the hydraulic cylinder 12. 
     Fourth means 46 provides a fluid pathway from the pump 16, through the control valve 18, through the circulating conduit 44, through a portion or all of the second means 24, back through the control valve 18, and through the third means 26 to the reservoir 14 at the first position of the spool 36, as shown in FIG. 2. At the second position of the spool 36, the fourth means 46 provides for blocking the fluid pathway from the pump 16 to the circulating conduit 44 and provides a fluid pathway from the pump 16, through the control valve 18 and to the hydraulic cylinder 12 for the actuation thereof. 
     The fourth means 46 is provided by constructing the valve with openings such as holes, slots, or grooves at preselected locations relative to the travel limits of the spool 36. 
     Referring to FIGS. 2 and 3, a first opening 48, here a curvilinear hole, opens at one end on a side of the spool 36, extends through a portion of the spool 36, and opens on the spool first end 50. The first opening 48 can be a groove, an orifice, or other configuration, but is of a construction sufficient for communicating said first and fourth ports 30,40 through said first opening 48 at the first position of the spool (FIG. 2) and spacing said first opening 48 from the first port 30 at the second position of the spool 36 (FIG. 3) for blocking fluid communication of the first port 30 with the fourth port 40. 
     A second opening 52, such as a groove for example, is positioned on a middle portion 54 of the spool 36. The second opening is of a construction sufficient for communicating the second and third ports 32,34 at the first position of the spool 36 (FIG. 2) and, at the second position of the spool 36, spacing said second opening 52 from the third port 34 while communicating said first and second ports through said second opening 52. 
     In the embodiment shown in the drawings, the control valve 18 has a housing 56 having a removable end cap 58 with said fourth port 40 extending through said end cap 58. 
     During use of the hydraulic system, fluid from the pump 16 is circulated through the control valve 18, conduits 24,44 and back to the reservoir during periods when fluid is not being delivered to cylinder 12. Therefore, the fluid in the conduits between the control valve 18 and the cylinder 12 is not in a quiescent, exposed position for detrimentally increasing the viscosity thereof in response to chilling. 
     Other aspects, objects, and advantages of this invention will become apparent from a study of the specification, claims, and the drawings.