Self-pressurization system for gearboxes and the like
BACKGROUND OF THE INVENTION 
     This invention relates generally to a means for pressurizing a housing so that liquid may be more readily withdrawn from the sump of the housing and supplied to drive assembly components within the housing for lubrication and control purposes. 
     Aircraft drive assemblies, such as gear boxes and constant speed drives, often employ a scavenge pump in the housing to withdraw liquid from the sump. An example of such a mechanism is Gantzer U.S. Pat. No. 3,365,981, entitled &#34;Combined Fluid and Mechanical Drive,&#34; assigned to the assignee of the present invention. These devices do not scavenge well at high altitudes if the pressure in the drive housing or case is not above the ambient pressure. Baits et al U.S. Pat. No. 3,600,106, entitled &#34;Self-Pressurizing Scheme,&#34; also assigned to the assignee of the present invention, shows a means for pressurizing the housing or case when the ambient pressure exceeds the case pressure by a predetermined amount. Therein, air-entrained liquid was transmitted to the sump by employing an injector and a differential pressure valve. One problem encountered with this type of scheme is that the system has a tendency to become &#34;air bound&#34; at very high altitudes, i.e., low atmospheric pressure, due to bubbles coming out of the driving fluid. 
     One method of suitably pressurizing the case would be to employ a displacement pump for supplying air thereto. However, the pump cannot be allowed to run dry without lubrication, and provision must be made to avoid overpressurization of the case, preferably without blowing anything overboard. 
     Reeve U.S. Pat. No. Re. 28,428, while not relating to a case pressurization system, shows a pump which will pump one liquid until outlet pressure builds up and shifts a valve to connect an air passage to the pump intake so that liquid and air are delivered to the pump. The valve operates on both oil and air intake conduits. However, the amount of oil cannot be regulated so that at least a minute amount of oil is always delivered to the pump. 
     In a pressurization system as disclosed herein, the air from atmosphere should be cut off completely when the housing pressure reaches a predetermined level. 
     Bales U.S. Pat. No. 2,650,577 and Stone U.S. Pat. No. 2,782,773 relate to engine pressurization systems including pressurization of the crankcase. Bales shows an oil pump, but includes no valve for regulating air or oil intake into the crankcase. Stone shows the use of a diaphragm which builds up pressure in the crankcase. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the invention to enhance the practicability of using a displacement pump to pressurize a drive housing. 
     In accordance with the invention, a pump having its discharge port connecting to the housing is employed to pressurize the housing with ambient air drawn through a conduit communicating with the atmosphere. A valve responsive to a pressure differential between the housing and atmosphere regulates the amount of liquid, such as lubricating oil, delivered to the intake port of the pump in order to control the amount of ambient air delivered by the pump. When housing pressure relative to atmospheric pressure is high, a substantial amount of oil is delivered to the pump so as to exclude ambient air. As a result, there is positive exclusion when the desired pressure in the housing is reached. When the pressure differential is low, only a small amount of oil is delivered to provide constant lubrication to the pump and wet the clearances of the pump for efficient operation and thereby permit air to be drawn into the system. 
     In such a system, a positive constant displacement pump may be used without overpressurizing the housing and having overboard flow. 
     The pressurization system of the invention includes an air intake conduit between the atmosphere and the pump intake port having a check valve therein, a conduit between a source of oil and the inlet to the pressure-responsive valve, and a pair of conduits between the valve outlet and the pump intake port, one of which has an orifice restricting oil flow to less than the pump displacement. The valve has one pilot port communicating with the housing. The total pressure created by a biasing spring and atmospheric pressure opposes the housing pressure. When the housing pressure is high relative to atmospheric pressure, the valve delivers oil through the unrestricted conduit, thereby closing the check valve; if low, oil is delivered through the conduit having the orifice to limit the oil flow, thereby permitting ambient air to be drawn into the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a schematic illustration of a hydraulic circuit permitting self-pressurization of a housing. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     An example environment in which this circuit may generally be employed is shown in U.S. Patent No. 3,600,106, cited earlier. Therein, the drive assembly has a housing, drive means in the housing including input and output shafts, a sump in the housing, and pump means operatively associated with the drive means for withdrawing liquid, such as oil, from the sump and supplying oil to a reservoir. As will become apparent, the self-pressurization system herein described may be substituted for the injector and the differential pressure valve utilized therein. The details of the drive assembly form no part of the invention disclosed herein. 
     Referring to the drawing, a self-pressurization system for a drive assembly housing 10 such as shown in the above referred to patent is seen to primarily include a valve 12, a positive constant displacement pump 14, an orifice 16, and a check valve 18. These components may be disposed within the housing 10 which is airtight. 
     The valve 12 has an inlet 20, outlets 22 and 24, and pilot ports 26 and 28. The inlet 20 is connected by a conduit 32 to a source of oil 30 which may be a reservoir in the housing 10. The source of oil 30 may be the lube pressure in the return conduit which delivers makeup, lubricating and control fluid for the system. This could be a connection to line 63 of the above referred to patent. The pilot port 26 communicates with the housing 10 including the sump via a conduit 34 so that pressure in the housing 10 is transmitted to the valve 12. The pilot port 28 is connected to an air intake conduit 36 by a conduit 38. 
     The check valve 18 is in the conduit 36 between the end open to the atmosphere and the conduit 38. The check valve 18 has a valve member 42 which is biased to a closed position by a small centering spring 44. The force provided by the spring 44 is relatively low and acts only to return the valve member 42 to its seat when the conduit 36 is sealed from the atmosphere. 
     The valve 12 includes a spool 46 slidable within a bore 48 and is biased towards the pilot port 26 by a spring 50 providing a reference pressure or force. A typical reference pressure would be 5 p.s.i. The reference pressure is augmented by atmospheric pressure delivered through conduit 38 from the air intake conduit 36. An aneroid reference could be used to regulate to an absolute pressure, but this is usually not necessary. 
     The spool 46 is selectively movable to a first position, as shown in the drawing, communicating the inlet 20 with the outlet 22 and the conduit 52 and a second position (not illustrated) closing this connection. The orifice 16 is in the conduit 54 to always provide a small flow of oil into the pump 14. Whenever the valve 12 is moved to its second position, air is drawn into the housing 10 and, at the same time, the pump 14 is lubed and the clearance gaps wet. 
     The pump 14 is a positive constant displacement pump having an intake suction port 56 connected to a conduit 58 which communicates with the conduits 36, 52 and 54 at a junction 60. The pump has a discharge port 62 communicating with the housing 10 through a conduit 64. The pump acts to reduce the pressure in its suction chamber whereby pressure is reduced at its intake port so that external pressure (herein, the pressure normally acting on the oil source 30) pushes oil into the pump. 
     The amount of ambient air the pump 14 will draw in can be regulated by controlling the amount of oil supplied to the pump 14. If sufficient oil is provided to the pump 14, its suction port pressure will rise above ambient and no air will be drawn in. The supply of oil to the pump 14, and hence to the housing 10, is in turn controlled by the valve 12. 
     When the housing pressure is less than 5 p.s.i. more than ambient atmospheric pressure (minus minor loss in the check valve 18), the spring 50 controls, and the valve spool 46 is moved to its second position to starve the pump 14 so that air flowing from the conduit 36 to the pump 14 is drawn into the system to pressurize the housing 10. The orifice 16 provides for lubrication flow for the pump 14 at all times the unit is operated. 
     When housing pressure is over 5 p.s.i. more than the ambient atmospheric pressure, the housing pressure acts through the conduit 34 against the spring 50 and shifts the valve spool 46 upwardly, as shown in the drawing, whereby oil flows into the conduit 52. As a result, the pump intake port 56 is pressurized, which along with the back pressure from the valve 12 acts against the valve member 42 to pressurize the check valve 18, close it and block off the air supply. Thereafter, air is prevented from being drawn into the housing 10, and only oil is circulated in the system to terminate continued pressurization of the housing 10. When the relative pressure differential between the housing and atmosphere is lowered, atmospheric pressure opens the check valve 18 and acts through conduits 36 and 38 to shift the valve 12 to its second position once again.