Abstract:
An air compressor assembly of the rotary screw type. The air compressor assembly comprises a housing having an inlet end and a discharge end. An internal working chamber extends within the housing and terminates in a discharge end face at the discharge end of the housing. At least one rotor is mounted for rotation and axial movement within the working chamber. The rotor has a discharge end surface having a step defined thereon. A thrust piston extends from the rotor and is positioned within a thrust piston chamber. A pressure source is associated with the thrust piston chamber and is controllable between a high pressure condition and a reduced pressure condition to control the position of the rotor relative to the discharge end face. A method of mounting a rotor with a desired end clearance is also provided.

Description:
BACKGROUND 
     The present invention relates to air compressors. More particularly, the present invention relates to an improved screw-type air compressor. 
     Rotary screw-type air compressors generally include a pair of complementary rotors mounted within an internal working chamber of the compressor housing. Each rotor has a shaft supported for rotational movement by a pair of opposed radial bearings. Air enters through an airend inlet and is compressed by the rotating rotors as it moves toward a discharge port at the discharge end of the chamber. The spacing between the end surfaces of the rotors and the discharge end face of the housing is referred to as the discharge end clearance. This discharge end clearance has a substantial effect on the performance of the compressor. Accordingly, it is desirable to precisely set and maintain an operating discharge end clearance of a given air compressor to achieve a desired performance. 
     Current methods of mounting the rotors with a desired operating end clearance generally require extensive, very precise machining of the rotors and the housings. Bearings must also be accurately manufactured to provide not only radial support, but also axial support. Even with precise machining, the desired end clearance is often not achieved without extensive assembly procedures, for example, precision measuring and calculating of relative housing and rotor assembly measurements and the inclusion of compensating components, including shim plates or like. In addition to precise machining and assembly, other factors, for example, the internal rotor gas forces, must also be calculated and compensated for. 
     SUMMARY 
     The present invention provides an air compressor assembly of the rotary screw type that provides accurate discharge end clearances with minimized manufacturing and assembly requirements. The air compressor assembly comprises a housing having an internal working chamber that extends within the housing and terminates in a discharge end face at the discharge end of the housing. At least one rotor is mounted for rotation and axial movement within the working chamber. The rotor has a discharge end surface having a step defined thereon. The step is preferably machined to a height precisely equal to the desired discharge end clearance. A thrust piston extends from the rotor and is positioned within a thrust piston chamber. A pressure source is associated with the thrust piston chamber and is controllable between a high pressure condition and a reduced pressure condition. In the high pressure condition, a high thrust pressure is created such that the thrust piston is moved axially toward the discharge end and the rotor step abuts the housing discharge end face to precisely position the rotor with the desired discharge end clearance. This condition is generally referred to as the “loaded” condition during which the airend generally delivers compressed air to the intended application. In the reduced pressure condition, the thrust pressure is reduced and the rotor step moves away from the discharge end face to allow the rotor to freewheel. This condition is generally referred to as the “unloaded” condition during which compressed air is not delivered to the intended application by the airend. 
     A method of mounting a rotor with a desired end clearance in accordance with the present invention is also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic longitudinal cross-sectional elevation view of an air compressor assembly in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a partial, exploded view of the discharge end of the air compressor of FIG.  1 . 
     FIG. 3 is a longitudinal cross-sectional elevation view of a preferred thrust piston chamber valve of the present invention in the closed position. 
     FIG. 4 is a longitudinal cross-sectional elevation view of the thrust piston chamber valve of FIG. 3 in the opened position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an air compressor assembly  10  that is a preferred embodiment of the present invention is shown. The air compressor assembly  10  includes a housing  20  having an inlet end  22  and a discharge end  24 . An internal working chamber  26  is defined between the ends  22  and  24  and terminates in a discharge end face  27  adjacent the discharge end  24 . An airend inlet  28  and an oil inlet  30  extend into the working chamber  26  toward the inlet end  22  of the housing  20 . A discharge port  32  exits the working chamber  26  adjacent the discharge end  24 . The air/oil mixture exiting the discharge port  32  generally travels to a separation tank  34 . Oil separated from the air/oil mixture is returned from the separation tank  34  to the air compressor assembly  10  via the oil inlet  30 . The compressed air is delivered from the separator tank  34  via a conduit  35  to an intended application, for example, a pneumatic tool. The housing  20  may be cast, machined or the like and is preferably manufactured from aluminum, but may be manufactured from other materials, for example, cast iron. 
     Preferably, a pair of complementary rotors  40  and  50  are supported within the working chamber  26 . While a pair of rotors  40 ,  50  is preferred, it is also contemplated that more or fewer rotors may also be utilized. Each rotor  40 ,  50  has a rotor shaft  42 ,  52  supported in a pair of radial bearings  44 ,  54  at opposite ends of the housing  20 . The radial bearings  44 ,  54  are preferably hydrodynamic bearings, but other bearings, for example, rolling element bearings, may also be utilized. The radial bearings  44 ,  54  support the respective rotor shafts  42 ,  52  for rotation and axial movement. One of the rotor shafts  42  extends from the housing  20  and engages a drive mechanism (not shown) which provides the desired rotational movement of the rotors  40 ,  50 . 
     One end of each rotor shaft  42 ,  52  terminates in a thrust piston  46 ,  56  positioned within a respective thrust piston chamber  48 ,  58 . As illustrated in FIG. 1, the thrust chambers  48 ,  58  may be located at opposite ends of the housing  20 . Such positioning allows the thrust pistons  46 ,  56  to have maximized diameters without interfering with one another. However, other configurations, including side by side thrust pistons may also be used. Each chamber  48 ,  58  is supplied with oil via an oil supply path  72  extending from an oil reservoir  70  adjacent the discharge end  24  of the housing  20 . The oil reservoir  70  may be formed integral with the housing  20  or may be formed as a separate component. The oil supply path  72  enters each chamber  48 ,  58  such that oil at discharge pressure is supplied to the chamber  48 ,  58 . Conduits  61 ,  62  vent the thrust chambers  48 ,  58  on the opposite sides of the thrust pistons  46 ,  56  to inlet pressure such that a net differential force is generated by each thrust piston  46 ,  56 , thereby forcing the respective rotors  40 ,  50 , toward the discharge end  24  of the housing  20 . Each thrust piston  46 ,  56  has a pressure surface  47 ,  57  of sufficient area such that when the air compressor assembly  10  is in a loaded condition, the thrust force on each piston  46 ,  56  in the direction of the discharge end is greater than the opposing rotor gas forces A, B created by the rotating rotors  40 ,  50 . The thrust forces thereby drive the respective rotors  40 ,  50  axially until each rotor discharge end  41 ,  51  abuts the housing discharge end face  27 . 
     Referring to FIG. 2, each rotor  40 ,  50  is formed with a step  43 ,  53  extending from its discharge end surface  41 ,  51 . The steps  43 ,  53  are formed with a height equal to the desired discharge end clearance  60 , the distance between the non-stepped portion of each rotor discharge end surface  41 ,  51  and the housing discharge end face  27 . As such, the thrust pistons  46 ,  56  force the rotors  40 ,  50  axially until the steps  43 ,  53  contact the housing discharge end face  27 , thereby accurately defining the desired discharge end clearance  60  for each rotor  40 ,  50 . In addition to defining the discharge end clearance  60 , the steps  43 ,  53  also define a thrust bearing surface of minimal area. That is, the diameter of each step  43 ,  53  is substantially less than the diameter of the respective rotor discharge end surface  41 ,  51 . Oil flowing within the thrust piston chambers  48 ,  58  flows through the respective bearings  44 ,  54  and between the thrust faces  45 ,  55  and the discharge end face  27 , forming a hydrodynamic thrust bearing having a minimized contact surface for each rotor  40 ,  50 . While an aluminum housing  20  is preferred since it provides a proper bearing surface for faces  45 ,  55 , the continuous oil coating allows for a wear free bearing even when other materials are used for the thrust surface. 
     Referring again to FIG. 1, by applying or relieving thrust piston pressure, the rotors  40 ,  50  move toward or away from the discharge end face  27  of the housing  20  and thereby either pump air (loaded condition) or freewheel (unloaded condition). To facilitate the changing conditions, the preferred compressor assembly  10  includes a discharge port check valve  80  and an oil stop valve  84 . The discharge port check valve  80  is configured to close the discharge port passage  32  when the rotors  40 ,  50  are in the unloaded condition, thereby trapping the high pressure air in the separator tank  34  and allowing the rotors  40 ,  50  to freewheel at atmospheric pressure. Such unloading reduces the power requirement of the compressor assembly  10 . 
     The oil stop valve  84  is configured to close the oil inlet  30  when the rotors  40 ,  50  are in the unloaded condition to prevent oil flooding in the working chamber  26 . However, whether the compressor assembly  10  is operating in a loaded or unloaded condition, it is necessary to maintain oil flow in the rotor radial bearings  44 ,  54 . While oil flow about the thrust bearings  45 ,  55  is beneficial, it is generally not required in the unloaded condition since the rotors  40 ,  50  move away from the housing discharge end face  27  as will be described in more detail hereinafter. The desired oil flow is provided by the oil reservoir  70 . During loaded operation, the high pressure air/oil mixture passes out the discharge port  32  with oil filling the oil reservoir  70  and excess oil traveling with the air/oil mixture to the separator tank  34 . The entrance to the oil reservoir  70  is preferably on the bottom of the discharge port  32  such that oil flowing through the discharge port  32  drains by gravity into the oil reservoir  70 . Oil in the reservoir  70  travels through the oil supply paths  72  to the thrust piston chambers  48 ,  58 . The oil entering each chamber  48 ,  58  flows to the radial bearing  44 ,  54  respectively adjacent the chamber  48 ,  58 . Additionally, a secondary oil path  74  extends from each chamber  48 ,  58  to the adjacent bearing  44 ,  54  of the other rotor shaft  42 ,  52 . That is, one secondary oil path  74  allows oil to flow from thrust piston chamber  58  to airend bearing  44  and the other secondary oil path  74  allows oil to flow from the thrust piston chamber  48  to the discharge end bearing  54 . When the compressor assembly  10  is unloaded, the discharge port check valve  80  and the oil stop valve  84  close and the rotors  40 ,  50  freewheel at atmospheric pressure. Although the oil reservoir  70  is also at atmospheric pressure, it is located above the thrust piston chambers  48 ,  58  and bearings  44 ,  54  such that gravity causes the oil to flow to the chambers  48 ,  58  and bearings  44 ,  54 . Oil passing through the bearings  44 ,  54  into the working chamber  26  is thrown toward the discharge port  32  by the rotating rotors  40 ,  50  such that it flows back into the reservoir  70  from where it can be recirculated. 
     Referring to FIG. 1, a preferred embodiment of the discharge port check valve  80  and the oil stop valve  84  is shown. The valves  80  and  84  are provided by a single rod  86  and valve head assembly  88 . The valve head  88  is attached to the rod  86  which extends adjacent the discharge port  32  and the oil inlet  30 . To close both valves  80  and  84 , the rod  86  moves axially such that the rod  86  closes off the oil inlet  30  and the valve head  88  moves into the path of and closes off the discharge port  32 . When the rotors  40 ,  50  are in the unloaded condition, the pressure in discharge port  32  is lower than the pressure in separator tank  34 . As air tries to flow from the separator tank  34  back through the port  32 , it forces the valve head  88  into the closed position. A spring or the like (not shown) may be provided to bias the rod  86  toward the closed position. Both valves  80  and  84  are held open in the loaded condition by air flow from the discharge port  32  forcing valve head  88  into the open position. 
     Having described the components of the preferred compressor assembly  10 , its operation will be described with reference to FIGS. 1 and 2. Loading and unloading of the compressor assembly  10  is controlled by controlling the pressure in the thrust piston chambers  48  and  58 . To unload the compressor assembly  10 , the chambers  48  and  58  are vented to the inlet end  22  of the compressor housing  20 . The pressure in the chambers  48 ,  58  is at atmospheric pressure, such that the rotor gas force A, B is greater than the thrust piston pressure whereby the rotors  40  and  50  move away from the discharge end face  27 , thus increasing the discharge end clearance  60 . Even though the discharge end clearance  60  is relatively large, the pressure at the discharge port  32  is greater than the inlet pressure. To load the compressor assembly  10 , the vent lines to chambers  48  and  58  are closed and the higher discharge end pressure is applied to the oil reservoir  70 , and in turn, to the chambers  48  and  58 . The increase in pressure in the thrust chambers  48  and  58  increases the thrust forces which causes the rotors  40 ,  50  to begin to move axially toward the discharge end face  27 , thereby decreasing the discharge end clearance  60 . The reduced discharge end clearance  60  causes a greater discharge port pressure which increases the oil reservoir pressure, and in turn, the pressure in the chambers  48 ,  58 . The process continues until the compressor assembly  10  is fully loaded with the steps  43  and  53  against the discharge end face  27 , thereby precisely defining the desired discharge end clearance  60 . 
     A preferred valve assembly  100  utilized in venting the thrust piston chambers  48 ,  58  is shown in FIGS. 3 and 4. An individual valve assembly  100  may be utilized for each chamber  48 ,  58 , or a common valve assembly may be utilized to simultaneously control both chambers  48 ,  58 . The valve assembly  100  includes a valve housing  102  having an internal chamber  104 . An inlet passage  106  from the thrust piston chamber  48 ,  58  extends into the valve chamber  104  in alignment with an outlet  108  from the chamber  104  to the compressor airend inlet  28 . A spool member  110  including a passage area  111  is positioned in the chamber  104  between the inlet passage  106  and the outlet  108 . The spool member  110  is axially moveable within the chamber  104  such that the passage area  111  can be aligned with (open) or offset from (closed) the inlet passage  106  and outlet  108 . A spring  112  or the like biases the spool member  110  to the offset, closed position. A second inlet  114  from the separator tank enters the valve chamber  104  on the side of the spool member  110  opposite the spring  112 . The spring  112  is selected such that it will prevent axial movement of the spool member  110  until the pressure in the separator tank  34  reaches a preselected value. Once the separator tank pressure reaches the preselected value, the spring force is overcome and the spool member  110  moves to the aligned, open position (see FIG. 4) whereby the thrust piston chamber  48 ,  58  vents to the airend inlet  28 . With this configuration, the compressor assembly  10  can be controlled to store a desired pressure within the separator tank  34  and freewheel until the pressure is relieved by air utilization, at which time the valve  100  will close and the compressor assembly  10  will return to loaded operation.