Abstract:
A double acting pneumatic motor having a cylinder, a piston within the cylinder that divides the cylinder into two cavities that vary in volume with movement of the piston, a source of fluid under pressure, and a valve assembly for alternately directing fluid to one cavity while exhausting the other cavity to make the piston reciprocate. The valve assembly resides in a housing, and includes a cylindrical spool that moves axially between two positions with at least one enlarged diameter portion between smaller diameter portions at each end. The pneumatic motor may be provided with a cover for attenuating sound from the exhaust.

Description:
BACKGROUND 
     This invention relates generally to pneumatic motors, and more particularly concerns a piston and cylinder device in which a pneumatic valve automatically causes the piston to reciprocate by alternately directing flow of air to and from each side of the piston. 
     Reciprocating piston and cylinder devices, commonly used as pumps and motors, are generally either single acting or double acting. In single acting piston and cylinder devices, fluid under pressure is selectively directed to only one side of the piston in a forward stroke and means such as a return spring return the piston to its original position. In double acting piston and cylinder devices, fluid under pressure is selectively directed to one side of the piston to drive it in a forward stroke and alternately to the opposite side of the piston to drive it in a return stroke. Usually the control of the pressurized fluid is performed by a main directional valve that alternately directs the fluid to one of two supply passages connected to opposite ends of the cylinder. The side of the piston that is not being pressurized is exhausted to the environment. Air is often the pressurized fluid, and such devices are generally referred to as air motors. 
     Because single acting piston and cylinder devices rely on mechanical components such as springs and poppet valves actuated by mechanical toggles or trips to function, they are subject to wear. In addition to requiring maintenance, these devices create noise when the toggles are contacted. 
     Double acting piston and cylinder air motors typically include an air valve assembly within an independent air valve cylinder. The air valve assembly may include a spool-shaped member as the main valve that controls the direction of airflow. At any given time, the spool is positioned towards either one end of the air valve assembly or the other. Two pilot pistons control the position of the spool. Some devices require that the main piston, pilot pistons, or both have differential cross-sectional surface areas in order to create the differential force required to shift the spool. This significantly increases the overall manufacturing cost of the device. 
     In devices with two identical pilot pistons, the pilot pistons respond to pressurized air flowing through passages originating from pilot ports in the main piston cylinder near the end of the main piston stroke. When the main piston moves past a pilot port to place the pilot port on the high-pressure side of the main piston, a pilot piston will move and shift a corresponding pilot valve. Pilot valves, reciprocally disposed in a through-bore in the spool, selectively vent air chambers at opposite ends of the spool. This results in shifting the spool to direct pressurized air to the opposite side of the main piston, reversing the direction of movement of the main piston. 
     Conventional double acting devices often malfunction at the cold temperatures incidental to use of compressed gasses. The usual cause of the malfunction is moisture in the air freezing around the spool and the pilot valves. The ice causes the air motor to slow and occasionally stop by clogging passages. Such malfunctioning may occur within five minutes of continuous use. The device cannot be used again until the ice melts. Conventional devices also have airline oil lubrication. This type of lubrication, which is standard industry practice, washes out the grease in the air motor, causing premature failure. 
     Pneumatically controlled and driven piston and cylinder devices are often used as the motive force for pumping of viscous fluids. For example, such devices are used in manufacturing facilities and commercial automotive maintenance shops to deliver grease, motor oils, gear oils, hydraulic oils, and automatic transmission fluid from original refinery drums or tanks to the location of use. 
     Noise attenuation is a concern with air motors because the air exhausting at high velocity from the motor can produce excessive sound levels, often in environments such as manufacturing facilities and maintenance shops where workers are in the immediate vicinity on a prolonged basis. Mufflers are the most popular method of noise attenuation. Mufflers are most often canister-type devices that are mounted externally to the air motor. The muffler receives the exhaust air from the motor and expands the air, thereby reducing the air velocity before discharging the air to the environment. One type of muffler design includes an expansion chamber that requires the muffler to be quite large, sometimes as large as the air motor itself, increasing the cost of the air motor. More complex designs include filtering or baffling systems that also increase the cost. Compact mufflers are generally less effective in attenuating noise than is desirable. 
     Accordingly, there is a need for a pneumatic motor that functions well under a variety of environmental conditions, has durable parts and a long life, is relatively quiet and compact, and is relatively low in cost to manufacture. 
     SUMMARY 
     Accordingly, it is an object of the present invention to provide a pneumatic motor that operates reliably for extended periods. 
     Another object of the present invention is to provide a pneumatic motor that is durable and has low maintenance requirements. 
     Still another object of the present invention is to provide a pneumatic motor that runs quietly. 
     A yet further object of the present invention is to provide a pneumatic motor that is compact and relatively inexpensive to manufacture. 
     According to the present invention, a double acting pneumatic motor is provided that comprises a cylinder, within the cylinder a piston that divides the cylinder into two cavities that vary in volume with movement of the piston, a source of fluid under pressure, and a valve assembly for alternately directing fluid to one cavity while exhausting the other cavity to make the piston reciprocate. 
     The valve assembly resides in a housing with a cylindrical inside surface, and includes a substantially cylindrical spool, or main valve member, that moves axially between two positions. The housing is in continuous fluid communication with each cavity in the cylinder by way of two passages. The spool alternately directs pressurized air to one passage and exhausts the other passage to the environment, and has a central through-bore that at least a part of which continuously communicates with the pressurized fluid source. The spool has at least one portion with an enlarged diameter in between a smaller diameter portion at each end. 
     Two variable volume fluid pressure chambers are on opposite sides of the enlarged diameter portion of the spool. A pilot valve substantially within the central bore of the spool also moves axially between two positions that alternately pressurize or exhaust the pressure chambers adjacent to the enlarged diameter portion of the spool. The position of the pilot valve determines the position of the spool, and the position of the spool determines which cavity in the cylinder is pressurized, which cavity is exhausted, and the direction of motion of the piston. 
     The present invention further comprises a pilot piston at each end of the pilot valve. The pilot pistons are responsive to pressurized air from the cylinder, and cause the pilot valve to shift between its two positions. 
     A frame within the housing is provided in and around which the spool, pilot valve, and piston valves move. The frame comprises two exhaust adapters and two pilot adapters. The valve assembly parts are exemplarily made of acetal resin. 
     Also according to the present invention, a pneumatic motor may be provided with a cover for attenuating sound from the exhaust of the pneumatic motor. The cover includes a curved portion that is radially spaced from the curved outer surface of a cylindrical member outside the valve assembly to form an exhaust flow path. The curved portion directs exhaust flow in a substantially tangential direction along the surface of the cylindrical member. The cover may further comprise an expansion chamber and a muffler. Where the cylinder and valve assembly are incorporated into one substantially cylindrical body, the body serves as the curved surface over which exhaust flow is directed. The cover, expansion chamber, and exhaust flow path may all be substantially the same length as the body. 
     The present invention features a spool and pilot valve that each have two positions, combining to create a four-way valve. The spool, exhaust adapters, pilot adapters, and housing define seven pressure chambers. In one embodiment, two of the chambers are continuously pressurized, three are continuously exhausted, and two are alternately either pressurized or exhausted resulting in a force on the spool that impels the spool to move. Alternatively, in another embodiment, three of the chambers are continuously pressurized, two are continuously exhausted, and two are alternately either pressurized or exhausted resulting in a force on the spool that impels the spool to move. The components of the valve assembly are arranged to create chambers, ports, or passages that direct the flow of pressurized air to and from each side of the piston. Movement of parts changes the available passages in which air can flow by realigning the various passages through the parts and the chambers that are formed by the recessed areas of the parts. The cover improves the diffusion of exhaust and sound attenuation by taking advantage of a phenomenon known as the Coanda effect. 
     The pneumatic motor has only four moving parts to alternately direct pressurized fluid to the piston chambers. No mechanical levers or springs are required. The piston reciprocates rapidly until the flow of pressurized air is stopped. The pneumatic motor minimizes or eliminates occasions when ice forms around the pilot valve, which can cause the motor to malfunction, such as the main valve stopping between its two positions and allowing air to flow directly from supply to exhaust, with no effect on the piston. Little maintenance is required. The pneumatic motor is also compact and operates relatively quietly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings and described below. 
     FIG. 1 is a partial longitudinal section view of a pneumatic motor according to the present invention; 
     FIG. 2 is a schematic cross-section view of a body, cover, and muffler of the pneumatic motor shown in FIG. 1, as viewed from the left end of FIG. 1; 
     FIG. 3 is an exploded perspective view of a portion of the pneumatic motor shown in FIG. 1; 
     FIG. 4 is an exploded perspective view of a body, muffler, and cover of the pneumatic motor shown in FIG. 1; 
     FIG. 5 is a front elevation view of a cover of the pneumatic motor shown in FIG. 1; 
     FIG. 6 is a partially schematic longitudinal section view of the pneumatic motor shown in FIG. 1; 
     FIG. 7 is a side elevation view of an exhaust adapter of the pneumatic motor shown in FIG. 1; 
     FIG. 8 is a cross-section view of an exhaust adapter taken along line  8 — 8  of FIG. 7; 
     FIG. 9 is a cross-section view of an exhaust adapter taken along line  9 — 9  of FIG. 7; 
     FIG. 10 is a side elevation view of a spool of the pneumatic motor shown in FIG. 1; 
     FIG. 11 is a side elevation view of a pilot adapter of the pneumatic motor shown in FIG. 1; 
     FIG. 12 is a side elevation view of a pilot valve of the pneumatic motor shown in FIG. 1; 
     FIG. 13 is a side elevation view of a pilot piston of the pneumatic motor shown in FIG. 1; 
     FIGS. 14A-14D are schematic views showing a sequence of operation of the pneumatic motor shown in FIG. 1; 
     FIG. 15 is a partial longitudinal section view of another embodiment of a pneumatic motor according to the present invention; 
     FIG. 16 is a schematic cross-section view of a body, cover, and muffler of the pneumatic motor shown in FIG. 15, as viewed from the left end of FIG. 15; 
     FIG. 17 is a partially schematic longitudinal section view of the pneumatic motor shown in FIG. 15; and 
     FIGS. 18A-18D are schematic views showing a sequence of operation of the pneumatic motor shown in FIG.  15 . 
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings, a pneumatic motor referred to as an air motor  20  having features of the present invention is shown in FIG.  1 . The air motor  20  comprises a body  22 , a cover  23 , a piston and cylinder assembly  24 , and an air valve assembly  26  (FIG.  3 ). The air motor body  22  is substantially cylindrical except where it enlarges to include the air valve assembly  26 , and has three bores that extend through the full length of the body, shown most clearly in FIG.  2 . The largest bore  28  defines a cylindrical void  29  of the piston and cylinder assembly  24 . The next largest bore  34  is a cylindrical air valve chamber  36 , which houses the air valve  26  components. The smallest through-bore  38  is a cylindrical compressed air conduit  40 . In addition, one conduit  42  drilled from the top end  43  of the body  22  connects ports  44  and  46  while another conduit  48  drilled from the bottom end  49  of the body  22  connects ports  50  and  52 . 
     As shown in FIGS. 1 through 5, the cover  23  slides and locks onto the body  22  with a locational clearance fit. The cover  23  has internal protrusions  54 ,  55  that mate with a slot  56  and a protrusion  57  on the body  22  respectively, forming an expansion chamber  58  for the exhausting air. The cover  23  and expansion chamber  58  extend for the full length of the body  22 . The expansion chamber  58  houses a muffler  60  to attenuate exhaust noise. A curved portion  62  of the cover  23  is radially spaced from the outside diameter of the body  22 . The remainder of the cover  23  is substantially shaped like three sides of a rectangle. At the intersection of the curved portion  62  with the expansion chamber  58 , there is an internal protrusion  63  on the cover  23  that secures the muffler  60  in position. The body  22  and the curved portion  62  of the cover  23  form a curved path  65  for the exhausting air. As the exhaust air exits, the Coanda effect causes it to diffuse around the body  22  resulting in further noise reduction. The Coanda effect is the term given to the observation that a free jet emerging from a nozzle will tend to follow and “attach to” a nearby curved or inclined surface. The jet will come in contact with and flow along the surface if the curvature or angle of inclination is not too sharp. The Coanda effect is further described in U.S. Pat. No. 4,756,230, the contents of which are hereby incorporated by reference. 
     The curved path  65  causes flow to be generally tangential to the curved surface of the body  22  where the exhaust exits the cover  23 . The curved portion  62  may be any length; as measured in degrees around the cylindrical body  22 , any length greater than zero degrees will assist in directing the airflow to be tangential to the curved body  22  surface, and will therefor encourage attachment of the exhaust to the body  22  surface to promote the Coanda effect. The desirable length will vary with the curvature of the body  22  surface, but exemplarily the curved portion  62  is about  45  degrees along the cylindrical surface of the body  22 . 
     The top end cap  66  and bottom end cap  68  fit over the respective ends  43 ,  49  of the body  22  and cover  23 . The top end cap  66  is closed, while the bottom end cap  68  has an opening to allow the translation of motion from the piston and cylinder assembly  24 . The end caps  66 ,  68  are fastened in the preferred embodiment with four bolts  70  and associated nuts  71 , one pair of which is fully shown in FIG. 1, that pass through chambers  72 ,  74 , and  76 . Angle members  69  that hold the curved portion  62  of the cover  23  to the body  22  extend from the end caps  66 ,  68  and are located only at the ends  43 ,  49  of the body  22 . This results in an opening between the cover  23  and the body  22  that runs the full length of the cover  23  and body  22 , rather than individual ports that restrict flow and would therefor be less effective at reducing pressure and attenuating sound. The curved portion  62  of the cover  23  is reduced in thickness at its free end  77  in order to fit in the void created by the angle members  69 . 
     As shown in FIG. 1, the piston and cylinder assembly  24  comprises a piston  78 , a rod  80 , and a piston guide  82 . The piston  78  is reciprocally mounted in the cylinder bore  28  and is attached to the rod  80 . The rod  80  is slidably mounted in a through-bore with an annular seal in the guide  82  for the purpose of aligning the rod  80  and the piston  78  for linear reciprocation. The piston  78  has an annular sealing ring  84  that sealingly engages the cylinder bore  28  to divide the cylinder  29  into two distinct variable volume pressure chambers  30 ,  32 . Reciprocating pumping action is produced by first pressurizing a chamber on one side of the piston  78  while simultaneously exhausting the chamber on the opposite side, and then reversing the sides of the piston  78  that are pressurized and exhausted. The reciprocating movement of the piston  78  can cause a pump to which the piston  78  is connected (not shown) to move fluid. While the air motor  20  is a general utility reciprocating piston air motor, one particular application is to drive a material handling reciprocating piston pump. 
     In general, the body  22 , end caps  66 ,  68 , and the components of the piston and cylinder assembly  24  are made of aluminum alloy, such as ANSI 380.0, but as with all the structural elements of the air motor  20 , any materials of sufficient strength to withstand the forces encountered in use may be used. The cover  23  is exemplarily made of aluminum alloy 6005-T5. Exemplarily, fasteners such as bolts  70  and other hardware are made of steel, and seals such as the sealing ring  84  are elastomeric. The muffler  60  is made of an open-celled material to allow through-flow of exhaust. The body  22 , end caps  66 ,  68 , and the components of the piston and cylinder assembly  24  may be formed by any suitable known process, including but not limited to extrusion, machining, or casting. The scope of the invention, however, is not intended to be limited by the materials or fabrication methods listed herein, but may be carried out using any materials and fabrication methods that allow the construction and operation of the described air motor. The body  22  exemplarily has a length of 6.5-inches, an outside diameter of its cylindrical portion of 4.3-inches, a bore  28  for the piston and cylinder assembly  24  of 3.0-inches, and a bore  34  for the air valve chamber  36  of 1-inch. The dimensions of all of the components of the air motor  20 , however, are based on the particular application as may be determined by someone of ordinary skill in the art. A compressed air source supplies pressurized air to the air motor  20 , exemplarily in the range of up to 150 pounds per square inch operating pressure. 
     As shown in FIG. 3, the air valve  26  comprises a spool  90 , pilot pistons  92 ,  94 , a pilot valve  96 , pilot adapters  98 ,  100 , and exhaust adapters  102 ,  104 . The components are all generally cylindrical, and are shown installed in the air valve chamber  36  in FIG.  6 . The spool  90 , pilot adapters  98 ,  100 , and exhaust adapters  102 ,  104  each have axially central through-bores. In selected locations these components have passages or ports through their walls to provide fluid communication between the respective through-bore and the exterior of the component. The pilot adapters  98 ,  100 , exhaust adapters  102 ,  104 , and end caps  66 ,  68  together with the bore  34  form a stationary frame in and around which the other components move. The moving components of the air valve  26  are the spool  90 , pilot pistons  92 ,  94 , and the pilot valve  96 , which are constructed to slide along the stationary components. The components of the air valve  26  are arranged to create smaller chambers, ports, or passages that direct the flow of air in order to control the pressure in the pressure chamber  30 ,  32  on each side of the piston  78 . Movement of parts changes the available passages in which air can flow by realigning the various passages through the parts and the chambers that are formed by the recessed areas of the parts. Passages are sized such that the flow of air is not significantly restricted. 
     As shown in all figures, seals are used in conjunction with the parts to produce nonleaking pressurized chambers. Each component sealingly engages concentric adjacent components. Sealing is accomplished by elastomeric sealing rings  106  (FIG. 3) in the grooves of internal components at lands on each part. The chambers shown on one side of the longitudinal axis of the air valve chamber  36  are the same as the corresponding chamber on the opposite side of the axis. In general, chambers in the figures are substantially annular in shape. For example, in FIG. 6 chamber  86  is the same as chamber  86 ′; chamber  86  is a generally annular shape. Likewise, chamber  88  is the same chamber as chamber  88 ′. The remaining chambers are not numbered in this manner, but should be understood to allow fluid communication through their generally annular shapes around the entirety of the air valve chamber  36 . 
     The exhaust adapters  102 ,  104  sealingly and fixedly engage the air valve bore  34  with O-ring seals at lands  108  (FIGS. 7,  8 , and  9 ). The spool  90  is the main valve member and controls the direction of airflow to and from each piston chamber  30 ,  32 . The spool  90  includes seals at lands  110 ,  112 ,  114 ,  116 ,  118 ,  120  (FIG.  10 ). The diameter of the spool  90  increases at the two lands  114 ,  116  on the longitudinally central portion of the spool  90 , and the seals at these lands  114 ,  116  slide along the cylindrical walls of the air valve bore  34 . The central through-bores of the air valve  26  components are shown in FIG.  6 . At each end of the spool  90  the diameter is reduced and seals at lands  110 ,  112 ,  118 ,  120  on the spool  90  each slide within the central through-bores  122  of the respective exhaust adapters  102 ,  104 . The central through-bore  124  of the spool  90  receives the stem portion  126  (FIG. 11) of the pilot adapters  98 ,  100  at each end of the air valve chamber  36 . The pilot adapters  98 ,  100  widen at one end  128  where they fixedly engage the exhaust adapters  102 ,  104  at land  130 . The seals at lands  132  on the stem portion of the pilot adapters  98 ,  100  sealingly engage the through-bore  124  of the spool  90 , and allow the spool  90  to slide along the lands  132 . 
     The pilot valve  96  (FIG.  12 ), including lands  136 , is reciprocally and slidably mounted to the through-bore  134  of the pilot adapters  98 ,  100 . The pilot pistons  92 ,  94  (FIG. 13) have a stem portion  138  coaxial with and located adjacent to each end of the pilot valve  96  extending from the bottom of a cylindrical portion  140 . The enlarged cylindrical portion  140  of each pilot piston  92 ,  94  is hollow and has an open end facing toward the respective end of the air valve chamber  36 , and has a land  142  that sealingly and slidably engages the respective exhaust adapter  102 ,  104 . A slightly enlarged diameter at the opposing end of the pilot pistons  92 ,  94  serves as a guide  144  to center the pilot pistons  92 ,  94  within the through-bore  134  of the respective pilot adapter  98 ,  100 . 
     The air valve  26  components are exemplarily made of an acetal resin, such as Delrin® 570 (Delrin is a registered trademark of E.I. du Pont de Nemours and Company), but as with all the structural elements of the air motor  20 , any materials of sufficient strength to withstand the forces encountered in use may be used. Desirable characteristics of acetal resins include: high tensile strength, impact resistance, and stiffness; good fatigue endurance; resistance to moisture and solvents; and natural lubricity. 
     As shown in FIG. 6, the top end cap  66  and the bottom end cap  68  each have a protuberance  146 ,  148  that is loosely received within the respective pilot piston  92 ,  94 . The fit is such that the end of the pilot piston  92 ,  94  will not contact the respective end cap  66 ,  68 , and therefore a chamber  150  exists even when the pilot piston  92  is in the extreme top position, and a chamber  152  exists when pilot piston  94  is in the extreme bottom position. 
     The air valve chamber  36  has a main supply port  154  to which a source of pressurized fluid, such as compressed air, may be connected. The main supply port  154  is connected to the compressed air conduit  40 , which communicates through passages  156  and  158  with the air valve chamber  36 . The air valve chamber  36  communicates with the piston pressure chambers  30 ,  32  through main passages  160 ,  162  and reset passages  164 ,  166 . The air valve chamber  36  also communicates through pilot passages  46 ,  52  with the pressure chambers  30 ,  32  through conduits  42 ,  48  and then passages  44 ,  50  respectively. The air valve chamber  36  vents to the environment through passages  168 ,  170 ,  172  and the expansion chamber  58  defined by the cover  23  (FIG.  2 ). 
     The spool  90  defines seven chambers  174 ,  176 ,  86 ,  178 ,  180 ,  182 ,  184  with the pilot adapters  98 ,  100  and the exhaust adapters  102 ,  104 . Passages  156  and  158  continuously pressurize, or direct fluid, to chambers  174  and  184  respectively. Chambers  176 ,  178 , and  182  are continuously exhausted by passages  168 ,  170 , and  172  respectively. Chambers  86  and  180  are alternately pressurized or exhausted. 
     The spool  90  and the pilot valve  96  each have two positions and combine to create a four-way valve. The position of the spool  90  determines which chamber  30 ,  32  within the piston cylinder  29  is pressurized and which is exhausted. The position of the pilot valve  96  determines which chamber  86 ,  180  is pressurized or exhausted, and this in turn determines the position of the spool  90  based on a differential force on each end of the spool  90 . Pressurized fluid within these chambers  86 ,  180  exerts a force on the longitudinally central portion of the spool  90  with the increased diameter at the lands  114 ,  116  that causes the spool  90  to try move to the end of the air motor  20  that is opposite the force. 
     FIGS. 14A-14D show how the air motor  20  operates. FIG. 14A shows an initial position in the sequence of operation. Compressed air, being air supplied to the air valve  26 , and exhaust air, being air leaving the air valve  26 , will be hereafter referred to as “air” and “exhaust” respectively. For simplicity, some chambers on the perimeter of the air valve  26  are not shown in FIGS. 14A-14D. For example, chamber  88  (FIG. 6) which connects to passage  186  is not shown in FIG.  14 A. It should be understood, however, that corresponding passages on opposite sides of the longitudinal axis of the air valve  26  are in fluid communication through the generally annular chambers that are not shown. For example, in FIG. 14A passage  186  is in fluid communication with passage  186 ′, even though the chamber  88  that provides this fluid communication is not shown here. As previously noted, the moving parts  90 ,  92 ,  94 ,  96  move in and around a stationary frame  185  formed by the pilot adapters  98 ,  100 , exhaust adapters  102 ,  104 , and end caps  66 ,  68 . 
     As shown in FIG. 14A, air enters the air motor  20  through main supply port  154 . Air is routed via passage  156 , passage  188 , chamber  174 , passage  190 , and passage  160  to pressurize chamber  30 . Simultaneously, chamber  32  exhausts via passages  162 ,  186 ,  192 , chamber  182 , and passage  172  to atmosphere. The pressure in chamber  30  is therefore higher than that in chamber  32 . This net pressure differential moves the piston  78  down towards the bottom  49  of the air motor  20 . Air also communicates via passage  156 , passage  188 , chamber  174 , and passages  194 ,  196 , and  198  to chamber  86 . Simultaneously, chamber  180  exhausts via passages  200 ,  202 ,  204 , and  206 , chamber  178 , and passage  170  to atmosphere. Because chambers  174  and  86  are pressurized on the top end of the spool  90  and chamber  184  is the only chamber pressurized on the bottom end of the spool, there is a net downward force on the spool, causing the spool to remain in the position toward the bottom  49  of the air motor  20 . 
     Pilot air from chamber  30  communicates via passages  46 ,  42 ,  44 , and  208  to pressurize pilot chamber  150 . Pilot air from chamber  30  also communicates via passages  164  and  212  to pressurize chamber  214 . Pilot piston  92  therefore does not move because there is equal pressure on both sides of the piston. Simultaneously, chambers  152  and  216  both exhaust into chamber  32 . Chamber  152  exhausts via passages  210 ,  50 ,  48 , and  52 , while chamber  216  exhausts via passages  218  and  166 . Therefore, pilot piston  94  has no net force on it, and also does not move. 
     FIG. 14B shows the piston  78  after having moved down towards the bottom  49  of the air motor  20 , and having moved across passage  52 . Immediately after the piston  78  crosses passage  52 , air communicates via passages  52 ,  48 ,  50 , and  210  to pressurize chamber  152 , causing pilot piston  94  to push the pilot valve  96  up towards the top  43  of the air motor  20 . Since the upper pilot piston  92  is pressurized on both sides in chambers  150  and  214 , there is no resistance to that pilot piston moving towards the top  43 . Air is then routed via passage  158 , passage  220 , chamber  184 , and passages  222 ,  202 , and  200  to pressurize chamber  180  while chamber  86  simultaneously exhausts via passages  198 ,  196 ,  204 ,  206 , chamber  178 , and passage  170  to atmosphere. With chambers  180  and  184  pressurized on the bottom side of the spool  90  and only chamber  174  pressurized on the top side of the spool, the upward force on the spool is greater than the downward force. 
     FIG. 14C shows that the overall upward force causes the spool  90  to move up toward the top  43  of the air motor  20  and reroute air via passage  158 , passage  220 , chamber  184 , and passages  186  and  162  to pressurize the chamber  32  while simultaneously exhausting chamber  30  via passages  160 ,  190 ,  224 , chamber  176  and passage  168  to atmosphere. The differential pressure on the piston  78  resulting from the chamber  32  being at a greater pressure than the chamber  30  causes the piston to change direction and move upward toward the top  43  of the air motor  20 . Chamber  150  now exhausts to atmosphere via passages  208 ,  44 ,  42 ,  46 , and chamber  30 , which exhausts to atmosphere as described above. Chamber  214  also exhausts to atmosphere via passages  212 ,  164  and chamber  30 , while chamber  152  exhausts to atmosphere via passages  210 ,  50 ,  48 ,  52 , and chamber  30 . Chamber  216  is pressurized by air from chamber  32  that communicates via passages  166  and  218 . 
     FIG. 14D shows that the pilot piston  94 , under the differential pressure resulting from a pressurized chamber  216  and an exhausted  152  in FIG. 14C, next moves down toward the bottom  49  of the air motor  20 . Immediately after the piston  78  crosses passage  52 , air communicates with chamber  152  via passages  52 ,  48 ,  50 , and  210  and pilot piston  94  becomes pressurized on both sides. The piston  78  continues to move up toward the top  43  of the air motor past passage  46  and then returns to the center of the air motor  20 , experiencing a sequence similar to that described above, completing a cycle. The piston  78  moves in a rapid reciprocating manner until the supply of air is terminated. 
     The best mode embodiment of an air motor  20   a  according to the present invention is shown in FIGS. 14,  15 , and  16 . Where a feature designated with a number is modified between figures, a letter is added after the feature number to distinguish that feature from a similar feature in a previous figure. 
     Although the previously described air motor  20  improves on the performance of known designs, during prolonged use, moisture in the exhausting air may still freeze and ice may form around the pilot valve  96 , mixing with grease in the air valve  26 . Although the air motor  20  continues to function, the ice can abrade the sealing rings  106  on the pilot valve  96 , shortening their life. By reversing the path of the compressed air into the air valve  26  such that the air inlet becomes the exhaust and air outlet becomes the air supply, the annular void defined by the central portions of the bore  124  of the spool  90  and the pilot valve  96  are continuously pressurized. This deters ice accumulation and eliminates abrasion of the sealing rings  106 , extending the life of the air motor  20   a  yet further. 
     To reverse the air path within the air valve  26 , air inlet passages  156  and  158  in the air motor body  22  are replaced by passages  230  and  234  that are located between passages  160  and  162 , while exhaust passages  168  and  172  are replaced by passages  226 ,  227 ,  228 , and  229  that are located to the outside of passages  160  and  162 . The pilot exhaust passage  170  is replaced by passage  232  that is located between passages  230  and  234  such that passage  232  becomes an air inlet instead. The modified body  22   a  results. 
     Similarly to FIGS.  6  and  14 A- 14 D, in FIGS.  17  and  18 A- 18 D the chambers on one side of the longitudinal axis of the air valve  26  are the same as the corresponding chambers on the opposite side of the axis. As in FIGS. 14A-14D, in FIGS. 18A-18D some of the chambers on the perimeter of the air valve  26  are not shown, but it should be understood that corresponding passages on opposite sides of the axis are in fluid communication because of the generally annular chambers that exist throughout the air valve chamber  36 . 
     As shown in FIGS.  17  and  18 A- 18 D, again similarly to FIGS.  6  and  14 A- 14 D, the spool  90  defines seven chambers  174 ,  176 ,  86 ,  178 ,  180 ,  182 ,  184  with the pilot adapters  98 ,  100  and the exhaust adapters  102 ,  104 . The compressed air conduits and the exhaust passages, however, have been reversed. Passages  226 ,  227 ,  228 , and  229  continuously exhaust chambers  174  and  184  respectively. Chambers  176 ,  178 , and  182  are continuously pressurized, or in other words have fluid directed to them, by passages  230 ,  232 , and  234  respectively. Chambers  86  and  180  are alternately pressurized or exhausted. 
     FIG. 18A shows the initial position in a sequence of operation of the air motor  20   a . As shown in FIG. 18A, air enters the air motor  20   a  through main supply port  154 . Air is routed via passage  230 , chamber  176 , and passages  236 ,  190  and  160  to pressurize chamber  30 . Simultaneously, chamber  32  exhausts via passage  162 , passage  186 , chamber  184 , and passages  220  and  228  to atmosphere. The pressure in chamber  30  is therefore higher than that in chamber  32 . This net pressure differential moves the piston  78  down, or towards the bottom  49  of the air motor  20   a . Air also communicates via passage  232 , chamber  178 , and passages  206 ,  204 ,  202 , and  200  to pressurize chamber  180 . Simultaneously, chamber  86  exhausts via passages  198 ,  196 ,  194 , chamber  174 , and passages  188  and  226  to atmosphere. Because chamber  180  is the only chamber pressurized that exerts a net axial force on the spool  90  and is on the bottom end of the spool, there is a net upward force on the spool, causing the spool to remain in the position toward the top end  43 . 
     Pilot air from chamber  30  communicates via passages  46 ,  42 ,  44 , and  208  to pressurize chamber  150 . Pilot air from chamber  30  also communicates via passages  164  and  212  to pressurize chamber  214 . The pilot piston  92  therefore does not move because there is equal pressure on both sides of the pilot piston  92 . Simultaneously, chambers  152  and  216  both exhaust into chamber  32 . Chamber  152  exhausts via passages  210 ,  50 ,  48 , and  52 , while chamber  216  exhausts via passages  218  and  166 . Therefore, the pilot piston  94  has no net force on it, and also does not move. 
     FIG. 18B shows the piston  78  after having moved down towards the bottom  49  of the air motor  20   a , and having moved across passage  52 . Immediately after the piston  78  crosses passage  52 , air communicates via passages  52 ,  48 ,  50 , and  210  to pressurize chamber  152  causing the pilot piston  94  to push the pilot valve  96  up toward the top end  43 . Air through passage  232  is then routed via chamber  178  and passages  206 ,  204 ,  196 , and  198  to pressurize chamber  86  while chamber  180  simultaneously exhausts via passages  200 ,  202 ,  222 , chamber  184  and passages  220  and  228  to atmosphere. Only the pressurized air in chamber  86  exerts a net axial force on the spool  90 . 
     FIG. 18C shows that the downward force from chamber  86  causes the spool  90  to move down and reroute air from passage  234  via chamber  182 , passages  192 ,  186  and  162  to pressurize chamber  32  while simultaneously exhausting chamber  30  via passages  160 ,  190 , chamber  174 , and passages  188  and  226  to the atmosphere. The differential pressure on the piston  78  resulting from the chamber  32  being at a greater pressure than the chamber  30  causes the piston  78  to change direction and move upward toward the top end  43 . Chamber  150  now exhausts to atmosphere via passages  208 ,  44 ,  42 ,  46  and chamber  30 . Chamber  214  also exhausts to atmosphere via passages  212 ,  164 , and chamber  30 , while chamber  152  exhausts to atmosphere via passages  210 ,  50 ,  48 ,  52 , and chamber  30 . Chamber  216  is pressurized by air from chamber  32  that communicates via passages  166  and  218 . 
     FIG. 18D shows that the pilot piston  94 , under the differential pressure resulting from a pressurized chamber  216  and an exhausted chamber  152  as shown in FIG. 18C, next moves down toward the bottom end  49 . Immediately after the piston  78  crosses passage  52 , air communicates with chamber  152  via passages  52 ,  48 ,  50 , and  210  and pilot piston  94  becomes pressurized on both sides. The piston  78  continues to move up past passage  46  and then returns to the center of the air motor, experiencing a sequence similar to that described above, completing a cycle. The piston  78  moves in a rapid reciprocating manner until the supply of air is terminated. 
     The air motor  20 ,  20   a  of the present invention has many advantages, including providing a reliable reciprocating air valve that functions well, and does not run erratically or malfunction as the result of ice formation in the valve. In addition, use of an airline oil lubricator is not necessary. When an airline lubricator is used in accordance with standard industry practice on a conventional air motor, the airline lubricator will wash out the grease within the air motor, which can cause premature failure of the air motor. If an airline lubricator is used on the air motor  20 ,  20   a  according to the present invention, the grease will be removed from the air motor  20 ,  20   a , but this will not affect the performance of the air motor  20 ,  20   a , and will not lead to premature failure. The new air motor requires minimal maintenance and runs quietly. 
     Although the present invention has been shown and described in considerable detail with respect to only two exemplary embodiments thereof, it should be understood by those skilled in the art that the invention should not be limited to these embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. For example, the passages and chambers may vary in size, shape, location, and number, and modifications may be made to the size, shape and number of the internal components. Accordingly, it is intended to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.