Abstract:
An apparatus converts rotary motion of a motor drive shaft into oscillatory motion of a work piece, for example, in a power tool. The apparatus comprises a planetary gear assembly connected to the motor for rotating an output gear. The output gear meshes with a drive gear including a drive pin. The drive pin fits in a yoke slot for reciprocation of the yoke upon rotation of the drive gear. The yoke is operatively connected to the work piece. An air flow system cools the power tool housing. The housing has an inlet aperture in the handle portion and an outlet aperture for placing the interior of the housing in communication with the atmosphere. A wall integral with the handle portion engages the body of the motor housing between a first end and openings in the body. The wall provides a barrier to minimize air flow between an intake air portion of the interior of the housing and an exhaust air portion. A fan on the motor drive shaft induces air flow from the intake air portion and exhaust air portion of the interior chamber into the motor housing through openings in the end walls. Air is discharged from the openings in the body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 11/299,837, filed Dec. 12, 2005 now U.S. Pat. No. 7,523,843, which is a continuation of application Ser. No. 10/249,900 filed May 15, 2003 now U.S. Pat. No. 7,004,357, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to power tools and, more particularly, to portable, electrically-powered power tools, such as, for example, for pumping fluids as in a handheld battery-powered grease gun. 
     A conventional handheld battery-powered grease gun generally comprises a housing including a head portion and a handle portion extending transversely from the head. A cylindrical barrel holding a supply of grease is removably secured to the head and extends from the head alongside the handle. The head portion includes a pump mechanism including a piston that reciprocates in a bore that forms a pump cylinder. The head portion has an inlet port in communication with the bore and the material in the barrel and an outlet port at one end of the bore to a flexible hose for delivering grease to a point of lubrication. 
     An electric motor is accommodated in the housing and a gear transmission mechanism is provided between the motor and the pumping mechanism for changing the rotating motion of the motor output shaft to the linear reciprocating motion of the piston while reducing the rotational speed and increasing torque. The transmission of these type mechanisms usually ends in a rotary crank plate having an eccentrically located crank pin that is drivingly disposed within a slot of a reciprocating yoke coupled to the piston. This arrangement has been used primarily in jigsaws, which are the most common type of power tool employing reciprocating drive mechanism. Batteries to power the motor and the switch to control the operation of the power tool are also found in the housing. 
     In a battery-powered grease gun, the transmission is needed for dispensing grease under pressure. In order for the grease gun to perform satisfactorily, significant force must be exerted. This requirement has led to the development of large, heavy power transmission mechanisms, resulting in awkward and difficult to handle grease guns. The power requirement also reduces the life cycle of the rechargeable battery. Moreover, since the transmission drive system includes numerous components, the manufacturing is relatively complicated and costly. 
     Another problem that affects all power tools, including a battery-powered grease gun, is heat build-up within the housing during use of the power tool. Heat build-up can shorten the life of the motor and other moving parts, and is particularly a problem when a housing is made of plastic. Thus, care must be taken to ensure good heat dissipation. For this reason, the electric motor used in power tools typically includes a fan for cooling air circulation. Conventionally, the fan is mounted on the motor armature shaft for generating air flow through openings in the motor and the tool housing. Vents in the tool housing facilitate air flow between the interior of the housing and the atmosphere. The need for good cooling air flow around the motor necessitates placing the motor in a position in the housing to allow sufficient air flow around and through the motor. Unfortunately, the motor position necessitates an arrangement that results in an unfavorably located center of gravity which does not facilitate overall handling of the power tool. 
     For the foregoing reasons, there is a need for a power transmission which is compact, yet efficient and powerful enough to be used in power tools, such as battery-powered grease gun. There is also a need for improved cooling in power tools so as to allow more convenient placement of the motor. Ideally, the motor could be positioned in the handle to further reduce the size and improve the handling of the power tool. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an apparatus is provided for converting rotary motion of a drive shaft of a motor into oscillatory motion of a work piece. The motion converting apparatus comprises a planetary gear assembly operatively connected to the motor for rotating an output gear. The output gear meshes with a drive gear including a drive pin eccentrically mounted on a face of the drive gear. The drive pin is received in a cam slot of a yoke for reciprocation of the yoke by the drive pin upon rotation of the drive gear. The yoke is adapted to be operatively connected to the work piece. 
     Also according to the present invention, an apparatus is provided for dispensing a fluid. The fluid dispensing apparatus comprises a housing having a bore forming a pump cylinder. An inlet passage, in fluid communication with the fluid supply, and an outlet passage extend from the exterior of the housing and open into the bore. The outlet passage opens into the bore at a point axially spaced in a first direction from the opening of the inlet passage into the bore. An electric motor is disposed within the housing and means for energizing the motor is provided. A transmission is disposed within the housing and operably connected to the motor. The transmission comprises a planetary gear assembly including an output gear. The output gear meshes with a drive gear including a drive pin eccentrically mounted on a face of the drive gear. The drive pin is received in a cam slot of a yoke for reciprocation of the yoke by the drive pin upon rotation of the drive gear. A piston is fastened to the yoke at one end and the other end is slidably disposed in the bore for reciprocal movement relative to the housing. The piston is movable between a first position axially spaced in a second direction from the opening of the inlet passage into the bore and a second position past the inlet passage opening in the first direction. The piston moves through a pumping stroke toward the outlet passage opening in the first direction for forcing the fluid in the bore out through the outlet passage. In a return stroke, the piston moves away from the outlet passage opening and past the inlet passage opening in the second direction for priming the bore. 
     Further according to the present invention, a grease gun is provided comprising a housing including a handle portion and a head portion. The head portion has a bore forming a pump cylinder, and an inlet passage and an outlet passage extending from the exterior of the head portion and opening into the bore. The outlet passage opens into the bore at a point axially spaced in a first direction from the opening of the inlet passage into the bore. A grease supply cylinder is sealingly secured to the head portion so that the bore is in fluid communication with the grease in the supply cylinder. An electric motor is provided as well as means for energizing the motor. A transmission is operably connected to the motor. The transmission comprises a planetary gear assembly including an output gear and a drive gear meshing with the output gear. The drive gear includes a drive pin eccentrically mounted on a face of drive gear. The drive pin is received in a cam slot in a yoke for reciprocation of the yoke by the drive pin upon rotation of the drive gear. A piston is fastened to the yoke at one end and the other end of the piston is slidably disposed in the bore for reciprocal movement relative to the housing. The piston is movable between a first position axially spaced in a second direction from the opening of the inlet passage into the bore and a second position past the inlet passage opening in the first direction. The piston moves toward the outlet passage opening in the first direction through a pumping stroke for forcing the grease in the bore out through the outlet passage opening. The piston moves away from the outlet passage and past the inlet passage opening in the second direction through a return stroke for priming the bore. 
     According to another aspect of the present invention, an air flow system is provided for a power tool housing defining an interior chamber and including a handle portion. The housing has at least one air inlet aperture in the handle portion and at least one air outlet aperture for placing the interior chamber in communication with the atmosphere. The air flow system comprises an electric motor adapted to be disposed in the handle portion of the housing. The motor comprises a motor housing having a first end wall, a second end wall and a body interconnecting the end walls. A fan is fixed to the drive shaft within the motor housing for drawing air through the motor housing. A wall integral with the handle portion of the housing extends inwardly into the interior chamber. The wall engages the body of the motor housing between the first end wall and openings in the body so that the interior chamber is divided into a first intake air portion, wherein the openings in the first end wall are in fluid communication with the atmosphere through the at least one air inlet aperture, and a second exhaust air portion, wherein the openings in the body and the second end wall are in fluid communication with the atmosphere through the at least one air outlet aperture. The wall provides a barrier to minimize air flow between the intake air portion of the interior chamber and the exhaust air portion. In use, the fan induces air flow from the intake air portion of the interior chamber into the motor housing through openings in the first end wall and from the exhaust air portion of the interior chamber into the motor housing through openings in the second end wall. Air is discharged from the openings in the body. 
     Also according to another aspect of the present invention, a power tool is provided comprising a housing defining an interior chamber and including a handle portion. The housing has at least one air inlet aperture in the handle portion and at least one air outlet aperture for placing the interior chamber in communication with the atmosphere. An electric motor is disposed in the handle portion of the housing. The motor comprises a motor housing having a first end wall, a second end wall, a body interconnecting the end walls, and a drive shaft extending from the second end wall. A fan is fixed to the shaft within the motor housing for drawing air through the motor housing. A wall integral with the handle portion of the housing extends inwardly into the interior chamber and engages the body of the motor housing between the first end wall and openings in the body. The wall divides the interior chamber into a first intake air portion, wherein the openings in the first end wall are in fluid communication with the atmosphere through the at least one air inlet aperture, and a second exhaust air portion, wherein the openings in the body and the second end wall are in fluid communication with the atmosphere through the at least one air outlet aperture. The wall provides a barrier to minimize air flow between the intake air portion of the interior chamber and the exhaust air portion. In use, the fan induces air flow from the atmosphere through the at least one air inlet aperture in the handle portion and into the motor housing through openings in the first end wall and from the exhaust air portion of the interior chamber into the motor housing through openings in the second end wall. Air is discharged out of the openings in the body. 
     Further according to another aspect of the present invention, a grease gun is provided comprising a housing defining an interior chamber and having at least one air inlet aperture and at least one air outlet aperture for placing the interior chamber in communication with the atmosphere. The housing includes a handle portion having the at least one air inlet aperture, and a head portion extending transversely from one end of the handle portion. The head portion has a bore forming a pump cylinder. An inlet passage and an outlet passage extend from the exterior of the head portion and open into the bore, the outlet passage opening into the bore at a point axially spaced in a first direction from the opening of the inlet passage into the bore. An electric motor is disposed in the handle portion of the housing. The motor comprises a motor housing having a first end wall, a second end wall, a body interconnecting the end walls, and a drive shaft extending from the second end wall. A fan is fixed to the shaft within the motor housing for drawing air through the motor housing. A wall integral with the handle portion of the housing extends inwardly into the interior chamber and engages the body of the motor housing between the first end wall and openings in the body. The wall divides the interior chamber into a first air intake portion, wherein the openings in the first end wall are in fluid communication with the atmosphere through the at least one air inlet aperture, and a second exhaust air portion, wherein the openings in the body and the second end wall are in fluid communication with the atmosphere through the at least one air outlet aperture. The wall provides a barrier to minimize air flow between the intake air portion of the interior chamber and the exhaust air portion. A grease supply cylinder is sealingly secured to the head portion so that the bore is in fluid communication with the grease in the supply cylinder. The cylinder extends laterally from the head portion such that the longitudinal axis of the cylinder is substantially parallel to the longitudinal axis of the handle. A battery is received within the handle portion for energizing the motor. In use, the fan induces air flow from the atmosphere through the at least one air inlet aperture in the handle portion and into the motor housing through openings in the first end wall and from the exhaust air portion of the interior chamber into the motor housing through openings in the second end wall. Air is discharged out of the openings in the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is a perspective view of a battery-powered grease gun according to the present invention; 
         FIG. 2  is an exploded view of the battery-powered grease gun shown in  FIG. 1 ; 
         FIG. 3  is an exploded perspective view of an electric motor and mounting plate for use in the battery-powered grease gun according to the present invention; 
         FIG. 4  is a perspective view from the other side of the mounting plate shown in  FIG. 3 ; 
         FIG. 5  is a perspective view of an electric motor mounted in the battery-powered grease gun according to the present invention, with surrounding components cut-away; 
         FIG. 6  is a side elevation fragmentary view of the battery-powered grease gun shown in  FIG. 1  with the right handle part removed; 
         FIG. 7  is a partial cross-section fragmentary view of the battery-powered grease gun shown in  FIG. 1 ; 
         FIG. 8  is an end elevation view of the battery-powered grease gun shown in  FIG. 1  with the handle portion of the housing removed; 
         FIG. 9  is a side elevation view of a left hand handle part for use in the battery-powered grease gun according to the present invention; 
         FIG. 10  is a cross section of the handle portion of the housing of the battery-powered grease gun shown in  FIG. 1  and taken along line  10 - 10  of  FIG. 11 ; and 
         FIG. 11  is a side elevation view of the battery-powered grease gun shown in  FIG. 1  with the internal components shown in phantom to depict air flow through the housing during operation of the battery-powered grease gun according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. For example, words such as “upper,” “lower,” “left,” “right,” “horizontal,” “vertical,” “upward,” and “downward” merely describe the configuration shown in the Figures. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. 
     Referring now to the drawings, wherein like reference numerals designate corresponding or similar elements throughout the several views, an embodiment of the present invention is shown in the form of a battery-powered grease gun, designated generally at  20 . It is understood that, although the present invention will be described in detail herein with reference to the exemplary embodiment of the battery-powered grease gun  20 , the present invention may be applied to, and find utility in, other portable, hand-held power tools. As described above, electric motors are used in a wide variety of applications involving power tools such as, for example, drills, saws, sanding and grinding devices, yard tools such as, for example, edgers and trimmers, and the like. Further, although the present invention will be described in detail herein as embodied in a power tool wherein rotating motion of the electric motor is converted to linear reciprocating motion, it is not intended to be so limited. The present invention may be used in rotary power tools, such as power drills, screw drivers, and the like, and in kitchen appliances such as, for example, mixers and blenders. Thus, the present invention has general applicability to any device powered by an electric motor wherein improvements in efficiency and cooling are desired. 
     Referring now to  FIG. 1 , the grease gun  20  comprises a housing  22 , including a rear handle portion  24  and a front head portion  26 . The housing  22 , as viewed from the side, is generally L-shaped with the handle  24  extending transversely from an upper end of the head  26 . The handle  24  is generally tubular and of a length somewhat greater than the width of a human hand, and of a girth such that the handle  24  may be readily grasped in the hand of the user. The handle  24  may be contoured so that the handle  24  may be grasped comfortably. A rechargeable battery pack  28  is mounted to the housing  22  at the rear end of the handle  24 . A manually operated trigger  30  extends from an opening in the side of the handle  24 . In this position, the trigger  30  can be selectively operated by manual manipulation by the user gripping the handle  24  to control the flow of electric current from the battery pack  28  to an electric motor (not shown in  FIG. 1 ) in the housing  22 . 
     A tubular reservoir  32  is removably secured to the housing  22  at a lower end of the head  26  for holding a supply of grease. The reservoir  32  is aligned substantially parallel with the longitudinal axis of the handle  24 . It is understood that the term substantially parallel as used in this context throughout this specification means more parallel than not. A discharge hose  33  extends from the lower end of the head  26  for delivering grease to desired points of lubrication. It is understood that a wide variety of fluids other than grease, or other lubricant, can be dispensed according to the present invention, such as, for example, sealants such as caulk, glue, cake frosting as well as other high viscosity fluids or semi-solid materials that require high pumping pressure to achieve adequate flow rates. 
     As best seen  FIG. 2 , the handle  24  may be formed as two complementary, symmetric parts such that the handle  24  is in effect split in half along a central longitudinal plane forming a right hand handle part  36  and a left hand handle part  38 , as viewed by the user holding the gun in his right hand with the head  26  at the top. The two handle parts  36 ,  38  are joined together in a conventional manner using fasteners, such as screws, an adhesive, welding, or a combination thereof. As depicted in the present embodiment, screw holes  40  are formed in the two handle parts  36 ,  38  for fastening the handle parts together. The handle  24  may be made of various materials, including plastics or metals. Preferably, the handle  24  is made of an electrically insulating material with low heat conductivity, such as hard plastic. 
     The head  26  is generally a rectangular shape with rounded corners and parallel side walls extending between and interconnecting irregular front and rear walls. The upper end of the head  26  has a cylindrical passthrough opening  42 . The head  26  is preferably a metal casting. 
     Referring now to  FIGS. 2 and 3 , the electric motor  50  includes a substantially cylindrical motor housing  52  having a side wall  54  with an external surface, a front end wall  56  and a rear end wall  58 . The side wall  54  has two diametrically opposed, circumferentially-extending air ports  60  opening at the external surface of the motor housing  52 . The front end wall  56  has four spaced air ports  62 . An axial rotary output shaft  66  extends from the front end wall  56  of the motor housing  52 . A fan (not shown) is located within the motor housing  52  and attached to the motor shaft  66 . Preferably, the fan is an impeller type fan. 
     A circular mounting plate  70  is provided for securing the motor  50  in the handle  24 . The mounting plate  70  has front surface  72  and a rear surface  74  ( FIG. 4 ). The rear surface  74  of the mounting plate  70  has four circumferentially spaced recesses  76  having a substantially triangular shape. The mounting plate  70  is adapted to be fastened to the front end wall  56  of the motor housing  52  with the motor shaft  66  extending through a central opening  78  in the mounting plate  70 . The mounting plate  70  is positioned relative to the motor housing  52  such that the apertures  76  in the rear surface  74  of the mounting plate  70  are aligned with the air ports  62  in the front end wall  56  of the motor housing  52 . The mounting plate  70  is fastened to the motor housing  52  using screws  80  which pass through holes  82  in the mounting plate  70  and are received in threaded openings  84  in the front end wall  56 . The motor housing  52  with attached mounting plate  70  is aligned with the opening  42  in the upper end of the head  26 , as best seen in  FIGS. 5 and 6 . The mounting plate  70  is fastened to the head  26  using screws  86  which pass through openings in ears  88  circumferentially spaced on the periphery of the mounting plate  70  and are received in threaded openings in the head  26 . The mounting plate  70  is large enough to cover the opening in the head portion. A washer  89  ( FIG. 2 ) is positioned between the mounting plate  70  and head  26 . As seen in  FIG. 5 , the rear end wall  58  of the motor housing  52  has four spaced air ports  64 . When the motor  50  is operating, the fan rotates to draw air through the air ports  62 ,  64  in the front and rear end walls  56 ,  58  of the motor housing  52  to cool the motor  50 . Warmed air is exhausted from the motor housing  52  through the side wall air ports  60 . A suitable motor for use in a power tool according to the present invention is available from Johnson Electric Engineering Ltd. of Hong Kong, and sold as model number HC683LG. 
     The motor  50  drives a transmission that drives a pump assembly for pumping grease under pressure from the reservoir  32  through the discharge hose  33 . In one embodiment of the present invention, the transmission comprises a planetary gear reduction system, preferably a two-stage planetary gear reduction system housed in the opening  42  in the upper end of the head  26 , which serves as a gear housing. Referring to  FIGS. 2 and 7 , a first planetary gear set of the planetary gear system includes three planet gears  96  (only one of which is shown in  FIG. 2 ) rotatably mounted on pins  98  extending from a rear surface  99  of a first carrier  100 . A pinion gear  102  is press fit onto the distal end of the motor shaft  66  and forms a part of the transmission. The pinion gear  102  fits between and meshes with the three planet gears  96  on the first carrier  100 . The three planet gears  96  also mesh with an orbit gear  104  fixed in the opening  42  in the head  26 . The orbit gear  104  has four longitudinal ridges  106  ( FIG. 2 ) circumferentially spaced about the periphery. The head  26  has corresponding longitudinal slots  108  formed in the wall defining the upper opening  42  for non-rotatably receiving the orbit gear  102 . 
     A sun gear  110  is axially mounted to a front surface of the first carrier  100  for rotation with the first carrier. The sun gear  110  meshes with and drives three planet gears  114  (only one of which is shown in  FIG. 2 ) of a secondary planetary gear set of the transmission. The second set of planet gears  114  are rotatably mounted on pins  116  extending from a rear surface  118  of a second carrier  120  and also mesh with the orbit gear  104 . An axial reduced diameter shoulder  122  extends forwardly from a front surface of the second carrier  120 . A roller bearing  126  is positioned between the cylindrical peripheral surface of the shoulder  122  and the interior surface of the head  26 . An output gear  128  is fixed to the second carrier  120  at a front surface of the shoulder  122  for rotation with the second carrier  120 . An axial shaft  130  extends from the spur gear  128  and is received in a bore in a semi-circular bracket  132  fastened to the front wall of the head  26 . The shaft  130  is supported for rotation in the bracket  132  by a needle bearing  134 . 
     A drive gear  140  is provided at the forward end of the transmission. An axial shaft  146  extends from a rear surface  144  of the drive gear  140  and is rotatably received in a passthrough axial bore  148  in the head  26  below the upper opening  42 . The axial shaft  146  is supported in the bore  148  by a needle bearing  150  and a ball bearing  151  positioned between two retaining clips  153 . The front clip  153  fits in a groove in the interior surface of the bore  148  for maintaining the ball bearing  151  in the bore  148 . The rear clip  153  fits in a groove in the shaft  146  for maintaining the axial position of the shaft  146 . An eccentrically mounted shaft  152  extends transversely from the front surface of the drive gear  140 . A hollow cylindrical drive pin  154  is mounted for rotation on the eccentric shaft  152  between two washers  155 . A retaining clip  156  fits into a groove  157  in the end of the shaft  152  to hold the drive pin  154  in place. 
     As shown in  FIGS. 7 and 8 , a yoke  160  is positioned adjacent to the front surface of the drive gear  140 . The yoke  160  is substantially heart-shaped and has a front face  160   a , a rear face  160   b , and a pair of opposite sides  160   c . A curved oblong cam slot  166  is formed in the yoke  160  and extends in a side-to-side direction with respect to the yoke. The cam slot  166  is dimensioned to receive the drive pin  154 , allowing sufficient room to enable the drive pin  154  to slide freely through the cam slot  166  from end to end. As depicted in the Figures, a scotch yoke design having a track configuration that minimizes the side load forces imposed on the yoke  160  is preferred. However, it is understood that the configuration of the cam slot  166  may be straight, with the length of the slot  166  equal to the diameter of the circle traced by the drive pin  154 . 
     Referring again to  FIG. 7 , the pump assembly comprises a pump chamber  168  including the lower end of the head  26 . The pump chamber  168  defines a cylindrical bore  170  which, as will be described below, is in fluid communication with the reservoir  32  of grease and the discharge hose  33 . The pump chamber bore  170  receives a piston or plunger  172  in sliding engagement with the interior surface of the bore  170 . The plunger  172  extends upwardly through an opening in the pump chamber  168 . The distal end of the plunger  172  is received in an opening in the yoke  160  and secured in place with a pin  174 . A resilient seal  176  is positioned in an annular recess in the opening in the pump chamber  168  and surrounds the plunger  172  for sealing the pump chamber  168 . 
     The pump chamber  168  has a circular threaded flange  178  that is internally threaded for receiving an externally threaded open end of the reservoir  32 . A gasket  179  is seated between the head  26  and the reservoir  32  for sealing the connection. The operation of the grease reservoir  32  may be typical of a conventional grease gun that is either manually or battery-powered. Therefore the interior of the reservoir  32  is not shown in the drawings. The grease supply in the reservoir  32  is in fluid communication with the bore  170  via an inlet passage  180  formed in the pump chamber  168  and extending from the recess  178  and opening into the bore  170 . An outlet passage  182  is spaced downward from the inlet passage  180  and extends from the bore  170  to a fitting  184  to which the discharge hose  33  is connected. A ball check valve assembly  186  is positioned in the pump chamber  168  at the end of the bore  170 , and is held in place by a threaded plug  188 . 
     As shown in  FIGS. 2 and 7 , the handle  24  has an opening  190  to accommodate the trigger  30 . The trigger  30  has transverse arms  192  that rotatably fit into opposed bosses  194  in the handle  24  so that the trigger  30  will pivot relative to the handle  24 . A paddle  200  extends forwardly from one of the trigger arms  192 . An electrical switch  196  is mounted in the handle  24  adjacent to the trigger  30 . A torsion spring  198  is mounted around one of the trigger arms  192 . One end of the spring  198  engages the trigger  30  and the other of the spring engages the interior of the handle  24  for biasing the trigger  30  outwardly of the handle and away from the switch  196  in an off position. Two wires carry power from the battery pack  28  to the motor  50 . When the trigger  30  is actuated by the user, the trigger  30  pivots inwardly against the biasing action of the spring  198 . The paddle  200  contacts the switch  196  for moving the switch to an on position. When the user releases the trigger  30 , the spring  198  operates to pivot the trigger  30  back to the off position. 
     In use, the user grips the handle  24  and manually manipulates the trigger  30  to energize the motor  50 , rotating the motor shaft  66  and pinion gear  102 . Rotation of the pinion gear  102  is transmitted through the transmission causing the drive gear  140  to rotate at a reduced speed of rotation and at an increased torque from that of the pinion gear  102 . The rotation of the drive gear  140  is transmitted to the yoke  160  by the action of the drive pin  154  engaging the inside peripheral surface of the cam slot  166  for reciprocating the yoke  160  and plunger  172 . 
     The plunger  172  reciprocates in the bore  170  of the pump chamber  168  through a pressure stroke and a return stroke. On the pressure stroke, the plunger  172  moves in the bore  170  in a downward direction, as seen in  FIG. 7 , past the inlet passage  180  and toward the outlet passage  182 . Grease in the bore  170  is thus pushed toward the outlet passage  182 . Pressure on the grease increases until the ball check valve  186  is unseated and grease under pressure passes through the outlet passage  182  and is discharged through the hose  33 . Once the pressure stroke has been completed, the plunger  172  is retracted upward, as seen in  FIG. 7 , away from the outlet passage  182  and back across the inlet passage  180  thereby allowing more grease to enter into the bore  170 . 
     As best seen in  FIGS. 7 and 8 , the bracket  132  partially closes the opening in the upper end of the head  26 . This maintains the axial relationship of the components of the transmission and resists any tendency of the drive gear  140  to tilt or skew relative to its central axis due to forces exerted by the yoke  160  against the drive pin  154  during rotation of the drive gear  140 . As described above, it is understood that other power tools may use this transmission arrangement, including tools with rotating drives wherein rotary movement may continue through to a chuck which is adapted to drive a suitable bit or implement that comes into engagement with the work. 
       FIG. 9  shows the interior of the left hand handle part  38 . It is understood that the interior of the right hand handle part  36  is a mirror image of the left hand handle part  38 . The left hand handle part  38  includes inwardly projecting integral walls. An upper battery socket wall  202  and a lower battery socket wall  204  are formed at the rear end of the handle parts  38  and are configured to accommodate the battery pack  28 . The inner ends of the battery socket walls  202 ,  204  have transverse slots  206  for receiving a contact assembly  208  for connection to the battery pack  28 . A partition wall  210  extends radially inward in the handle  24  forward of the battery socket walls  202 ,  204 . The partition wall  210  has a central semi-circular cutout  212 . The internal walls of the handle  24  provide strength and rigidity to the handle  24 . 
     When the handle parts  36 ,  38  are assembled, the entire length of the battery socket walls  202 ,  204  and the straight lengths  214  of the partition walls  210  contact one another. The semi-circular cutouts  212  of the partition wall  210  define a central opening in the handle  24  which surrounds and seals against the periphery of the side wall  54  of the motor housing  52 . As best seen in  FIGS. 7 and 10 , the partition walls  210  seal against the motor housing  54  adjacent to and rearward of the air ports  60  in the side wall  54 . A slot  216  is formed in the partition wall  210  for allowing the wires  199  from the contact assembly  208  to pass. The slot  216  is sized to minimize clearance between the wires  218  and the partition  210  ( FIG. 9 ). 
     The partition wall  210  divides the interior of the handle  24  into two separate chambers when the handle parts  36 ,  38  are joined. Specifically, a chamber  220  is formed in the rear portion of the handle  24  upstream of the motor  50 . The upstream chamber  220  is defined by the interior surface of the rear portion of the handle  24 , the partition wall  210  and the upper and lower battery socket walls  202 ,  204 . Forward of the partition wall  210  is an exhaust air chamber  222  defined by the interior surface of the forward portion of the handle  24 , the partition wall  210  and the exterior surface of the head  26 . Air vents  224 ,  226  are formed in the front portion and rear portion of the handle  24 . The exhaust chamber  222  and the upstream chamber  220  are in communication with the atmosphere via the vents  224 ,  226  in the front and rear portions of the handle, respectively. 
     Air flow through the housing  22  is generated by rotation of the fan on the motor shaft  66  when the motor is running. Referring to  FIGS. 1 ,  5 ,  6  and  11 , dotted lines with arrows in the Figures show the direction of air flow through the housing  22 . The motor fan draws ambient air from outside of the housing  22  through the inlet air vents  226  in the rear portion of the handle  24 . Air flows forwardly into the inlet ports  64  in the rear end wall  58  of the motor housing  52 . The partition wall  210  directs substantially all of the air drawn into the rear portion of the handle  24  into the motor housing  52  for cooling the motor  50 . As described above, air flows through the motor  50  and is discharged from the ports  60  in the side wall  54  into the exhaust chamber  222 . 
     A portion of the warmed air exiting the motor housing  52  through the exhaust ports  60  flows along the outer surface of the motor housing  52 , into the ports  62  in the front end wall  56  and again through the motor housing  52  to be discharged from the side wall ports  60 . The remainder of the exhausted air flows lengthwise of the handle  24  cooling the exhaust chamber  222  of the handle  24  and head  26  prior to exiting the housing  22  through the outlet vents  224  to the ambient. The periphery of the motor housing  52  and mounting plate  70  do not extend to the interior surface of the handle  24  for defining an annular space between the periphery of the motor housing  52  and mounting plate  70  and the interior surface of the handle  24  for the passage of air. The partition wall  210  substantially prevents the recirculated exhaust air from returning to the upstream chamber  220  in the rear portion of the handle  24 . Where the cross-sectional area of the mounting plate  70  is greater than the cross-sectional area of the motor housing  52 , as in the embodiment of the present invention shown in the Figures, the mounting plate  70  may function to deflect a portion of the motor exhaust air radially of the motor housing  52  ( FIG. 6 ). The recesses  76  in the mounting plate  70  induce a portion of the deflected air to flow into the ports  62  in the front end wall  56 . 
     It is understood that the structure of the interior of the handle  24  generally confines the air flow through the housing  22  to the path described. The channeled air flow acts to cool the motor  50  to a sufficiently low temperature to prevent excessive heat buildup in the motor housing  52  and handle  24 , which would otherwise be detrimental to either the motor  50  or the housing  22 , particularly a plastic housing, and cause discomfort to the user. This extends the life of the tool components. Moreover, this arrangement allows the placement of the motor  50  in the handle  24  of the grease gun  20 . Placing the motor  50 , battery pack  28 , and transmission in the handle provides a compact, balanced design for any power tool. In a grease gun, wherein the handle  24  is oppositely positioned relative to the grease reservoir  32 , a weight distribution is now available that enables the grease gun  20  to be more easily manipulated while gripping the handle  24 . 
     Although the present invention has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to limit the invention to the 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, any number of planetary gear stages could be used in the transmission depending on the motor construction. Further, the handle construction channeling air flow can be used in a power tool that does not use a planetary gear system in the transmission. Accordingly, we intend 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.