Abstract:
A battery-powered grease gun is provided for enhancing the pressure applied to a blocked grease fitting. The grease comprises a circuit interconnecting the battery and an electric motor. The circuit includes a switch operable by a user and a thermal protector for breaking the circuit at a predetermined temperature. When applying grease to a blocked grease fitting, actuating the switch until the thermal protector reaches the predetermined temperature breaks the circuit until the motor cools and the thermal protector resets for completing the circuit and again energizing the motor. Continuing to maintain the switch in the actuated position until the thermal protector resets causes the pressure in the grease gun to increase with each cycle of the thermal protector.

Description:
BACKGROUND  
       [0001]     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.  
         [0002]     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.  
         [0003]     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 a 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.  
         [0004]     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.  
         [0005]     Another problem that effects 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.  
         [0006]     Yet another problem that effects all grease guns, whether battery-powered or manual, is blocked, or “frozen”, grease fittings. The frozen fitting will not allow grease from the grease gun to reach a desired point of lubrication. Occasionally, the frozen fitting can be cleared if enough pressure can be generated by the grease gun. However, a conventional battery-powered grease gun generates only from about 2900 psi to about 6000 psi of pressure, which often is not sufficient to overcome the frozen fitting. As a result, the defective fitting is usually removed and cleaned or replaced.  
         [0007]     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. There is also a need for a battery-powered grease gun which generates a high output pressure for potentially overcoming frozen grease fittings.  
       SUMMARY  
       [0008]     According to the present invention, a grease gun is provided for enhancing the pressure applied to a blocked grease fitting. The grease gun comprises 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 a battery for energizing the motor and a circuit interconnecting the battery and the motor. The circuit includes a switch operable by a user for manually activating the motor when the switch is actuated and a thermal protector for breaking the circuit at a predetermined temperature. 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. When the switch is continuously actuated, the thermal protector cycles between an open circuit condition and a closed circuit condition when the discharge hose is connected to a blocked grease fitting. The pressure in the bore and discharge hose increase each time the thermal protector resets to the closed circuit condition up to a maximum pressure.  
         [0009]     Also according to the present invention, a method is provided for operating the grease gun. The grease gun operating method comprises the steps of providing a thermal protector for breaking the circuit at a predetermined temperature, actuating the switch, maintaining the switch in the actuated position until the thermal protector reaches the predetermined temperature and breaks the circuit, and continuing to maintain the switch in the actuated position until the thermal protector resets for completing the circuit and again energizing the motor. This method causes the pressure in the grease gun to increase with each cycle of the thermal protector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     For a more complete understanding of the present invention, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings:  
         [0011]      FIG. 1  is a perspective view of a battery-powered grease gun according to the present invention;  
         [0012]      FIG. 2  is an exploded view of the battery-powered grease gun shown in  FIG. 1 ;  
         [0013]      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;  
         [0014]      FIG. 4  is a perspective view from the other side of the mounting plate shown in  FIG. 3 ;  
         [0015]      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;  
         [0016]      FIG. 6  is a side elevation fragmentary view of the battery-powered grease gun shown in  FIG. 1  with the right handle part removed;  
         [0017]      FIG. 7  is a partial cross-section fragmentary view of the battery-powered grease gun shown in  FIG. 1 ;  
         [0018]      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;  
         [0019]      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;  
         [0020]      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 ;  
         [0021]      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;  
         [0022]      FIG. 12  is a perspective view of a thermal protector for use in a battery-powered grease gun according to the present invention; and  
         [0023]      FIG. 13  is a side elevation fragmentary view of the thermal protector shown in  FIG. 12  mounted in the battery-powered grease gun according to the present invention.  
     
    
     DESCRIPTION  
       [0024]     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.  
         [0025]     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.  
         [0026]     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 .  
         [0027]     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.  
         [0028]     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.  
         [0029]     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 pass through opening  42 . The head  26  is preferably a metal casting.  
         [0030]     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.  
         [0031]     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.  
         [0032]     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 .  
         [0033]     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 .  
         [0034]     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 pass through 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.  
         [0035]     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. A curved oblong cam slot  166  is formed in the yoke  160 . 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  154  pin.  
         [0036]     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 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 .  
         [0037]     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 .  
         [0038]     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 end 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.  
         [0039]     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 .  
         [0040]     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 .  
         [0041]     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.  
         [0042]     In another embodiment of a battery-powered grease gun according to the present invention, a thermal protector is used to enhance the pressure generated by the grease gun  20 . A thermal protector for this purpose is shown in  FIG. 12  and generally designated at  250 . The thermal protector  250  includes two terminals  252  at one end for electrically connecting the thermal protector  250  to wire leads. Referring to  FIG. 13 , the thermal protector  250  is shown in position in the left hand handle part  38  of the handle  24  upstream of the motor  50  in the chamber  220  formed in the rear portion of the handle  24 . The thermal protector  250  is electrically connected between the contact assembly  208  and the motor  50  to control the flow of electric current from the battery pack  28  to the motor  50 . Specifically, one of the wires carrying power from the contact assembly  208  to the battery pack  28  leads through the thermal protector  250  to the motor  50 . The other wire leads from the contact assembly  208  to the switch  196  and from the switch  196  to the motor  50 .  
         [0043]     The operation of the battery-powered grease gun  20  according to this embodiment is, as described above, by manual manipulation of the trigger  30  for moving the switch  196  to an “on” position to energize the motor  50 . The motor  50  causes rotation of the drive gear  140  which rotation is transmitted to the yoke  160  by the action of the drive pin  154  engaging the inside peripheral surface of the cam slot  162  in the yoke  160 . The yoke  160  is connected to the plunger  172  which reciprocates in the bore  170  forcing grease under pressure through the outlet passage  182 . The grease is discharged through the hose  33  to a desired point of lubrication. However, if the discharge hose  33  is connected to a blocked grease fitting (not shown), grease will not flow, causing pressure to build in the bore  170  and the discharge hose  33 . Because the electric motor  50  has to work against the pressure, the current passing through the thermal protector  250  increases, thereby increasing the temperature of the thermal protector  250 . Eventually, the thermal protector  250  reaches a pre-calibrated temperature at which point the thermal protector  250  functions to open the circuit. It is understood that this temperature is reached during a fault condition caused by an increase in electric current flowing through the thermal protector  250  and not an increase in the ambient temperature. In the present configuration of the grease gun  20  described herein, the thermal protector  250  opens the circuit shortly after the pressure in the grease gun  20  reaches about 7000 psi.  
         [0044]     After the thermal protector  250  breaks the circuit, the motor  50  stops and the thermal protector  250  cools. When the temperature of the thermal protector  250  is again below the pre-calibrated temperature, the thermal protector  250  will automatically reset. Because the ambient temperature is below the pre-calibrated temperature, the thermal protector  250  cools quickly and will reset within several seconds. If the user continues to actuate the trigger  30  for maintaining the switch  196  in the on position, power will again be delivered through the circuit for energizing the motor  50 . If the grease fitting remains blocked, the current increases rapidly and the thermal protector  250  again opens the circuit. The thermal protector  250  will cool quickly and reset, and this cycle will repeat indefinitely. Importantly, each time the circuit cycles between the open condition and the closed condition, the pressure in the bore  170  and the discharge hose  33  of the grease gun  20  increases. As the thermal protector  250  continues to cycle, the pressure will continue to increase up to about 10,000 psi, after which there will be no further increase in pressure even if the thermal protector  250  continues to cycle. Moreover, it is understood that the high pressure generated by the cycling of thermal protector  250  increases the likelihood of clearing the blocked grease fitting.  
         [0045]     It has been observed that when the thermal protector  250  opens the circuit, the high pressure in the bore  170  causes the plunger  172  to move upwardly in the bore  170  from a position where the plunger  172  has stalled, which always occurs during the pressure stroke. Since this is opposite to the direction of movement of the plunger during the pressure stroke when the grease gun  20  is powered, upward movement of the plunger  172  back drives the drive gear  140 , transmission and electric motor  50  by the action of the drive pin  154  in the cam slot  162  of the yoke  160 . As the plunger  172  retreats, additional grease is drawn into the bore  170  through the inlet passage  180 . When the thermal protector  250  resets, the plunger  172  is driven downward in the bore  170 , although the plunger  172  may or may not reach the previous stall position.  
         [0046]     A suitable thermal protector for use in the battery-powered grease gun according to the present invention is available from Texas Instruments and sold under device code 7AM029A5-YYY. This thermal protector is pre-calibrated to open the circuit at a temperature of 110° C.+/−5° C. However, as described above, when the grease gun  20  is used with a blocked grease fitting, the fault condition of the thermal protector  250  is typically reached due to an increase in current rather than as a result of the ambient temperature reaching the pre-calibrated temperature. Thus, the cycling of the thermal protector  250  for building pressure in the grease gun  20  may be achieved using thermal protectors having a broad range of pre-calibrated opening temperatures. Preferably, the thermal protector  250  is selected to have a pre-calibrated opening temperature that is not reached during normal operation of the grease gun  20  in the absence of a blocked grease fitting.  
         [0047]      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 .  
         [0048]     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 ).  
         [0049]     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.  
         [0050]     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 .  
         [0051]     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 re-circulated 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 .  
         [0052]     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 .  
         [0053]     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 crew may be equivalent structures.