Abstract:
A portable grease gun with a dual pressure output capability which can be selectively changed by the user with little effort. The larger diameter plunger is slidable within a plunger retainer. The larger diameter plunger has a bore through them in which a smaller diameter plunger is slidable. The smaller diameter plunger moves when an actuating lever is operated to compress grease in the grease gun. The larger diameter plunger may be adjusted to remain static during operation of the actuating lever or it may be adjusted to move during all or part of the compression stroke of the actuating lever. A high pressure can be generated by movement of the smaller diameter plunger alone, or a high flow volume can be generated by movement of the smaller and larger diameter plungers in unison.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of application Ser. No. 11/106,417 titled Two Stage Grease Gun, filed on Apr. 14, 2005, with the United States Patent and Trademark Office. The disclosure of application Ser. No. 11/106,417 is hereby incorporated in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention pertains to portable grease guns, whether manually operated or driven by air power or by electric batteries. Typical hand operated grease guns include a piston which forces grease from a nozzle, the grease being fed to the compression chamber from an attached mounted barrel containing bulk grease or a grease cartridge. Most existing grease guns provide only one output pressure of the grease expressed from the gun.  
         [0003]     Frequently the user of a grease gun encounters a grease zerk which will not accept grease because it is seized or clogged. In that case, little can be done without replacing the zerk to correct the problem. One prior art grease gun requires changing position of the handle to adjust the mechanical advantage. This device cannot provide the 4:1 ratio pressure increase needed to overcome a resistant zerk. Another prior art grease gun uses a complex labyrinth and a mechanical switch to engage or disengage a ball check valve to vary pressure of grease exiting the grease gun. This device does not permit supply to the grease gun from an attached barrel and therefore the portability and versatility of the device is limited.  
         [0004]     The foregoing problems are addressed by the present two stage grease gun invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     A portable grease gun is provided with a dual pressure output capability which can be selectively changed by the user with little effort. The grease gun includes a plunger assembly which includes a larger diameter volume plunger which is selectively coupled longitudinally to a plunger extension by screw threads. The length of the volume plunger with the plunger extension attached may be varied by inserting the extension into the bore of the volume plunger a greater or lesser amount. This is accomplished by rotating the plunger extension relative to the volume plunger thereby moving the coupled end of the plunger extension in and out of the screw threads within the bore of the volume plunger. The volume plunger and its plunger extension are received in a longitudinal passageway of a plunger retainer which limits the longitudinal movement of the volume plunger, and prevents rotational movement of the volume plunger relative to the plunger retainer. A smaller diameter pressure plunger is slidable within the bore through the volume plunger and the attached plunger extension. The plunger assembly is longitudinally received within a bore of the grease gun body.  
         [0006]     An actuating lever is pivotal upon the grease gun body which includes the plunger assembly within the longitudinal bore through the body. The lever includes a push block which abuts the plunger assembly to urge the pressure plunger and selectively the volume plunger toward the output orifice of the grease gun.  
         [0007]     The desired pressure of the grease expelled from the grease gun body is controlled by selective rotation of the plunger extension to shorten or extend the effective length of the volume plunger and plunger extension combination. When the plunger extension is maximally received into the bore of the volume plunger, the volume plunger is prevented from longitudinal travel. However, the smaller diameter pressure plunger may slide along the bores of the volume plunger and the plunger extension, traveling into the compression chamber to pressurize grease at a high pressure and low volume. When the plunger extension is minimally received within the bore of the volume plunger, the effective length of the volume plunger and plunger extension combination is increased, allowing the volume plunger to be moved into the compression chamber when the push block of the actuating lever applies force to the end of the pressure plunger and the plunger extension, causing both the volume plunger and the pressure plunger to move in concert.  
         [0008]     When the plunger extension is moved to an intermediate insertion within the bore of the volume plunger, the actuation of the lever will first cause movement of the pressure plunger alone, providing a high pressure grease delivery during the first part of the stroke of the actuating lever and subsequently a low pressure grease delivery when the push block begins to abut the plunger extension and the pressure plunger together, thereby urging both the pressure plunger and the volume pressure together toward the output body of the grease gun. The grease gun can therefore be used to apply high pressure grease to a resistant zerk during the first part of the stroke of the actuating lever and then to deliver high volume grease at low pressure after the resistance of the zerk is overcome.  
         [0009]     It is a primary object of the invention to provide a portable grease gun which can be adjusted to deliver grease at high pressure and low volume or at low pressure and high volume. It is a further object of the invention to provide a portable grease gun which can be adjusted to overcome a clogged or seized zerk and then to inject grease into the corrected zerk at lower pressure and higher volume. It is also an object of the invention to provide a grease gun which is easily adjusted for pressure output by the user without tools. It is an additional object of the invention to provide an easily manufactured grease gun which will expel grease at more than one pressure. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0010]      FIG. 1  is an exploded view in perspective of a preferred embodiment grease gun according to the present invention.  
         [0011]      FIG. 2  is a cross section of the preferred embodiment of the invention shown with the actuating lever fully retracted and with the plunger assembly adjusted for delivery of grease at high pressure and low volume as the actuating lever is moved toward the handle.  
         [0012]      FIG. 3  is a cross section of the preferred embodiment of the grease gun adjusted as in  FIG. 2  but with the actuating lever abutted to the handle of the grease gun and the pressure plunger advanced to the outlet check valve.  
         [0013]      FIG. 4  is a cross section of the preferred embodiment of the invention shown with the actuating lever fully retracted and with the plunger assembly adjusted for delivery of grease at low pressure and high volume as the actuating lever is moved toward the handle.  
         [0014]      FIG. 5  is a cross section of the preferred embodiment of the grease gun adjusted as in  FIG. 4 , but with the actuating lever abutted to the handle of the grease gun and with both the pressure plunger and the volume plunger fully advanced into the compression chamber of the grease gun.  
         [0015]      FIG. 6  is a cross section of the preferred embodiment of the invention shown with the actuating lever fully retracted and with the plunger assembly adjusted for delivery of grease at high pressure during the beginning portion of the stroke of the actuating lever and at low pressure during the remainder of the stroke.  
         [0016]      FIG. 7  is a cross section of the preferred embodiment of the grease gun adjusted as in  FIG. 6  with the actuating lever actuated part way such that the high pressure portion of the stroke is completed and the plunger assembly is in position for delivery of grease at high volume and low pressure for the remainder of the stroke of the actuating lever toward the handle.  
         [0017]      FIG. 8  is a cross section of the preferred embodiment of the grease gun adjusted as in  FIG. 6  with the actuating lever shown abutted to the handle with the pressure plunger advanced maximally into the compression chamber and with the volume plunger advanced partially into the compression chamber.  
         [0018]      FIG. 9  is an isometric view of a grease gun according to an alternative embodiment of the present invention.  
         [0019]      FIG. 10  is a side elevational view of the grease gun of  FIG. 9 .  
         [0020]      FIG. 11  is a front elevational view of the grease gun of  FIG. 9 .  
         [0021]      FIG. 12  is an exploded view of the grease gun of  FIG. 9 .  
         [0022]      FIG. 13  is a partial cross-section view of the grease gun of  FIG. 9  adjusted to a high pressure mode with the actuating lever in a retracted neutral position.  
         [0023]      FIG. 14  is a partial cross-section view of the grease gun of  FIG. 9  adjusted to a high pressure mode with the actuating lever in a closed position.  
         [0024]      FIG. 15  is a partial cross-section view of the grease gun of  FIG. 9  adjusted to a high volume mode with the actuating lever in a retracted neutral position.  
         [0025]      FIG. 16  is a partial cross-section view of the grease gun of  FIG. 9  adjusted to a high volume mode with the actuating lever in a closed position.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Reference is drawn to  FIGS. 1-8  and specifically to  FIG. 1 , initially.  FIG. 1  discloses a grease gun body  1  according to the present invention with the plunger assembly  27  thereof shown in exploded view. Plunger assembly  27  is received within longitudinal bore  28  of grease gun body  1 . Plunger assembly  27  comprises a volume plunger  3  which is slidably received within an elongate hollow cylindrical plunger retainer  9 . Plunger retainer  9  is provided with an opening  29  on its sidewall  30  into which guide pin  17  is received. Volume plunger  3  includes an elongate slot  31  into which guide pin  17  may extend such that volume plunger  3  is prevented from rotation relative to plunger retainer  9 , while remaining free to move longitudinally within plunger retainer  9  over a range limited by a stop within plunger retainer  9 .  
         [0027]     O-ring  23  and backup ring  25  are received on inner end  32  of plunger retainer  9 . Opposing outer end  33  of plunger retainer  9  is threaded to be received in threaded bore  28 . Slots  56  are provided on outer end  33  to aid threading plunger retainer  9  into bore  28 .  
         [0028]     Plunger extension  11  is received in longitudinal bore  36  of plunger retainer  9  and is coaxially retained to volume plunger  3 , at a selected depth, by selective insertion of inner end  34  of plunger extension  11  into passageway  35  of volume plunger  3 . Threads  55  on inner end  34  of plunger extension  11  mate with internal threads  54  within passageway  35  of volume plunger  3 . Plunger extension  11  with volume plunger  3  attached coaxially thereto is biased toward push block  4  by volume plunger spring  12 . Bearing ring  18  provides a sliding surface between shoulder  53  of plunger extension  11  and volume plunger spring  12 .  
         [0029]     By rotating plunger extension  11  clockwise or counterclockwise, the effective length of the coaxially connected plunger extension  11  and volume plunger  3  may be selected. Thumb wheel  48  on plunger extension  11  facilitates rotation thereof by the user.  
         [0030]     Pressure plunger  2  is slidably received within the longitudinal bore  37  of plunger extension  11  and the longitudinal passageway  35  of volume plunger  3  and is biased toward push block  4  by pressure plunger spring  13 . Pressure plunger  2  comprises inner end  41  from which extends needle  42  having pressure face  43  on free end  44  thereof.  
         [0031]     Volume plunger  3  is slidable over a limited range within the longitudinal bore  36  of plunger retainer  9 . Guide pin  17  is received in sidewall  30  of plunger retainer  9  and rides in slot  31  of volume plunger  3 .  
         [0032]     The range of longitudinal retroactive movement of volume plunger  3  is limited by an internal stop  57  (see  FIG. 6 ) spaced apart from inner end  32  of plunger retainer  9 . Volume plunger  3  comprises a counterbore  50  extending from outer end  49  part way therethrough. Counterbore  50  accommodates inner end  34  of plunger extension  11  while the remainder of passageway  35  is reduced in size to accommodate body  38  of pressure plunger  2  for sliding movement therealong.  
         [0033]     To reach into and be slidable within passageway  35  of volume plunger  3 , pressure plunger must pass along bore  37  of plunger extension  11 . Pressure plunger spring  13  biases head  39  of pressure plunger  2  longitudinally outward along bore  37  while volume plunger spring  12  biases the interconnected plunger extension  11  and volume plunger  3  longitudinally outwardly along bore  36 . Inward longitudinal travel of volume plunger  3  is limited by contact of the thumb wheel  48  against outer end  33  of plunger retainer  9 .  
         [0034]     Referring now to  FIGS. 2 and 3 , grease gun  26  further comprises an actuating lever  5  pivotally retained to grease gun body  1  by hinge pin  46 . Push block  4  is retained to actuating lever  5  by spring pin  10  allowing push block  4  to rotate about spring pin  10  in order to maintain a flat surface against outer end face  40  of pressure plunger  2 .  
         [0035]     Handle  8  depends from grease gun body  1  at an approximate perpendicular to the longitudinal axis of grease gun body  1 . Output body  6 , which may be a nozzle, is received within longitudinal bore  28  grease gun body  1  at output end  22  and includes an output check ball  20  biased by output spring  15  which is retained in output body  6  by output check screw  16 .  
         [0036]     Grease gun body  1  further comprises a barrel mount  47  which houses inlet valve body  7 , which includes valve spring  14  which biases inlet check ball  21 . A barrel  24  may be selectively mounted to grease gun body  1  at barrel mount  47 . Grease gun  26  is shown with the inner end  34  of plunger extension  11  substantially fully inserted into counterbore  50  of volume plunger  3 . In this condition, grease gun  26  is adjusted to deliver grease at a high pressure.  
         [0037]     In  FIG. 2 , the grease gun  26  as shown is ready to begin to expel grease from compression chamber  45  through output body  6  at high pressure, that is, in the range of about 10,000 psi. Actuating lever  5  has been retracted from handle  8  allowing pressure plunger spring  13  to urge pressure plunger  2  to follow push block  4  as it retreats from grease gun body  1  as it is carried on spring pin  10  on actuating lever  5 . Volume plunger  3  is maximally retracted from compression chamber  45  while needle  42  of pressure plunger  2  remains partially within compression chamber  45 . Compression chamber  45  may be filled with grease which passes inlet check ball  21  from barrel  24 .  
         [0038]      FIG. 3  discloses the position of the plunger elements after actuating lever  5  has been moved back to rest against handle  8 . Volume plunger  3  remains in the same position as in  FIG. 2  while pressure plunger  2  has further protruded into compression chamber  45  with needle  42  thereof inserted within the output duct  51  thereby forcing grease from compression chamber  45  past outlet check ball  20 . The force upon outer end face  40  of pressure plunger  2  exerted by push block  4  compresses pressure plunger spring  13 .  
         [0039]     Referring now to  FIGS. 4 and 5 , grease gun  26  is illustrated in condition to expel a large volume of grease at a low pressure.  FIG. 4  shows the location of pressure plunger  2  and volume plunger  3  when actuating lever  5  is maximally retracted from handle  8 . In the condition of grease gun  26  in  FIGS. 4 and 5 , plunger extension  11  has been retracted from counterbore  50  of volume plunger  3  by selective rotation of plunger extension  11  by manipulation of thumb wheel  48  thereof effectively unscrewing inner end  34  of plunger extension  11  from counterbore  50 . This effectively extends the length of the combined volume plunger  3  and plunger extension  11 , allowing outer end  52  of plunger extension  11  to extend from longitudinal bore  28  and from plunger retainer  9 . As should be understood, as actuating lever  5  starts to move toward handle  8 , push block  4  forces both pressure plunger  2  and volume plunger  3  toward output body  6  across compression chamber  45 . This movement causes volume plunger  3  combined with pressure plunger  2  to force a large volume of grease at low pressure past output check ball  20  against output spring  15 .  
         [0040]     After actuating lever  5  has been fully rotated into abutment on handle  8 , as shown in  FIG. 5 , volume plunger  3  as well as pressure plunger  2  have traversed compression chamber  45  toward output duct  51  causing low pressure grease to pass along output duct  51  and pass output check ball  20  of output body  6 .  
         [0041]      FIGS. 6-8  illustrate the adjusted position of grease gun  26  in which plunger extension  11  is mounted to volume plunger  3  midway into counterbore  50  thereof. By selective rotation of plunger extension  11  by manipulation of thumb wheel  48 , inner end  34  may be moved into an intermediate position along counterbore  50  of volume plunger  3 . In this position grease gun  26  is capable of delivering grease at high pressure or at high volume depending on the extent of retraction of the actuating lever  5 . Specifically, in  FIG. 6 , actuating lever  5  is shown fully retracted such that grease gun  26  is ready to expel grease at high pressure, such as when a resistant zerk must be overcome. Volume plunger  3  is stationed at a rest position determined by the retractive urging created by volume plunger spring  12  as limited by internal stop  57  of plunger retainer  9 . Pressure plunger  2  is retracted from output body  6  by the urging of pressure plunger spring  13  but such retraction is limited by the abutment of head  39  thereof against push block  4 .  
         [0042]     As seen in  FIG. 7 , as actuating lever  5  has moved from a fully retracted position partway toward handle  8 , pressure plunger  2  has been urged into compression chamber  45  while volume plunger  3  has remained in its rest position as seen in  FIG. 6 . As needle  42  of pressure plunger traverses compression chamber  45 , grease therein is forced at high pressure past output check ball  20  to be expelled into a zerk (not shown) which would be inserted into a coupler and extension (not shown) attached to output body  6  proximal to output check screw  16 . Therefore, high pressure grease could be forced into a resistant zerk to overcome its clog or seizure. From the position of actuating lever  5  seen in  FIG. 7 , grease gun  26  is in condition to deliver grease at low pressure as actuating lever  5  continues to be moved toward handle  8 .  
         [0043]      FIG. 8  illustrates the position of volume plunger  3  and pressure plunger  2  after actuating lever  5  has completed its travel and come into abutment with handle  8 . Volume plunger  3  has advanced substantially into compression chamber  45  coincident with movement of pressure plunger  2 , with plunger assembly  27  propelling grease from compression chamber  45  through output body  6  at high volume and low pressure.  
         [0044]     Therefore it should be understood that grease gun  26  may be adjusted to deliver grease at high pressure and low volume when inner end  34  of plunger extension  11  is maximally inserted into counterbore  50  of volume plunger  3 . However, when inner end  34  of plunger extension  11  is minimally inserted into counterbore  50  of volume plunger  3 , grease gun  26  will expel grease at low pressure and high volume. Furthermore, plunger assembly  27  may be adjusted such that a first portion of the stroke of actuating lever  5  from its fully retracted position will deliver grease at high pressure while a remaining portion of the stroke of the actuating lever  5  will deliver grease at low pressure. This adjustment is made by varying the depth of insertion of inner end  34  of plunger extension  11  into counterbore  50  of volume plunger. By selective adjustment, the proportion of stroke of the actuating lever  5  which causes the grease gun  26  to deliver high pressure grease may be varied, that is, the shorter the effective length of the combination of the volume plunger  3  and the plunger extension  11 , the larger the proportion of stroke of the actuating lever  5  from its fully retracted position which will deliver high pressure grease from grease gun  26 . As the effective length of the volume plunger  3  and plunger extension  11  increases, the smaller the proportion of the stroke of actuating lever  5  from full retraction will be which causes the grease gun  26  to deliver high pressure grease, with the remainder of the compression stroke of actuating lever  5  toward handle  8  causing low pressure grease to be expelled from grease gun  26 .  
         [0045]     An additional embodiment of a two-stage grease gun  100  according to the present invention is illustrated in  FIGS. 9-16 . This gun  100  may be fed with a reusable barrel that may be bulk loaded with grease (typically about 16 ounces) or loaded with a disposable cartridge (typically containing about 14.5 ounces). This gun  100  can develop over 10,000 psi of pressure in a high pressure, or boost mode. The high pressure is sometimes required to unblock a plugged grease fitting (zerk). Once the blockage is overcome, the gun  100  can be switched to a low pressure high volume mode to deliver the nominal amount of grease—preferably about one ounce of grease in twenty-two (22) strokes. The gun  100  uses two diameters of pistons with the smaller alternately able to slide inside the larger or engage the larger and translate both in unison in order to accomplish the two-stages. This gun  100  preferably has a handle  140  long enough to permit two-handed operation.  
         [0046]     As best seen in  FIGS. 9-12 , the gun  100  includes a gun body  101  that has an outlet valve  109  for dispensing grease. The gun body  101  may be formed from die cast aluminum, injection molded plastic, or by other known materials and mechanisms. A grease reservoir  150 , such as a reusable barrel or a disposable cartridge is connected to generally to a lower portion of the body  101 . An actuating lever  102  is pivotally attached at the rear of the gun body  101 . Preferably the handle  140  is long enough to permit two-handed operation. The actuating lever  102  is pinned to the body  101  by pivot member  130 . A toggle lever  121  may be adjusted up or down about sixty degrees (60°) to move between the boost mode and the volume mode. In the embodiment shown in  FIGS. 9-16 , the gun  100  is in the boost mode when the toggle lever  121  is adjusted to the upper position shown in  FIG. 9 , and can be adjusted to the volume mode by moving the toggle lever  121  downwardly.  
         [0047]     The inner working of the gun  100  can best be seen in  FIG. 12 , which provides an exploded view of the gun  100  from  FIG. 9 . Outlet valve  109  and inlet valve  115  attach to front and lower portions of the body  101  respectively. According to one embodiment, the outlet valve  109  and inlet valve  115  are metal fittings that are molded into a plastic body  101 . Alternatively, the outlet valve  109  and inlet valve  115  may be threaded such that they screw into their respective positions. A piston housing  104  is provided within the gun body  101  to house a piston assembly. According to one embodiment, the housing  104  is molded into the body  101  and is threaded for mated engagement with the outlet valve  109 .  
         [0048]     A volume piston  108  is slidably provided within piston housing  104 . The volume piston  108  has an opening in its front face. Boost piston  107  is also slidably provided within housing  104 , and has a front portion that extends through the opening in the front face of the volume piston  108 . A retaining ring  120  is snap fit on the end of the boost piston  107  to retain the front portion of the boost piston  107  on the front, or outlet, side of the front face of the volume piston  108 . Stop ring  119  is threaded into the housing  104  to limit rearward travel of the volume piston  108 . A piston spring  105  is provided between the stop ring  119  and a shoulder formed on the inner piston  107  to urge the inner piston  107  rearwardly away from the outlet  109 . The rearward travel of the inner piston  107  is constrained by the volume piston  108  bearing against the stop ring  119  and the retaining ring  120  bearing against the volume piston  108 .  
         [0049]     The rear portion of the volume piston  108  is provided with a slot or recessed portion  141 . The inner piston  107  is provided with a key portion  142  that is milled flat, such that when the inner piston is rotated to a mating alignment, the key portion  142  will fit into the recessed portion  141 , permitting the inner piston  107  to slide forward relative to the volume piston  108 . When the key portion  142  is not in mating alignment with the recessed portion  141 , the key portion  142  will press against the rear of the volume piston  108 , causing the inner piston  107  and volume piston  108  to move forward together as a unit. The inner piston  107  can be rotated within the housing  104  by adjusting toggle lever  121 . Pin  118  engages housing  104  and volume piston  108  to prevent rotation of the volume piston  108  relative to the housing. The volume piston  108  is provided with a longitudinal slot to receive the pin  118 , such that longitudinal sliding the volume piston  108  relative to the housing  104  is permitted.  
         [0050]     Additional structure of the gun  100  can be seen in the cross-section view of  FIG. 13 . A gasket  103  may be provided to seal the grease reservoir  150  (not shown in  FIG. 13 , see FIG.  10 ) with the gun body  101 . The inlet valve body  115  is sealed to the housing  104  by a seal ring  116 . A ball  114  and spring  113  are used to provide a one-way valve arrangement within inlet valve body  115 . The outlet valve  109  works as described above with respect to the other embodiments of the present invention. In particular, a set screw  112  is used to adjust the resistance of spring  111  bearing against ball  110 . A seal ring  117  is provided to seal the outlet valve body  109  to the housing  104 .  
         [0051]     Sealing ring  106  seals the inner piston  107  with the volume piston  108 . Likewise, sealing rings  124  and  125  seal the volume piston  108  with housing  104 .  
         [0052]     Bearing  126  is mounted to actuating lever  102  by pin  127  to bear against the rear face of inner piston  107 . Preferably the bearing  126  is rotatably mounted to the lever  102 .  
         [0053]      FIGS. 13 and 14  show the gun  100  adjusted into a boost mode. In  FIG. 13 , the actuating lever  102  is in a neutral position, and the inner and volume pistons  107  and  108  are withdrawn to their rearward most positions, such that the cavity behind the outlet  109  contains a full load of grease. In  FIG. 14 , the lever  102  has been squeezed (in a clockwise direction as shown), to press the bearing  126  against the inner piston  107 , causing the inner piston to translate forward towards the outlet  109 . Because the toggle switch  121  is in the boost mode, the key portion  142  of the inner piston  107  is matingly aligned with the recessed portion of the volume piston  108 , and the inner piston  107  will slide through the volume piston  108 , driving the front portion of the inner piston  107  towards the outlet  109 . A volume of grease corresponding to the volume displaced by the inner piston  107  is forced through the outlet  109 . Because the force of the actuating lever  102  is transmitted through the small surface area of the front face of the inner piston  107 , a high outlet pressure can be generated if a clogged zerk is encountered. If the actuating lever is released, the spring  105  will return the inner piston  107  and the actuating lever  102  to the position of  FIG. 13 . It should be appreciated that as the inner piston  107  withdraws back to the neutral position, the resulting vacuum will draw in additional grease through the inlet valve  115 .  
         [0054]      FIGS. 15 and 16  show the gun  100  adjusted into a high volume mode. In  FIG. 15 , the actuating lever  102  is in a neutral position, and the inner and volume pistons  107  and  108  are withdrawn to their rearward most positions, such that the cavity behind the outlet  109  contains a full load of grease. In  FIG. 16 , the lever  102  has been squeezed (in a clockwise direction as shown), to press the bearing  126  against the inner piston  107 , causing the inner piston to translate forward towards the outlet  109 . Because the toggle switch  121  is in the volume mode position, the key portion  142  of the inner piston  107  bears against the rear of the volume piston  108 , causing the two pistons  107  and  108  to move forward as a unit. As shown, the outer diameter of the volume piston  108  is about twice that of the inner piston  107 . Accordingly, the combination of the volume piston  108  and the inner piston  107  displace about four times as much volume during a single stroke as compared to the inner piston  107  alone. Correspondingly, the combination of the volume piston  108  and the inner piston  107  can generate about one fourth of the pressure based on the same input force applied by the actuating lever  102 . If the actuating lever  102  is released, the spring  105  will return the inner piston  107 , the volume piston, and the actuating lever  102  to the position of  FIG. 15 .  
         [0055]     Accordingly, grease gun  100  may be fed from a 16 oz barrel, and can develop over 10,000 psi of pressure. This pressure is sometimes required to unblock a plugged grease fitting. Once the blockage is overcome, the gun  100  can be switched to the volume mode to deliver the nominal amount of grease—preferably about one ounce of grease in 22 strokes. The feature that allows this is primarily the use of two diameters of piston with the smaller alternately able to slide inside the larger or engage the larger and translate both in unison.  
         [0056]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations of the embodiments are possible in light of the above disclosure or such may be acquired through practice of the invention. The embodiments illustrated were chosen in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and by their equivalents.