Patent Publication Number: US-6655570-B2

Title: Constant volume valve for a combustion powered tool

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a constant volume valve for a combustion-powered tool, such as a power framing tool. More specifically, it relates to a constant volume valve assembly that measures a volume of a fluid before allowing it to flow into a combustion chamber. 
     This invention also relates to a pneumatically powered, combustion-powered, or other rapidly acting, fastener-driving tool of a type utilizing collated fasteners. Typically, as exemplified in Nikolich U.S. Pat. Re. 32,452, Nikolich U.S. Pat. No. 4,522,162; Nikolich U.S. Pat. No. 4,483,474; Nikolich U.S. Pat. No. 4,403,722 and Wagdy U.S. Pat. No. 4,483,473, which are herein incorporated by reference, a combustion-powered, fastener-driving tool comprises a combustion chamber, which is defined by a cylinder body and by a valve sleeve arranged for opening and closing the combustion chamber. Generally, similar combustion-powered, nail- and staple-driving tools are available commercially from ITW-Paslode (a unit of Illinois Tool Works Inc.) of Vernon Hills, Ill., under its IMPULSE trademark. 
     In such a tool, it is beneficial to apply a constant force during the driving stroke to each fastener as it is driven into the workpiece. Measurement of the amount of fuel to the combustion-powered tool, or the amount of compressed gas to a pneumatically powered tool, helps provide a constant force. A combustion powered fastening tool is described in U.S. Pat. No. 4,721,240 to Cotta that measures fuel by opening a valve for a length of time defined by movement of a cam. Fuel passes through a fuel valve to a combustion chamber conduit, the amount of which is equal to the volume that passes through a needle valve during the time the fuel valve is open. Measurement of the flow of a fluid by time allows the amount of fluid supplied to the tool to vary as flow rates of the fluid change. As a fuel cylinder is emptied, the flow rate of the fluid changes as the cylinder pressure drops. Similarly, pressure or flow variations in a common supply of pneumatic fluid will also result in differences in the amount of power supplied on each charge of the cylinder. 
     Control of fuel into a combustion chamber by valve assemblies is shown in U.S. Pat. Nos. 655,996 and 1,293,858. Both references disclose a pressurized fluid inlet valve and fluid outlet valve that bracket a machine-supply passage. High-pressure fluid is fed to a machine to supply power via the inlet valve, and is discharged through the outlet valve when it returns from the machine following expulsion of its power. Neither reference teaches the use of such a system to supply a constant measurement of fluid. Further, following combustion of a fuel or expansion of a high-pressure fluid, the fluid is no longer useful to supply power to a tool and measurement at that point is ineffective. 
     U.S. Pat. No. 4,913,331 to Utsumi describes an apparatus that drives a piston with an internal combustion engine that utilizes a measuring chamber to dispense a constant volume of fuel. A fuel piston containing the measuring chamber is reciprocally moveable within a fuel cylinder. The fuel inlet channel and the fuel outlet channel are positioned such that the measuring chamber is filled and emptied by movement of the piston between the inlet and outlet channels. Seals are located on either side of the chamber between the fuel piston and the cylinder, preventing leakage of fuel from the pressurized fuel supply to the combustion chamber. Steady movement of the piston would cause rapid wear on these seals, since they are constantly in contact with the cylinder surface. 
     One operational drawback of conventional combustion-powered tools, is that when operated at relatively low temperatures, such as below 32° F., the pressure of the pressurized fuel falls, causing a greater pressure differential between the atmosphere and the fuel. At this lower pressure, the fuel does not dissipate as rapidly through the appropriate passageways and into the combustion chamber. This condition causes a delay in the combustion, which interferes with the operational efficiency of the tool. 
     It is, therefore, an object of this invention to provide an improved constant volume measurement of a fluid to an apparatus, such as a combustion-powered tool, to produce a constant driving force. 
     It is yet another object of this invention to provide an improved constant volume measurement of fluid in a compact space. 
     It is still another object of this invention to provide an improved constant volume valve assembly, whose seals are not constantly wearing against a sealing surface. 
     It is a further object of the present invention to provide an improved constant volume valve assembly that facilitates the movement of fuel even when fuel pressure drops, such as when the tool is exposed to low temperatures. 
     SUMMARY OF THE INVENTION 
     These and other objects are met or exceeded by the present device for metering a constant volume of fluid to provide constant energy to a tool. This apparatus is most useful in a portable fastening tool powered either pneumatically or by an internal combustion engine. In the preferred embodiment, configuration of the valves and control mechanism also provides a delay between the closing of one valve and the opening of another, ensuring that fluid is metered before moving downstream to the combustion chamber. 
     More specifically, the present invention provides a constant volume metering chamber and valve assembly for use with a pressurized fluid supply. The assembly includes housing defining a metering chamber having an inlet and an outlet. A first spring-biased valve is disposed in a housing to control fluid flow to the inlet. A second spring-biased valve is disposed in the housing to control fluid flow to the outlet. An actuator assembly is connected to the two valves, and is sequentially operable from a first position, in which the first spring-biased valve is open and the second spring-biased valve is closed, to a second position, in which the first and second spring-biased valves are both closed, and to a third position, in which the first spring-biased valve is closed and the second spring-biased valve is open. The valve assembly is configured and arranged so that a volume of fluid entering the chamber from the inlet in the first position is collected in the metering chamber, sealed within the metering chamber in the second position, and released from the metering chamber in the third position to provide a constant volume of fluid from each sequential movement of the actuator from the first position to the third position. 
     The present constant metering valve produces a constant driving force by a fastener-driving tool because it provides a consistent quantity and quality of fuel or hydraulic fluid each time the tool is fired. The fluid supply to the power tool of this invention is measured by volume, not by time, providing a more accurate and more consistent supply of power to the tool. As pressure varies, the fluid density changes in either system because the molecules become more or less densely packed. However, in a flow system, flow rates will also change if the pressure drop across the metering valve fluctuates. Change in flow rate will have no effect in a constant volume system as long as the constant volume chamber is filled in the time the inlet valve to the metering chamber is open. 
     Further, arrangement of the metering chamber and the spring-biased valves in the present invention leads to compact use of space, as would be useful in a compact, portable tool. Collinear placement of the valves and the oblique angle of the combustion chamber passageway features a shorter distance from the pressurized fluid supply to the combustion chamber, compared to other designs. 
     Using spring-biased valves to control fluid flow is also advantageous. The seat of the valve that forms the seal with the inlet and outlet of the metering chamber is in contact with the walls of the chamber only for a relatively short time. As the valves open and close, there is no constant rubbing of the seals with adjacent walls. This leads to longer life for the seals. 
     Another advantage of the present valve assembly is that a disk is preferably provided to at least one of the spring-biased valves which facilitates the flow of fuel into a combustion chamber passageway even in operational conditions when fuel flow is impaired, as when outside operational temperatures fall below freezing. 
     Still another feature of the present valve assembly is that the actuator assembly is configured to provide an inherent delay in the operation of the upper and lower spring-biased valves to ensure that a designated volume of fuel will be retained in the metering chamber before the lower valve releases the fuel to the combustion chamber. In the preferred embodiment, this delay is achieved in part by a deliberately loose mating engagement between a tongue of an actuator pivoting link arm and a notch in an actuator control arm. This loose engagement ensures that, while the pivoting link arm travels a continuous motion due to the engagement of the tool upon a workpiece, the actuator control arm is not continuously moved, resulting in a slight “pause” in the operation of the spring-biased valves. In this manner, the consistency of the volume of fuel temporarily held in the metering chamber is maintained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a back view of the present constant volume valve assembly as attached to a fuel canister; 
     FIG. 2 is a front vertical sectional view of the present constant volume valve assembly; 
     FIGS. 3A-3C are a series of fragmentary sectional views of the present constant volume valve assembly depicting three valve positions as the actuator assembly moves through an operational sequence; 
     FIG. 4 is a fragmentary sectional view of the present constant volume valve shown equipped with a disk for facilitating the movement of fuel from the metering chamber into the combustion chamber; and 
     FIG. 5 is a fragmentary sectional view of an alternate embodiment of the present constant volume valve showing the sealing connection between the valve and the interior nozzle of a pressurized fuel cartridge. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a constant volume valve assembly and metering chamber is, generally designated  10 . In the following description, the terms “upper” and “lower” refer to the assembly in the orientation shown in the drawings. However, it is contemplated that the present assembly may be used in a variety of positions as is well known in the art. The present valve assembly  10  is particularly useful in a pneumatic or combustion powered tool (not shown), having a valve housing  12  in which the fluid to be metered is injected under pressure. The valve assembly  10  provides a fixed amount of fuel to the combustion chamber (not shown) of the tool. Alternatively, it is contemplated that the present valve assembly  10  may also meter pressurized air, which expands to provide power, to the pneumatic tool. The present valve assembly  10  is usable in any tool or device that would benefit from a steady, uniform supply of a pressurized fluid. 
     The housing  12  of the valve assembly  10  includes at least two spring-biased valves, a first spring-biased valve  16  and a second spring-biased valve  18  that respectively control the fluid flow to an inlet  20  and an outlet  22  of a metering chamber  24 . The metering chamber  24  is defined by the housing  12 , and optionally has one or more ports in addition to the inlet  20  and outlet  22 , as will be discussed below. Neither the shape of the metering chamber  24 , nor the position of the inlet  20  or outlet  22  is particularly important. However, it is preferable to place the inlet  20  and the outlet  22  at diametrically opposed ends of the metering chamber  24 . In this configuration, the spring-biased valves  16 ,  18  are preferably approximately axially collinear, conserving space. In this preferred configuration, fluid flow through the metering chamber  24  will flow from the inlet  20  to the outlet  22 , generally parallel to the axes of the spring-biased valves  16 ,  18 . 
     The metering chamber  24  may be any type of chamber capable of providing a constant volume space for measurement of the fluid, meaning that the volume of fluid collected in the metering chamber is equal to the volume of fluid released from the metering chamber. While the fluid is sealed within the metering chamber  24 , the pressure remains constant. The metering chamber  24  may be a separate vessel or it may simply be a cavity  24  within the housing  12 . The housing  12  will generally also be used to support other components of the propulsion system, such as a pressurized fluid canister  28  (shown in FIG. 1) and the spring-biased valves  16 ,  18 . Preferably, the metering chamber  24  is stationary relative to the housing  12 . 
     The volume of the metering chamber  24  while preferably fixed, is optionally adjustable by, for example, placement of a movable wall or opening of valves to additional chambers (not shown). However, its usefulness for metering purposes depends upon the ability of the chamber  24  to remain at a constant volume until some setting, valve or adjustment is purposely changed. 
     The spring-biased valves  16 ,  18  each include a preferably conical seat  30 ,  32 , a rod  34 ,  36 , and a spring  38 ,  40 , respectively. Although discussed in terms of the first spring-biased valve  16 , it is to be understood that the following description also applies to the corresponding parts of the second spring-biased valve  18 . The seat  30  is sized and configured to sealingly engage with the inlet  20  of the metering chamber  24  when the spring-biased valve  16  is in a closed position. Movement of the seat  30  between an open position and the closed position, is controlled by the rod  34 . Although the spring  38  is an economical method of biasing the valve, use of other biasing devices is contemplated. The spring  38  is used to bias the valve  16  toward the closed position. Each of the springs  38 ,  40  has an anchored end  42 ,  44  and a movable end  46 ,  48 , respectively. The movable end  46  exerts a force against the seat  30  tending to move it in the direction of the metering chamber  24  by the force of the spring  40  pushing against the anchored end  42 . Although the anchored end  42  may be anchored directly to the housing  12 , preferably, the anchored end is seated within a compartment described in greater detail below. 
     Fluid is supplied to the housing  12  under pressure. It is generally desirable that the tool is portable, and in such a case, the fluid is delivered from the pressurized canister  28  that fits within or attaches to the tool. In the case where the tool is to be used in a shop or other location where a large supply of pressurized fluid is available, the fluid is preferably available to the tool through a hose or similar device (not shown). The valve assembly  10  of the present invention is useful in either of these situations, and use in either setting is contemplated. Since temperature and pressure affects the density of any fluid, these factors should be kept as constant as possible to minimize variation in the amount of fluid supplied. 
     Before entering the valve assembly  10 , the fluid preferably flows through a filter  50  (FIG. 2) to minimize unwanted contaminants. The filter  50  is preferably disposed at one end of a nipple  51 , which matingly and sealingly engages the canister  28 . After passing the filter  50 , the fuel travels into an upper passageway  52 . The upper passageway  52  leads from the source of the pressurized fluid, such as the pressurized canister  28 , to the inlet  20  of the metering chamber  24 . To achieve the most consistent amount of fluid, the upper passageway  52  is preferably sufficiently wide to consistently achieve supply pressure before closure of the first spring-biased valve  16 . 
     In some cases, it is desirable to provide an upper chamber  54  for accumulation of pressurized fluid. Where, for example, the flow rate of the fluid is low, fluid accumulates in the upper chamber  54 , providing a burst of fluid to enter the metering chamber  24  when the inlet  20  is opened. Fluid released from the metering chamber  24  flows into a lower chamber  56 . Metering is accomplished through opening and closing of the first and second spring-biased valves  16 ,  18  by an actuator assembly  60 . The actuator assembly  60  is any mechanism capable of causing the opening and closing of the first and second spring-biased valves  16 ,  18  in a particular sequence to allow measurement of the fluid in the metering chamber  24 . While a mechanical linkage is the preferred form of the actuator assembly  60 , a computer controlling one or more cams is an example of an acceptable alternative configuration. 
     In the preferred embodiment, the actuator assembly  60  includes a C-shaped actuator arm with an upper arm  62 , which is connected to the rod  34  of the first spring-biased valve  16 , and a lower arm  64 , which is connected to the rod  36  of the second spring-biased valve  18 . The upper arm  62  and the lower arm  64  are connected to each other by a control arm  66  (FIG.  1 ). A notch  67  in the control arm  66  is engaged by a pivoting link arm  68  which is pivotally engaged to the housing  12  at a point  68   a . The specific engagement between the link arm  68  and the notch  67  is via a tongue  69 . The control link arm  68  is operated through movement of the nosepiece valve linkage (not shown), the construction and operation of which is disclosed in the Nikolich patents incorporated by reference here. 
     An important feature of the present actuator assembly  60  is that a delay is created in the movement of the control arms  62 ,  64 ,  66  and their actuation of the upper and lower spring-biased valves  16 ,  18  so that a constant volume of pressurized fluid is momentarily retained in the metering chamber  24 . This delay is created in part by a loose mating engagement between the tongue  69  and the notch  67 . In the preferred embodiment, the tongue  69  is provided with a reduced area compared to the notch  67 , so that the control link arm  68  can move slightly along its arcuate travel path without causing movement of the control arms  62 ,  64 , and  66 . The looseness or “sloppiness” of the engagement between the tongue  69  and the notch  67  can vary with the application, as can the specific configuration of the mating engagement, including having the notch on the arm and the tongue on the control arm  66 . 
     The actuator assembly  60  moves the first and second spring-biased valves  16 ,  18  in either a first valve sequence or a second valve sequence, depending on which valve is to be opened and which valve is to be closed. The valve sequence is determined according to the combustion cycle, in the case of a combustion tool, or the impact cycle of a pneumatic tool. 
     Turning now to FIGS. 3A-3C, the valve sequences are described. The beginning of the first valve sequence is defined when the tool is in between uses. In this position, the tool is powered up and ready to be used, but is not yet in contact with the workpiece into which a fastener is to be driven. At this time, the actuator assembly  60  is in the first position as depicted in FIG. 3A, the arm  62  is spaced a maximum distance from an opposing wall of the housing  12 . The first spring-biased valve  16  is in an open position and the second spring-biased valve  18  is closed. The metering chamber  24  is thus filled with fuel or fluid due to communication with the cartridge  28  through the passageway  52 . 
     During the first valve sequence, the first spring-biased valve  16  moves from an open position to a closed position and the second spring-biased valve  18  opens, but the second valve does not begin to open until first valve is completely closed. This first valve sequence will generally be triggered by some stimulus in preparation for firing of the tool. To have power to drive a fastener, the metered fluid is moved into position to deliver that power; i.e., fuel is moved into the combustion chamber or air into an expanding cylinder. The sequence is preferably initiated by any preparatory mechanism, such as contact of the tool with a workpiece, beginning to squeeze the trigger mechanism and the like. If a combustion powered framing tool is used, priming of the combustion chamber preferably takes place when a workpiece contact element comes in contact with the workpiece, allowing the fuel to flow from the metering chamber  24 , through the lower chamber  56 , into a combustion chamber passageway  70  and ultimately to the combustion chamber (not shown). In the depicted and preferred embodiment, the sequence is initiated by contacting the tool with a workpiece, which causes the pivoting link arm  68  to begin its arcuate path of travel represented by the arrow A (FIG.  1 ). 
     It is important to note that the metering chamber  24  is used solely for measurement of the fluid, and that there are no physical or chemical changes to the fluid while it is sealed in the chamber. To provide constant power, the fluid is preferably delivered at the same volume, temperature and pressure for each cycle. Fluids cannot be accurately measured while chemical or physical reactions are taking place, thus it is preferred that the fluid have the same chemical composition when it is released from the metering chamber  24  as when it entered the metering chamber. 
     Referring now to FIG. 3A, which corresponds to the first position in the preferred embodiment shown, in this position, fluid freely enters the metering chamber  24 . As the pivoting link arm  68  moves in an arc defined by the arrow A (FIG.  1 ), the tongue  67  moves in a reverse arcuate direction. As such, the former upward pressure exerted upon the first rod  34  by the upper arm  62  is released, allowing the spring  38  to bias the first seat  30  of the first valve  16  into engagement with the inlet  20  of the metering chamber  24 . 
     At this point, both spring-biased valves  16 ,  18  are closed, preventing flow of the fluid from the fluid supply canister  28  into and out of the metering chamber  24 . This position is depicted in FIG. 3B, and corresponds to the second position of the actuator assembly  60 . The metering chamber  24  is closed at both the inlet  20  and the outlet  22 , sealing the fluid within it and providing a measured volume of fluid within the chamber. 
     The loose mating engagement between the tongue  69  and the notch  67  described above results in a temporary delay in the opening of the second valve  18  while the pivoting link arm  68  continues its arcuate path defined by the arrow A (FIG.  1 ). Due to the loose engagement, as the pivoting link arm  68  moves, there is a delay while the upward bias opening the first valve  16  is released, and the control arm  66  has not been moved sufficiently to open the second valve  18 . This delay ensures that the volume of fuel in the metering chamber  24  will remain constant, and that unwanted additional amounts cannot enter the chamber, or that premature leakage from the outlet  22  into the lower chamber  56  cannot occur. 
     The third position of the actuator assembly  60  is shown in FIG. 3C, which is attained after the first valve  16  has completely closed and the second spring-biased valve  18  is opened. In this position, the fluid is released from the metering chamber  24 . In the preferred embodiment, the entire first valve sequence takes place as the actuator arm  60  moves continuously from the first position through the second position to the third position. 
     Following firing of the tool  12 , the second valve sequence is initiated, in which the lifting of the tool from the workpiece causes the pivoting linking arm  68  to move the actuator assembly  60  from the third position, through the second position, to the first position. This sequence closes off the outlet  22  of the metering chamber  24  from flow downstream, and reopens the inlet  20  to again allow flow of fluid into the metering chamber  24 . Any stimulus that follows firing of the tool  12  but precedes the first valve sequence may be used to start this sequence. 
     The second valve sequence moves the first and second spring-biased valves through the same steps as the first valve sequence, but in the reverse order. Starting with the third actuator assembly  60  position shown in FIG. 3C, the second spring-biased valve  18  is disengaged from the outlet  22 , preventing flow of the fluid from the metering chamber  24 . After the second valve  18  is completely closed, the second actuator assembly  60  position is obtained, as shown in FIG.  3 B. Here both valves  16 ,  18  are closed to prevent backflow of the fluid, and the metering chamber  24  contains only a residual amount of fluid. Finally, the first spring-biased valve  16  is disengaged from the inlet  20 , allowing free flow of the fluid from the fluid supply  28  into the metering chamber  24 , but that fluid is prevented from flowing freely from the pressurized fluid supply  28  through the inlet  20  and the outlet  22  of the metering chamber  24  to the combustion chamber passageway  70 . 
     In the preferred embodiment, this operation or valve sequence is controlled by the pivoting action of the link arm  68  which moves the actuator assembly  60  from a position where the upper arm  62  has a maximum spacing from the housing  12  (FIG.  3 A), to a position where the lower arm  64  has a maximum spacing from the housing  12  (FIG.  3 C). In the preferred embodiment, in addition to the loose mating engagement between the notch  67  and the tongue  69 , the actuator assembly  60  also includes a delay mechanism also operating between the closing of one of the valves  16 ,  18  and the closing of the other valve  18 ,  16 . Any type of delay mechanism is suitable, such as an electrical delay, electronic means of a mechanical delay mechanism. In the most preferred mechanical delay mechanism, the actuator assembly  60  is slidably connected to each of the rods  34 ,  36 . The first rod  34  has a first opener  71  such as a ‘C’-clip secured to the rod  34  and the second rod  36  has a second opener  72 . Spacing of the openers  71 ,  72  on the rods  34 ,  36  are preferably used to create a delay in the closing of one valve  16 ,  18  before the opening of the other valve  18 ,  16 . 
     In the preferred delaying mechanism, the control arm  66  of the actuator assembly  60  is longer than the housing  26  in which the valve assembly resides. The excess length is sufficient to allow the upper arm  62  and the lower arm  64  to sandwich the housing  12  between them with excess space between the housing, and the actuator arms  62 ,  64 . In response to the stimulus that triggers the valve sequences, the control arm  66  moves up and down (directions relate to the tool, as oriented in FIG.  3 ). 
     Referring now to FIG. 3A, as the actuator assembly  60  moves through the first valve sequence, the upper arm  62  begins in contact with the first opener  71 . As the control arm  66  moves downward, release or expansion of the first spring  38  holds the first opener  71  against the upper arm  62  until the first seat  30  comes into contact with the inlet  20  of the metering chamber, closing the first spring-biased valve. Once the control arm  66  moves sufficiently so that the upper arm  62  is disengaged from the first opener  71  (as shown in FIG.  3 B), the first spring  38  biases the valve  16  into the closed position. During this movement from the first position (FIG. 3A) to the second position (FIG. 3B) of the control arm  66 , the lower arm  64  has slid along the second rod  36 , partially, but not totally decompressing the second spring  40 . Next, in moving from the second position (FIG. 3B) to the third position (FIG. 3C) of the control arm  66 , the lower arm  64  slides along the second rod  36  and finally contacts the second opener  72 , compressing the second spring  40 , and opening the second spring-biased valve  18 . The second valve sequence similarly reverses the above steps, introducing a delay between the closing of the second spring-biased valve  18  and the opening of the first spring-biased valve  16 . 
     Seals are used where suitable to prevent flow of the fluid into the area outside the valve assembly  10 , the metering chamber  24 , and the housing  12 . The exact number, shape and placement of such seals depend on the exact configuration of the valve assembly  10  for a specific application. In the preferred embodiment shown, a removable insert  74  is optionally used to surround the rod  34 ,  36  of each of the spring-biased valves  16 ,  18  as the rod passes through the housing  26  and contacts actuator assembly  60 . O-rings  76 , gaskets or similar devices, are preferably used to prevent leakage between the removable insert  74  and the housing  12  or the rods  34 ,  36 . In some applications, it will be preferable for the length of the spring  38 ,  40  to exceed the dimensions of the upper chamber  54  or the lower chamber  56 . When this is desirable, the removable insert  74  includes a hollow compartment  78  that is sized and configured to receive a portion of the length of the spring  38 ,  40 , and to receive the anchored end  42 . The removable insert  74  also provides easy access to the spring-biased valves  16 ,  18  and their component parts when replacements are installed. 
     Referring now to FIG. 4, it is preferred that the present valve assembly  10  be provided with a mechanism for facilitating the movement or evacuation of fuel from the metering chamber  24  through the outlet  22  and ultimately into the passageway  70  leading to the combustion chamber. As described above, it has been found that when combustion-powered tools of this type are operated at cold temperatures, such as below  32 ° F., the fuel pressure drops and it becomes more difficult to move the fuel into the combustion chamber. To address this problem, the present valve assembly  10  is preferably provided with a disk  80  secured to the valve  18 , specifically at the end of the rod  36  disposed in the metering chamber  24 . The disk  80  is preferably located closer to the inlet  20  when the valve  18  is closed. To that end, the disk  80  is secured to a pedestal  82  which in turn is secured to the conical seat  32 . In the preferred embodiment, the disk  80  is made of brass or equivalent rigid, heat resistant material, and the pedestal  82  is made of rubber or similar resilient polymeric or plastic material. However, other materials are contemplated. Preferably, the disk  80  is friction fit to the pedestal  82  through a frictional mating engagement between a lug  84  on the pedestal and an axial bore  86  in the disk. However, other ways of fastening the disk  80  to the pedestal  82  are contemplated, including but not limited to ultrasonic welding, insert molding, adhesives or other mechanical fasteners. The disk  80  is dimensioned to have a diameter which approximates, but is less than the diameter of the metering chamber  24 . 
     In operation, as the valve  18  opens, as described above in relation to FIG. 3C, the disk  80  moves with the seat  32  from its rest position near the inlet  20  of the metering chamber  24 , (best seen in FIG. 4) to a location closer to the outlet  22 . This movement will push any residual fuel from the metering chamber  24  through the outlet  22  and ultimately into the passageway  70  leading to the combustion chamber. In this manner, the fuel is mechanically moved from the metering chamber  24 . However, since the problem of low fuel pressure is temperature-related, an alternate solution would be to provide a supplemental exhaust passageway  88  through which hot exhaust from the combustion chamber heats up the metering chamber during operation of the tool. An equivalent arrangement is the provision of an electric heating element powered by a resistor or other known arrangement which maintains a satisfactory temperature in the metering chamber  24  to maintain fuel pressure. 
     Referring now to FIG. 5, the connection between the valve  10  and the fuel canister  28  is shown in greater detail. It is important that a sealing relationship be established between the valve  10  and the fuel canister  28  to prevent loss of fuel, as well as avoid unwanted combustion. The fuel canister  28  is provided with an internal stem  90  which defines an outlet for the fuel contained in the canister under pressure, as is known in the art. As is well known in the art, and exemplified by U.S. Pat. No. 5,115,944 which is incorporated by reference, the stem  90  is secured to, and is circumscribed by an endcap  92  which encloses the end of the canister  28  and forms a rolled seam  94  thereover. 
     An adapter  96  frictionally engages the endcap  92  and circumscribes and protects the projecting stem  90 . An axial passageway  98  is defined by the adapter  96  and accommodates the stem  90 . In the preferred embodiment, the adapter also includes a frangible end membrane  100  which blocks the passageway  98 , and provides a visible indication of whether or not the canister  28  has been used. The membrane  100  is configured to be pierced upon mating engagement with the nipple  51 . Accordingly, the passageway  98  is dimensioned for accommodating the nipple  51 . 
     By the same token, the nipple  51  is preferably generally cylindrical in shape, and has a diameter or cross-sectional parameter dimensioned to slidably and matingly engage the passageway  98 , and a length dimensioned to engage an end  102  of the stem  90  to effect fluid communication between the canister  28  and the valve  10 . In the preferred embodiment, the nipple  51  is cylindrical, however, other non-circular cross-sectional shapes are contemplated depending on the application, and including oval, square, rectangular and polygonal shapes. 
     In the preferred embodiment, the nipple  51  and the stem  90  are configured so that, upon operational engagement as depicted in FIG. 5, a sealing relationship is achieved. This relationship, designed to prevent unwanted loss of fuel, may be achieved through frictional contact between the end  102  of the stem  90  and an end  104  of the nipple  51 . However, it is preferred that some sort of sealing formation be provided to at least one of the nipple  51  and the stem  90 . In the preferred embodiment, the sealing formation is a resilient O-ring  106  provided to the nipple  51 . However, other known types of sealing formations are contemplated, including but not limited to ring seals, molded seals and flat washers. 
     Also, the present nipple end  104  defines a chamber  108  for receiving or capturing a resilient sealing member such as the O-ring  106 . More specifically, the end  104  is tapered or chamfered for both retaining the O-ring  106  and also for facilitating insertion of the nipple  51  into the adapter passageway  98 . The tapered end  104  more easily pierces the membrane  100 , especially when the nipple  51  is fabricated of metal such as brass, which is preferred, however other suitably rigid and durable materials are contemplated. 
     To further enhance the sealed relationship of the engaged nipple  51  and the stem  90 , the end  102  of the stem is configured for matingly engaging or accommodating the O-ring  106 . As such, the end  102  is preferably provided with an annular groove  110 . Naturally, it is contemplated that the O-ring  106  or other resilient sealing member may be alternately mounted to the stem  90 , or that it may be attached to the nipple end  104  by adhesive, in a groove (not shown) or other known type of O-ring attachment technology. 
     It is also contemplated that, depending on the application, if fluid communication with the canister  28  is required for any reason, a connector maybe provided in the form of the nipple  51  which, at the end opposite to the end  104 , is in fluid communication with a fluid container or reservoir as desired. 
     In use, the canister  28  is inserted into the combustion tool so that the nipple  51  matingly engages the adapter  96 . The canister  28  is pressed upon the nipple  51  so that the membrane  100  is pierced and the nipple end  104  enters the passageway  98  until contact is made with the stem end  102 . As described above, as sealing relationship is preferably obtained, and it is contemplated that other locking apparatus may be employed to secure the canister  28  in this position. 
     Thus, it will be seen by those skilled in the art that the present valve assembly and metering changer provide a simple method of providing a constant volume of fluid to a power fastening tool. The two spring-biased valves  16 ,  18  control the inlet and the outlet to the constant volume metering chamber  24 , measuring a constant amount of fluid, independent of in fluctuations in the fluid flow rate. The actuator assembly  60  manipulates opening and closing of the valves  16 ,  18 , receiving the fluid from the pressurized source  28  and metering it before it flows downstream to a combustion or expansion chamber. This arrangement of the valves  16 ,  18  minimizes wear on the seals, reducing maintenance. 
     While a particular embodiment of the constant volume valve assembly and metering chamber has been shown and described, it will be appreciated by those skilled in the art that changes and modifications maybe made thereto without departing from the invention in its broader aspects and as set forth in the following claims.