Abstract:
An apparatus for regulating and discharging gas includes a differential piston with first and second faces of unequal area. The differential piston reciprocates within a housing between a storage chamber and a discharge chamber, and includes an aperture extending between the first and second faces to allow the storage chamber to communicate with the discharge chamber. A spring positioned against the piston face with the smaller surface area biases the differential piston in the direction of the piston face with the larger surface area. The use of the apparatus allows a pneumatic device such as a pellet gun or a nail gun to rapidly recharge the firing chamber between firing cycles.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/976,641 filed Oct. 1, 2007, the entire contents of which are incorporated herein. 
     
    
     BACKGROUND 
       [0002]    The present invention is directed to an improved differential piston for a pneumatic device such as an air gun. 
       SUMMARY OF THE INVENTION 
       [0003]    In particular, the invention is directed to a modified differential piston of the type disclosed in U.S. Pat. Nos. 6,581,585, 6,701,908, and 6,957,646, all of which are incorporated by reference. 
         [0004]    The goal of the present invention is to provided a modified differential piston that is able to rapidly recharge the firing chamber while maintaining the power characteristics of the patented technology which include:
       1. A substantial increase in power output over common methods which rely solely on the expansion of a gas to derive power.   2. The precise metering of a compressed gas at a set pressure and volume.       
 
         [0007]    The modified differential piston is especially useful in applications such as nail guns which should be able to cycle through fire and recharge operations several times per second. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention will be more fully understood form the following detailed description taken with the accompanying drawings in which: 
           [0009]      FIG. 1 . is a sectional view of a prior art differential piston; and 
           [0010]      FIGS. 2 and 3  are sectional views of differential pistons according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    According to the invention, a pneumatic power plant used in connection with an air gun accomplishes two major improvements over the prior technology:
       1. It establishes a fairly constant force on the object being driven (for example, a nail). With other pneumatic plants, the force degenerates logarithmically as the volume behind the driving piston increases.   2. When fully charged, the device stores a precise volume of compressed gas at the same pressure for each firing round (volumetric regulation).       
 
         [0014]    Air gun power plants are disclosed in the patents mentioned above, and while such power plants provide exceptional results when used with air rifles, and provide quite promising results when used with other air-powered devices such as pneumatic nail guns, one drawback is that the system does not recycle between firing and recharging very rapidly. 
         [0015]    According to the patents mentioned above, the design included an air regulator installed along the center axis of the differential piston. This device served a worthwhile purpose: It allowed the output energy of the power plant to be adjustable, based on the air relief pressure setting of the regulator. The regulator was a simple spring-loaded device which allowed compressed air into the firing chamber (larger diameter side) of the differential piston. An example of such a design is illustrated at FIG. 5 of U.S. Pat. No. 6,701,908 (“the &#39;908 patent”) and  FIG. 1  of the present application. 
         [0016]    During the charging cycle, the differential piston  2  moves toward the high pressure storage chamber  4  as the gas expands into the firing chamber  6  to store a firing charge at one end of the piston (the firing side) and establishing a “pneumatic spring” at the other side of the piston (the high pressure storage chamber side.) Movement of the piston is initiated due to the piston having a larger cross sectional area of the firing side piston face  8  on the firing side than the high pressure piston face  10  on the high pressure storage chamber side. The air regulator  12  causes a slightly lower pressure in the firing chamber than in the high pressure storage chamber. Because the firing chamber has a larger area for its pressure to act upon, the piston moves in the direction of this force until the forces equalize. 
         [0017]    However, it has been discovered that the air regulator prohibits the rapid response of the piston to the incoming pressure. Air only enters the firing chamber when there is sufficient differential pressure between the two chambers to overcome the regulator&#39;s spring force. In particular, the air regulator has an air passage which decreases as the forces across the piston approach equilibrium. Although initial movement of the piston may be relatively rapid, movement becomes progressively slower as the air regulator begins to close. 
         [0018]    This problem is exacerbated by a key element in the design of the power plant. In particular, the overall efficiency of the system increases as the cross-sectional area of the smaller side of the differential piston approaches the area of the larger side (firing chamber side). This larger area provides a more powerful pneumatic spring while increasing the volume of gas stored in the high pressure storage chamber. Power output will increase substantially as the ratio of the smaller area to the larger area increases, but there will be progressively less difference in area between the two piston heads to generate the force necessary to charge the system. This lengthens the time required to recharge the system. As the cross-sectional areas of the pistons approach one another, the more restrictive the regulator&#39;s orifice becomes; lengthening the time required to fully balance the forces across the piston and for movement to stop. Furthermore, as the piston movement slows, the regulator tends to “hunt” as it attempts to establish its rated pressure differential. Consequently, with the air regulator in place, the system&#39;s recharge rate becomes intolerable as the system&#39;s power output becomes most advantageous. 
         [0019]    While the air regulator is the source of the problem, it cannot be completely removed as its removal would cause the piston to move into the fully charged position regardless of the charging pressure, and the advantages in power and volumetric regulation would not be realized. Therefore, rather than removing the regulator altogether, according to the present invention, the pressure regulator is replaced with a resistive orifice. 
         [0020]    Referring to  FIG. 2 , a housing  22  is provided as a cylinder with two different diameters. A floating differential piston  24  is provided within the housing and separates the housing into a high pressure storage chamber  26  of the housing, and a firing chamber  28  of the housing. The differential piston has two piston faces, each with a different diameter corresponding to the different diameters of the housing. According to this embodiment, a first piston face  32  faces the high pressure storage chamber of the housing while a second piston face  34  faces the firing chamber of the housing. For this embodiment, the second piston face is larger than the first piston face. Pressurized air enters the high pressure storage chamber from a pressurized air source  36  through a check valve  38 . A spring  42  provides resistance against the first piston face, and may be a stiff steel spring. Proper seals are maintained through the use of seals such as o-rings  43 . 
         [0021]    An aperture  44  extends through the differential piston between the first and second piston faces, and may include a resistive orifice  46 . The orifice is “resistive” in the sense that the orifice does not provide a short circuit upon firing for compressed air from the high pressure storage chamber through the differential piston into the firing chamber, but not so resistive as to prohibit the free flow of compressed air into the firing chamber during the recharge portion of the cycle. The proper design of a few key elements of the system will greatly enhance this goal. 
         [0022]    The purpose of a resistive orifice is to restrict the flow of air from the high pressure storage chamber side of the piston to the firing chamber side of the piston. For high pressure air rifles operating at about 1,500 psig, the resistive orifice can be about 0.040″ in diameter. As the device&#39;s operating pressure decreases, a larger resistive orifice will be required. Nail guns operate at about 100 psig and the resistive orifice must be large enough to permit rapid firing (about 2-3 shots per second). Also, a large quantity of air must enter the firing chamber to allow proper firing. In this case, the orifice needs to be larger. In one low pressure embodiment of the invention such as for a nail gun, the resistive orifice is about ⅛″ in diameter. 
         [0023]    While the size of the resistive orifice is an important design consideration, the piston will move quite rapidly under the combined forces of the stiff steel spring and the compressed air on the smaller piston head. Therefore, regardless of the diameter of the resistive orifice, both of these forces will work toward maintaining the pressure in the firing chamber by reducing the chamber&#39;s volume as air is discharged from the chamber. 
         [0024]    Another key to the design is the placement and proper selection of the spring inserted into the high pressure storage side (small end of this embodiment) of the differential piston. With the removal of the air regulator, the pressure on both sides of the piston will always be equal. Without the spring, the piston will almost instantaneously move to the full recharge position when very little pressure is applied. The inclusion of the spring provides a linearly increasing force which matches the required increase in pressure as the firing chamber fills. The amount of energy stored in the system therefore becomes a function simply of the spring constant (K) and can be set at different levels by increasing of decreasing the amount of “preload” applied to the spring. “Preload” refers to the length the spring is compressed during assembly and before any air pressure is applied to the system. It should be noted that this spring is similar to the large compression spring of the device of the &#39;908 patent, though in this application the spring is a necessity to establish a resistive force against which the differential piston moves. The spring may be made of any suitable material and design. 
         [0025]    In general, the stiffness of the spring is determined by the ratio of the diameters of the two piston heads. For example, in a nail gun where the larger piston head is 4″ in diameter and the smaller piston head is 3.85″ in diameter, there is a difference in piston areas of about 0.9248 sq. in. If the operating pressure is 100 psig and the piston has a total travel of 2.5″, then the spring coefficient, K of the spring is estimated as follows: 
         [0000]      0.9248 in 2 *100 lb f /in 2 =2.5 in*K, or K=37 lb f /in. 
         [0000]    Under these conditions, when the operating pressure reaches 100 psig, the piston will have traveled 2.5 inches if the spring constant (K) is 37 lb f /in. 
         [0026]    In one embodiment of the invention, the spring is preloaded during assembly so that there is substantial pressure in the device before the piston starts to move. The basic idea is that the spring constant (K) is dependent on the sizes of the two piston heads, operating pressure, the total distance the differential piston will travel, and any preload (pre-compression) which is to be applied to the spring. 
         [0027]    Relatively little energy is provided by the spring. Its primary purpose is to provide a means to allow the pressure in the firing chamber to rapidly build to the air gun&#39;s operating pressure while fully charging the device. However, it serves a secondary purpose: On firing, it assists greatly in maintaining a constant pressure in the firing chamber as the differential piston moves forward. This provides a constant force upon the nail being driven. The spring needs to be selected such that its maximum compression does not exceed the manufacturer&#39;s recommended limits as the spring cannot exceed the yield point of its material without altering the listed spring constant. Also, the spring must be selected such that its total number of cycles will not exceed the manufacturer&#39;s recommendation. 
         [0028]    The device is fully charged when the differential piston has moved toward the high pressure storage chamber a precise distance. At this point, the pressure and volume stored in the firing chamber will always be exactly the same between shots. This is also referred to as volumetric metering. Upon discharging, the pneumatic spring and stiff spring combine to move the piston forward, decreasing the volume occupied by the firing chamber and maintaining this charge at a nearly constant pressure. This results in a nearly constant force being applied to the driven object, for example, the nail in a nail gun. 
         [0029]    Other advantages to the invention are apparent. For example, in a nail gun application, it is common for a number of nail guns and other pneumatic tools to run off a single compressor. Consequently, the operating pressure of a particular nail gun will vary with the combined load from all pneumatic tools. The differential piston of the present invention has the capability of precisely setting the operating pressure of an individual nail gun. This is accomplished by simply shutting off the air supply when the differential piston has reached a pre-determined maximum travel. As described above, the firing charge for the gun will, at this point, be at a precise pressure and volume. Additionally, the spring energy (of the combination of the pneumatic spring and the stiff spring) will be constant between cycles. As long as the device has an operating pressure requirement no higher than the minimum anticipated, the nail gun will perform identically over the entire operating range of the air compressor. 
         [0030]    Additionally, the power output of the gun may be altered simply by changing the pre-load on the spring. This can be accomplished by any one of known methods. For example, spacers can be placed between the spring and the rear spring housing. Alternatively, external adjustment can be achieved, for example, by use of a set screw used to raise or lower the spring tension by compressing or decompressing the stiff spring using known methods. 
         [0031]    The size of the faces of the differential piston can be varied in different embodiments. The diameter of the large side of the piston, plus the total travel length, determine the amount of compressed air which is available as energy by expanding this air (to do work). The diameter of the smaller side determines how much energy will be available to act as a pneumatic spring. This air spring functions to decrease the volume in the firing chamber side as the device fires. If the volume can remain constant, the pressure will also be constant, and the work accomplished will be roughly double the work which would be available if the air was allowed to simply expand, which is the typical method for air-powered equipment. This is a key advantage with the device and there is a substantial power increase over the existing method of simply allowing a set volume of compressed air to expand against a piston. 
         [0032]    In one embodiment of the invention, the volume of the firing chamber is set to about 50% of the total volume to be displaced. This produces a constant force on the item being driven for the first 50% of the item&#39;s travel and a standard decreasing force thereafter as the air expands logarithmically until the item is fully discharged. 
         [0033]    Referring to  FIG. 3  another embodiment of the invention is illustrated. According to this embodiment, a housing  52  is provided as a cylinder with two different diameters. A floating differential piston  54  is provided within the housing and separates the housing into a high pressure storage chamber  56  of the housing, and a firing chamber  58  of the housing. The differential piston has two piston faces, each with a different diameter corresponding to the different diameters of the housing. According to this embodiment, a first piston face  62  faces the high pressure storage chamber of the housing while a second piston face  64  faces the firing chamber of the housing. For this embodiment, the second piston face is larger than the first piston face. Pressurized air enters the high pressure storage chamber from a pressurized air source  66  through a hollow rod  68  with one or more lateral air passages  70 . A first spring  72  (not shown in full length in order to better illustrate other features of this embodiment) provides resistance against the first piston face, and may be a stiff steel spring. Proper seals are maintained through the use of seals such as O-rings  73 . For this embodiment, rather than an aperture with a restrictive orifice, a novel type of spring-loaded valve is provided that may be referred to as a “snap-action” or quick opening and quick closing valve  74  is located within the differential piston between the first and second piston faces. The snap-action valve is biased in an open position by a second spring  75 . 
         [0034]    A third spring  76  is anchored at a standard firing mechanism  77  in the firing chamber, and is biased against a stem  78  of the snap-action valve to assist in holding the snap-action valve in a fully open position as the system charges with compressed air. When the system is completely charged, the hollow rod abuts the valve, compressing the second spring and seating the valve within the orifice of the differential piston. In this position, the snap-action valve remains closed due to the air pressure on the first piston face until the system is fired. Once the system is fired, the snap-action valve opens again for the charge cycle. 
         [0035]    According to this embodiment, there is virtually no restriction in air flow from the pneumatic spring chamber into the firing chamber during the charging process. Additionally, there is virtually no transmittal of compressed air into the firing chamber from the pneumatic spring chamber during the firing process. With the exception of the operation of the-snap action valve, the device functions as indicated above. 
         [0036]    The differential piston and various embodiments of the present invention described allow the piston to respond almost instantaneously to an air charge, moving a set and predictable distance precisely when the design operating pressure for the system is achieved. Furthermore, the device acts as a precise regulator which provides the same pressure and volume of gas with each shot. Moreover, the elimination of the pressure regulator results in a device that is simpler to build and operate with fewer moving parts. Such a device can be used not only in various air guns such as pellet rifles or nail guns, but may also be used in other pneumatic devices such as pneumatic jack hammers.