Abstract:
A method and means of minimizing the effect of elastic valve recoil in impact applications, such as percussive drilling, where sliding spool valves used inside the percussive device are subject to poor positioning control due to elastic recoil effects experienced when the valve impacts a stroke limiting surface. The improved valve design reduces the reflected velocity of the valve by using either an energy damping material, or a valve assembly with internal damping built-in, to dissipate the compression stress wave produced during impact.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 12/364,600 filed Feb. 3, 2009 now U.S. Pat. No. 8,006,776, which is incorporated herein by reference. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to control of percussive hammer devices, such as pneumatic percussion drills and rock breakers. 
     A downhole pneumatic hammer is, in principle, a simple device consisting of a ported air feed conduit, more commonly known as a feed tube, check valve assembly above the feed tube to preventingress of wellbore fluids into the drill, a reciprocating piston, a case, a drill bit, and associated retaining hardware. The typical valveless device, for example, possesses on the order of 15 components. The reciprocation of the piston is accomplished by sequentially feeding high-pressure air to either the power chamber of the case (the volume that when pressurized moves the piston towards the bit shank) or return chamber of the case. The regulation of the air flow can be accomplished either by use of passages (e.g., slots, grooves, ports) machined into the feed tube, piston body, or hammer case; or a combination of active valving and porting through either the piston, the case, or an additional sleeve. 
     However, existing designs do not provide the most efficient use of the total air energy available because they have built-in inherent inefficiencies. The present invention greatly reduces these inefficiencies. 
     Impact applications, such as percussive drilling, that utilize sliding valves to control fluid flow (usually a gas) within the device are subject to control difficulties if the valve is not properly located relative to port positions during a cycle. Misalignments and mis-positionings of the valve can result in poor regulation of the device pressure chambers. Standard valve materials, such as steels or high strength plastics, are stiff and have very little internal damping, leading to predominantly elastic impact collisions in which almost all of the impact velocity of the component is preserved in rebound. 
     A typical configuration consists of an air feed conduit (tube), a reciprocating piston, and a spool valve within the piston. During operation, the air feed conduit is stationary, the piston reciprocates bi-directionally along the feed conduit axis, and the valve moves within the piston covering radial ports in the piston at different points in the cycle to regulate air flow that is used to control the piston&#39;s motion. In applications where rapid velocity reversals of the piston occur (e.g., hammer drilling), the valve within the piston tends to recoil elastically off the position-limiting surfaces of the piston. This recoil often causes the valve to unintentionally cover, or expose, the incorrect ports, leading to control or performance problems. 
     Against this background, the present invention was developed. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and means of minimizing the effect of elastic valve recoil in impact applications, such as percussive drilling, where sliding spool valves used inside the percussive device are subject to poor positioning control due to elastic recoil effects experienced when the valve impacts a stroke limiting surface. The improved valve design reduces the reflected velocity of the valve by using either an energy damping material, or a valve assembly with internal damping built-in, to dissipate the compression stress wave produced during impact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form part of the specification, illustrate various examples of the present invention and, together with the detailed description, serve to explain the principles of the invention. 
         FIG. 1A  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 1B  shows a schematic side view of the exterior of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 1C  shows an isometric, solid-shaded, cut-away view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 2  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 3  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 4  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 5  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 6  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 7  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 8  shows a schematic cross-section view of a standard sliding valve. 
         FIG. 9A  shows a schematic cross-section view of a reduced-impact sliding valve, according to the present invention. 
         FIG. 9B  shows a cross-section photomicrograph of a reticulated network of silicon carbide foam. 
         FIG. 10  shows a schematic cross-section view of another reduced-impact sliding valve, according to the present invention. 
         FIG. 11  shows a schematic cross-section view of another reduced-impact sliding valve, according to the present invention. 
         FIG. 12  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 13  shows a schematic cross-section view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
         FIG. 14  shows a schematic cross-section view of another pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is of a reduced-impact sliding feed tube pressure control valve for reciprocating hammer drills that is more efficient and produces more drilling power. Typically these are pneumatic (air) percussive drills, but could also use other motive fluids (such as water, steam or gas other than air). 
       FIG. 1A  shows a schematic cross-section side view of the present invention, which comprises outer casing  10 , reciprocating piston  12 , front end face  8 , rear end face  9 , air feed tube  14 , air feed slot  16  (two of them, 180 degrees apart), rear supply port side-hole  18 , rear supply port  20 , front supply port side-hole  22 , front supply port  24 , central piston bore  25 , return chamber  26  (also known as the forward/front chamber), power chamber  28  (also known as the rear chamber), sliding valve  30 , rear piston inner shoulder  32 , and front piston inner shoulder  34 . This device comprises a mechanical means of regulating the flow of air or other motive fluid to the power and return chambers of a percussive hammer device (e.g., hammer drill); although, in principle, this regulation scheme can be applied to any application where control over the reciprocation of a piston-like element is desired based on its stroke position. The device provides the ability to regulate the flow of air into both the power and return chamber. 
     The mechanical form of the regulating mechanism (i.e., valve  30 ) is a “spool” or a “sleeve” that is positioned between the piston  12  of the device and air distributor  14  (or “feed tube”, as it is called in downhole hammer drilling devices). The spool valve  30  acts to cover (partially, or fully) and, thereby isolate, the two side ports  18  and  22  that convey motive fluid to the device&#39;s rear (power) chamber  28  and forward (return) chamber  26 , respectively. 
     The position of the spool is controlled by the application of fluid pressure to the spool&#39;s exposed end faces  60  and  62 . End faces  60  and  62  can be rounded, as illustrated in  FIG. 1 , or square-ended, or chamfered, or slanted. The pressure is determined by controlled dimensioning of the sliding spool valve  30 , and controlled location of the porting (air feed slot  16 ) in the air distributor or “feed tube”  14 . The piston&#39;s side-holes  18  and  22  (ports perpendicular to the main axis) can be oversized, elongated slots. 
       FIG. 1B  shows a schematic side view of the exterior of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. Piston  12  has an elongated slot-shaped side-hole (port)  18  with axial length, B, and circumferential width, E, and full-radius ends. Sliding spool valve  30  can be seen through side-hole  18 . The other side-hole,  22 , is hidden in this view because it is located 180° around on the opposite side of piston  12 . In one embodiment, B can equal two times E. 
       FIG. 1C  shows an isometric, solid-shaded, cut-away view of a pneumatic hammer device with a reduced-impact sliding valve, according to the present invention. The part numbers match those of  FIGS. 1A and 1B . 
       FIG. 12  shows that, in one embodiment, the axial length, B, of elongated rear supply port side-hole  18  is greater than the axial length, A, of elongated front supply port side-hole  22 . For example, the axial length, B, of the rear supply port side-hole  18  can be 1.5 times longer than the axial length, A, of the front supply port side-hole  22 . The different lengths (i.e., A not equal to B), allows for asymmetric timing between the power and return cycles. Alternatively, A can equal B. Alternatively, A can equal 1.5 times E. Generally, the circumferential width (E) of the side-holes  18  and  22  are the same. 
     In  FIG. 12 , the spool valve  30  does not completely overlap the side-holes  18  and  22  at the ends of its travel, thereby permitting fluid flow around it, and, hence, pressure to be applied to the valve&#39;s end surfaces to move it when it is at its extreme limit positions. This reduces the impact forces. Valve  30  shuttles/slides back and forth in-between a pair of hard limit stops: rear piston internal shoulder  32 , and front piston internal shoulder  34 . 
       FIG. 13  shows that when valve  30  is at its rearward most position (limited by rear piston internal shoulder  32 ), the distance “C” is greater than zero. The gap distance “C” is determined by the position at which the rear piston internal shoulder  32  is located relative to the rearward-most extent of front supply port side-hole  22 . In one embodiment, the distance “C” can be about ⅓ of the distance “A” (see  FIG. 12 ). This means that valve  30  leaves about ⅓ of the area of side-hole  22  open/uncovered when valve  30  is at its rear limit position. With this configuration, valve  30  is not able to completely close and block airflow through front supply port side-hole  22 . This permits the air pressure to apply a forward-facing force to the valve&#39;s rearward facing end surface  60 , which changes the motion and timing of the valve  30  during a piston cycle. In other words, the purpose of the inner shoulder(s) is to provide a pathway for air to pressurize the valve&#39;s face(s) when it is seated. Without the gap under the valve, there is no way for air to get under the valve; unless it has already started to unseat. 
       FIG. 14  shows that, in another embodiment, when valve  30  is at its forward most position (limited by front piston internal shoulder  34 ), the distance “D” is greater than zero. In one embodiment, the distance “D” can be about ¼ of the distance “B” (see  FIG. 12 ). This means that valve  30  leaves about ¼ of the area of side-hole  18  open/uncovered when valve  30  is at its forward most limit position. In other words, valve  30  is not able to completely close and block airflow through rear supply port side-hole  18 . This permits the air pressure to apply a rearward-facing force to the valve&#39;s forward facing end surface  62 , which changes the motion and timing of the valve  30  during a piston cycle. The gap distance “D” is determined by the position at which the front piston internal shoulder  34  is located relative to the forward-most extent of rear supply port side-hole  18 . 
     A complete cycle is shown in  FIGS. 2-7 .  FIG. 2  shows the minimum piston position during power stroke—pressure inside rear supply port  20  and rear supply port side-hole  18  forces spool  30  to cover most of forward supply port side-hole  22 , partially blocking front supply port  24 .  FIG. 3  shows the piston&#39;s middle position during power stroke; air supply to rear chamber  28  continues.  FIG. 4  shows beginning rear vent during power stoke—the spool is still blocking forward supply port; and rear chamber begins to vent.  FIG. 5  shows to spool shifted forward up against the front piston shoulder  34 , prior to piston impact at the top (e.g., impacting on a drill bit  6 ). In  FIG. 5 , the feed tube slot  16  begins to supply the non-overlapped area of the forward chamber supply port  24 , which shifts the spool forward, along with impact, and allows full pressurization of forward chamber  26 . Note that the rear supply port  20  is simultaneously partially blocked, changing the point in the stroke at which the feed tube will connect with this port.  FIG. 6  shows continuing to supply air to the forward chamber  26  on initiation of return stroke. Finally,  FIG. 7  shows beginning supply air to the rear supply port  20  on return stroke; when the feed tube slot passes the spool, the spool shifts back to the rear (limited by the rear piston inner shoulder  32 ), to supply the rear chamber  28 , and to partially cover the front chamber supply port  22  and  24 . Note: this occurs closer to the rear than on the power stroke, because of the shifted spool position. 
     The intention of this approach is threefold: (1) to prevent pressurization of forward chamber  26  during power stoke; (2) to increase length of pressurization of rear chamber  28  during power stroke; and (3) to decrease length of pressurization of rear chamber  28  during power stroke (to increase overall stroke length). 
     The spool valve  30  can be inserted after counter-boring the rear side of the piston, and installing an end cap tube to create the confining surface. 
     Impact applications, such as percussive drilling, that utilize sliding valves to control fluid flow within the device are subject to control difficulties if the valve is not properly located relative to port positions during a cycle. Misalignments and mis-positionings can result in premature fatigue damage and breakage of the valve, control tube, or other parts inside the drill. Standard valve materials, such as steels or high strength plastics (see, e.g.  FIG. 8 ), are stiff and have very little internal damping, leading to predominantly elastic impact collisions in which almost all of the impact velocity of the component is preserved in rebound. 
     A reduced-impact spool valve, according to the present invention, involves the use of either energy damping material or an energy damping valve assembly to reduce rebound velocity (and, hence, impact forces). Three examples of improved designs are given. 
     One design for reducing valve recoil is to fabricate the valve from a material with high internal energy damping (see  FIG. 9A ), such as a metallic or ceramic foam core  40  (e.g., Aluminum or SiC reticulated foam made by near-net shape chemical vapor infiltration techniques, see  FIG. 9B ), with or without a solid skin. 
     A second design, shown in  FIG. 10 , comprises an external shell  42  filled with small particles or pellets/balls  44 ; such that when impact occurs, the impact stress wave propagating through the interior will be dissipated by interaction between the particles. Additionally, the interior of shell  42  can also be filled with a fluid, such as a high viscosity oil, to provide further damping. 
     A third design, shown in  FIG. 11 , comprises an external shell  42  filled with a high viscosity, damping fluid  46  (e.g., an oil) and a freely-moving sliding internal sleeve  48  disposed inside of the shell  42 . In this embodiment, recoil reduction is accomplished through: (a) energy dissipation/damping between the sliding sleeve  48  and viscous oil  46 , and (b) through using the internal sleeve  48  to provide a counter-impact (delayed impact) to the external shell  42 , after the shell  42  strikes either of the piston&#39;s internal shoulder stops  32  or  34  (see  FIG. 1A ). With respect to the latter, internal sleeve  48  and external shell  42  both move in the same direction, with the same velocity, prior to valve impact. After valve impact, the external shell&#39;s velocity is reversed, while the sliding internal sleeve  48  continues to move in the same direction; until it impacts the shell&#39;s end. Because the external shell  42  and internal sleeve  48  have momentum values of opposite sign, the net momentum of the entire two-part assembly is reduced, and the velocity of the assembly  30  after impact is significantly reduced. In this sense, the internal sleeve  48  acts as a counter-weight. Internal sleeve  48  can be made of a heavier (more dense) material, such as steel. 
     Alternatively, in  FIG. 11 , internal sleeve  48  can have longitudinal or circumferential or spiral-running vanes, ribs, grooves, or knurled surface on the outer or inner surfaces (or both), to modify the friction between sleeve  28  and damping fluid  46 . Alternatively, sleeve  48  can have a pattern of small holes drilled through the sleeve to also affect the friction. Alternatively, sleeve  48  can be made of a porous ceramic or metal material (as described above) to also affect the friction. 
     The scope of the invention is defined by the claims appended hereto.