Patent Publication Number: US-8109447-B2

Title: Intrinsically safe valve for a combustion spray gun and a method of operation

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to systems and methods for applying coatings, and more specifically to an intrinsically safe, multi-ported valve for controlling a hand-held combustion spray gun. 
     2. Discussion of Background Information 
     Combustion wire spray systems and processes are known for providing coatings on objects for various purposes. A typical combustion wire spray system includes a hand-held combustion spray gun that mixes oxygen, fuel gas, and air to melt a metal wire and spray the molten metal as a coating onto a target object. For example, a conventional combustion spray gun has a set of drive rolls powered by an air turbine that draw one or more metal wires into the gun. Oxygen and fuel gas are mixed in the gun and ignited to create a flame. Common fuel gases include acetylene, hydrogen, propane, methylacetylene-propadiene, or natural gas. Compressed air is used to shape and accelerate the flame at an air cap which includes an outlet nozzle. The metal wire is fed into the flame where it is melted and atomized. In this manner, molten droplets of the metal are propelled toward the object to be coated. 
     The sprayed-on molten metal solidifies on the object to form a coating that provides the surface of the object with one or more performance-enhancing characteristics. For example, combustion wire spray coating may be used for many applications, including but not limited to: corrosion protection, wear protection, surface restoration, electrical/thermal conductivity, decorative surfaces, etc. 
     Conventional air-powered combustion spray guns typically have a valve core for controlling the flow of various gasses, e.g., oxygen, fuel gas, and compressed air, through the gun. The valve core can be a rotary element that has various ports and passageways that, depending on the rotational position of the valve core, selectively control flow of the different gasses within the gun. For example, the valve core may be ported such that a plurality of gasses can be controlled simultaneously to achieve controlled flows for off, idle/ignite, and full flow operating conditions. Particularly, by rotating the valve core to a predefined position, ports within the valve core line up with gas passages in other parts of the gun. The diameters of the various ports in the valve core dictate the flow of each gas by acting as a flow-regulating orifice depending upon the supply pressure of each gas. 
     Typically, the position of the valve core is set by the operator using a valve core handle that extends from the combustion spray gun. When the operator rotates the valve core handle to a predetermined position corresponding to an operating condition, e.g., off, idle/ignite, full flow, etc., the valve core is positioned within the gun to provide a precise mixture of gasses for that operating condition. Generally, a spring-loaded detent mechanism maintains the valve core in each predetermined position. In this manner, the operator can set the operating condition of the gun using the valve core handle, and then release the valve core handle and use both hands for controlling the motion of the gun. Because the valve core stays in position once set, the gun will continue to emit a high velocity flame and molten metal until the operator turns the valve core handle to the off position. This can present a safety hazard, for example, in the case of a dropped gun that is operating at full flow. 
     Conventional air-powered combustion spray guns, such as those described above, do not have a mechanism for automatically stopping gun operation in case of an operator accident. Many electrically powered devices, from robots to hand-held power tools, have a safety “deadman” switch that cuts off electrical power to the device or tool when the switch is released by an operator. However, since hand-held air-powered combustion spray guns do not utilize electrical power, electrical deadman switches are not applicable to such combustion spray guns. 
     Moreover, known electrical emergency cutoff devices only provide for an on and off position. Hand-held air-powered combustion guns, on the other hand, require multiple position settings for different gas flow states. Additionally, since hand-held air-powered combustions guns should be lightweight to minimize operator fatigue, adding extra safety valves to an existing gun is not desirable. 
     Accordingly, there exists a need in the art to overcome the above-noted deficiencies. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings. In accordance with a first aspect of the invention, there is an apparatus configured to provide a safety mechanism for a combustion spray gun. The apparatus includes a torsion element rotatable relative to a housing of the combustion spray gun to a charged position. The apparatus also includes a biasing element applying a force to the torsion element, which force urges the torsion element to move a valve core to an off position. The apparatus further includes an engagement mechanism configured to selectively engage and hold the torsion element in the charged position. 
     In an embodiment, the engagement mechanism holds the torsion element in the charged position when at least a predetermined force is applied to the engagement mechanism, whereas the engagement mechanism disengages from the torsion element when less than the predetermined force is applied to the engagement mechanism. Also, the valve core may be rotatable relative to the torsion element and the housing while the torsion element is held in the charged position by the engagement mechanism. 
     In a particular implementation, the torsion element has an engagement surface, and the engagement mechanism has a pawl that is structured and arranged to engage the engagement surface. The engagement surface may be formed in an indentation at an outer portion of the torsion element. The apparatus may additionally include a trigger fixedly connected to the pawl, wherein the trigger is structured to move the pawl relative to the torsion element. Application of a trigger force to the trigger that is greater than or equal to a predetermined force maintains the pawl in engagement with the engagement surface and prevents the biasing element from rotating the torsion element. 
     The biasing element may be a spring that biases the torsion element to rotate relative to the housing. Also, the combustion spray gun may be a hand-held, air powered combustion spray gun. 
     In accordance with another aspect of the invention, there is a combustion spray gun having a gas head assembly including a housing and a valve core rotatably disposed within the housing. The valve core is selectively positionable between an off position and a full flow position. The gun also includes a biasing element positionable to bias the valve core toward the off position, and an engagement mechanism configured to selectively counteract a biasing force associated with the biasing element. 
     In an embodiment, the gun additionally has a handle and a trigger moveable relative to the handle. The engagement mechanism counteracts the biasing force when at least a predetermined force is applied to the trigger, whereas the biasing force is transmitted to the valve core when less than the predetermined force is applied to the trigger. 
     In another embodiment, the gun includes a torsion element rotatably connected to the housing. The torsion element selectively transmits the biasing force to the valve core. In a further embodiment, the engagement mechanism counteracts the biasing force by engaging the torsion element in a charged position and holding the torsion element in the charged position. In an even further embodiment, the engagement mechanism holds the torsion element in the charged position when at least a predetermined force is applied to a trigger of the engagement mechanism, and the engagement mechanism disengages from the torsion element when less than the predetermined force is applied to the trigger of the engagement mechanism. The valve core may be rotatable relative to the torsion element and the housing while the torsion element is held in the charged position by the engagement mechanism. 
     In accordance with another aspect of the invention, there is a method of operating a combustion spray gun. The method includes charging a torsion element into a charged position and releasably grasping a trigger to selectively maintain the torsion element in the charged position. The method also includes adjusting a gas flow to a nozzle while the torsion element is selectively maintained in the charged position and cutting the gas flow to the nozzle when the trigger is released. 
     In an embodiment, the charging of the torsion element includes rotating a valve core from an off position to a flow position against a force of a biasing element, and the cutting of the gas flow comprises the torsion element moving the valve core to an off position under the force of the biasing element. 
     The method may additionally include igniting the gas flow and feeding a metal wire into the ignited gas flow. In a particular embodiment, the releasably grasping of the trigger inserts a pawl into a notch formed in the torsion element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIGS. 1-3 ,  4 A- 4 F,  5 A- 5 F,  6 A- 6 F, and  7 - 12  show aspects of a safety mechanism for use with a combustion spray gun in accordance with aspects of the invention; 
         FIG. 13  shows a combustion spray gun equipped with a safety mechanism in accordance with aspects of the invention; 
         FIGS. 14-16  show aspects of an alternate embodiment of a safety mechanism for use with a combustion spray gun in accordance with aspects of the invention; and 
         FIG. 17  shows aspects of an alternate embodiment of a safety mechanism for use with a combustion spray gun in accordance with aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. 
     The invention relates generally to systems and methods for applying coatings, and more specifically to an intrinsically safe, multi-ported valve for controlling a hand-held combustion spray gun. In accordance with aspects of the invention, a safety mechanism is provided for a hand-held, air-powered combustion spray gun. In an embodiment, the safety mechanism operates as a deadman switch that automatically halts operation of the combustion spray gun in the event that the operator releases a trigger. 
     In a particular embodiment, the safety mechanism includes a torsion ring that is urged by a biasing element to move the valve core to the off position. The torsion ring may be rotated by the operator to a charged position and held in the charged position by application of force to a trigger located near the handle of the combustion spray gun. While the torsion ring is held in the charged position, the operator is free to rotate the valve core to any desired operational position, including, for example, full flow, idle/ignite, and off. However, when the operator releases the trigger, the biasing element automatically shuts off the flow of oxygen and fuel gas by rotating the valve core to the off position. In this manner, implementations of the invention provide a deadman switch for a hand-held, air-powered combustion spray gun. 
     Embodiments of the invention are described herein with respect to a hand-held combustion wire spray gun. However, the invention is not limited to use with a combustion wire spray gun. Thus, implementations of the invention may be utilized with any hand-held, gas-powered device in which the rotational position of a rotary valve controls operation of the device, including but not limited to: combustion powder spray guns, high velocity oxygen fuel (HVOF) spray systems, welding systems, etc. 
       FIG. 1  shows a cutaway view of an embodiment of a gas head assembly  10  of a combustion spray gun in accordance with aspects of the invention. More specifically, the gas head assembly  10  includes a housing  15  having various ports and passageways  20  for routing oxygen, fuel gas, and compressed air through the combustion spray gun. The gas head assembly  10  also includes a safety mechanism  25  for automatically shutting off the flow of gasses through the combustion spray gun under certain circumstances. 
     In an embodiment, the safety mechanism  25  includes a torsion ring  30  that is rotatably aligned with a valve core  40  and rotatable relative to the housing  15  about axis “A”. With reference to  FIGS. 1 and 2 , the torsion ring  30  has a hole  35  that accommodates a valve core  40 . The valve core  40  is rotatable relative to both the housing  15  and the torsion ring  30  about axis “A”. Although not depicted in  FIGS. 1 and 2 , the valve core  40  includes suitable ports for precisely controlling the flow of gasses within the gas head assembly  10  of the combustion spray gun. For example, as is known such that further explanation is not believed necessary, ports may be provided in the valve core  40  and housing  15  to define flow conditions for oxygen, fuel gas, and compressed air for at least three operating conditions including, but not limited to, off, idle/ignite, and full flow conditions. 
     As depicted in  FIG. 2 , a valve core handle  45  is connected to the valve core  40 . The valve core handle  45  permits an operator to selectively set the rotational position of the valve core  40 . Although the valve core handle is shown at an end  50  of the housing  15  opposite the torsion ring  30 , the invention is not limited to this configuration. Rather, the valve core handle  45  and torsion ring  30  may be located in any desired locations relative to the housing  15 . For example, the valve core handle can be located at any circumferential position around the torsion ring  30 , including on a same side of the housing  15  as the torsion ring  30 . 
     In a particular embodiment, an end cap  55  is fixedly connected to the valve core  40 , such that the valve core  40 , valve core handle  45 , and end cap  55  rotate together as a single unit. The end cap  55  may be affixed to the valve core  40  using a screw  60  having a head  65  that is countersunk into a cavity  70  of the end cap  55 . However, the invention is not limited to this configuration, and other connecting arrangements, including press/friction fit, splines, adhesive, etc., may be used to affix the end cap  55  to the valve core  40 . 
     Still referring to  FIG. 2 , a pin  75  has a first end  80  fixedly held in a hole  85  in the end cap  55 . A second end  90  of the pin  75  is slidingly received in a slot  95  in the torsion ring  30 . As depicted in  FIG. 1 , the slot  95  has an arcuate shape having a first slot end  100  and a second slot end  105 . By virtue of the first end  80  of the pin  75  being fixed to the valve core  40  via the end cap  55  and the second end  90  of the pin  75  being slidably engaged in the slot  95 , the slot  95  defines a range of relative rotational movement that can occur between the valve core  40  and the torsion ring  30 . 
     In one implementation, the safety mechanism  25  further includes a biasing element  110  that biases the torsion ring  30  to rotate relative to the housing  15  in a first direction, e.g., clockwise as depicted by arrow “B”. In a particular embodiment, the biasing element  110  is composed of a torsion spring having a first spring end  115  engaged in a first anchor hole  126  in the torsion ring  30  and a second spring end  125  engaged in a second anchor hole  127  in the housing  15 . The biasing element  110  can be any desired type of spring suitable to urge relative rotational movement between the torsion ring  30  and the housing  15 . Preferably, the biasing element  110  is a constant force torsion spring. However, the invention is not limited to a constant force torsion spring, but rather, any suitable spring, including but not limited to a plain torsion spring, round spring, etc., can be used. 
     Due to the interaction between the slot  95  and the pin  75 , the biasing element  110  also biases the valve core  40  to rotate relative to the housing  15  in the first direction “B” under certain conditions. More specifically, when the biasing element  110  causes the torsion ring  30  to rotate relative to the housing  15  in the first direction “B”, the first slot end  100  abuts and pushes against the pin  75 , which causes the valve core  40  to rotate with the torsion ring  30  relative to the housing  15  in the first direction “B”. At least one of the valve core  40  and the housing  15  may be provided with a mechanical stop (not shown) that prevents rotation of the valve core  40  in the first direction “B” beyond a predetermined position. In a preferred embodiment, the predetermined position corresponds to the off operating positioning of the combustion spray gun, e.g., where oxygen and fuel gas are prevented from flowing through the gas head assembly  10 . 
     Although the biasing element  110  urges the torsion ring  30  in the first direction “B”, the valve core handle  45  may be used to rotate the valve core  40  and torsion ring  30  in a second direction opposite the first direction “B”, e.g., counterclockwise as depicted by arrow “C”. For example, an operator may apply sufficient force to the valve core handle  45  to overcome the force of the biasing element  110 , such that the pin  75  abuts and pushes against the first slot end  100 , thereby causing the valve core  40  and torsion ring  30  to rotate together relative to the housing  15  in the second direction “C”. 
     The amount of force required to overcome the biasing element  110  may be set at any desired value through careful selection of the biasing element  110 . In an embodiment, about 17 to 18 inch-pounds of force is required at the valve core handle  45  to overcome the biasing element  110  and cause rotation of the valve core  40  and torsion ring  30  to rotate together relative to the housing  15  in the second direction “C”. However, the invention is not limited to this value, and any desired force may be set through selection of materials and geometries of the various parts. 
     Referring to  FIG. 1 , the safety mechanism  25  includes a pawl  130  that can be brought into engagement with a contact surface  117  of an indentation  120  formed in the torsion ring  30  when the valve core handle  45  is used to rotate the torsion ring  30  in the direction “C” to a predetermined position, e.g., the charged position. In an embodiment, the pawl  130  is fixedly connected to an axle  135  by, for example, a set screw  140  or other suitable fastening structure. The axle  135  is rotationally disposed within a holder  145  that is fixed to the housing  15  by a bracket  147 . In this manner, the pawl  130  can be rotated toward and away from the torsion ring  30 . Particularly, by way of the axle  135  and holder  145 , the pawl  130  can be rotated into and out of engagement with the indentation  120  in the torsion ring  30 . 
     In a further embodiment, the safety mechanism  25  also includes a lever  150  fixedly attached to the axle  135  and a trigger  155  fixedly attached to the lever  150 . When the gas head assembly  10  is included as part of a combustion spray gun, as partially depicted in  FIG. 3 , the trigger  155  may be located sufficiently close to a handle  160  of the combustion spray gun to permit an operator of the combustion spray gun to hold the handle  160  and manipulate the trigger  155  with a single hand. Although the trigger  155  is depicted in  FIGS. 1 and 3  as having a cylindrical or rod shape, the invention is not limited to this shape, and any suitable shape may be used for the trigger  155 . 
     As described above, the biasing element  110  biases the valve core  40  to an off position in which air, oxygen and fuel gas cannot flow through the gas head assembly  10 . The torsion ring  30  is configured relative to the valve core  40  and the housing  15  such that the indentation  120  is located away from the pawl  130  when the valve core  40  is in the off position. 
     To bring the combustion spray gun into a working mode, e.g., idle or full flow, in which air, oxygen and fuel gas flow through the gas head assembly  10 , an operator applies sufficient force to the valve core handle  45  in the direction “C” to overcome the biasing element  110 . As described above, such a force causes rotation of both the valve core  40  and torsion ring  30  in the direction “C” relative to the housing  15 . This rotation of the torsion ring  30  causes the indentation  120  to be rotated toward the pawl  130 . When the indentation  120  is rotated into alignment with the pawl  130 , the operator may pull the trigger  155  toward the handle  160 , which causes the axle  135  to rotate within holder  145 , which in turn advances the pawl  130  into engagement with the contact surface  117  of the indentation  120 . 
     In accordance with an aspect of the invention, the safety mechanism  25  is configured such that a predetermined force applied to the trigger  155  will maintain the pawl  130  in engagement with the indentation  120 , thereby holding the torsion ring  30  stationary in the charged position. Particularly, when the pawl  130  is engaged with the indentation  120 , rotation of the torsion ring  30  in the direction “B” may be prevented by applying a sufficient force to the trigger  155 . As described in greater detail below, when the torsion ring  30  is held fixed in the charged position by the trigger  155  and pawl  130 , the valve core  40  may be rotated relative to the torsion ring  30  and housing  15  to any desired operational position of the combustion spray gun. 
     On the other hand, when less than the predetermined force is applied to the trigger  155 , the urging force of the biasing element  110  in conjunction with the geometry of the surfaces of the indentation  120  and pawl  130  causes the torsion ring  30  to rotate and disengage the pawl  130 . Therefore, when the trigger  155  is released by the operator, the biasing element  110  rotates the torsion ring  30  in the direction “B”, which in turn rotates the valve core  40  to the off position. In this manner, the safety mechanism  25  operates as a deadman switch for the combustion spray gun. 
     The force of the biasing element  110  and the geometry of the pawl  130  and contact surface  117 , among other things, will affect what amount of force at the trigger  155  is required to maintain the pawl  130  in engagement with the indentation  120 . Accordingly, the predetermined force may be tailored to any desired value through selection of materials and geometries of parts. In a preferred embodiment, the predetermined force is relatively small in order to reduce operator fatigue. For example, the predetermined force may be in a range of about 1 ounce to 5 pounds, preferably in a range of about 3 ounces to 4 ounces. In a further embodiment, the contact surface  117  and a corresponding surface of the pawl  130  are arranged at an angle in a range of about 10° to 30° relative to a radial axis of the torsion ring  30 , with a preferred range of about 15° to 20°, and particularly preferably at an angle of about 17°. By using surfaces arranged at this angle, the urging force of the biasing element  110  causes the contact surface  117  to push the pawl  130  out of the indentation when the trigger  155  is released. However, the invention is not limited to this geometry, and any desired geometry may be utilized within the scope of the invention. 
     In a particularly advantageous embodiment, the arcuate slot  95  and valve core  40  are configured such that the valve core  40  may be rotated to any desired operational position while the torsion ring  30  is held fixed in the charged position. As should be apparent from  FIGS. 1 and 2  and the foregoing description, the valve core  40  is free to rotate relative to the torsion ring  30  and the housing  15  while the torsion ring  30  is held stationary by engagement of the pawl  130  and indentation  120 . Particularly, the pin  75  that is connected to the valve core  40  by way of the end cap  55  is free to slide within the arcuate slot  95  while the pawl  130  prevents rotation of the torsion ring  30 . Accordingly, by appropriately sizing and locating the arcuate slot  95  and the various ports within the valve core  40  and housing  15 , the valve core  40  may be rotated via the valve core handle  45  to any desired operational position, e.g., off, idle/ignite, full flow, when the torsion ring  30  is held fixed by the pawl  130 . 
     For example, when the pawl  130  is in engagement with the indentation  120 , a full flow position of the valve core  40  may correspond to a position in which the pin  75  is abutting the first slot end  100 , an off position of the valve core  40  may correspond to a position in which the pin  75  is abutting to the second slot end  105 , and an idle/ignite position of the valve core  40  may correspond to a position in which the pin  75  is at a substantial midpoint between the first slot end  100  and second slot end  105 . It is noted that the invention is not limited to these operational positions of the valve core  40 . Instead, any desired number of operational positions of the valve core  40  may be defined at any desired positions of the pin  75  within the slot  95  when the pawl  130  is in engagement with the indentation  120 . Additionally, one or more detent mechanisms (not shown) may be used within the housing  15  to hold the valve core  40  in the respective operational positions. Of course, when detent mechanisms are utilized, the biasing element  110  should be configured to provide sufficient rotational force to overcome the detent mechanisms when the trigger  155  is released. 
     In one implementation, the torsion ring  30  is made of plastic or aluminum, the pawl  130  is made of steel, for example hardened steel, and the trigger  155  is made of aluminum. Aluminum parts may be anodized. However, the invention is not limited to these materials, and the elements and parts described herein may be constructed of any suitable materials while remaining within the scope of the invention. 
     In another implementation, wear pads  165  are provided on at least one of the pawl  130  and the engagement surface  117  to reduce wear on these parts. The wear pads  165  may be made of hardened steel, or any other suitable material. 
     As described herein, the gas head assembly  10  and safety mechanism  25  may be used in combination with a hand-held, air-powered combustion spray gun. In such an implementation, the safety mechanism  25  biases the combustion spray gun to a default off state. In an exemplary method of operation, an operator may grasp the handle  160  of the combustion spray gun, in which the valve core  40  is in the default off position, and apply a force to the valve core handle  45  in the direction “C”. When the force applied to the valve core handle  45  overcomes the biasing element  110 , the valve core  40  and torsion ring  30  rotate relative to the housing  15  in the direction “C” until a stopping point is reached where a mechanical stop prevents further rotation of the valve core  40  and torsion ring  30  relative to the housing  15 . At this point, the biasing element is “charged” and the torsion ring is in the charged position. At the charged position, the indentation  120  is substantially aligned with the pawl  130 , such that the operator may move the trigger  155  in an appropriate direction and with sufficient force to bring the pawl  130  into engagement with the indentation  120 . 
     In an embodiment, the valve core  40  is in the full flow operational position when torsion ring  30  is in the charged position and the pin  75  abuts the first slot surface  100 . Thus, by rotating the valve core  40  and torsion ring  30  relative to the housing  15  in the direction “C” to the stopping point, the operator permits air, oxygen and fuel gas to flow at full flow rates through the gas head assembly  10 . Full flow of the gases before ignition serves to purge the gas lines, which is beneficial for avoiding backfires. 
     After a sufficient amount of purge time at the full flow position, and while maintaining sufficient force on the trigger  155  to prevent rotation of the torsion ring  30 , the operator rotates the valve core handle  45  in the direction “B” until the valve core  40  is located in the idle/ignite position. In the idle/ignite position, the valve core  40  permits a reduced amount of oxygen and fuel gas to flow through the gas head assembly  10 . In this manner, the operator may ignite the gas mixture of the combustion spray gun in a controlled manner. 
     After igniting the gas mixture, and while maintaining sufficient force on the trigger  155  to prevent rotation of the torsion ring  30 , the operator rotates the valve core handle  45  in the direction “C” until the valve core  40  is located again at a desired position, e.g., the full flow position. At the full flow position, the operator may use the combustion spray gun in a conventional manner to apply a coating to an object. That is to say, metal wire may be fed into the ignited gas mixture such that the metal is melted, atomized, and propelled out of a nozzle of the gun. At any time while the operator is maintaining sufficient force on the trigger  155  to prevent rotation of the torsion ring  30 , the operator may move the valve core handle  45  to any desired position defined within the slot  95  in order to reposition the valve core  40  in any desired operational position, e.g., full flow, idle/start, and off. 
     However, should the operator release the trigger  155 , such as, for example, by intentionally releasing the trigger, accidentally dropping the gun, etc., then the biasing element  110  automatically causes the valve core  40  to rotate in the direction “B” to the off position where air, oxygen and fuel gas are prevented from passing through the gas head assembly  10  and the gun is essentially shut off. In this manner, the safety mechanism  25 , which includes the torsion ring  30 , biasing element  110 , pawl  130 , and trigger  155 , acts as a deadman switch for automatically shutting off a hand-held, air-powered combustion spray apparatus under certain circumstances. Beneficially, the safety mechanism  25  does not interfere with the normal operation of the combustion spray gun, but rather, once engaged, permits the combustion spray gun to operate at any desired operational state. 
       FIGS. 4A-4F ,  5 A- 5 F, and  6 A- 6 F show various views of another embodiment of the invention, in which like reference numerals represent elements already described herein. More specifically,  FIGS. 4A-4F  show a gas head assembly  10  having a housing  15  and a valve core handle  45 . A safety mechanism  25  in accordance with an aspect of the invention includes a torsion ring  30 , pawl  130 , axle  135 , lever  150 , and trigger  155 . As depicted in  FIGS. 4A and 4E , the pawl  130  is disengaged from the torsion ring  30 . Accordingly, the valve core (not visible in  FIGS. 4A-4F ) is depicted as being in the default off position by virtue of the torsion ring  30 , end cap  55 , and screw  60 . 
       FIGS. 5A-5F  depict the apparatus where the valve core handle  45 , and consequently the valve core and end cap  55  are arranged at the full flow position. Also in  FIGS. 5A-5F , the torsion ring  30  is shown in the charged position where the indentation  120  is aligned with the pawl  130 . Additionally,  FIGS. 5A and 5E  depict where the trigger  155  has been moved in the direction “D”, which has caused the pawl  130  to come into engagement with the indentation  120  of the torsion ring  30 . 
     The state shown in  FIGS. 5A-5F  may correspond to, for example, the situation where an operator has rotated the valve core handle  45  from the default off position to move the torsion ring  30  into the charged position, and then grasped the trigger  155  to bring the pawl  130  into engagement with the indentation  120 . The torsion ring  30  will remain fixed relative to the housing  15  so long as the operator maintains sufficient force on the trigger  155 . While the torsion ring  30  is held stationary relative to the housing, the operator may move the valve core handle  45  to any desired position within the confines of the arcuate slot (not shown). However, should the operator release the trigger, then the biasing element will rotate the torsion ring  30  relative to the housing  15 , which will force the valve core  40  to the off position. 
       FIGS. 6A-6F  depict the apparatus where the torsion ring  30  is held in the charged position by the pawl  130  and the trigger  155 , while the valve core handle  45  is arranged in the idle/start position. As described above with respect to  FIGS. 1-3 , the valve core handle  45  is free to rotate in either direction “B” or direction “C” when the torsion ring  30  is held fixed in the charged position by the pawl  130  and trigger  155 . However, when the trigger  155  is released, the biasing element (not visible in  FIGS. 4-6 ) urges the torsion ring  30 , and consequently the valve core, to the off position. 
       FIGS. 7-12  depict different views of an embodiment of the invention in which like reference numerals represent elements already described herein. More specifically,  FIGS. 7-12  show various views of a gas head assembly  10  including a housing  15 , valve core  40 , and valve core handle  45 . The apparatus also has a safety mechanism  25  that includes a torsion ring  30 , biasing element  110 , pawl  130 , axle  135 , lever  150 , and trigger  155 . An end cap  55  is attached to the valve core  40  via a screw  60 , and a slot  95  defines a range of relative rotational movement between the torsion ring  30  and the valve core  40 . 
     Also depicted in  FIGS. 7-12  is an optional cover  200 . In an embodiment, the cover  200  includes a first portion  202  that covers and protects the torsion ring  30  and end cap  55 . The first portion  202  may be substantially cylindrical, although the invention is not limited to this shape and any desired shape may be used. In a further embodiment, the cover  200  includes a second portion  203  that at least partially covers and protects the pawl  130 . The first portion  202  and second portion  203  may be integral, or may be removably connected to each other. In a particular embodiment, the cover  200  is removably connected to the housing  15  by at least one mechanical fastener, such as, but not limited to, at least one screw  204 . 
     Additionally shown in  FIGS. 7-12  are inlet ports  205 ,  210 , and  215  where oxygen, fuel gas, and compressed air may be input into the gas head assembly  10 . Also shown are numerous internal ports  220   a ,  220   b ,  200   c , etc., in the valve core  40  and numerous internal ports  230   a ,  230   b , etc., in the housing  15  for routing oxygen, fuel gas, and compressed air through the gas head assembly  10 . In one embodiment, the housing  15  includes a flange  240  or other mounting structure for connecting an air cap of the combustion spray gun. 
       FIG. 13  shows a combustion spray gun  250  in accordance with aspects of the invention. In an embodiment, the combustion spray gun  250  includes a gas head assembly  10  and safety mechanism  25  as described herein. For example, the gas head assembly  10  includes a housing  15  having inlets  205 ,  210 ,  215  for oxygen, fuel gas, and process air. Portions of the safety mechanism  25  are contained within the cover  200 , which is removably connected to the housing  15  by screws  204 . Valve core handle  45  extends from one side of the housing  15  for adjusting the position of the valve core. 
     In an embodiment, the combustion spray gun  250  includes a handle  160 . The trigger  155  extends near the handle  160  such that an operator can manipulate the trigger  155  while holding the combustion spray gun  250  via the handle  160 . The combustion spray gun  250  may also include an air cap  255  that functions to mix the oxygen and fuel gas. 
     In a further embodiment, the combustion spray gun  250  includes an air turbine  260  that is powered by the compressed air. The air turbine  260  drives an internal wire drive roll, via a reduction gearing system, to draw a metal wire into the wire inlet  265  and move the metal wire into the air cap  255  where the metal is melted, atomized, and entrained in the flow of gasses exiting the outlet nozzle  270 . 
       FIGS. 14-16  show views of another embodiment of a safety mechanism for a hand-held combustion spray gun according to an implementation of the invention. More specifically,  FIGS. 14-16  depict a gas head assembly  310  having a housing  315  and valve core handle  345 , which may be similar to gas head assembly  10 , housing  15  and valve core handle  45  described with respect to  FIG. 1 . The gas head assembly  310  may also include a valve core  340  that operates to control flow of oxygen, fuel gas, and compressed air using ports and rotational positions, similar to the valve core  40  described with respect to  FIG. 1 . 
     In contrast to the embodiment shown in  FIG. 1 , the valve core  340  includes a plurality of engagement surfaces  370   a ,  370   b ,  370   c  that are selectively engagable with a pawl  375  attached to a trigger  385  by a linkage  380 . In a particular embodiment, each respective engagement surface  370   a ,  370   b ,  370   c  corresponds to a predetermined operational position of the valve core  340 . Additionally, a biasing element  365  is connected between the valve core  340  and the housing  315  to urge the valve core  340  toward the off position, e.g., represented by valve core handle  345  position “G”. The biasing element  365  may be, for example, a torsion spring similar to that described above with respect to  FIGS. 1 and 2 . However, there is no torsion ring in the embodiment shown in  FIGS. 14-16 . Instead, the biasing element  365  acts directly on the valve core  340  to urge the valve core to rotate relative to the housing at all times. Alternatively, a torsion ring that is fixed to the valve core  340  may be used, where the biasing element  365  is arranged between the torsion ring and the housing  315  and there is no relative movement between the torsion ring and the valve core  340 . 
     In using the apparatus shown in  FIGS. 14-16 , an operator holds the combustion spray gun by the handle  390  and moves the valve core handle  345  to any desired position, such as, for example, the full flow position “H”. When the valve core handle  345  is set in a predetermined operational position, the operator grasps the trigger  385  to bring the pawl  375  into engagement with a respective one of the engagement surfaces  370   a ,  370   b ,  370   c  that corresponds to the selected operational position of the valve core  340  and valve core handle  345 . 
     To move the valve core  340  between operational positions, the operator grasps the valve core handle  345 , releases the trigger  385 , rotates the valve core handle  345  to the new position, and then grasps the trigger again to bring the pawl  375  into engagement with another one of the engagement surfaces  370   a ,  370   b ,  370   c . If the operator releases the trigger for whatever reason without simultaneously controlling the valve core handle  345 , then the biasing element  365  causes the valve core  340  to automatically rotate to the off position. In this manner, the apparatus depicted in  FIGS. 14-16  provides a deadman switch with reduced weight and bulk, e.g., by eliminating elements such as the torsion ring, end cap, etc. 
       FIG. 17  depicts another embodiment of a safety mechanism for a hand-held combustion spray gun according to an implementation of the invention. Alternatively to a mechanically driven pawl, such as those described above with respect to  FIGS. 1-16 , the pawl may be pneumatically or electrically driven. More specifically,  FIG. 17  shows a safety mechanism having a pneumatically driven pawl  475  that is moveable into and out of engagement with engagement surfaces  470   a ,  470   b ,  470   c  of a valve core  440 . The valve core  440  may be similar to valve core  340  described above with respect to  FIGS. 14-16  in that it operates to control flow of oxygen, fuel gas, and compressed air using ports and rotational positions, and includes a plurality of respective engagement surfaces  470   a ,  470   b ,  470   c  that correspond to predetermined operational positions. Additionally, the valve core  440  may be rotatably held in a housing of a gas head assembly of a combustion spray gun similar to that described above with respect to  FIG. 1 . 
     In an embodiment, a biasing element  465  is connected between the valve core  440  and the housing to urge the valve core  440  toward the off position. The biasing element  465  may be, for example, a torsion spring similar to that described above with respect to  FIGS. 1 and 2 . However, there is no torsion ring in the embodiment shown in  FIG. 17 . Instead, the biasing element  465  acts directly on the valve core  440  to urge the valve core  440  to rotate relative to the housing at all times. Alternatively, a torsion ring that is fixed to the valve core  440  may be used, where the biasing element is arranged between the torsion ring and the housing and there is no relative movement between the torsion ring and the valve core  440 . 
     In the implementation shown in  FIG. 17 , the pawl  475  is brought into and out of engagement with the engagement surfaces  470   a ,  470   b ,  470   c  by way of a pneumatic assembly. More specifically, the pawl  475  is formed as part of a piston  515  that is slidably retained in a cylinder  520 . A spring  525  inside the cylinder  520  acts on a head  530  of the piston  515  to urge the piston  515  away from the valve core  440 , e.g., to urge the pawl  475  out of engagement with any of the surfaces  470   a ,  470   b ,  470   c.    
     The piston  515  may be moved against the force of the spring  525 , e.g., to move the pawl  475  toward the valve core  440 , by providing sufficient pressure in a chamber  535  of the cylinder  520  on a side of the head  530  located opposite the spring  525 . More specifically, when the pressure in the chamber  535  acting on the surface area of the head  530  exceeds the force of the spring  525 , the piston  515  is extended out of the cylinder  520  such that the pawl  475  is driven toward the valve core  440 . Alternatively, when the pressure in the chamber  535  acting on the surface area of the head  530  does not exceed the force of the spring  525 , the piston  515  is retracted into the cylinder  520  such that the pawl  475  is pulled away from the valve core  440 . 
     Pneumatic pressure is selectively provided to the chamber  535  via a trigger  540  and switch  545  that are operatively connected between a compressed air source  550  of the combustion spray gun and the cylinder  520 . More specifically, the trigger  540  moves the switch  545  into a first state when the trigger  540  is depressed toward the handle  555 . The switch  545  is configured such that, in the first state, a vent  560  is closed and the compressed air source  550  is placed in communication with a conduit  565 . Accordingly, when the switch  545  is in the first position, compressed air flows from the compressed air source  550 , through a conduit  565 , and into the chamber  535 , thereby overcoming the force of the spring  525  and moving the pawl  475  toward the valve core  440 . Thus, when the trigger  540  is depressed toward the handle  555 , the pawl  475  is extended toward the valve core  440 . 
     On the other hand, the trigger  540  places the switch  545  in a second state when the trigger  540  is moved away from the handle  555 , such as for example, if the combustion spray gun is accidentally dropped. In the second state, the vent  560  is open to atmosphere and the compressed air source  550  is blocked, e.g., taken out of communication with the conduit  565 . In this manner, air from the chamber  535  and conduit  565  is permitted to bleed out of the vent  560 . Accordingly, the force of the spring  525  retracts the piston  515  into the cylinder  520 , thereby pulling the pawl  475  out of engagement with the valve core  440 . 
     In using the apparatus shown in  FIG. 17 , an operator holds the combustion spray gun by the handle  555  and moves the valve core handle to any desired position, such as, for example, the full flow position where engagement surface  470   a  aligns with the pawl  475 . When the valve core handle is set in the desired operational position, the operator pulls the trigger  540  toward the handle  555 . This brings the pawl  475  into engagement with the respective one of the engagement surfaces  470   a ,  470   b ,  470   c  that corresponds to the selected operational position of the valve core  440  and valve core handle. 
     To move the valve core  440  between operational positions, the operator grasps the valve core handle, releases the trigger  540 , rotates the valve core handle to the new position, and then grasps the trigger again to bring the pawl  475  into engagement with another one of the engagement surfaces  470   a ,  470   b ,  470   c . If the operator releases the trigger  540  for whatever reason without simultaneously controlling the valve core handle, then the pawl  475  will move out of engagement with the valve core  440 , and the biasing element  465  will cause the valve core  440  to automatically rotate to the off position. In this manner, the apparatus depicted in  FIG. 17  provides a deadman switch that is pneumatically operated. 
     In another embodiment, the pneumatically operated pawl  475  depicted in  FIG. 17  can be used with the safety mechanism depicted in  FIGS. 1 and 2 . For example, the trigger  540 , valve  545 , piston  515 , cylinder  520 , spring  525 , conduit  565 , and air source  550  could be used to selectively move the pawl  475  into engagement with the indentation  120  of the torsion ring  30 , instead of using trigger  155  and pawl  130 . Alternatively, the trigger  540 , valve  545 , piston  515 , cylinder  520 , spring  525 , conduit  565 , and air source  550  could be positioned to selectively apply a force to the pawl  130  to selectively move the pawl  475  into engagement with the indentation  120  of the torsion ring  30 . 
     A solenoid or other electrical actuator could be used instead of the pneumatic arrangement shown in  FIG. 17 . For example, depression of a trigger may energize a solenoid to a first position that moves a pawl into engagement with an engagement surface of a valve core or torsion ring, while releasing the trigger energizes the solenoid to a second position that moves the pawl out of engagement with the valve core or torsion ring. 
     Additionally or alternatively, an emergency stop button may be provided that is electrically connected to the solenoid, but that is remotely located with respect to the hand-held combustion spray gun. In an embodiment, depressing the emergency stop button energizes the solenoid to the second position. In this manner, a person who is far away from the combustion spray gun may turn off the combustion spray gun by depressing the emergency stop button. 
     Additionally or alternatively, a computerized controller may be provided that is electrically connected to the solenoid, but that is remotely located with respect to the hand-held combustion spray gun. For example, the computerized controller may be provided with sensors that detect operational parameters of the combustion spray gun. When a predefined condition is detected, the computerized controller may energize the solenoid to the second position to disengage the pawl from the valve core and permit the torsion spring to automatically rotate the valve core to the off position. 
     The safety mechanisms described herein can be added or retrofitted to existing hand-held, gas-powered devices that use rotary valve cores. More specifically, the safety mechanisms described herein may be added externally without modification of the interior ports and gas passageways of the valve core and/or housing. 
     It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.