Patent Publication Number: US-2006011457-A1

Title: Deadman switch

Description:
This invention relates to a deadman switch, particularly for use in the sand blasting industry.  
      In heavy industrial sand blasting the handling of the sand blasting hose and output nozzle is a very dangerous practice. The high pressure in a sand blasting hose means that if the hose is accidentally dropped, it has a tendency of performing an uncontrollable whipping motion. The size and weight of the hose and nozzle together with the pressures involved means that the random motion of the end of the hose is extremely dangerous and it has been known to cause serious injury and even death.  
      One means of preventing dangers of this kind is to install a deadman switch onto the hose that causes the pressure to be immediately cutoff once the switch is released. Thus, deadman switches have been incorporated on the end of the hose. These switches have to be held down by the operator and are activated once the operator releases the end of the hose.  
      Unfortunately, conventional deadman switches have very poor ergonomics which cause users to either remove the switches totally or to put the switches into the hands of another party. If another party is holding the deadman switch there is often a delay between an accident and that party activating the switch. Both these practices are clearly undesirable and it is viewed that there is a need for a much more ergonomically friendly deadman switch that can be formed with the connector in a manner that it can not be detached.  
      It is these issues that have brought about the present invention.  
      In accordance with one aspect of the present invention there is provided a deadman switch adapted to form an integral part of a hand held end connector of a hazardous hand tool, the deadman switch comprising a housing integrally formed with the connector, the housing including switching means arranged to control operation of the tool, a handle pivotally supported by the housing to be displaceable from an off position to an on position in which the handle activates the switching means to control operation of the tool, and means to urge the switching means and handle to the off position, whereby in use the connector is hand held to depress the handle to activate the switching means and cause operation of the tool.  
      Preferably the switching means comprises a valve assembly within the housing, the valve assembly comprising a valve member located within a valve chamber adapted to be connected to air inlet and outlet lines, whereby depression of the handle displaces the valve member to cause operation of the tool.  
      Preferably, the switch includes a releasable latch that operates to hold the handle in the off position.  
      The handle is preferably ergonomically designed to run parallel to the tool so that the tool can be used single handedly with the hand holding the tool also holding down the handle.  
      In a preferred embodiment the deadman switch is adapted to form part of the end connector of a high pressure sand blaster. 
    
    
      An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:  
       FIG. 1  is a schematic illustration of a sand blasting circuit,  
       FIG. 2  is a side elevational view of a nozzle connector incorporating a deadman switch in the off position,  
       FIG. 3  is a side elevational view of the connector and deadman switch in the on position,  
       FIG. 4  is a plan view of the connector and deadman switch in the off position,  
       FIG. 5  is a plan view of the connector and deadman switch in the on position,  
       FIG. 6  is a detailed elevational view a valve forming part of the deadman switch in the on position,  
       FIG. 7  is a elevational view the valve in the off position,  
       FIG. 8  is a side elevational view of the connector,  
       FIG. 9  is another side elevational view of the connector,  
       FIG. 10  is a plan view of the connector,  
       FIG. 11  is an end elevational view of the connector,  
       FIG. 12  is a cross sectional view taken along the lines A-A of  FIG. 11 , and  
       FIG. 13  is a side elevational view of a handle forming part of the deadman switch. 
    
    
      As shown in the accompanying drawings, a deadman switch has been designed for particular use with high pressure sand blasting equipment. As shown in  FIG. 1 , equipment of this kind incorporates a high pressure air compressor C and a sand or other aggregate hopper  5  that are joined to a mixer M where the sand and air is mixed. The output of the mixer M is coupled to a miser valve V that controls flow to a flexible hose (not shown). The flexible hose terminates in a connector  10  on which suitable nozzles N can be attached in dependence on the requirements of the sand blaster.  
      The user of the equipment holds the connector  10  and points the nozzle N at the object that is to be sand blasted. It is commonplace for the hose to be held under the arm of the user so that the user can withstand the sideways movement that the high pressure air source imparts to the hose. A deadman switch  5  is designed to be part of the connector  10  and in essence comprises a handle  20  that when pressed down places a switch in the on position. When the handle  20  is released the switch automatically returns to the off position which has the effect of closing off a feedback line F to the miser valve V to in turn immediately close off the main pressure that drives the sand blasting hose. It is understood that the switch can be electrical or pneumatic.  
      As shown in  FIGS. 8-13 , the connector  10  includes a cylindrical housing  11  with a elongate bore  12  through which the high pressure sand and air mixture passes. The forward end of the connector includes an internally threaded bore  13  that is adapted to threadedly engage the externally threaded end of a variety of nozzles. The opposite end  14  of the connector is adapted to be firmly secured to the end of heavy duty high pressure hose. Integrally formed with the connector body is a deadman switch mount  15  that is adapted to support the handle  20  of the switch  5  whilst at the same time house a valve chamber  30  and an air inlet  31  and air outlet  32  which are in turn coupled to low pressure feedback lines F 1  and F 2  that are in turn coupled to the miser valve V back at the source of the high pressure air and aggregate mixture. The deadman switch handle  20  is shown in  FIG. 13  and comprises a U-shaped forward end  21  with downwardly extending flanges  22  that sit on either side of the mount  15  of the connector  10  and are secured to the connector by fasteners (not shown) that allow the handle  20  to pivot about the connector  10 . The forward end  21  of the handle has a rearwardly trailing lever portion  23  that terminates in an upturn toe  24  which has been designed to prevent the likelihood of a user&#39;s hand sliding off the rear of the handle  20 . The underside of the handle has a horizontal portion  25  adjacent the toe  25  which then merges into an inclined portion  26  which extends up to the U-shaped forward end  21 . The inclined portion  26  serves as an abutment surface for actuating a valve member  50  that is located in a vertical bore  33  in the valve chamber  30 . The vertical bore  30  joins two spaced air conduit chambers  31 ,  32  and the valve member  50  is displaceable from an on position in which air flows between the chambers and through the bore and a closed position in which air can not pass from one conduit to the other.  
       FIGS. 6 and 7  show the valve member  50  in greater detail. The valve member  50  comprises a valve support member  51  that has a head  52  joined to an end flange  53  by a cylindrical shank  54 . Two spaced shoulders  55  and  56  separate the shank  54  into upper and lower portions  54   a  and  54   b . A frustoconical rubber seal  57  with an internal bore  58  is located on each shank  54   a  and  54   b  with the narrower ends of the seals facing each other. The rubber seals are held into position by the head  52  and shoulder  55  on the one hand and the shoulder  56  and the end flange on the other.  
      In the off position shown in  FIG. 7 , the air pressure from the inlet conduit acts on the underside of the valve member to urge it upwardly to the position shown in  FIG. 7  where the frustoconical side wall  59  of the valve engages the wall of the bore  33  to prevent air flow into the valve chamber. When the head  52  of the valve is depressed by the handle to the position shown in  FIG. 6  the lower valve member is moved clear of the wall of the bore  33  allowing air to flow past the valve member and into the center of the valve chamber and out through the outlet passage. Escape of air up through the top of the valve chamber  33  is stopped by the upper valve member as its frustoconical wall  59  engages the top of the wall of the bore  33 . Once the air flows from the inlet to the outlet, the miser valve operates to operate the sand blasting equipment. The air pressure on the base of the valve member  50  urges the valve member up and assists in returning the handle to the off position.  
      As shown in FIGS.  2  to  5 , the mount  15  for the handle  20  also includes a horizontal bore  60  which is located at a press-stud  61  that is screwed onto the end of the bolt  62  with a spring  63  therebetween acting on an internal shoulder  64  within the bore  60 . The spring  43  urges the press-stud  61  out and the press-stud  61  has a frustoconically tapered head  65  that fits into an aperture  66  of one flange  21  of the handle  20  so that, as shown in  FIGS. 2 and 4 , when the handle  20  is in the off position the stud  61  locates within the hole  66  in the flange  21  preventing downward depression of the handle  20 .  
      To release the deadman switch  5  it is necessary to first push in the head  65  of the stud  61  clear of the aperture  66  and then press down on the handle  20 . This in turn moves the valve member  50  to cause flow of air through the feedback loop to in turn activate the high pressure miser valve V. If the handle  20  is accidentally released, the handle is immediately urged upwardly by both the air pressure that acts on the valve member  50  which pushes the handle up as shown in  FIG. 2 ,and the tapered head  65  on the stud  61  also has the effect of imparting a vertical force on the handle  20  that causes it to move upwardly to the off position.  
      Thus, any release of the handle  20  immediately causes the deadman switch  5  to assume the off position which in turn causes immediate cessation of the high pressure delivery of particulate material and air.  
      The handle  20  of the switch  5  is ergonomically designed to be comfortable and easy to use. The upturned end  24  prevents the hand from sliding off and it is of sufficient length that it does not cause any major inconvenience to the user of the sand blasting equipment. The connector  10  is also designed in a manner that it is impossible to tamper with the deadman switch. Any attempt to remove the handle causes the valve to automatically assume the off position so there is no possibility of disarming the deadman switch or moving the switch to another position. If the operator wishes to use the equipment then it is essential that the deadman switch is depressed for operation to take place.  
      Obvious disabling of the switch, such as using tape or other means to hold the handle down, would be highly conspicuous and would contravene a variety of health and safety regulations.  
      The handle is designed so that instead of extending rearwardly in a direction parallel to the hose, it could extend forwardly of the connector in situations where the nozzle component (not shown) needs to be held. The length of the handle would vary in dependence on the size of the nozzle and it is understood that the componentry of the deadman switch would be either injection moulded in plastics or die cast in suitable metals. It is further understood that to prevent the possibility of grit or dirt impeding the motion of the release stud  61 , a small O-ring seal can be provided on the head  65  of the stud  61  to prevent the ingress of particulate material.  
      Whilst a pneumatic valve is the preferred form of switching means it is understood that the connector could include an electrical switch with either a wired or wireless connection to the miser valve V.  
      It is further understood that the deadman switch could be used on a variety of other hazardous hand tools.  
      In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.