Patent Publication Number: US-6901841-B2

Title: Gas bleed system with improved control

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to control of gas flows, and more particularly to a bleed system for releasing pressurized gas from a chamber at a selectively adjustable flow rate. The bleed system is described primarily herein for application in a runaway control for an air motor which shuts off the motor when its speed exceeds a predetermined threshold speed. However, it is understood that the bleed system may be applied in any device where pressurized gas is released at a controlled rate. 
   It is difficult to accurately control a release of gas from a higher pressure region to a lower pressure region, particularly for an adjustable, low-level release. Due to manufacturing tolerances, conventional bleed valves frequently exhibit inconsistent or varying sizes of flow passages. Adjustments which are intended to produce minor incremental changes in flow rates can result in large step changes. Consequently, the valves are over-sensitive, cannot be accurately calibrated, and are not repeatable in setting a selected rate of flow. 
   A runaway control of the prior art is disclosed in U.S. Pat. No. 5,349,895, entitled “Air Motor Control,” which is hereby incorporated by reference. That patent discloses an air motor with an expansible chamber having a reciprocable piston driving a pump for pumping materials such as lubricants, sealants, or inks. A problem of pump runaway is at times encountered, due for example to breakage of a discharge line or running out of the material being pumped. The load on the motor is reduced such that the motor speeds up and drives the pump at very high speeds. That can damage the pump and cause expensive and time-consuming spills of material. A runaway control is provided for cutting off operation of the air motor under these circumstances. The control can be adjusted so that it activates to cut off the air motor at a selectable and predetermined threshold speed (e.g., between 5 and 50 cycles per minute) which depends generally on the viscosity of the material being pumped. Adjustment of cut-off speed is effected by a bleed valve for adjusting the rate of flow from a chamber having a pressure which varies in proportion to the speed of the motor. The bleed may be adjusted to vent a maximum quantity of air from the chamber when operating the motor at an increased speed, or adjusted to vent a minimal quantity of air when operating the motor at a nominal speed. 
   A drawback to runaway controls is that bleed valves are prone to be over-sensitive to adjustments, as described above. It is difficult to accurately calibrate the bleed valve to obtain a desired cut-off speed or to repeat previously obtained settings. 
   SUMMARY OF THE INVENTION 
   Among the several objects and features of the present invention may be noted the provision of a gas bleed system for releasing gas at a selectively adjustable flow rate; the provision of such a system which may be calibrated to obtain repeatable flow rates; the provision of such a system for use in a runaway control of an air motor for stopping the motor if it should start to run away; and the provision of such a system which is efficient and durable in use and cost-efficient to construct. 
   In general, the present invention involves an improved air motor of the expansible chamber type comprising an air cylinder, a piston reciprocable therein, a valve mechanism shiftable alternately to effect supply of air to and venting of air from opposite sides of the piston to reciprocate the piston, and a runaway control operable on increase in speed of the air motor above a speed limit to stop the motor. The control includes a pressure-responsive mechanism comprising an air chamber for air under pressure, a movable mechanism movable away from a first position in response to increase in air pressure in the chamber above a predetermined pressure limit to a second position, and movable back to the first position on reduction of pressure in the chamber below the pressure limit. The movable mechanism when in its first position enables operation of the air motor and when in its second position cuts off operation of the motor. An air pump is interconnected with the motor for operation simultaneously with the motor for delivering air under pressure to the chamber at a rate related to the speed of the motor. The improvement comprises a bleed mechanism for bleeding air from the chamber at a controlled rate. The pressure in the chamber is controlled by the rate of delivery of air under pressure to the chamber and the bleed of air from the chamber. On increase in speed of the motor above the speed limit, the pump, operating at increased speed, delivers air under pressure at an increased rate to the chamber over and above the capability of the bleed to bleed off the increase, and on ensuing increase in air pressure in the chamber above the pressure limit, the movable mechanism moves to its second position to cut off the motor. The bleed mechanism comprises a plurality of bleed flow paths of different lengths providing varying resistance to the flow of air. Each bleed flow path has an inlet communicating with the chamber and an outlet. A bleed path selector mechanism is movable between a plurality of different settings corresponding to the plurality of different flow paths. The bleed path selector mechanism communicates in each of the settings with the outlet of one of the bleed flow paths and allows the bleed flow path to vent for bleeding air from the chamber while sealing the outlets of the other bleed flow paths, whereby the speed limit at which motor cuts off can be adjusted by moving the selector mechanism to the desired setting. 
   In another aspect, a bleed system according to the present invention vents pressurized gas from a chamber to a vent opening at a selectively adjustable flow rate to control change of pressure in the chamber. The bleed system comprises at least two passageways each adapted for venting gas from the chamber and thereby tending to reduce gas pressure in the chamber, each of said at least two passageways having a length. A selector mechanism is adapted for establishing fluid communication between the chamber and vent opening via a selected one of the passageways such that gas is vented from the chamber through the selected passageway to the vent opening. The passageways have different lengths such that selection of one passageway results in flow of gas from the chamber to the vent opening at one flow rate and selection of the other passageway results in flow of gas from the chamber to the vent opening at a different flow rate. 
   In yet another aspect, a runaway motor control system according to the present invention is for an air motor. The control system comprises a pressure-responsive mechanism operable on increase in speed of the air motor above a speed limit to stop the motor. The mechanism comprises an air chamber for air under pressure, a movable mechanism movable away from a first position in response to increase in air pressure in the chamber above a predetermined pressure limit to a second position, and movable back to the first position on reduction of pressure in the chamber below the pressure limit. The movable mechanism when in its first position enables operation of the air motor and when in its second position cuts off the operation of the motor. An air pump is interconnected with the motor for operation simultaneously with the motor for delivering air under pressure to the chamber at a rate related to the speed of the motor. A bleed mechanism is for bleeding air from the chamber at a controlled rate. The pressure in the chamber is controlled by the rate of delivery of air under pressure to the chamber and the bleed of air from the chamber, whereby on increase in speed of the motor above the speed limit, the pump, operating at increased speed, delivers air under pressure at an increased rate to the chamber over and above the capability of the bleed to bleed off the increase. On ensuing increase in air pressure in the chamber above the pressure limit, the movable mechanism moves to its second position to cut off the motor. The bleed mechanism comprises a plate having a series of channels in a face thereof providing bleed flow paths of varying resistance to the flow of air. 
   In one more aspect, a bleed system of the invention vents pressurized gas from a chamber to a vent opening at a selectively adjustable flow rate to control change of pressure in the chamber. The bleed system comprises a passageway establishing fluid communication between the chamber and vent opening for venting gas from the chamber and thereby tending to reduce gas pressure in the chamber, the passageway having an inlet, an outlet, and a flow path extending between the inlet and the outlet. An adjustment mechanism is for selectively adjusting a length of the path to change a rate of flow of gas from the chamber to the vent opening. 
   In yet one more aspect, a bleed system of the invention is for venting pressurized gas from a chamber to a vent opening at a selectively adjustable flow rate to control change of pressure in the chamber. The bleed system comprises a passageway establishing fluid communication between the chamber and vent opening for venting gas from the chamber and thereby tending to reduce gas pressure in the chamber. An adjustment mechanism is for selectively adjusting a size of the passageway to change a rate of flow of gas from the chamber to the vent opening. The adjustment mechanism comprises a conical plug movable within a conically-shaped bore. 
   Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a vertical section of an air motor and runaway control of the prior art; 
       FIG. 1B  is a side elevation of the air motor and runaway control of  FIG. 1A  with portions broken away; 
       FIG. 1C  is an enlarged section of an air pump shown in  FIG. 1A ; 
       FIG. 1D  is an enlarged section of a pressure-responsive system shown in  FIG. 1A ; 
       FIG. 2  is a vertical section of a runaway control and bleed system according to the present invention for mounting at one side of the air motor of  FIG. 1A ; 
       FIG. 3  is an exploded perspective of the runaway control and bleed system of  FIG. 2 ; 
       FIG. 4A  is a view showing a face of a plate of the bleed system and its passageways; 
       FIG. 4B  is an enlarged section along line  4 B— 4 B of  FIG. 4A ; 
       FIG. 5A  is a view similar to  FIG. 4A  showing a face of a plate of a second embodiment; 
       FIG. 5B  is an enlarged section along line  5 B— 5 B of  FIG. 5A ; and 
       FIGS. 6-8  are schematic sections of third, fourth and fifth embodiments, respectively. 
   

   Corresponding reference characters indicate corresponding parts throughout the views of the drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings and in particular to  FIGS. 2 and 3 , a bleed system according to the present invention for venting pressurized gas at a selectively adjustable flow rate is indicated generally at  2 . The bleed system  2  may be used, for example, in a runaway control for an air motor  1  of the type disclosed in  FIGS. 1A-1D , although the system may be applied in any device wherein gas is released at a controlled rate. 
   The air motor  1  of the prior art, and its runaway control system, are now described with reference to  FIGS. 1A-1D . In one embodiment, the bleed system  2  of the present invention is intended to replace a conventional bleed valve  171  ( FIG. 1D ) of the runaway control. The air motor  1  comprises a cylinder  3  which as generally used occupies a vertical position as shown in FIG.  1 A and which has first and second end heads  5  and  7  at first and second ends thereof, the first being the upper and the second being the lower end head. The heads are secured on the upper and lower ends of the cylinder by bolts or tie rods (not shown) as in U.S. Pat. No. 4,846,045, which issued Jul. 11, 1989 and is entitled “Expansible Chamber Motor”. A motor piston  9  is reciprocable up and down in the cylinder, having an O-ring seal as indicated at  11 . A piston rod  13  extends down from the piston through the lower end head  7 , an O-ring seal for the rod being indicated at  15 . The piston rod is adapted for connection in conventional manner at its lower end to the plunger of a pump (not shown) for pumping materials such as lubricants, sealants, and inks. 
   A valve generally designated  17  is mounted on the upper end head  5  for controlling supply of pressure air from a source thereof to and exhaust of air from opposite ends of the cylinder  3 . The valve comprises an elongate metal block  19  (e.g. a cast aluminum block) suitably secured on top of the upper end head having a cylindric bore  21  extending from one end thereof to the other and end heads  23  and  25  closing the ends of the bore. A valve member  27 , more particularly a valve spool, is axially slidable in the bore between a first position toward the right end of the bore as shown in  FIG. 1A , for effecting delivery of pressure air from a source to the upper end of the cylinder and exhaust of air from the lower end of the cylinder for driving the piston down, and a second position toward the left end of the bore for effecting delivery of pressure air from the source to the lower end of the cylinder and exhaust of air from the upper end of the cylinder for driving the piston upwardly. Pressure air is supplied from a suitable source to pressure supply ports  29 L and  29 R in the upper end head  5  which are in communication with the bore  21  in the valve block. At  31  is indicated an exhaust port in communication with the bore and with the ambient atmosphere. Delivery to and exhaust of air from the upper end of the cylinder (i.e., the chamber in the cylinder above the piston) is via passaging in the upper end head indicated at  33 . Delivery to and exhaust of air from the lower end of the cylinder (i.e., the chamber in the cylinder below the piston) is via passaging indicated at  35 . The valve spool is constructed as illustrated with annular grooves  37   a ,  37   b ,  37   c  and  37   d  between lands  39   a ,  39   b ,  39   c  and  39   d  to establish communication between ports  29 R and  33  and between ports  35  and  31  when in its right-hand position of FIG.  1  and to establish communication between ports  29 L and  35  and between ports  33  and  31  when in its left-hand position. The lands have seals such as indicated at  41 . 
   The valve spool  27  is movable from its right-hand position of  FIG. 1A  to its left-hand position on delivery of pressure air to the right end of the bore  21  through passaging indicated at  43  in the upper end head  5  and in the valve block end head  25 , and exhaust of air from the left end of the bore via passaging indicated at  45  in the left end head  23  of the valve block  19 . The valve spool  27  is movable from its left-hand position to its right-hand position on delivery of pressure air to the left end of the bore  21  via passaging  45  and exhaust of air from the right end of the bore via passaging  43 . The supply of air to and exhaust of air from the opposite ends of the bore  21  are under control of an air-operated relay valve  47  (desribed in U.S. Pat. No. 5,349,895) to establish communication for pressure air from pressure supply port  29 R in the upper end head  5  of the cylinder  3  via a passage  59  to a port  61  and thence via passaging  45 , and for exhaust of air from the right end of the bore  21  via passaging  43  to an exhaust port. 
   For operation of the relay valve  47 , means indicated generally at  71  is provided for delivery of air under pressure to and exhaust of air from the left end of the relay valve and means indicated generally at  73  is provided for delivery of air under pressure to and exhaust of air from the right end of the relay valve. The means  71 ,  73  comprise a first pilot valve  75  ( FIG. 1B ) and a second pilot valve  101 , respectively, housed in a recess of a block  79  mounted at one side of the cylinder. The first pilot valve  75  is a pressure responsive valve in communication by passaging as indicated at  81  with the relay valve  47 . The pilot valve is also in communication by passaging as indicated at  83 ,  85  with the cylinder  3 . The second pilot valve  101  is mounted in a recess  103  of the block  79 . The pilot valve is also a pressure responsive valve in communication by passaging as indicated at  105  to the relay valve  47 , and by passaging as indicated at  107 ,  109  with the cylinder  3 . Further description of the pilot valves is provided in U.S. Pat. No. 5,349,895. A cover member  80  encloses the outer surface of the block  79  such that the required passaging between means  71 ,  73  and the relay valve  47  is located between the block and the cover member. 
   An air pump, generally designated  121  in  FIGS. 1A and 1C  operates as a slave to the air motor and is housed in a recess  123  in the side of the block  79 . Air pump  121  comprises a cylinder  124  having a first chamber constituting a motor chamber  127  and a second chamber constituting a pump chamber  131 , and a piston  125  reciprocally movable in the motor chamber, and a plunger  129  movable conjointly with the piston in the pump chamber. As shown, the plunger  129  has a smaller diameter than piston  125  and extends from and is integral with the piston. O-rings maintain an air-tight seal between the piston and the motor chamber and between the plunger and the pump chamber so that pressurized air in the chambers does not escape. 
   The motor chamber  127  is in communication with the bottom of the cylinder  3  via passaging  137  ( FIG. 1C ) located at the left end of the motor chamber, and in communication with top of the cylinder via passaging  139  located at the right end of the motor chamber. Upon increase of pressure in the bottom of cylinder  3 , pressurized air is delivered to the air pump  121  through passaging  137  thereby forcing the piston  125  to the right to a first position, and upon an increase of pressure in the top of the cylinder  3 , pressurized air is delivered to the air pump  121  through passaging  139  thereby forcing the piston  125  back to the left to a second position. When the piston  125  moves to its second position, the plunger  129  draws in atmospheric air into the pump chamber  131  through a vent  141 . And, upon moving to its first position, the plunger  129  forces the air in the pump chamber  115  through a passageway  143 . A ball check  145  engagable with a seat  147  is provided in vent  141  for preventing air in the pump chamber  131  from flowing back through the vent when the plunger  129  moves from its first position to its second position, and likewise, an identical ball check  149  engagable with a seat  151  is provided in passageway  143  for preventing the plunger  129  from drawing air into the pump chamber  131  when the plunger moves from its second position to its first position. 
   Passageway  143  connects the air pump  121  to pressure-responsive system, indicated generally at  161 , which is located in a recess  163  in block  79  adjacent the air pump. The pressure-responsive system  161  comprises a first diaphragm  165  located at the right side of the recess and a second diaphragm  167  proximate the first diaphragm  165  and to the left thereof. As shown in  FIG. 1D , the space between diaphragms  165  and  167  defines a chamber  169  which receives pressurized air from the air pump  121  via passageway  143 . Upon delivery of pressurized air from the air pump  121  to the chamber  169 , the air is vented from the chamber by a conventional bleed valve  171  in communication with the chamber via a passageway  173  at a rate consistent with the predetermined operating speed of the air motor. Bleed valve  171  is adjustable for varying the rate of bleed from chamber  169 , i.e., the bleed may be adjusted to vent a maximum quantity of air when operating the air motor at an increased rate, or adjusted to vent a minimal quantity of air when operating the motor at a nominal rate. 
   Pressure-responsive system  161  further comprises a pressure-responsive valve  175  (“movable means”) movable within the recess  163  upon an increase in pressure in chamber  169 . Valve  175  includes a valve stem  177  which is connected at its right-hand end to the second diaphragm  167  and at its left-hand end to a ball valve member  179 , the ball valve member being engagable with a first valve seat  181  located to the left of the ball valve member and a second valve seat  183  located to the right of the ball valve member. The space between valve seats  181 ,  183  defines a passage chamber  185  which is in communication with passaging  107  such that air traveling through the passaging must enter into and exit from the chamber  185  as the air travels to relay valve  47 . The pressure-responsive valve  175  is movable from a first position in which the ball valve member  179  engages the second valve seat  183  (and spaced from the first valve seat  181 ) such that air flows through passaging  107  to maintain communication between cylinder  3  and pilot valve  101 , and in response to increase in air pressure in the chamber  169  above a predetermined limit to a second position in which the ball valve member  179  engages the first valve seat  181  for blocking passaging  107 , and therefore blocking flow of air to pilot valve  101 . On blocking of passaging  107 , the pilot valve  101  is unable to operate, thereby disabling the operation of the relay valve  47  which in turn disables the valve spool  27  for stopping the movement of piston  9  and cutting off the operation of the motor  1 . The pressure-responsive valve  175  is movable back to its first position on reduction of pressure in the chamber  169  below the limit. 
   A spring  187 , engageable with a washer  189  positioned adjacent the second diaphragm  167 , biases the second diaphragm to maintain the pressure-responsive valve  175  in its stated first position. Upon increase of speed of the motor above a predetermined operating speed (e.g., 50 cycles per minute as set by bleed  171 ), the air pump  121 , operating at increased speed, delivers air under pressure at an increased rate to the chamber  169  over and above the capability of the bleed  171  to bleed off the increase and over and above the resistance of the spring  187  on the second diaphragm  167 . On the ensuing increase in air pressure in the chamber above the limit, the second diaphragm moves to the left against the bias of the spring so that the pressure-responsive valve  175  moves to its second position thereby blocking passaging  107  and cutting off the motor. A vent  191  exhausts built-up air pressure to the left of the second diaphragm to the atmosphere. 
   The previously described arrangement is such that the length of time from when the air motor reciprocates at the predetermined speed limit to when the air motor is cut off depends upon how much over the speed limit the air motor is reciprocating. The greater the speed of the motor, the shorter the length of time for increasing air pressure within chamber  169  over and above the resistance of spring  187  for moving pressure-responsive valve  175  to its second position. And conversely, a speed only marginally above the speed limit delivers pressurized air to chamber  169  at a slower rate, thereby increasing the amount of time needed to move the valve  175  to its second position. 
   The first and second diaphragms  165 ,  167  are interconnected at their respective centers by a member  193 . The first diaphragm  165  is also biased by the spring  187  (via the force of the spring transmitted through diaphragm  167  and member  193 ) against the right-hand wall of the recess  163  to block a passageway  195  which is connected to the source of pressure air for supplying pressurized air on diaphragm  165  (which constitutes an auxiliary valve member). The pressurized air assists in moving the pressure-responsive valve  175  to its second position. On the initial movement of the pressure-responsive valve  175  to its second position (as a result of increased pressure in chamber  169 ), the first diaphragm  165  moves away from the passageway  195  to an open position and pressurized air exerts pressure on the first diaphragm for facilitating the movement of the pressure-responsive valve to its second position. Only by closing the air supply and venting the air trapped in the recess  163  to the right of the first diaphragm may the pressure-responsive valve move back to its first position. 
   A trip indicator, indicated generally at  201 , located on the exterior of the block  79  is in communication with the recess  163  to the right of the first diaphragm  165  by another passageway  197  and is activated upon increased pressure to the right of the first diaphragm as a result of pressurized air being supplied by the air supply. As shown in  FIG. 1D , the trip indicator  201  includes a valve stem  203  slidable within a bore  205 . Upon increased air pressure on the left-hand portion  207  of valve stem, it moves towards the right so that a narrower right-hand portion  209  of the stem extends through an opening  211  formed in the block  79 . A spring  213  maintains the trip indicator  201  towards the left in the bore and only upon delivery of supply air on the left hand portion  207  of the stem  203  is the stem able to move against the bias of the spring. The right-hand and portion  209  of the stem  203  is colored red so that it may be visible to the operator. Upon activating the trip indicator  201 , the operator knows that the air motor runaway control has been activated and that the air motor needs to be reset by shutting off the pressurized air. 
   During operation of the air motor, piston  9  is movable up and down in cylinder  3  in response to pressurized air delivered by valve  17 . Piston  9  drives the plunger of the pump (not shown) connected at the lower end of the piston rod  13  for pumping materials such as sealants. In the event of the discharge line of the pump breaking, or exhaustion of the supply of the material being pumped, the piston  9  will tend to reciprocate in the cylinder  3  at high speed which can cause significant damage to the pump. In response to the increased speed of the piston  9 , the air pump  121  of the runaway control operates at an increased speed since the air pump operates as a slave to the air motor. The air pump  121  in turn delivers pressurized air at an increased rate to chamber  169  of the pressure-responsive system  161 . The pressure in the chamber  169  is controlled by bleed  171  via which the air entering the chamber from air pump  121  is vented from the chamber at a rate consistent with the predetermined operating speed of the air motor. In response to increase of air pressure in the chamber  169  over and above the capability of the bleed  171  to bleed off the increase, the pressure-responsive valve  175  moves to its second position in which its ball valve member  179  engages the first valve seat  181  for blocking passaging  107 . The first diaphragm  165  also moves to a position away from passageway  195  thereby allowing air pressure to be delivered on the first diaphragm for maintaining the blockage of passaging  107 . 
   By blocking passaging  107 , the second pilot valve  101  is incapable of allowing the delivery of pressurized air to passaging  105  for moving the relay valve  47  because pressurized air from the lower end of the cylinder entering passaging  107  above the piston when the piston is in its substantially down-stroke position is blocked from entering the second pilot valve  101 . Since the relay valve  47  is incapable of moving, the valve spool  27  of the valve means  17  is incapable of moving to its left-hand position. Pressurized air from supply port  29 R continues to be supplied to passaging  45  which keeps valve spool  27  in its right-hand position. With the valve spool  27  maintained in its right-hand position, pressurized air from supply port  29 R continues to be supplied to the top of the cylinder  3  via passaging  33  above piston  9  thereby holding the piston in its down-stroke position. 
   By shutting off the air supply (which applies pressure on diaphragm  165 ) and opening the bleed  171  for venting the built-up air pressure in the chamber  169 , the air motor is reset for operation. Upon releasing the built-up air pressure in the chamber  169 , the pressure-responsive valve  175  moves back to its first position under the bias of spring  187 . Before the air motor is restarted, however, the cause for the air motor runaway must be attended to, e.g., the broken discharge line should be replaced, or the material being pumped should be resupplied. 
   Reference is made to U.S. Pat. No. 5,349,895 for further detail regarding the runaway motor control. 
   The bleed system  2  of the present invention is now described with reference to  FIGS. 2 ,  3 ,  4 A, and  4 B. In the embodiment shown in the drawings, the bleed system includes a thin, flat plate  220  having channels  230  formed in one face of the plate, two gaskets  240  for sealing engagement with opposite faces of the plate, an inner housing member  242 , an outer housing member  244 , and a selector mechanism indicated generally at  250 . The inner and outer housing members  242 ,  244  are clamped together and attached to the air motor  1  by suitable fasteners such as ten cap screws  252 , with the gaskets  240  and plate  220  held in sandwiched position between the inner and outer housing members. The plate  220  has ten holes  254  for permitting the cap screws  252  to pass through the plate, and one fastener hole  256  for permitting a fastener bolt  258  of the selector mechanism to pass through the plate. The housing members  242 ,  244  are made of a suitable rigid material, such as cast steel or aluminum. 
   Each gasket  240  comprises a piece of a suitable rubber or plastic for generally airtight sealing against a face of the plate  220  and closing open sides of all channels  230  on the plate. Other types of sealing arrangements do not depart from the scope of this invention. The gasket has holes  254 ,  256  corresponding with those on the plate  220  for registering alignment therewith. As shown in  FIG. 4B , the plate  220  has channels formed in a front face  260  of the plate, with a back face  262  being smooth. Consequently, the bleed system  2  requires only one gasket  240  to close the open side of the channels  230  by face-to-face engagement with the plate and thereby seal each channel (although twogaskets may be installed without any detriment). 
   Significantly, the channels  230  in the plate define a plurality of alternate air bleed flow paths or passageways having different lengths and providing different resistances (e.g., friction) to flow of gas through the passageways. Consequently, selection of one passageway results in flow of gas at one flow rate and selection of another passageway results in flow of gas at a different flow rate. It is understood that systems of other forms, including but not limited to channels formed in other objects (which are neither thin nor flat) or channels comprising stand-alone pipes, conduits, or passageways, do not depart from the scope of this invention. 
   Referring to the embodiment of  FIG. 4A , the channels  230  defining the passageways are arranged in series. The channels are formed in this particular embodiment by a single continuous groove as indicated generally at  266  which has a sinuous shape, also referred to as forming a tortuous path, comprising a series of down-and-back segments or reaches  268   b - 268   g  extending along the length of the plate  220 . Each bleed flow path or passageway has an inlet and an outlet. A hole in the plate indicated at  270  comprises a common inlet for all passageways, and it is in communication with passageway  173  and the pressurized chamber  169  of the runaway control from which air is to be continuously released at a controlled rate. The plate  220  has a plurality of outlet holes  272   a - 272   g  spaced at intervals around the fastener hole  256 , seven such outlet holes being shown in  FIG. 4A  for purposes of illustration. (It will be understood that the number of outlet holes  272  or inlet holes  270  may vary.) Each outlet hole  272  is connected to the continuous groove  266  by means of a connector channel  274 . The intersections  276  between the connector channels  274  and the continuous groove  266  are spaced along the groove at different distances from the common inlet  270  to form a plurality of passageways (bleed flow paths) of different lengths, the length of each such passageway being the distance along the groove  266  between the inlet  270  and a respective intersection  276  plus the distance along a respective connector channel  274  from the intersection to the respective outlet hole  272 . It is understood that a system with separate channels of differing resistances, such as a distinct channel of selected length for each outlet, does not depart from the scope of this invention. 
   The plate  220  of  FIGS. 4A and 4B  has a shape and size suitable for attachment and use with the air motor runaway control. In one embodiment, the plate is rectangular with approximate dimensions of 7.5×2.4×0.015 inches. Larger plate sizes can permit longer passageways. The plate  220  is formed of a suitable material such as brass, stainless steel, or aluminum, and has a smooth surface finish. It is understood that there can be multiple plates or other objects containing channels which are placed adjacent to one another, or stacked together, without departing from the scope of this invention. Each channel  230  is arranged in spaced relation from adjacent channels and holes  254  to avoid close proximity to other channels and holes and thereby provide flat areas or lands  278  immediately adjacent each channel for the gasket(s)  240  to seal against and ensure airtight sealing of the channels and holes. These spacings preferably range from {fraction (1/16)} to {fraction (3/16)} inch. 
   The channels  230  are formed on the face of the plate  220  with a conventional chemical etching process, as known to those skilled in the art, such as by exposing the plate to an acid to remove material from selected locations. In one embodiment, all channels  230 ,  274  have a uniform cross-sectional area which remains uniform along an entire length of each channel. Although a variety of cross-sectional sizes or shapes are possible, in a preferred embodiment each channel has a width of about 0.015 inch, a depth of about 0.006 inch, and features a generally rounded shape with a flat bottom as shown in FIG.  4 B. It is understood that systems with passageways of other shapes and configurations do not depart from the scope of this invention. Further, a system wherein resistance to air flow is varied by passageway cross-sectional area (instead of length), or by a combination of cross-sectional area and length, does not depart from the scope of this invention. 
   The selector mechanism  250  ( FIGS. 2 and 3 ) is movable between different settings corresponding to the different passageways, and it places a selected one of the outlets (outlet holes  272 ) into communication with the inlet  270 , with other outlets remaining closed. Each of a plurality of valve members  280  (seven in the embodiment shown in the drawings) is movable between a closed position in which it is seated at a respective outlet to seal the outlet and an open position to permit airflow through the outlet. A selector device  282  holds the valve members  280  in their closed positions. 
   In one embodiment, the valve members  280  comprise spherical balls and the selector device  282  comprises a knob which is rotatable by a user for moving one valve member to its open position and thereby placing one selected outlet in communication with the inlet. Referring to  FIG. 3 , the outer housing member  244  has a recess for receiving the knob  282  and seven holes in the recess for receiving the balls  280  and holding the balls in registering alignment with respective outlet holes  272  of the plate. An o-ring seal  284  is positioned beneath each ball  280  forming a seat for the ball. The fastener bolt  258  attaches the knob  282  to the outer housing member  244  at a position such that the bottom surface of the knob presses the balls  280  into airtight engagement with the o-rings  284  while being movable relative to the balls to maintain the ability to rotate the knob. The bottom surface of the knob has one recess  286  ( FIG. 2 ) which permits one ball  280  to move into the recess, as it is urged by the resiliency of its o-ring  284  and air pressure, to an unseated position such that air can flow out from a respective passageway of the plate  220  through the corresponding outlet hole  272 . Rotation of the knob  282  places the recess  286  over a selected outlet hole  272 . Air from the groove  266  is then free to exit the plate  220  via the selected flow path and outlet, passing by the ball  280  and flowing out around the knob  282  to the surrounding air at atmospheric pressure. 
   The selector mechanism  250  also includes a cover  288 , a cover fastener  290 , and a washer  292  placed between the bolt  258  and knob  282 . Preferably, the washer  292  is a Belleville type washer which augments pressing of the knob against the sealing balls  280 . A conventional tactile detent  294  ( FIG. 2 ) is held in the knob, such as a spring-loaded, extendable pin. The pin is receivable in indentations  296  ( FIG. 3 ) of the outer housing member  244  for alignment purposes. The detent  294  provides the user with a difference in sensed resistance or noise (e.g., a click) to indicate that the knob  282  is set at a position wherein one of the balls  280  is unseated. Other types of selector mechanisms do not depart from the scope of this invention. 
   In use, the bleed system  2  of the present invention provides a release of gas at a selectively adjustable flow rate. The rate of flow varies generally inversely with the resistance. By rotating the knob  282  of the selector mechanism, the user selects a longer or shorter passageway through which air must pass and a correspondingly larger or smaller resistance to flow. When the first outlet  272   a  is selected, air has a minimum distance to traverse on the plate. Specifically, air travels only through one connector channel  274 , from the inlet  270  to the outlet  272   a . When the second outlet  272   b  is selected, air must travel one down-and-back reach  268   b  along the plate. Successive outlets provide additional length, with incremental addition of one successive down-and-back reach  268  in series for each outlet. When the final (seventh) outlet  272   g  is selected, air must traverse all six of the down-and-back reaches  268   b - 268   g  on the plate, thereby providing maximum frictional resistance and minimum flow rate. It is understood that other arrangements, such as passageways having distinct inlets or which are not arranged in series, alternative patterns, orientations, channel densities or spacings, or a fewer or greater number of channels, inlets, or outlets, do not depart from the scope of this invention. The reaches may have equal lengths resulting in approximately equal increments in resistance to airflow, or alternatively may vary, as shown by the final (sixth) reach  268   g  on  FIG. 4A  which is relatively shorter, to vary incremental resistance. 
   The bleed system  2  of the present invention is repeatable because flow paths and cross-sectional areas do not change from one use to the next. There is no variation due to manufacturing tolerances as with conventional bleed valves. Consequently, the bleed system may be accurately calibrated for flow rate or other variable of interest. For use with the air motor  1 , settings of the knob  282  may be calibrated corresponding to specific speed limits so that the maximum, predetermined cut-off speed of the air motor can be selected, e.g., 50 or 75 cycles per minute. The bleed system is attached to the air motor as shown in  FIG. 2 , and it replaces the bleed valve  171  of the prior art. With the exception of that replacement and the improvements resulting therefrom as described above, the air motor  1  and runaway control are unchanged. The bleed system is reliable, durable, and inexpensive to produce. 
   The bleed system can be used in other applications, e.g., compressors, valve systems, fuel injectors, power generator systems, medical and dental devices, and spraying systems. 
   A second embodiment of the invention ( FIGS. 5A and 5B ) includes a plate  300  with channels  230  formed in both opposing faces  260 ,  262 . The plate  300  has connecting holes indicated at  302  for passing air from a channel  230  on the front face  260  to a channel on the back face  262 . The arrangement is such that turning the selector knob  282  to successive outlet holes  272  increments the distance traveled by the air by one, two or more down-and-back reaches  268  of grooving. This configuration increases the length of passageway on the plate compared to a plate  220  of the first embodiment having single face grooves, and is done so without increasing the size of the plate or density of grooves. For example, when the second outlet  272   b  is selected on the plate of  FIG. 5A , air must travel two reaches  268  along the length of the plate, whereas only one reach is traversed on the plate of FIG.  4 A. Thus, the plate  300  of the second embodiment of  FIG. 5A  provides additional length and air resistance. Two gaskets  240  are required for sealing passageways on both faces of the plate  300 . 
   A third embodiment of the invention, shown schematically in  FIG. 6 , includes a bleed system indicated generally at  330 . The system  330  comprises a bleed valve  332  which replaces the prior art bleed valve  171  of FIG.  1 D. The valve  332  is in communication with passageway  173  and adjustable for varying the rate of bleed from chamber  169  to change the cut-off speed of the air motor  1 . The bleed valve  332  includes a piston  334  movable in a bore  336 . Advantageously, the piston and bore have circular cross-sections, although other shapes are acceptable. The piston  334  has a size which is slightly smaller than the bore  336  thereby providing a clearance fit. An annular gap between the piston and bore defines a passageway  338  for flow of gas. The gas is then vented through passages  340  to the surrounding air. An inlet  342  of the passageway  338  is positioned adjacent a distal end of the piston (the left hand end in FIG.  6 ), and an outlet  344  is defined along the bore  336  at the beginning of each passage  340 . 
   An adjustment mechanism comprising a knob  346  is provided for varying a length L of the passageway  338 . The knob is firmly connected to piston  334  by a rod  348  such that translation of the knob moves the piston in the bore. A locking mechanism (not shown) is provided to fix the knob  346  and piston  334  at selected positions. 
   Movement of the piston varies the length L of the passageway  338  between inlet  342  and outlet  344  to vary resistance to flow of gas through the passageway. The location of inlet  342  varies with movement of the piston, while location of outlet  344  is fixed, such that the length of the path is adjustable. Significantly, the length L of the path is continuously adjustable (i.e., is not limited to discrete increment or decrement units) as the piston is moved to any selected position. Consequently the cut-off speed of the air motor  1  may be adjusted with improved resolution. 
   A fourth embodiment of the invention, shown in  FIG. 7 , includes a bleed system indicated generally at  360 . A bleed valve  362  comprises a threaded screw  364  received in a threaded bore  366  defining a gas passageway  368  extending in a helical path along the threads. Rotation of the screw adjusts the length of the passageway to vary resistance to flow of gas through the passageway. An inlet  372  of the passageway  368  is positioned adjacent a distal end of the screw (the left hand end in FIG.  7 ), and an outlet  374  is defined along the bore  366  at an expanded opening. An adjustment mechanism comprising a knob  376  is provided for rotating the screw and varying the length of the passageway  368  between the inlet  372  and the outlet  374 . A locking mechanism (not shown) is provided to fix the knob  376  and screw  364  at selected positions. The length of passageway  368  is continuously adjustable as the screw is moved to any selected position so that the cut-off speed of air motor  1  may be adjusted with improved resolution. 
   A fifth embodiment of the invention, shown in  FIG. 8 , includes a bleed system indicated generally at  380 . A bleed valve  382  comprises a plug  384  having a conical shape movable within a conically-shaped bore  386  having a taper generally corresponding with the shape of the plug. Advantageously, the plug  384  comprises a frustum although other shapes are suitable. A gas passageway  388  is defined in an annular gap between the plug and the bore. Movement of the plug  384  by adjustment knob  396  changes both clearance between the plug and bore  386  and length of passageway  338 , thereby adjusting resistance to flow of gas by a combination of cross-sectional flow area and length. A locking mechanism (not shown) is provided to fix the knob  396  and plug  384  at selected positions. The area of passageway  388  is continuously adjustable as the plug is moved to any selected position so that the cut-off speed of air motor  1  may be adjusted with improved resolution. 
   In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
   When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
   As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description as shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.