Abstract:
A limit switch having a housing carrying one or more components that are individually field replaceable by an operator so that an operator can make an on-the-go field repair any faulty component in the limit switch. The field replaceable components can be frictionally or mechanically secured by a finger operable latch or the like so that an operator can remove and quickly replace a faulty component while the limit switch is connected a pneumatic conveying system or the like.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a limit switch and, more particularly, to a field serviceable limit switch that can provide an operator with information on the condition of a component or system such as on a pneumatic conveying system.  
       CROSS REFERENCE TO RELATED APPLICATIONS  
       [0002]     none  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0003]     None  
       REFERENCE TO A MICROFICHE APPENDIX  
       [0004]     None  
       BACKGROUND OF THE INVENTION  
       [0005]     The concept of limit switches for indication of the status of a component is generally known in the art In general, indicators with electrical switches and cam members are packaged as a unit When the limit switch malfunctions in the field one has to replace the entire limit switch. As the replacement can be time consuming as well as costly it is desirable to eliminate the system down time as well as the cost to replace an entire limit switch.  
         [0006]     The present invention provides a field serviceable limit switch that includes a status indicator that visually alerts an observer to the condition of a particular component or components of a system such as a valve or the like in a pneumatic conveying system. The limit switch can also be used to control various other components in the pneumatic system as well as to indicate the presence of a faulty or dangerous condition in a system such as in a pneumatic conveying system.  
         [0007]     Typically, the field serviceable limit switch can include such items as field replaceable electrical switches, field replaceable terminal blocks, field replaceable cams and field replaceable cam holders, field replaceable transducers, field replaceable indictors and a field replaceable transparent housing. By field replaceable part it is meant that an individual part can be quickly replaced by an operator in the field with few or no hand tools and without the need to replace the entire limit switch.  
       SUMMARY OF THE INVENTION  
       [0008]     Briefly, the present invention comprises a limit switch having a housing carrying one or more components that are individually field replaceable by an operator so that an operator can make an on-the-go field repair any faulty component in the limit switch. The field replaceable components can be frictionally or mechanically secured by a finger operable latch or the like so that an operator can remove and quickly replace a faulty component while the limit switch is connected a pneumatic conveying system or the like. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is an exploded view of the limit switch with an alerter;  
         [0010]      FIG. 2  is a side view of the limit switch in an assembled condition; and  
         [0011]      FIG. 3  is a partial side view of the of the cam with the cam rollers on electrical switches carried in the housing of the limit switch. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]      FIG. 1  shows an exploded view of a limit switch  10 . Limit switch  10  includes a base  11  for securing to a mount or the like. Base  11  carries a square shaped housing  12  having a chamber  13  therein. Housing  12  contains a set of snap latches  12   a  located on opposite sides for releasable securing a cover  65  thereto. Each of snap latches  12   a  include a fastener  12   c  for securing the snap latch  12   a  to the housing  12 . Located around the top peripheral region of housing  12  is a lip  12   b  that forms a shoulder for supporting cover  65  thereon. Base  11 , housing  12  and cover  65  are preferable made from a polymer plastic or the like.  
         [0013]     Located within the housing  12  are a number of replaceable components that are releasably secured to permit field replacement of the limit switch components thus avoiding the need to replace an entire limit switch if one component fails.  
         [0014]     Limit switch  10  includes an alerter or visual indicator  60  to provide an observer a status indication of the equipment being monitored. For example, the limit switch might be used to indicate whether a valve is in an open or closed condition as well as to control other components in a system such as a pneumatic conveying system.  
         [0015]      FIG. 1  shows a centrally positioned cam shaft  40  rotationally supported in housing  12 . Located on cam shaft  40  is a cam latch  40   a  for removably securing a cam  55  thereon. Cam  55  includes a cross shaped slot  55   a  for mechanical engagement with cam shaft  40  to permit the cam shaft  55  to rotate the cam  55 . Located on one side of cam  55  is a cam lobe  55   b  and on the other side is a cam lobe  55   c . In assembly of cam shaft  40  and cam  55  the cam  55  is slid longitudinally downward along shaft  40 , which forces the cantilevered resilient latch  40   a  to cantilever inward and allow cam  55  to slide therepast Once the cam  55  is past the cam latch  40   a  the resilient cam latch  40   a  snaps outward to prevent withdrawal of the cam  55  from the cam shaft  40   a  To remove and replace cam  55  one can squeeze cam latch  40   a  toward cam  40  thus allowing cam  55  to be slide upward and off the cam shaft  40 .  
         [0016]     The lower end of cam shaft  40  includes an end  40   b  for engaging with a device that connects to the system being monitored. For example, cam shaft end  40   b  can be connected to the pivot member on a butterfly type valve in a pneumatic conveying system to cause the cam shaft  40  to rotate as the butterfly valve is opened or closed. The top end  40   c  of cam shaft  40  engages a dome shaped visual indicator  60 . Dome shape indicator  60  includes a female receptacle (not shown) that mechanically connects to cam shaft  40  to cause the indicator  60  to rotate as the cam shaft  40  rotates. The dome shaped indicator  60  includes a first peripheral region  60   a  with a first readable visual message such as “CLOSED” and a second peripheral region  60   b  with a second readable visual message such as “OPEN”.  
         [0017]     In order for a person to obtain the information on the system or component status a masking cover  70  is provided that includes a first peripheral opening  70   a  that permits viewing one or the other of the readable message  60   a  and  60   b  therethrough. That is, on one condition the “OPEN” message can be visible through peripheral region  70   a  to let the operator no that the component being monitored is in an open condition. Similarly, in another condition the “CLOSED” message can be visible through opening  70   a  to let the observer know the component is in a closed condition.  
         [0018]     Indicator  60  provides dual signals or dual visual indicators, one that is visible in a lateral direction from the limit switch and one that is visible from above the limit switch. The second visual indicator comprises a color coded regions  60   c  which comprises truncated pie shaped segments on the top surface of indicator  60 . For example, regions  60   c  can be colored red so as to indicate that a component being monitored is in a closed condition. Spaced adjacent to color coded peripheral region  60   c  are two additional truncated pie shaped colored regions  60   d . Regions  60   d  can be color coded green to indicate the monitored component is in the open condition.  
         [0019]     Located on top of indicator drum  60  is masking cover  70  that includes a first truncated pie shaped opening  70   c  and a second diagonally positioned truncated pie shaped opening  70   b . In operation of the limit switch  10  the color coded regions  60   c  or  60   d  are visible through the truncated pie shape openings  70   b  and  70   c . For example, in one condition color coded regions  60   c , which may be red to indicate a closed condition, are visible though; the openings  70   b  and  70   c  to indicate the component is in a closed condition. In another condition the color coded regions  60   d , which can be green to indicate an open condition, are visible through the openings  70   c  and  70   b  to indicate the component is in an open condition.  
         [0020]     The use of a solid color coded region also provides a proportional signal. That is, if a portion of the red coded region  60   c  and a portion of the green coded region  60   d  are visible in the opening  70   c  and  70   b  one can determine the condition of the component. For example, one could determine that the component being monitored, such as a butterfly valve, is in a 50% open condition by observing that half of the green colored region  60   d  is visible and half of the red colored region  60   c  is visible through the openings  70   b  and  70   c.    
         [0021]     In order to protect the indicator  60  from the environmental conditions there is provided a cup shaped transparent member  75  that fits over the masking cover  70  and the visible indicator  60  to permit an observer to observe the visual signals from the indicator  60  from either a lateral position with respect to the limit switch  10  or a position above the limit switch  10 .  
         [0022]      FIG. 2  shows the transparent member  75  and masking cover  70  can be secured directly to the top of cover  65  along the circumferential edge  65   a  on cover  65 . This allows one to assemble the cover  65  and transparent member  75  as a unit to the housing  12 . A set of four identical tabs  65   b  located on opposite sides (only two are shown) allow one to use snap latches  12   a  to secure the cover  65  to the unit to form a closed and protected limit switch with alerter (see  FIG. 1 ) . Thus a person positioned laterally of limit switch  10  can view the indicator  60  thorough the peripheral opening  70   a  in masking cover  70 .  
         [0023]     The cam shaft  40   a  and indicator  60  in cooperation with cover  65 , housing  12  and transparent cover  75  can provide a visual indication of the status of a component through the rotational displacement of the cam shaft  40 . A further feature of the invention is that the cam shaft  40  can be used to control an electrical circuit. In order to appreciate the control of an electrical circuit reference should be made to  FIG. 1 , which shows a switch holder  30  comprising a box like member extending upwardly in the housing  12 . Switch holder  30  contains a chamber  30   a  for receiving a first electrical switch  31  having a cantilevered roller arm  31   a  and second electrical switch having a cantilevered rolling arm  33   b . Such switches are well known in the art and are readily available as off-the-shelf items. The switches have two conditions, an off condition and an on condition. In one type of switch the off condition occurs when the roller arm is depressed and in another type of switch the on condition occurs when the roller arm is depressed. Each of the switches  31  and  32  are peripherally held in position by the sidewalls of the box like switch holder  30  so that no screws or the like are required to secure the switches in an operating condition proximate cam  55 .  
         [0024]      FIG. 3  shows an isolated view of the switch holder  30  and the cam  55  in engagement with each other. In this embodiment two switches  31  and  33  are removably secured in switch holder  30  by a cover  30   a  having a lip or latch  30   b  that engages switch holder  30  to secure the switches  31  and  33  in a fixed condition in switch holder  30 . In this condition the cantilevered resiliently mounted roller arms  31   a  and  33   b  protrude outward from switch holder  30  and are in an operable position to engage the cam  55 . In the position shown a cylindrical cam surface  55   c  is in contact engagement with roller arm  33   b  and the cam lobe  55  is in contact engagement with roller arm  31   a . Additional cam lobes  55   d  and  55   e  provide for additional contact with the roller arms. As cam shaft  40  rotates the different cam lobes  55   d  and  55   e  are brought into contact with the roller arms on the switches  31  and  33  to bring the switches  31  and  33  to proper on or off condition. Thus, a feature of the invention is that a conventional electrical switch can be quickly inserted or removed from switch holder  30  by merely engaging or disengaging snap latch cover  30   a  which includes ears for engaging the peripheral lip extensions  30   d  on switch holder  30 . In the embodiment shown switch holder  30  includes a base  30   e  that can be secured to extension  12   e  in housing  12  to allow for removal and replacement with few or no hand tools. (See  FIG. 1 )  
         [0025]     A further feature of the invention is the inclusion of an electrical terminal block holder  20  which is shown in  FIG. 1 . Terminal block holder  30  comprises a frame like member having a set of cylindrical pegs  20   c  that extend upward. The lower portion of block holder  20  includes an extension  20   a  for frictionally engaging extension  12   f  in housing  12  to hold electrical terminal block holder  20  thereon. Once the terminal block holder  30  is secured to housing  12  the terminal block  25  is positioned on the cylindrical pegs  20   c  to prevent lateral movement thereon. The side member  20   b  of block holder  20  comprises a resilient member having a lip (not shown) that latchingly engages one end of the terminal block  25  to hold one end of the terminal block in position in housing  12 . An identical side member (not shown) is located on the opposite end of block holder  20  to hold the opposite end of the terminal block  25  in position.  
         [0026]     An interface device  15 , which can be a transducer or the like that relates a pneumatic condition to an electrical condition, can be secured to housing  12  through an extension  15   a  to enable a pressure signal to be transferred through connection  12   c  to transducer  15  though the sidewall of housing  12 . The interface device  15  includes a set of electrical leads  15   b  for connection to the electrical terminal block  25 . The electrical leads are omitted for purposes of clarity, but in operation the electrical leads  15   b  can extend from the interface device  15  to the terminal block  25  and from the terminal block  25  to the connector lugs (not shown) on the electrical switches  31  and  33 . Thus a further feature of the invention is that the terminal block is removably mounted in housing  12  and can be replaced as well as the terminal holder  20 . In addition, the chamber  13  in housing  12  can carry both electrical components as well as mechanical components with either or both of the electrical and mechanical components field replaceable.  
         [0027]     A further feature of the invention is that extending partially around cam shaft  40  and extending vertically upward is a wire guard  50  that is fixedly supported in housing  12 . Wire guard  50  extends upward and circumferentially to prevents wires in housing  12  from being accidentally caught by the rotating cam shaft  40   b.    
         [0028]     In operation of the limit switch an operator can quickly open the limit switch by releasing the latch  12   a  to allow removable of the cover  65 . Once the cover  65  is removed the operator can obtain access to the components that are held in the housing  12 . An operator can then quickly replace the faulty component and replace the cover thus allowing the system to be up and running with a minimum of delay.