Abstract:
A calibration mechanism for setting the tripping point of a microswitch in a pressure switch has a lower collar fixed to the upper end of the pressure switch push rod, and an upper collar disposed above the lower collar and adapted to actuate the trigger of the microswitch when raised a sufficient distance above the lower collar. The upper and lower collars each have a threaded bore, each with a different thread pitch, plus guide means for keeping the threaded bores aligned for receiving a double-threaded adjustment screw having an upper section threaded to engage the upper collar&#39;s threaded bore, and a lower end threaded to engage the lower collar&#39;s threaded bore. Due to the different thread pitches, rotation of the adjustment screw will cause gradual movement of the upper collar either toward or away from the lower collar, thus facilitating fine adjustment of the microswitch trigger point. The calibration mechanism may also include a reset mechanism, operable on similar principles to set the mieroswitch&#39;s reset point.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to pressure-actuated switches for interrupting an electrical circuit in response to changes in pressure in a fluid flow line or vessel, and relates in particular to mechanisms for calibrating and resetting such switches. 
       BACKGROUND OF THE INVENTION 
       [0002]    Pressure switches are widely used in industrial applications relating to process fluids. They are used to regulate pump and compressor operation as well as liquid levels in tanks within specific predetermined pressure ranges. These pressure switches typically have two set points: a high (or tripping) pressure, and a low (or reset) pressure. 
         [0003]    Oilfield pumps and vessels containing liquid are common applications for pressure switches. Oilfield pumps are set up with a pressure switch located in the discharge line to detect high and low pressures. When the pressure within the discharge line senses the tripping pressure, the switch triggers the pump to shut down. When the line pressure drops below the reset pressure, the switch will trigger the pump to resume operation. 
         [0004]    Another common application for pressure switches is in association with a tank containing liquid, where a pump is required to fill or empty the vessel. A pressure switch allows the operator to set the pump to operate automatically within specific liquid head pressures. 
         [0005]    As may be seen by way of example in U.S. Pat. No. 5,554,834 and U.S. Pat. No. 5,670,766, a conventional pressure switch typically features an electrical enclosure at its upper end and a spring body and process connection at its lower end, which is mountable to an opening in a pipeline, vessel, or other component containing a fluid. A metal push rod is slidably mounted within the spring body housing, with its lower end operatively engaged with a metal piston and metal diaphragm assembly which closes off the process connection, such that the diaphragm will be exposed to pressure from the pipeline or vessel. The upper end of the push rod extends into the electrical enclosure. An electrical microswitch is disposed within the electrical enclosure and securely fastened to a mounting bracket. The microswitch has conventional contacts for wiring to whatever electrically-actuated device the pressure switch, is intended to control. The microswitch also has, on its lower side, a plunger or trigger which when pressed into the microswitch (i.e., upward relative to the enclosure) will trip the microswitch. 
         [0006]    The assembly described above is configured such that upward movement of the piston is transferred to the push rod in response to external fluid pressure applied to the diaphragm, such that the push rod trips the microswitch. The specific mechanism used to translate push rod movement into trigger movement may vary from one manufacturer to the next. 
         [0007]    A conventional pressure switch typically incorporates a spring assembly including a helical spring of suitable stiffness, disposed around the push rod and extending between the diaphragm end of the push rod and an upper abutment within the enclosure. This spring assembly provides a resistive force necessary to maintain a specific range of pressure to both trip and reset the device. Accordingly, a higher tripping pressure will entail a higher degree of spring, compression. To facilitate adjustment of the spring compression, the aforementioned abutment is longitudinally movable within the pressure switch housing. 
         [0008]    One of the critical challenges in the design of pressure switches is to provide for accurate and reliable pre-setting of desired tripping and reset pressures, which essentially boils down to finely controlled adjustment of the gap between the microswitch and push rod assembly. 
         [0009]    In some pressure switch designs, the microswitch could be tripped by effectively direct actuation of the trigger by the upper end of the push rod; in such designs, however, accurate control of the gap between the push rod and microswitch trip button would be difficult, since it would require the components to be machined to unrealistic tolerances. 
         [0010]    In other designs, a deformable offset trip plate (typically made of steel) may be provided in association with the microswitch such that the trigger is laterally offset from the axis of the push rod, but upward movement of the push rod will raise the free end of the trip plate, in turn causing another portion of the trip plate to exert an upward force on the trigger. However, this involves a tedious and time-consuming trial-and-error procedure. With the switch partially disassembled, the trip plate must be bent into a trial position, whereupon the switch is reassembled and connected to an external pressure source to determine the actual tripping pressure that corresponds to the trip plate position. If the gap between the push rod and the microswitch is too wide or too narrow, the switch must be disassembled again so that the trip plate can be bent one way or the other into a new trial position, and then the switch is reassembled and tested again. This procedure is followed until the trip plate is in a position that produces; the appropriate gap between the push rod and the microswitch. 
         [0011]    For the foregoing reasons, there is a need for a pressure switch calibration mechanism that facilitates fine adjustment of the gap between the push rod and microswitch more easily and more quickly than is possible with typical conventional switches, and without need for trial-and-error methods. The present invention is directed to this need. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0012]    In one aspect, the present invention is a calibration mechanism for fine adjustment of the tripping point of a microswitch (or other suitable mechanically-actuated electrical switching device) associated with a pressure switch. The calibration mechanism includes a lower collar that is mountable to the upper end of the push rod of the pressure switch, and an upper collar that is vertically movable relative to the lower collar by means of a double-threaded calibration adjustment screw that has a lower section threadingly engageable with a threaded bore in the lower collar, and ah upper section threadingly engageable with a threaded bore in the upper collar. 
         [0013]    The pitches of the upper and lower threads of the lower section of the calibration adjustment screw are slightly different, such that rotation of the adjustment screw in a first direction (typically but not necessarily clockwise) will cause the upper collar to move toward the lower collar, and rotation in the opposite direction will move the upper collar away from the lower collar. If the thread, pitches were identical, the relative movement between the collars would be zero. Therefore, for a given amount of rotation of the calibration adjustment screw, the differing thread pitches will cause the collars to travel at different rates along the calibration adjustment screw as it is rotated. 
         [0014]    The differential thread pitch between the upper and lower collars thus facilitates fine adjustment of the position of the upper collar relative to the push rod. The precision with which relative movement of the upper collar can be controlled will depend on the absolute values of the two thread pitches as well as the difference between them. For example, if the pitch of the lower section of the calibration adjustment screw (i.e., the section that engages the lower collar) is 20 threads per inch (tpi), and the pitch of the upper section (which engages the upper collar) is 24 tpi, each full rotation of the calibration adjustment screw will change the distance between the two collars by 1/20 th  of an inch (0.05″) minus 1/24 th  of an inch (0.04166″), or only 0.00833 inches, even though the screw is withdrawn 0.05″ from the lower collar. If the lower and upper thread pitches are changed to 12 tpi and 16 tpi respectively, each full rotation of the calibration adjustment screw will change the distance between the two collars by 1/12 th  of an inch (0.08333″) minus 1/16 th  of an inch (0.06250″), or 0.02083 inches, which is considerably greater than in the first example even though the thread pitch differential is 4 tpi in both cases. Accordingly, the absolute and differential values of the thread pitches may be selected to suit the degree of calibration precision desired for a given switch application. 
         [0015]    The calibration mechanism is provided with guide means whereby the upper collar will remain aligned with the lower collar as it moves toward or away from the lower collar in response to rotation of the calibration adjustment screw. A preferably upset portion of the upper surface of the upper collar (the “switch contact area”) is configured or adapted for contacting the trigger on the lower side of the microswitch. 
         [0016]    To calibrate a pressure switch incorporating the calibration mechanism of the present invention, the lower end of the switch is connected to a pressure source corresponding to the desired tripping pressure for the switch. The calibration adjustment screw is rotated as required to minimize the gap between the upper and lower collars, and the longitudinal position of the push rod within the electrical enclosure is coarsely set such that the switch contact area is disposed slightly below the microswitch trigger (or in contact with the trigger without tripping it). Rotation of the Calibration adjustment screw in the appropriate direction will then raise the switch contact area of the upper collar so as to depress the trigger until the trigger actuates the microswitch, at which point rotation of the calibration adjustment screw is stopped. The pressure switch is how calibrated to trip at the desired tripping pressure. 
         [0017]    In preferred embodiments, the calibration mechanism also incorporates a reset mechanism for use in conjunction with a microswitch having a reset button on its upper side. As will be explained in greater detail further on in this specification, the reset mechanism employs functional principles similar to those used in the calibration mechanism. The reset mechanism includes a crossbar disposed transversely above the reset button and supported so as to move in accordance with longitudinal movements of the push rod. The crossbar has a threaded bore for engaging the upper section of a double-threaded reset adjustment screw. Also provided is a reset contact button disposed generally below the crossbar but with a transverse slot on its upper side, such that the crossbar fits within the slot while leaving the contact button free to move longitudinally relative to the crossbar, but at the same time substantially prevented from rotating relative to the crossbar. The contact button has a threaded bore extending downward from the bottom of the slot and alignable with the threaded bore of the crossbar when the crossbar is disposed within the slot. 
         [0018]    The thread pitch in the contact button is slightly different from the thread pitch in the crossbar, correspondingly, the lower section of the reset adjustment screw will have a thread pitch different from that of the upper section. Accordingly, rotation of the reset adjustment screw in a first direction (typically but not necessarily clockwise) will cause the contact button to move away from the crossbar and toward the microswitch, thus causing the contact button to depress the reset button on the microswitch. It follows that rotation of the reset adjustment screw in the opposite direction will move the contact button upward relative to the crossbar, such that continued upward movement of the contact button will cause it to move away from the microswitch reset button resulting in a wider reset point for the switch. This adjustment provides movement necessary to set a specific reset point for the microswitch, thus setting the desired “dead band” for a given application (i.e., the pressure range within which the controlled electrical device will not operate). 
         [0019]    If, for example, it is desired for a pressure switch to trip when the pressure in a pipeline to which it is mounted reaches 500 pounds per square inch (psi), and it is further desired for the microswitch to be automatically reset when the pipeline pressure falls to 300 psi, the pressure switch is calibrated as previously described, with the switch being exposed to a pressure source set at 500 psi. The pressure source is then reduced to 300 psi, whereupon the reset adjustment screw is rotated as required until it depresses the reset button and thus resets the microswitch. The pressure switch is then ready to enter service in an actual field application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0021]      FIG. 1  is a first longitudinal cross-section through a pressure switch having calibration and reset mechanisms in accordance with one embodiment of the present invention. 
           [0022]      FIG. 2  is a longitudinal cross-section through the pressure switch shown in  FIG. 1 , taken at 90 degrees to the cross-section of  FIG. 1 . 
           [0023]      FIG. 3  is a perspective view of calibration and reset mechanisms in accordance with an embodiment of the present invention, shown assembled in conjunction with a microswitch having a reset button. 
           [0024]      FIG. 4  is a perspective view of the calibration and reset mechanisms shown in  FIG. 3 , shown in isolation. 
           [0025]      FIG. 5  is a partially cutaway view of the calibration and reset mechanisms shown in  FIG. 4 , shown installed on the upper end of a pressure switch push rod. 
           [0026]      FIGS. 6A and 6B  are, respectively, an upward-looking view of the lower side of the upper collar of the calibration mechanism of  FIG. 4 , and a transverse cross-section through same. 
           [0027]      FIGS. 7A and 7B  are, respectively, a plan view of the upper side of the lower collar of the calibration mechanism of  FIG. 4 , and a transverse cross-section through same. 
           [0028]      FIG. 8  is a perspective view of a calibration adjustment screw for use in association with the calibration mechanism shown in  FIG. 4 . 
           [0029]      FIG. 9  is a perspective view of the crossbar of the reset mechanism shown in  FIG. 4 . 
           [0030]      FIGS. 10A and 10B  are, respectively, a perspective view of the contact button of the reset mechanism shown in  FIG. 4 , and a transverse cross-section through same. 
           [0031]      FIG. 11  is a perspective view of a reset adjustment screw for use in association with the reset mechanism of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]      FIGS. 1 and 2  are cross-sections (taken 90 degrees to each other) through a pressure switch  10  incorporating calibration and reset mechanisms in accordance with one embodiment of the present invention. Pressure switch  10  comprises a lower switch body  12  which encloses a longitudinally-oriented spring chamber  20  which is open at its lower end. A diaphragm assembly  28  is disposed within the lower region of spring chamber  20  so as to effectively close off the open lower end thereof. A suitable fitting  18  is provided at the lower end of lower switch body  12  for connection to a pressurized pipeline, pressure vessel, or the like, such that diaphragm assembly  28  will be exposed to whatever fluid pressure is in the pipeline or pressure vessel. A piston  26  is provided in association with diaphragm assembly  28 , with piston  26  being slidingly movable within spring chamber  20 . A push rod  22  is disposed within spring chamber  20 , with its upper end  20 U slidably projecting through the upper end of lower switch body  12 , and with its lower end  20 L engaging piston  26  such that transverse deformations of diaphragm assembly  28  in response to external pressure will cause corresponding longitudinal movement of push rod  22  relative to lower switch body  12  (in accordance with technologies well known in the art of pressure switches). 
         [0033]    A helical spring  24  is disposed around push rod  22  within spring chamber  20 , for regulating the amount of external pressure required to move the push rod (i.e., the required or desired spring compression will increase with the desired tripping pressure of pressure switch  10 ). In the pressure switch shown in  FIGS. 1 and 2 , the lower end of spring  24  bears against piston  26  and the upper end of spring  24  bears, against an upper spring abutment  27  that is movable longitudinally within spring chamber  20 . The degree of compression in spring  24  is adjustable by rotation of a spring adjustment sleeve  14  which is disposed around an upper region of lower switch body  12  and threadingly engaged therewith. Sleeve  14  is operatively linked with upper spring abutment  27  (in accordance with well-known methods) such that rotation of sleeve  14  in a first direction (typically but not necessarily clockwise) will compress spring  24 , and rotation of sleeve  14  in the opposite direction will relieve compression an spring  24 . 
         [0034]    The foregoing components of pressure switch  10  have been described in general and representative terms only, because the specific details of these components are not directly relevant to the present invention. The construction of lower switch body  12  and its various components can generally conform with known technology in the field of pressure switches without affecting the scope of the present, invention. What is important for specific purposes of the present invention is that pressure switch  10  has a push rod  22  which is slidable within lower switch body  12  in response to external pressure acting on diaphragm assembly  28 . 
         [0035]    Pressure switch  10  also comprises an upper switch body  16  which is engageable with lower switch body  12  and which has a removable cover section  17 . Upper switch body  16  and cover section  17 , when assembled, define a switch chamber  30  in which a microswitch  40  is mounted. As best seen in  FIG. 3 , microswitch  40  has multiple electrical contacts  42  for wiring to an electrical device (hot shown) controlled by pressure switch  10 . On its lower side, microswitch  40  has a trigger  44  which upon being upwardly depressed will trip microswitch  40  and thus disconnect the connected electrical device. Microswitch  40  may also have a reset button  45  which may be downwardly depressed to reset microswitch  40  after it has been tripped, thus allowing the controlled electrical device to be reset and resume operation (until such time as microswitch  40  is tripped again). 
         [0036]      FIG. 3  illustrates calibration and reset mechanisms in accordance with one embodiment of the present invention, mounted in conjunction with microswitch  40 . The individual components of the calibration and reset mechanisms are perhaps most readily understandable from  FIGS. 4 and 5 , which illustrate the calibration and reset mechanisms in isolation from microswitch  40 . In the embodiment shown in  FIG. 3 , microswitch  40  is mounted to a mounting block  46  which has vertical bores or holes to receive mounting bolts  47 , the lower ends  47 L of which are matingly engageable with threaded bores in a portion of lower switch body  12  (or a suitable appurtenance rigidly connected thereto). 
         [0037]    Referring now to  FIGS. 3 ,  4 ,  5 ,  6 A,  6 B,  7 A, and  7 B, the calibration mechanism of the invention comprises a lower collar  60 , preferably (but not necessarily) in the form of a generally disc-shaped element having an upper surface  60 U and a lower surface  60 L. In preferred embodiments, a centrally located (and preferably cylindrical) abutment  62  extends upward from upper surface  60 U, and a center bore  63  extends through the full thickness of lower collar  60  and abutment  62 . Additional features of lower Collar  60 , in its preferred embodiment, are illustrated in further detail in  FIGS. 7A and 7B . 
         [0038]    Lower collar  60  is connected to the upper end  22 U of push rod  22 . In the illustrated preferred embodiment, this correction is facilitated by providing a threaded section on upper end  22 U of push rod  22 , and by providing mating threads in at least a portion of center bore  63  of lower collar  60 , thus allowing lower collar  60  to be screwed securely onto upper end  22 U of push rod  22 . However, lower collar  60  could be connected to push rod  22  by other means (e.g., press fit; splined connection; welding) without departing from the present invention. A pair of guide rod holes  61  extend downward from upper surface  60 U into the thickness of lower collar  60 , One on either side of threaded bore  63 . Also provided is a threaded bore  66  extending downward from upper surface  60 U into the thickness of lower collar  60 , with threaded bore  66  having a first thread pitch. 
         [0039]    The calibration mechanism also includes an upper collar  50  which is preferably (but not necessarily) disc-shaped, with ah upper surface  50 U and a lower surface  50 L. As best seen in  FIGS. 6A and 6B , upper collar  50  preferably has a recess  54  set into its lower surface  50 L, sized and configured to receive abutment  62  of lower collar  60  in a sliding tolerance fit, thereby helping to keep upper collar  50  and lower collar  60  in coaxial alignment when upper collar  50  moves relative to lower collar  60 . Although not shown, a spring washer may optionally be; disposed between abutment  62  and the “roof” of cylindrical recess  54 , to help keep upper collar  50  generally parallel to lower collar  60 . 
         [0040]    Upper collar  50  also preferably has a centrally-positioned upstand  52  projecting above upper surface  50 U, and a centrally-positioned smooth bore  53  extending through upstand  52  and intercepting cylindrical recess  54 . Smooth bore  53  is provided to receive, in a sliding tolerance fit, an unthreaded section of upper end  22 U of push rod  22 , above the threaded portion of upper end  22 U. This feature is advantageous as further means to help keep upper collar  50  in true alignment with lower collar  60 , but it is optional and not essential to the invention. In alternative variants, upper end  22 U of push rod  22  need not extend above lower collar  60  so long as means are provided for keeping upper collar  50  in substantial alignment with lower collar  60 . 
         [0041]    Upper collar  50  has a pair of guide rod holes  51  extending through the full thickness of upper collar  50 , and sized and spaced to match guide rod holes  61  in lower collar  60 . Upper collar  50  also has a treaded bore  56  extending downward from upper surface  50 U and through the full thickness of upper collar  50 , with threaded bore  56  having a second thread pitch different from the previously mentioned first, thread pitch of threaded bore  66  of lower collar  60  (i.e., the first and second threads have different numbers of threads per inch). 
         [0042]    The assembly of the calibration mechanism can now be readily understood with reference to  FIGS. 3 ,  4 , and  5 . A pair of guide rods  90  are provided, each having a lower end  90 L and an upper end  90 U. Guide rods  90  are connected to lower collar  60  by securing their lower ends  90 L into guide rod holes  61  (by means of a threaded connection or a press fit, or other effective means), thus leaving guide rods  90  projecting upward from lower collar  60 . Upper collar  50  may then be slipped over guide rods  90  (which slide through guide rod holes  51 ), thus positioning upper collar  50  directly above lower collar  60  (with abutment  62  of lower collar  60  nested within cylindrical recess  54  of upper collar  50 , in the preferred embodiment) and with threaded bores  56  and  66  in axial alignment. A double-threaded calibration adjustment screw  58  may then be used to precisely adjust the position of upper collar  50  relative to lower collar  60 , as will be described below. 
         [0043]    The assembled calibration mechanism is mounted to pressure switch  10  by connecting lower collar  60  to upper end  22 U of push rod  22  (by twisting lower collar  60  onto the threaded portion of upper end  22 U in preferred embodiments, or by other effective means). Microswitch  40  is then mounted so as to be disposed between guide rods  90  generally as shown in  FIGS. 1 and 3 , with upstand  52  of upper collar  50  positioned below trigger  44  of microswitch  40 . As previously noted, and as may be seen in  FIGS. 1 and 3 , the mounting of microswitch  40  within pressure switch  10  may be facilitated by providing a mounting block  46  which has vertical bores to receive mounting bolts  47 , the lower ends  47 L of which are matingly engageable with threaded bores in a portion of lower switch body  12 . In the illustrated embodiment, mounting block  46  has an additional bore  48  which passes over one of the guide rods  90  when microswitch  40  is installed, thereby helping microswitch  40  maintain a fixed lateral position relative to the calibration mechanism. Notwithstanding the benefits of the configuration discussed above, however, the use of a mounting block is not essential to the invention. Persons skilled in the art will appreciate that other ways or means for installing microswitch  40  in operative association with the calibration mechanism of the invention can be readily devised. 
         [0044]    As best seen in  FIG. 8 , calibration adjustment screw  58  has an upper section  58 A threaded to mate with threaded bore  56  of upper collar  50 , and a lower section  58 B threaded to mate with threaded bore  66  of lower collar  60 . Accordingly, lower section  58 B has a different thread pitch than upper section  58 A. In the preferred and illustrated embodiment, the diameter of lower section  58 B is less than that of upper section  58 A—and the diameter of threaded bore  66  is therefore less than that of threaded bore  56 —so that lower section  58 B can pass through threaded bore  56  without interference in order to engage threaded bore  66 . The upper end of calibration adjustment screw  58  is provided with suitable drive means  59  (shown by way of example as a hex socket) whereby calibration adjustment screw  58  may be rotated to raise or lower upper collar  50  relative to lower collar  60 , in accordance with the direction of rotation. 
         [0045]    Although calibration adjustment screw  58  has been described and illustrated herein as having upper section  58 A larger in diameter than lower section  58 B (with threaded bore  66  being corresponding larger in diameter than threaded bore  56 ), this arrangement is not essential to the invention. Persons skilled in the art will appreciate that variant embodiments can be readily devised in which upper section  58 A and lower section  58 B are of the same diameter, or in which upper section  58 A is smaller in diameter than lower section  58 B. 
         [0046]    It would be possible to assemble a pressure switch that incorporates only the calibration mechanism of the present invention, in accordance with the foregoing description. In such variants, guide rods  90  could be considerably shorter than those shown in the drawings; they would only need to be long enough to maintain upper collar  50  in substantial alignment with lower collar  60  through the upper collar&#39;s range of movement relative to lower collar  60 . In preferred embodiments, however, a reset mechanism is also provided in conjunction with the calibration mechanism, and in order to accommodate the reset mechanism, guide rods  90  are extended to a suitable distance above microswitch  40  as shown in  FIGS. 3-5 . 
         [0047]    The reset mechanism of the invention employs functional principles similar to those used in the calibration mechanism, and its construction and operation may be understood with particular reference to the preferred embodiments illustrated in  FIGS. 3 ,  4 ,  5 ,  9 ,  10 A,  10 B, and  11 . The reset mechanism includes a crossbar  70  that spans transversely across the upper ends  90 U of guide rods  90 . Crossbar  70  is shown as being of rectilinear configuration, but this is hot essential; crossbar  70  could take other shapes without departing from the concept of the invention, hi the preferred configuration shown in  FIGS. 4 ,  5 , and  9 , crossbar  70  is provided with guide rod holes  71  for receiving upper ends  90 U of guide rods  90 , with upper ends  90 U each having a shoulder  91  for bearing against the underside of crossbar  70 , and having a threaded portion extending above crossbar  70  to receive a nut  93 , thereby securing crossbar  70  to guide rods  90 . However, this particular means of attachment is not essential to the invention; other effective ways of securing crossbar  70  to guide rods  90  may be devised in accordance with common general knowledge in the field of the invention. As best seen in  FIG. 9 , crossbar  70  has a threaded bore  73 , the purpose of which is described in further detail below. 
         [0048]    The reset mechanism also includes a reset contact button  80 , which as best seen in  FIGS. 10A and 10B  is preferably (but not necessarily) of a generally cylindrical configuration, and has an upper surface  81  and a lower surface  84 . Contact button  80  is formed with a transverse slot  82  formed into the upper face  81  of contact button  80  and sized such that crossbar  70  can be disposed within slot  82  while leaving contact button  80  free to move longitudinally (i.e., parallel to guide rods  90 ) relative to crossbar  70  but substantially prevented from rotating relative to crossbar  70 . Accordingly, the width  82 W of slot  82  will preferably be only slightly larger than the width  70 W of crossbar  70 . The depth  82 D of slot  82  is preferably approximately equal to the thickness  70 T of crossbar  70 , but this relationship is not critical or essential to the invention; i.e., in variant embodiments of contact button  80 , slot depth  82 D could be either greater or smaller than crossbar thickness  70 T, so long as the crossbar&#39;s aforementioned non-rotatability and freedom of longitudinal movement are maintained. 
         [0049]    Contact button  80  has a threaded bore  86  extending downward from the base  82 A of slot  82  and positioned for alignment with threaded bore  73  of crossbar  70  when crossbar  70  is disposed within slot  82 . The pitch of the threads in threaded bore  86  in contact button  80  is slightly different from the pitch of the threads in threaded bore  73  of crossbar  70 . To provide for selective movement of contact button  80  relative to crossbar  70 , the reset mechanism includes a double-threaded reset adjustment screw  88 , illustrated by way of example in  FIG. 11 . Reset adjustment screw  88  has an upper section  88 A threaded to mate with threaded bore  73  of crossbar  70  and a lower section  88 B threaded to mate with threaded bore  86  of contact button  80 . Reset adjustment screw  88  may also have a neutral, unthreaded section  88 C as shown in  FIG. 11 . Lower section  88 B has a different thread pitch than upper section  88 A. In the preferred and illustrated embodiment, the diameter of lower section  88 B is less than that of upper section  88 A—and the diameter of threaded bore  73  of crossbar  70  is therefore greater than that of threaded bore  86  in contact button  80 —such that lower section  88 B of reset adjustment screw  88  can pass through threaded bore  73  of crossbar  70  without interference in order to engage threaded bore  86  of contact button  80 . The upper end Of reset adjustment screw  88  is provided with suitable drive means  89  (shown by way of example as a hex socket) whereby reset adjustment screw  88  may be rotated to raise or lower contact button  80  relative to crossbar  70 , in accordance with the direction of rotation. 
         [0050]    Although calibration adjustment screw  88  has been described and illustrated herein as having upper section  88 A larger in diameter than lower section  88 B (with threaded bore  73  being corresponding larger in diameter than threaded bore  86 ), this arrangement is not essential to the invention. Persons skilled in the art will appreciate that variant embodiments can be readily devised in which upper section  88 A and lower section  88 B are of the same diameter, or in which upper section  88 A is smaller in diameter than lower section  88 B. 
         [0051]    The operation of the calibration and reset mechanisms of the present invention may be particularly well understood with reference to  FIG. 3 , which shows these mechanisms installed in association with a microswitch  40  as described above. Lower collar  60  is connected to the upper end  22 U of push rod  22 . Upper collar  50  is disposed above lower collar  60  and connected thereto by double-threaded calibration adjustment screw  58 , which is disposed to one side of microswitch  40  to permit access to drive means  59  of calibration adjustment screw  58 . Upstand  52  of upper collar  50  is disposed directly below trigger  44  of microswitch  40 . Reset contact button  80  is connected to crossbar  70  such that lower surface  84  of contact button  80  is disposed directly above reset button  45  of microswitch  40 . The lower end of pressure switch  10  is connected (by means of fitting  18 ) to a pressure source corresponding to the desired tripping pressure. Calibration adjustment screw  58  is rotated in the appropriate direction so as to raise upper collar  50  until upstand  52  trips trigger  45 , thus setting microswitch  40  to trip at the desired tripping pressure. The pressure source is then reduced to a desired reset pressure, and reset adjustment screw  88  is rotated as required to lower reset contact button  80  until lower surface  84  of contact button  84  depresses reset button  45  of microswitch  40 , thus setting microswitch  40  to be reset at the desired reset pressure. 
         [0052]    It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the essential concept of the invention, and all such modifications are intended to come within the scope of the present invention and the claims appended hereto. It is to be especially understood that the invention is not intended to be limited to illustrated embodiments, and that the substitution of a variant of a claimed element or feature, without any substantial resultant change in the working of the invention, will not constitute a departure from the scope of the invention. 
         [0053]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following that word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element.