Patent Publication Number: US-5298700-A

Title: Limit switch module and cam for use in the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a limit switch which is normally maintained in a closed position so that an electrically rotatable device, such as a valve, is moved in response to an electrical control signal. The limit switch is tripped at a predetermined position of the valve to open the circuit, and to thereby interrupt a supply of power to the movable device. 
     2. Discussion of the Background 
     As described in U.S. Pat. No. 4,939,320 to Graulty, it is a conventional practice to use micro or limit switches to control the positioning of a rotatable valve at a selected one of its opened and closed conditions. Typically, two angularly displaced cams are integrally formed or rigidly attached on a shaft which is coupled for rotation with a rotatable stem of a valve. As the shaft rotates in either a clockwise or counterclockwise direction with the valve stem, the actuating cams will move either clockwise or counterclockwise along a path which brings one or the other of the cams into contact with one or the other of the limit switches, thereby interrupting a supply of electrical current to a motor driven valve, and stopping movement of the valve at either its open or closed position. The same cams and limit switches can be used to turn on or turn off status signals which represent the open or closed status of the motor driven valve. 
     However, as also discussed in the patent to Graulty, most of the prior art arrangements require complicated procedures to set up the angular orientation of the cams relative to their mounting shafts to ensure actuation of the limit switches at the desired position of the valve being controlled. Also, the setup must periodically be examined, and readjusted if necessary. 
     The patent to Graulty discloses an invention including cam members which are frictionally mounted on a rotating shaft. In a first embodiment of the Graulty invention, the cam members are preferably composed of a plastic material while the shaft is made of a metal material. However, in high temperature or low temperature environments, the friction fit of the parts is altered due to their different thermal expansion coefficients. Thus, the Graulty invention is only suited to a narrow range of temperatures. Although Graulty discloses a second embodiment of his invention for use in high temperature environments, a complex arrangement of springs, washers, annular shoulders, and nuts is necessary to supply a frictional force sufficient to cause the cam members to rotate along with the shaft. 
     Consequently, a need exists for a simplified cam/shaft interface which supplies an appropriate frictional force between the shaft and the cam at a wide range of temperatures. 
     Additionally, a need exists for an apparatus which can easily be assembled and disassembled without a need for special tools, and which can be calibrated by manual adjustment after complete assembly. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of this invention is to provide a novel unitary cam having a spring collar formed of the same material as the cam and having a spring constant capable of frictionally engaging a rotating shaft at a wide range of temperatures. 
     It is another object of the invention to provide the spring collar with a smooth inner surface for contact with the shaft so that the cam is manually adjustable to an infinite number of angular positions with respect to the shaft. 
     It is another object of the invention to provide a limit switch and a cam which are assembled into a single module wherein assembly of the module does not require separate fasteners or tools. 
     A still further object of the invention is to provide a plurality of modules having an arrangement of interconnecting projections and recesses for combining the modules without the use of separate fasteners or tools. 
     In a preferred embodiment of the limit switch module of the present invention, the module includes upper and lower halves which are snap-fit together. A cam and a limit switch are sandwiched between the upper and lower halves at a fixed distance from one another. The cam includes a spring collar, integrally molded therewith, to tightly (but adjustably) grip a shaft. Preferably, the upper and lower halves, the cam, and the shaft are all of the same material. They may, for example, each be injection molded polyoxymethylene (polyacetal). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is an expanded view showing the assembly of a single limit switch module according to the present invention. 
     FIG. 2 is a perspective view showing how the upper half of the module of FIG. 1 may be snap-fit to the lower half of an adjacent module. 
     FIG. 3 is a bottom view of the upper half of the module shown in FIG. 1. 
     FIG. 4 is a perspective view showing one embodiment of a cam for use in the module of FIG. 1. 
     FIG. 5 is a top view of the cam of FIG. 4. 
     FIG. 6 shows the cam in cross section as viewed from the line VI--VI shown in FIG. 5. 
     FIG. 7A illustrates a first position of the cam wherein the limit switch is not actuated. 
     FIG. 7B shows a second position of the cam wherein the limit switch is actuated. 
     FIG. 8 is an expanded view illustrating how a plurality of limit switch modules are assembled on a single shaft. 
     FIGS. 9 and 10 show actuators employing two limit switch modules and four limit switch modules, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a limit switch module according to the present invention includes an upper half 2, a lower half 4, a cam 6, and a limit switch 8. The cam 6 and limit switch 8 are sandwiched between the upper and lower halves when they are snapped together, and a hole is formed in each of upper half 2, cam 6, and lower half 4 for receiving a shaft 10 therethrough. 
     FIG. 2 shows how the lower half 4 of an adjacent module is received by the upper half 2 shown in FIG. 1. 
     As shown in FIGS. 1 and 2, lower half 4 includes a first arrangement of projections including pegs 12 and 14, which are received in throughholes 16 and 18 of limit switch 8, respectively. Lower half 4 is further provided with a second arrangement of projections, which, in the preferred embodiment of the invention, includes a pawl member 20 having an enlarged end 21. Additionally, lower half 4 is formed with a cylindrical bearing 22 for receiving cam 6 and shaft 10. 
     FIG. 3 provides a bottom view of upper half 2. As can be seen in FIGS. 1 and 3, upper half 2 includes a pair of pegs 32, 34, similar to pegs 12 and 14 of lower half 4, which are received in throughholes 16 and 18 of limit switch 8, respectively. Upper half 2 is further provided with a pawl receiving member 36 having a rectangular throughhole 38 for receiving pawl member 20 of lower half 4 in an interlocking relationship. A semicircular projection 40 is provided on upper half 2 for holding cam 6 in place on cylindrical bearing 22 when the upper and lower halves are assembled. The semicircular projection 40 has a throughhole 42 formed therein for rotatably receiving the shaft 10 therethrough. 
     A short pin 44 and a long pin 46 are provided on an upper surface of upper half 2. As can be seen in FIG. 2, when two modules are to be interconnected, short pin 44 is received by a throughhole 48 formed through lower half 4 of the adjacent module. Likewise, long pin 46 is received in a throughhole 50. 
     FIG. 4 shows a perspective view of cam 6. In the preferred embodiment of the invention, cam 6 is injection molded as a single unitary piece. The cam can be formed, for example, from a resin material such as polyoxymethylene (polyacetal), as is available from DuPont under the trade name DELRIN. As can be seen in FIG. 4, the preferred embodiment of the cam is configured to have an outer ring 60 having a tear-drop shape. Spring collar segments 62a and 62b are connected to outer ring 60 by separate joining webs 64a and 64b, which can be seen in FIGS. 5 and 6. As shown in the cross section shown in FIG. 6, the separate joining webs 64a, 64b have a reduced axial height h compared to the axial height H of outer ring 60 and spring collar segments 62a, 62b. As can be seen in FIG. 5, separate joining webs 64a, 64b also extend only around a small portion of the periphery of spring collar segments 62a, 62b. Accordingly, spring collar segments 62a, 62b flex slightly when a shaft 10 is received between the spring collar segments. When the spring collar segments 62a, 62b are flexed, the separate joining webs 64a, 64b exert a restoring force, holding the spring collar segments in a tight frictional engagement with shaft 10. Accordingly, when cam 6 is placed on a shaft, rotation of the shaft will also result in rotation of cam 6. However, since the outer surface of shaft 10 and the inner surfaces of spring collar segments 62a, 62b contacting the shaft are smooth, cam 6 may also be rotated relative to shaft 10 to adjust the position of a large radius portion 65 of the cam 6 relative to the shaft 10. Preferably, the axial height h and the circumferential length of spring collar segments 62a and 62b are designed so that cam 6 may be slid axially along a shaft and angularly adjusted relative to the shaft without the need for special tools. However, it may also be desirable to provide outer ring 60 with a plurality of slots 60a-60c (see FIG. 4), into which a screw driver or a similar tool may be inserted to provide additional leverage for adjusting cam 6 angularly with respect to shaft 10. 
     FIGS. 7A and 7B illustrate the operation of the limit switch of the present invention. The limit switch 8 is a commercially available microswitch, typical of those available from several manufacturers. Limit switch 8 may, for example be an AV series subminiature switch, available from Matsushita Corporation of Japan. As used in the present invention, such a switch includes a common ground wire 81, a power supply wire 82, and an indicator wire 83. An actuating lever 85 is biased to the position shown in FIG. 7A. In this position, power flows in through common ground wire 81 and out through power supply wire 82, as indicated by the arrows in FIG. 7A. However, when cam 6 is rotated by shaft 10 to the position shown in FIG. 7B, the circuit between common ground wire 81 and power supply wire 82 is interrupted. Instead of flowing out through power supply wire 82, electricity is diverted through indicator wire 83, as shown by the arrows in FIG. 7B. Accordingly, power supply wire 82 can be used to supply electrical power to a motor which drives the shaft 10 shown in FIGS. 1 and 8-10. When shaft 10 rotates to a predetermined position indicated in FIG. 7B, large radius portion 65 of cam 6 moves actuating lever 85 of the limit switch 8, thereby interrupting the supply of current through power supply wire 82, and initiating a supply of current through indicator wire 83. Indicator wire 83 may, for example, supply current to a lamp which indicates that the cam 6 has reached the predetermined position. The predetermined position can be adjusted by rotating cam 6 relative to shaft 10, either manually or with a screw driver using slots 60a-60c. Thus, the apparatus may be easily calibrated so that the predetermined position of cam 6 is representative of a status of a piece of equipment driven by shaft 10. The piece of equipment may, for example, be a motor driven valve, whose opened and closed positions are separated by a 90° angular rotation of shaft 10. 
     FIG. 9 shows a valve actuator employing two limit switch modules of the present invention. Shaft 10 is driven by a motor 90 by way of gears 91 and 92. An upper cam 6 is adjusted so that power to motor 90 will be interrupted at a first position of shaft 10. A lower cam 6 is adjusted so that when the direction of rotation of shaft 10 is reversed, the cam will interrupt supply of power to motor 90 at a second predetermined position of shaft 10. One cam 6 can be positioned to represent an opened position of the valve, for example, and the other cam 6 can be positioned to represent a closed position of the valve. 
     FIG. 10 represents an actuator having four modules. Each of the four cams 6 can be adjusted relative to shaft 10 such that opened and closed or on and off positions of two separate devices can be controlled by a single shaft 10. 
     The configuration of the present invention is especially advantageous because the parts of the invention can be assembled and disassembled without the necessity of any special tools. Accordingly, the assembly of a limit switch module according to the present invention is now described. 
     In order to assemble the limit switch module, lower half 4 is positioned as shown in FIG. 1, and a limit switch 8 is pressed down onto the pegs 12, 14. A cam 6 is then positioned on the cylindrical bearing 22 such that the cylindrical bearing is received between the outer ring 60 and the spring collar segments 62a, 62b of the cam 6. The pegs 32, 34 of upper half 2 are then aligned with bores 16 and 18, and upper half 2 is pressed down onto lower half 4, while ensuring that the pawl member 20 is received within pawl receiving member 36. The shaft 10 is then inserted through throughhole 42, cam 6, and cylindrical bearing 22. The shaft 10 may also be fed through additional limit switch modules, as shown in FIG. 8. It should, at this time, be noted that the terms &#34;upper half&#34; and &#34;lower half&#34; have been used only for convenience, and that the entire module can be turned upside down when placed on shaft 10. Note that in FIGS. 8-10, each module is oriented with lower half 4 on top of upper half 2. As can also be seen in FIGS. 8-10, each lower half 4 includes an electrically insulating shield 400, which extends between adjacent sets of electrical connections. 
     As can be appreciated from the above disclosure, the entire apparatus of the present invention is assembled simply and easily without the use of tools and without worry with regard to the position of the cam 6 relative to shaft 10 during assembly. Calibration of the individual cams 6a-6d can be delayed until after an actuator is entirely assembled and connected to the device being controlled, and calibration also does not require special tools. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.