Abstract:
A pressure switch having a pressure responsive diaphragm with a backing plate for operating a snap-acting switch. The switch has a cantilevered contact blade spring with an adjustment screw providing a reaction support at the free end. The middle portion of the contact blade spring provides a preload bias against the diaphragm backing plate. The blade spring includes a moveable electrical contact on a tang formed in the middle region and includes an over-center spring for effecting a snap action of the moveable contact against stationary contacts. The actuation-deactuation differential is determined, independently of the pressure setting for switch actuation, by setting the position of the stationary contacts.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to pressure switches and particularly those of the type intended for operation at a relatively low gauge pressure on the order of 1 psig with a narrow or tightly controlled differential between trip and reset or between actuation and deactuation. Such switches find common application in automatic clothes washing machines where it is desired to control certain machine electrical functions in response to the water level or pressure head sensed in the washing receptacle or drum. Typically in mass produced automatic clothes washing machines a sensing port is provided in the washing tub or drum with a tube connected from the sensing port to the pressure switch. The pressure in the switch sensing cavity is commonly applied in such known designs to an elastomeric diaphragm which has a rigid backing member or actuator attached thereto for applying the sum of the pressure forces acting on the diaphragm to an electrical switching mechanism. 
     A known pressure switch used for automatic clothes washing machine water level sensing is illustrated in FIG.  7  and has a pressure responsive diaphragm  1  disposed in a housing  2  forming a pressure sensing cavity  3  connected to inlet port  4 . The diaphragm has a backing plate  5  for applying the summation of the pressure forces acting over the diaphragm  1  to a switch actuation rod  7  which in turn is connected for moving a switch actuation blade  6 . The known pressure switch of FIG. 7 has the preload or bias applied directly to the diaphragm backing plate via rod  7 . The actuator  5  is preloaded by the force of rod  7  which is acted upon by a preload spring  8  which is adjustable for varying the preload on the diaphragm for calibration purposes. The blade  6  has a separate tang formed therein with a double sided moveable contact  9  mounted thereon for switching between the moveable contact and stationary side contacts  10 ; and, an over-center spring  12  provides a snap action to the switch. 
     The problem encountered with the prior art pressure switches, such as switches of the design shown in FIG. 7, is that, upon calibration of the switch by adjustment of the preload on the diaphragm actuator, the differential between actuation and deactuation of the switch is varied as the preload was varied on the diaphragm by virtue the changed position of the actuator blade  6  and the tolerances on the location and spacing of the side contacts  10  and  11 . During assembly and calibration, it has been found difficult to control the differential between actuation and deactuation, or trip-to-reset, over a range of pressure calibrations and tolerances on assembly of the switch contacts into the housing. 
     Thus, it has long been desired to find a way or means of providing a pressure switch for low pressure gauge settings, such as encountered in automatic clothes washers for water level sensing, and to provide accuracy in calibration and trip to reset and yet accomplish these functions with a minimum manufacturing cost. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved pressure switch for relatively low gauge pressure actuation and deactuation with reduced differential or variance between trip and reset and provides for accurate setting of the trip-to-reset during manufacturing. 
     It is an object of the present invention to provide a relatively low gauge pressure sensing pressure switch which may be calibrated for pressure actuation and independently set for the differential between trip and reset separately without interaction or of one effecting the other. 
     The present invention provides a pressure switch having a pressure responsive member moveable in response to changes in pressure in the switch sensing cavity formed in the housing and operable to move a cantilevered switch contact blade spring having the middle portion contacting and biasing the pressure responsive member. The blade spring includes an over-center snap mechanism which causes a tang formed in the blade spring to open and close a moveable contact mounted on the tang against a stationary switch contact. An adjustment means on the housing is operable to change the position of the free end of the blade spring to vary the bias on the pressure responsive means for changing the pressure level at which the switch actuates. The positioning of the stationary contact with respect to the moveable contact determines the differential or trip-to-reset pressure for the switch independently of the actuation point setting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of a single pressure level actuation embodiment of the present invention; 
     FIG. 2 is an axonometric view of the assembled embodiment of FIG. 1 with portions of the housing cover broken away; 
     FIG. 3 is a cross-section of the assembled switch of FIG. 1; 
     FIG. 4 is a view similar to FIG. 1 of a multilevel-setting pressure switch embodiment of present invention; 
     FIG. 5 is an axonometric view of the assembled embodiment of FIG. 4 with portions of the housing cover broken away; 
     FIG. 6 is a cross-section of the assembled switch of FIG. 4; and, 
     FIG. 7 is a cross-section of a prior art pressure switch. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1,  3 ,  4  and  5 , the pressure switch assembly of the single setting embodiment of the present invention is indicated generally at  20  and includes housing means comprising an upper shell  22  and a base  24  with a pressure responsive elastomeric diaphragm  26  disposed therebetween. The lower open end of the shell  22  has an outwardly extending circular flange  28  which is clamped over the rim of the diaphragm  26  to seal the diaphragm in the base  24 . The upper shell  22  is retained on the base  24  by a plurality of barbed tabs  25  formed about the periphery of base  24  which are snap-locked over flange  28 . 
     A tube fitting having a pressure port  30  therein is provided on the base  24  for communicating with a pressure cavity  32  (see FIG. 3) formed in the base  24  beneath the diaphragm  26 . A rigid insert or backing member  34  is provided against the upper surface of the diaphragm and member  32  includes an actuator tower  36  extending upwardly therefrom with a reduced diameter pilot portion  37 . 
     Upper shell  22  has a polygonal twist lock mounting boss  38  with a resilient locking tab  40  for mounting and orienting the switch when mounted through a panel with the flange  38 . 
     A first terminal connector or bus bar  42  has a plurality of barbed tabs  43  integrally formed thereon at right angles thereto; and, a stationary contact terminal  44  is secured thereon preferably by riveting. The bus bar  42  is secured to the inner surface of the upper shell  22  by insertion of the barbed tabs  43  in appropriate slots (not shown) formed in the undersurface of the shell  22  thus securing the contact  44  as a stationary switch contact. 
     A second contact terminal or bus bar  46  also has a plurality of right angle barbed tabs  48  provided thereon which are also inserted in suitable slots (not shown) provided in the undersurface of the upper shell  22 . Terminal connector or bus bar  46  has a plurality of raised annular bosses  50  formed thereon extending downwardly therefrom and which are received through correspondingly disposed apertures  52  provided in a contact blade spring  54 . Blade spring  54  is preferably formed of tempered material such as, for example, heat treated beryllium copper. The bosses  50  are then staked or riveted over the blade  54  to retain the blade  54  assembled to the strip  46 . 
     Blade  54  has a flexible tang  58  formed therein which has a moveable electrical contact  60  attached therethrough, preferably by riveting to form a double sided contact for single pole double throw action. The end of the tang  58  is placed under compressive load by a compression spring  62  so as to provide an over-center action between the tang  58  and the remaining portion of the blade  54 . It will be understood that the blade contact  60 , blade  54  and terminal strip  46  comprise the common terminal of a switch. 
     Blade  54  has an aperture  64  formed generally centrally therein through which is received over reduced diameter pilot portion  37  of tower  36 ; and, the undersurface of blade  54  in the region of the rim of aperture  64  is registered against the shoulder  39  provided on tower  36 . 
     A third terminal strip or bus bar  66  has a plurality of barbed tabs  68  formed thereon and generally at right angles thereto; and, the strip  66  has a second stationary electrical contact  70  received thereon and secured thereto preferably by riveting. The barbed tabs  68  are also received in slots (not shown) provided in the undersurface of the upper shell  22  and are so disposed so as to locate the stationary electrical contact  70  directly below the moveable contact  60 . It will be understood that tabs  48  on bus bar  46  and tabs  43  on bus bar  42  are so disposed so as to locate moveable contact  60  directly below the stationary contact  44 . 
     Aperture  64  is located on the blade spring  54  intermediate the contact  60  and the free end  56  of blade spring  54 . An adjustable stop or reaction support member in the form of a screw  72  is threaded through a web  21  formed in the upper housing shell and positions the free end  56  of blade  54  to apply the desired downward force on the tower  36  to preload or bias the diaphragm  26  to provide over-center actuation of tang  58  and movement of contact  60  at a desired pressure in cavity  32 . 
     In the presently preferred practice, the distance between the lower stationary contact  70  and the upper stationary contact  44  is determined by the amount the barbs  68  on the lower terminal strip  66  are inserted into the slots (not shown) in the underside of the upper shell  22 . This latter adjustment may be made independently of the preload calibration of the blade spring  54  by adjustment screw  72 . 
     Adjustment screw  72  may be located in any of the plurality of threaded apertures  74  formed in the web  21  to locate the end of the screw  72  to a desired distance from the point of application of the bias or preload at aperture  64 . Thus, movement of the screw  72  in different locations of the aperture  74  is operative to change the spring rate of the blade spring  54  and thus the pressure response of the switching mechanism to signal pressures at the inlet port  30 . 
     Referring to FIG. 3, the adjustment screw  72  is shown in dashed outline as located in an alternate one of the holes  74  for providing the minimum spring rate of the blade spring  54 . 
     Referring to FIG. 4, another embodiment of the invention is indicated generally at  100  in the form of a multilevel setting pressure switch which has the components indicated by reference numeral  102  common to the embodiment of FIG.  1  and FIG. 4; thus, a repeated description of these parts will be omitted for the sake of brevity. 
     Referring to FIGS. 4,  5  and  6 , embodiment  100  has the upper shell of the housing  104  formed with the flange  106  which is similar to flange  28  of the embodiment of FIG.  1  and which is secured to the bias over the diaphragm in a manner similar to the embodiment of FIG.  1 . Upper shell  104  has a tower portion  108  extending upwardly therefrom of reduced diameter from the side of the shell  104 ; and, tower  108  has a mounting flange  110  provided at the top thereof similar to flange  38  of the embodiment of FIG. 1. A locating tab or lug  112  is provided for orienting the pressure switch when mounted through a panel by flange  110 ; and, the lug  112  is similar to the lug  40  in the embodiment of FIG.  1 . 
     The upper shell tower portion  108  has a central aperture  114  formed through the top of the tower  108  and has journalled therein an adjustment shaft  116  which has an enlarged diameter annular cam  118  formed on the lower or interior end of the shaft  116 . 
     An adjustment block or slider  120  is slidably disposed in the shell tower  108  and guided therein by oppositely disposed guide surfaces  122 ,  124  for vertical movement within the upper shell. Slider  120  has a cam follower surface  126  formed thereon which is engaged by cam  118  for changing the vertical position of the slider  120 . The block or slider  120  has threadedly received therein an adjustment screw  128  which extends through the slider block and has the end thereof contacting the switch blade spring in a manner similar to the screw  72  embodiment of FIG.  1 . In operation, user rotation of shaft  116  causes the cam  118  to move the block  124  up or down as desired to increase or decrease the bias of the switch blade spring on the diaphragm to change the pressure setting for actuation of the switch. 
     Although the present invention has been described hereinabove with respect to the illustrated embodiments, it will be understood that the invention is capable of modification and variation and is limited only by the scope of the following claims.