The present invention relates to devices for operating an electrical switch in response to changes in sensed pressure in a cavity, commonly known as pressure switches, and particularly relates to pressure switches of the type having a pressure responsive member, such as a piston or flexible diaphragm defining a wall portion of the sensing cavity and movable in response to changes in pressure in the cavity. Pressure switches of the type employing a flexible diaphragm for sensing pressure are commonly employed in household washing machines for sensing pressure in a tube connected to the bottom of the washing machine receptacle or tub such that the sensed pressure is directly related to the height or depth of the water in the tub. If the washing machine tub is filled above a predetermined level, the hydrostatic pressure of the water fill is applied to the pressure switch diaphragm, and actuates an electrical switch for disabling the water fill valve or mechanism.
In the typical diaphragm-type pressure switch, the diaphragm is resiliently biased and preloaded by a spring; and, the preload is adjusted for calibration by an adjustment screw provided in a movable adjustment lever provided on the housing structure with the screw bearing against a movable spring reaction member such as a cap over one end of the preload spring. The movable adjustment lever is typically provided to enable user-adjustment of the pressure settings for switch actuation. In washing machine applications for the pressure switch, the user adjustment of the pressure settings corresponds to different desired levels of water in the machine wash tub.
Where an adjustment screw is used to alter the position of a reaction member attached to the preload spring, it is necessary to secure the adjusted position of the screw such as with an adhesive or mechanical crimping or staking. This construction has resulted in a costly and troublesome operation in mass production of pressure switches for household washing machines, and has also resulted in problems in field service where the means for securing the screw adjustment have failed, causing the pressure switch to change calibration. Thus, it has long been desired to provide an improved, more reliable and lower cost way of providing for calibration of a pressure switch.