Patent Publication Number: US-4223190-A

Title: Mercury float switch

Description:
The primary object of this invention is to provide a practical, durable float switch for controlling the operation of bilge or sump pumps or suitable signals which comprises means embodying structural features to prevent on and off operation of a pump, or the like, by the normal movement of the surface of the water on which the float rest, such as by the moving or rocking of a boat. 
     Another object of the present invention is to provide a float switch structure which embodies protective means and seals to prevent deterioration of the switch structure due to leakage of moisture into the switch structure certain of said sealing means also serving to prevent breakage or cracking of the insulation on the wires to the pump or the like, the operation of which the switch controls. 
     With these and other objects in view, as may appear from the accompanying specification, the invention consists of various features of construction and combination of parts, which will be first described in connection with the accompanying drawings showing a Mercury Float Switch of a preferred form embodying the invention and the features forming the invention will be specifically pointed out in the claims. 
    
    
     In the drawings: 
     FIG. 1 is a vertical longitudinal section through the switch structure. 
     FIG. 2 is a vertical view of the switch structure taken at right angles to that shown in FIG. 1. 
     FIG. 3 is a horizontal section through the switch structure. 
     FIG. 4 is a side elevation of the switch structure. 
    
    
     Referring more particularly to the drawings, the switch assembly includes a mercury tube 1 in which a globule of mercury 12 is contained. The mercury tube 1 has two switch terminals or contacts 4 in one end thereof. A wire support means for the electrical lead wires is mounted to support the lead wires on the interior of the float body at least at a point of junction of the lead wires to the contacts 4 within mercury tube 1. Support means has a hollow configuration at least in part and is further structured and configured in joining in communicating relation to the mercury tube 1. Interconnection between lead wires 3, supported at least in part on the interior of the wire support means, and contacts 4 is provided through an open portion or aperture in the wire support means. In one embodiment the wire support means includes a hollow shaft 5 being substantially fixedly attached to and extending through one end of float 2 and is further disposed relative to support brackets 8 and 9 to allow pivotal movement of the float about the longitudinal axis of hollow shaft 5 and the lead electrical wires maintained therein. The hollow shaft 5 is attached intermediate its ends to the mercury tube 1 by a moisture proof sealing material 6. The hollow shaft 5 has an opening or slot therein intermediate its ends which opens laterally through the wall of the hollow shaft 5. Lead wires 3 which connect the switch assembly to a pump or signal (not shown) extend one into each end of the hollow shaft and out through the lateral opening in the shaft and are connected to the terminals or contacts 4. The epoxy sealing material 6 which serves to connect the mercury tube 1 to the hollow shaft 5 also encloses and seals the connections between the wires 3 and the contacts 4 from mositure and thus, serves to connect the tube 1 shaft 5 and lead wires 3 in a unitary structure. The hollow shaft 5 is preferably made of acrillic plastic or similar material which is impervious to corrosion. 
     The lead wires 3 are insulated with neoprene or flexible material that is resistant to salt water and oil. The wires 3 are also sealed at their exit point from the hollow shaft 5 by shrinkable rubber tubing 7 assembled over the ends of the hollow shaft 5 and shrunk onto the wire and shaft ends as shown in FIG. 3 to provide a primary atmospheric seal for preventing moisture entering the shaft 5. This shrink tubing 7 being flexible also acts to prevent severe bending of the lead wires as they enter the shaft 5. 
     The switch assembly is placed in a cavity formed in a suitable mold (not shown) with end portions of the hollow shaft 5 extending out of the cavity. The cavity in the mold determines the outside shape of the float body 2. After the switch assembly is placed in the cavity of the mold a polyurethane foam being a two part liquid which is premixed is introduced into the mold and which subsequently forms around the switch assembly filling the mold cavity and embedding the switch therein. The liquid polyurethane injected into the cavity (not shown) solidifies into a solid light weight body which will readily float on the surface of water. 
     The shaft 5 is rotatably supported by two L-shaped mounting brackets 8 and 9 as shown in FIGS. 2 and 3. Each bracket 8 and 9 includes a horizontal which are clamped together after the spacing include of the upright section has been adjusted for proper float side clearance and alignment of shaft 5. The bases or horizontal portions of the brackets 8 and 9 project laterally from one of the upright section a sufficient distance to provide suitable structure for mounting the floating switch in position for its ultimate use. The lateral projections defining the horizontal bases are mounted one upon the other and are then chemically welded together by applying methylene to their outer edges. When the weld is secure, the clamp is removed and two mounting holes drilled through the two bases to accommodate the mounting screws 10 for attaching the switch to the bottom of the bilge, sump or other sensing points. This feature facilitates assembly of the float assembly to its support brackets and permits easy installation of the switch assembly by giving the installer unrestricted access to the mounting holes for driving the mounting screws. 
     A button or lateral projection 11 is formed projecting from one side of the polyurethane float body 2 during its molding and is positioned to engage the edge of one of the upright sections of the L-shaped brackets 8 or 9 to prevent it from rotating past upper dead center of the brackets as shown in FIG. 4. 
     As clearly shown in the drawings the switch contacts 4 extend into the mercury tube 1 at one end of the tube in spaced relation so as to normally prevent the closing of the electrical circuit through them. However, when the float assembly 2 rises upwardly, the globule of mercury will engage both contacts 4 and close an electrical circuit through them, the wires 3 and the pump, or the like to which the wires are connected. 
     To prevent cutting on and off of operation of the bilge pump when the level of water in the bilge is not sufficiently high as to require its pumping, the mercury tube 1 of the present invention is bent intermediate its ends at an angle to what would be the normal straight line of the tube. This requires greater movement of the bent mercury tube before allowing the mercury to engage both contacts 4. Consequently the nuisance and excessive wearing on the bilge pump by the intermittently cutting out and in of its operation is eliminated. The mercury is allowed to move upwardly over the bent inclined portion of the tube 1 only when the water level in the bilge reaches its sufficient height to restore the float assembly towards a maximum vertical position. The mercury remains in such position as to maintain a mercury bridge over the contact and continue operation of the pump until the water level in the bilge or sump falls to its minimal level. 
     This invention is a simple, inexpensive method of producing a small, reliable, long lasting automatic bilge or sump switch wherein the &#34;glass tube&#34; nercury switch is essentially packaged in polyurethane foam making it indestructable under all normal operating conditions especially in salt water environment which limits the life of conventional switches and under road shock conditions encountered with trailering a boat. Polyurethane foam makes the ideal float material for this application as it is extremely light weight, has a &#34;closed cell&#34; structure that prevents absorption of moisture and is completely resistant to oil and salt water over a wide operating temperature range. 
     It is important that the lead wires be flexible by using a large number of very fine gauge copper wires as the conductor in order that they transmit no appreciable torque to the float assembly. Insulation material is usually neoprene. 
     By limiting the bend radius of the lead wires with the flexible shrink tubing the possibility of cracking the insulation due to repeated bending is virtually eliminated. Other designs that permit this severe bend radius have a very limited life in marine applications as this bending causes cracking of the insulation material permitting moisture intrusion with subsequent corrosion of the stranded wires and eventual failure of the switch. Also, as stated, the only corrosion points of this design are the two junctions of the lead wires with the switch terminals, which are sealed with an epoxy or marine sealant. However, moisture should never reach this point as the primary &#34;shrink tubing&#34; seal prevents moisture from entering the inside of the axis tube and the foam material adhering to the outside of the axis tube forms a seal which prevents moisture from penetrating around the outside. These are the only paths for moisture to penetrate to the switch wire junctions as the foam material itself completely seals out moisture. This feature virtually eliminates the possibility of switch failure due to corrosion from salt water or other moisture by requiring a &#34;double seal&#34; failure and preventing cracked lead wire insulation by eliminating all sharp bend radius. As the switch itself imbedded in the foam is essentially indestructable, this design should have an extremely long service life. 
     It will be understood that the invention is not to be limited to the specific construction or arrangement of parts shown, but that they may be modified within the invention defined in the claims.