Abstract:
The novel electric heating element is particularly useful as a water heating element, although its use is not so limited. The element comprises the usual metal-sheathed heater having electrical termination at one end of the sheath. The customary water heating element has a sheath of hair-pin formation, with the ends of the two legs of the sheath connected to structure which serves as a mounting member and an electrical connector. Such structure in the present application is of molded plastic with a metal insert.

Description:
BACKGROUND AND SUMMARY 
     Electric heaters for water tanks have been well known in the art for many years, and generally comprise a metal sheathed heater of hair-pin formation with a metal mounting member connected across the two legs of the sheath. The mounting member was either a plate which was bolted to the tank wall, or a screw plug which was threaded into a fitting on the tank wall. A plastic terminal block was carried by the mounting member and served to electrically insulate the electrical connections from the latter. 
     The prior art construction required expensive assembly operations and my invention reduces the cost of making a water heater element, and provides other advantages hereinafter described. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the drawings accompanying this specification and forming a part of this application, there are shown, for purpose of illustration, several embodiments which my invention may assume, and in these drawings: 
     FIG. 1 is an elevational view looking at the head of my improved heating element and showing it attached to a fragmentary part of the wall of a hot water tank, 
     FIG. 2 is an enlarged, fragmentary sectional view corresponding to the line 2--2 of FIG. 1, with the heating element detached from the tank wall, 
     FIG. 3 is a plan view of a metal insert, 
     FIG. 4 is a sectional view corresponding to the line 4--4 and showing the insert prior to its assembly with the legs of the heating element, 
     FIG. 5 is a perspective view of parts of a mold which may be used in the formation of my improved heating element, and a fragmentary portion of element parts, 
     FIG. 6 discloses, in section, another embodiment of my invention, showing parts in separated relation, 
     FIG. 7 is a plan view of a metal insert used in the embodiment of FIG. 6, 
     FIGS. 8 and 9 are elevational views showing the faces of two parts used in the embodiment of FIG. 6, and 
     FIG. 10 is a sectional view of another embodiment of my invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiment of my invention shown in FIGS. 1 through 5 is a screw-plug type water heater adapted to be threaded into a metal ring 15 which is projection welded to the metal wall 16 of the wall of a water tank, in position to surround the opening 17 in such wall. 
     The heating element as disclosed is in the form of the well-known hair-pin type and comprises a metal sheath 18 having a bight portion 19 and a pair of legs 20--20 disposed in side-by-side relation. A helically coiled resistance wire 21 is disposed within the sheath 18 and held in centered relation and out of contact with the sheath by compacted refractory material 22, such as compacted magnesium oxide. 
     Electrically and mechanically secured to each end of the coiled resistance wire is a metal terminal pin 23 and each pin extends outwardly of the end of a respective sheath leg 20, as seen in FIG. 2. In the embodiment disclosed in FIGS. 1 through 5, a mounting member 24 of plastic material is adapted to mount the heating element in position so that its active heating portion is immersed in the liquid contained within the water tank. 
     As a preliminary step, a disc-like insert 25 is secured to the legs 20 of the sheath, and the insert may be formed of any suitable metal, such as cold roll steel plated with a corrosive resistive material such as zinc, or the insert may be formed of copper or brass. The washer-like portion 26 of the insert has a fairly large opening 27. A pair of smaller ring-like portions 28 are connected to each other by an integral intermediate neck 29 and one of the portions 28 is connected to the wall defining the opening 27 by an integral end connector 30. As seen in FIG. 4, the ring-like portions 28 are angularly displaced from the plane of the insert 25 and the legs 20--20 of the heater sheath are inserted through the openings of the portions 28 a predetermined amount. Thereafter, force is applied to the portions 28 in the direction of the arrows 31 against a suitable back-up device (not shown) to force the angularly disposed portions 28 into the plane of the insert. This will force the margins defining the openings in the ring-like portions 28 to dig into the metal of the sheath legs and form an annular seal thereabout. 
     The heating element, as seen in FIG. 5, is now ready for insertion within a mold for molding the plastic mounting member 24 about the ends of the legs 20--20 of the heating element with the metal insert 25 embedded within the plastic mounting member. The mold is preferably formed in two complementary halves 35 and 36 which are clamped together and about the front portion of the heating element prior to the molding operation. One or both mold halves may have the customary opening through which flowable plastic material may be injected into the mold. 
     The plastic material is preferably a glass reinforced plastic capable of withstanding high temperatures and offering greater resistance to creep on plastic deformation. Examples of suitable materials for this purpose are Noryl, made by the General Electric Company, or Celcon made by the Celanese Corporation. 
     FIG. 5 discloses the cavity in the lower half 35 of the mold, the cavity in the upper half 36 being complementary. As shown, a pair of semi-circular openings 37--37 are formed in an end wall of the mold half 35, the openings being dimensioned to closely clamp about the legs 20--20 of the sheath to prevent the injected plastic material from squirting out of the mold. The openings are also closely dimensioned, center-to-center to bring the legs 20--20 into desired spaced relation, since during manufacture of the heating element such legs may not be spaced the desired amount. 
     The mold half 35 also has a thread-forming cavity 38 to form the exteriorly threaded plug portion 39 of the mounting member 24. Inward of the cavity 38 is a narrow cavity 40 to receive the peripheral portion of the insert 25. Adjacent to the cavity 40 is a cavity 41 of hex shape to provide the hexagonal portion 42 of the mounting member. Adjacent to the cavity 41 is a circular cavity 43 to provide the circular portion 44 of the mounting member. The interior surface of the wall 45 of the mold has recessed portions 46, 46 to form the segment-shaped bosses 47,47 and a central groove 48 to provide the barrier rib 49. Further, the wall 45 has a pair of semi-circular grooves 50--50 to closely fit about the terminal pins 23--23 of the electric heating element so that injected plastic does not squirt out of the mold around the pins. All the cavities, grooves, and recesses just mentioned are also formed in complementary manner in the top mold member 36 so that when the mold halves are closed on each other, a complete overall cavity is formed, with the end of the heating element disposed therein in proper position for the molding operation. The parting line of the mold is considered to be taken along the line 2--2 of FIG. 1. 
     After the plastic has been injected into the mold, and permitted to cure, the mold is opened and the completed mounting member for the heating element has been formed and is ready for use, although in some cases a certain amount of flash may have to be trimmed off. 
     The finished molded product is shown in FIGS. 1 and 2 wherein it will be seen that the entire mounting member is an integral unit, with plastic within and through the opening 27 of the insert 25 and integrally joined with plastic forming the plug portion 39 and hex portion 42. The outer peripheral portion of the insert 25 is not covered with plastic and is disposed to engage the face 15.1 of the metal ring 15 when the plug portion 39 is fully threaded into the ring 15, to thereby ground the sheath of the heating element to the tank. Grounding of the sheath is necessary since, if the sheath is completely isolated from electrical connection with the tank, corrosive erosion of the sheath will occur. Adjacent to the insert 25, the plug portion is formed with an annular land for the seathing of a sealing member, such as the rubber O-ring 51 shown in FIG. 2. The O-ring is adapted to fit within an annular groove 52 in the ring 15, and is compressed a proper amount to prevent water leakage when the plug portion 39 has been threaded into the ring to seat the insert 25 in grounding relation against the face 15.1 of the ring. 
     In addition to its grounding function, the insert 25 provides other desirable results. For example, when the insert is in engagement with the ring 15, it provides for conduction of heat from the heating element to the tank wall to prevent a heat build-up in the plastic support member. The insert 25 also provides a mechanical support for the plastic support member to prevent the normal tendency of plastic to creep away from highly stressed areas. 
     The insert 25 also helps prevent distortion of the molded part, especially when torque is applied by a wrench on the hex portion 42 to thread the plug portion 39 into the ring 15. Tests have shown that torque in the order of 120 foot pounds may be applied to the hex portion without distortion of the molded part, and this is far in excess of the normally applied torque of 20 foot pounds. The insert 25 also prevents the legs 20--20 of the sheath from pulling out of the molded parts, since the insert is locked to the sheath legs by the ring-like portions 28--28 and provides a broad area in its washer-like portion 26 to resist any pulling of the legs 20--20 from the molded part. 
     The molded plastic mounting member disclosed above has many advantages over the prior art steel plug, some of which are listed as follows: a separate plastic terminal block is eliminated; the problem of thread corrosion is reduced; a complete unit is formed in one molding operation to thereby reduce the cost of assembling the various parts of a steel plug; a weight reduction of about 20% over the steel plug is possible, and this is important from a shipping standpoint; the entire mounting member is molded around the ends of the sheath legs to thus reduce the possibility of water leakage in the event of burn-through of any portion of the sheath immersed in water. In this connection, attention is called to the annular seal formed by crimping the ring-like portions 28--28 of the insert around the legs 20--20 of the sheath to further reduce the possibility of water leakage because a longer flow path is provided. 
     After the mounting member 24 has been molded around the ends of the legs 20--20 of the sheath, the heating element may be completed for use in the usual manner. For example, metal terminal strips 55--55 may be applied over the ends of respective terminal pins 20--20 as shown in FIG. 2. Each of these strips is disposed in a spaced defined by facing surfaces of a segment shaped boss 47 and the barrier rib 49 and thus held against rotation about its terminal pin. Each terminal pin 20 is electrically connected to its terminal strip 55 by a heliarc weld 56. Each terminal strip has a threaded opening for the reception of a machine screw 57, the latter being for the purpose of holding a conductor wire to the respective terminal strip so as to provide for the supply of electrical energy to the heating element. 
     DESCRIPTION OF FIRST OTHER EMBODIMENT 
     In order to simplify the mold and reduce the cost thereof, especially in low quantity production of my improved electric heater assembly, the mounting member may be molded in two parts which are subsequently joined with each other and the metal insert. In such case, the embodiment disclosed in FIGS. 6 through 9 may be utilized. 
     As seen in FIG. 6, the mounting member may be formed from separately molded plug and head portions 60 and 61, the two providing all the features of the molded part heretofore described. In this case, the insert 25a is formed in the shape shown in FIG. 7, wherein the ring-like portions 28a, 28a are integrally connected by necks 29a--29a to the wall defining the opening 27a. As before, the ring-like portions are crimpled around the legs 20a--20a of the sheath. 
     One of the parts 60,61 is molded with a boss and the other part is molded with a recess to receive the boss. In the illustrated embodiment, the plug portion 60 is formed with a pair of bosses 62--62 which extend from a side face thereof. As seen in FIG. 8, the bosses 62 are shaped to the contour of the open spaces 63--63 of the insert 25a, to fit closely therethrough. As seen in FIG. 9, two grooves 64--64 are formed in the side face of the head portion 61 and these grooves are shaped to closely receive those parts of respective bosses 62--62 which extend beyond the insert 25a. Each of the bosses 62 is formed with a V-shaped ridge 65 for ultra sonic welding of the bosses to the base surface of respective grooves 64--64 to provide a unitary assembly comparable to the embodiment shown in FIGS. 1 through 5. 
     To assemble the parts of the embodiment shown in FIGS. 6 through 9, the plug portion 60 is first disposed over the legs of the sheath and may drop as far as the bight portion of the sheath so that it is out of the way. Next, the insert 25a is crimped to the sheath legs in proper location thereon. Thereafter, the plug portion 60 is brought to the insert 25a and the bosses 62--62 extended through the spaces 63--63 of the insert. Then, the head portion 61 is disposed over the sheath legs 20a--20a, and extending parts of the bosses 62--62 are seated within the respective grooves 64--64 of the head portion 61. Thereafter, the assembly is placed in an ultrasonic welder and operation of the latter causes the ribs 65--65 and the mating surfaces of the bosses and grooves to fuse to form a unitary assembly of the type shown in FIG. 2. 
     DESCRIPTION OF FURTHER EMBODIMENT 
     My invention is adaptable for use as a flange mounting as well as it is for the plug mounting hereinbefore described, and FIG. 10 discloses a flange mount type. 
     Instead of the hexagonal portion 42 of the embodiment of FIGS. 1 through 5, a flange portion 70 is molded as part of the mounting member, while the circular portion 44b remains the same. The flange portion 70 may be circular in plan, but usually it is in the form of a square plate having holes 71 in its four corner portions. 
     The insert 25b has the same internal construction as the insert 25 previously described except that instead of being circular in plan, it is preferably square to match the shape of the flange portion 70, and is provided with four holes to align with the holes 71. The shank of bolts 72 pass through the aligned holes in the flange portion 70 and the insert 25b and are threaded in corresponding threaded holes in the metal ring 15b. The flange portion 70 has an integral inwardly extending boss 73 to form the annular land for the O-ring 51b. It will be appreciated that the mold for producing the molded part shown in FIG. 10 will have a cavity of a suitable configuration to accommodate the flange portion 70, the circular portion 44b, boss portion 73, and the insert 25b.