US3622935A

US3622935A - Helical resistance heating element

Info

Publication number: US3622935A
Application number: US47386A
Authority: US
Inventors: Sterling A Oakley
Original assignee: Oakley Ind Inc
Current assignee: Oakley Ind Inc
Priority date: 1968-12-06
Filing date: 1970-06-18
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

An electrical heating apparatus comprising a resistance heating element having a helical end convolution and a terminal member havng one end defined by a plurality of convergent, longitudinally extending, radially spaced projecting ridges, said ridges being welded to said end convolution at radially spaced points lying on a circle spaced between a mean diameter and an inner diameter of said end convolution.

Description

United States Patent Inventor Appl. No.

Filed Patented Assignee Sterling A. Oakley Chicago, Ill.

June 18, 1970 Nov. 23, 197 1 Oakley Industries Incorporated Skokie, Ill.

Original application Dec. 6, I968, Ser. No. 781,883, now Patent No. 3,536,884. Divided and this application June 18, 1970, Ser. No. 47,386

HELICAL RESISTANCE HEATING ELEMENT 7 Claims, 6 Drawing Figs.

U.S. Cl

Int. Cl

338/329, 338/238, 338/331 HOlc 1/14 [50] Field of Search .6 338/238, 296. 329. 331

[56] References Cited UNITED STATES PATENTS 2,899,666 8/1959 Drugmand 338/33! X Primary Examiner-E. A. Goldberg An0rney-Mason, Kolehmainen, Rathburn & Wyss ABSTRACT: An electrical heating apparatus comprising a resistance heating element having a helical end convolution and a terminal member havng one end defined by a plurality of convergent, longitudinally extending, radially spaced projecting ridges, said ridges being welded to said end convolution at radially spaced points lying on a circle spaced between a mean diameter and an inner diameter ofsaid end convolution.

I-IELICAL RESISTANCE HEATING ELEMENT The present invention relates to a new and improved electrical heating apparatus, and is division of copending application, Ser. No. 781,883, filed Dec. 6, 1968, and now US. Pat. No. 3,536,884 issued Oct. 20, l970.

Electrical heaters of the type disclosed in US. Pat. No. 2,483,839, issued Oct. 4, I949, have come into widespread usage in many and varied applications through the years. Heaters of this type include metallic tubular sheath formed of heat-resistant material in which is mounted coiled electric resistance eating element which is insulated from the sheath by an electrically insulating, heat-conductive particulate material, such as magnesium oxide. Opposite ends of the coiled heating elements are connected to suitable terminal pins which project outwardly of the ends of the sheath and are insulated therefrom. The terminal ends are adapted for connection to an appropriate source of electric power for supplying energy to the coiled resistance heating element within the sheath.

Over the years, many improvements have been made in electric heaters of this type as new and better materials have become available and a number of improvements have been made in the apparatus and methods of fabricating these heating units.

One of the problems associated with the manufacture of heating units of the character described is that of making mechanically strong, low resistance welded connection between the ends of the coiled heating element and the terminal pins. Because the resistance heating wire used is normally relatively small in diameter, it is very easy during welding and/or assembly operations to burn or physically damage the end convolution of the resistance coil or the convolutions closely adjacent thereto; and such burning or damage usually results in a shorter useful life and early burnout or failure of the heater.

Moreover, new types of resistance heating wire are now being used for heating elements because of lower cost, and one of these types, produced by the Aktiebolaget Kanthal Company, of Sweden, has many desirable characteristics but, because the wire is fabricated from an aluminum-iron alloy or mixture, is extremely difficult to weld and becomes brittle and subject to breakage if overheated during a welding operation. The use of aluminum-iron alloy resistance heating wire in place of a nickel-chromium type of resistance wire is extreme ly advantageous because of the lower cost of the wire and better heating characteristics thereof. However, the problems encountered in welding aluminum-iron wire of this type have prevented wider usage because of the tendency of the wire to become brittle after welding. Brittle wire is more subject to breakage and not ductile enough for easy forming and drawing, as is required when the finished heater is coiled or bent into irregular'shapes for specific applications.

In my copending US. Pat. application Ser. No. 644,l7l, filed June 7, 1967 now US. Pat. No. 3,492,624, there is disclosed a new and improved heating apparatus and method of making the same suitable for use with resistance heating wire of iron-aluminum alloy or mixture and the present invention is an improvement thereon enabling a better weld to be obtained between the terminal pins and coiled heating element on an economical basis and reliable basis.

An object of the present invention is the provision of new and improved electrical heating apparatus comprising a resistance heating element having a coiled and convolution and a terminal member welded thereto at a plurality of radially spaced, separate points around the end convolution.

Another object of the present invention is the provision of new and improved electrical heating apparatus of the character described having a resistance heating element formed of an iron-aluminum alloy or mixture.

Yet another object of the present invention is to provide a new and improved electrical heating apparatus of the character described which can be produced at low cost on a mass production basis and one which is extremely reliable in operation and has a long and useful operating life.

The foregoing and other objects of the present invention are accomplished by a new and improved method of connecting a terminal member to an end convolution or coil of a resistance heating element comprising the steps of supporting the end convolution with radially inwardly directed compressive clamping force and relatively moving the end of a terminal member into engagement with the supported end coil or convolution making point contact therewith at a plurality of radially spaced, separate contact points on the interior portion of the surface of the coil. Welding current is passed between the terminal and the end coil to heat the areas of point contact to welding temperature without overheating other portions of the cell or terminal, thus providing a mechanically strong weld without burning or physical damage to the end coil or terminal member. The electrical heating apparatus is extremely reliable in operation and has a long and useful operational life. Convergent, radially spaced ridge portions at the welded end of the terminal member are firmly secured to the end coil of the resistance heating element providing an extremely strong welded connection therebetween. In accordance with the invention, a new and improved apparatus for connecting a terminal member to the end coil or convolution of a resistance heating element comprises a first clamping electrode means for holding the end coil with radially inwardly directed compression, and a second electrode is provided for engaging the terminal member to supply current for the weld. A terminal member holding means is provided to grip and move the terminal forcefully against the end coil held in the first cathode means so that point contact at a desired contact pressure is established at a plurality of radially spaced, separate contact points, defined by convergent, radially spaced ridges formed on the end of the terminal member. Means is provided for supplying welding current to said electrodes to flow across said contact points to effect heating and welding of the end coil' and terminal member.

For a better understanding of the present invention, reference should be had to the following detailed description taken in conjunction with the drawings and appended claims. The apparatus illustrated in the drawings is by way of illustration and not limitation and sets forth the best mode contemplated by the inventor for practicing the invention. In the drawings:

FIG. I is a longitudinal sectional view of a new and improved apparatus, in accordance with the present invention, for welding a terminal member to the end coil of a resistance heating element;

FIG. 2 is a transverse cross section through a primary electrode of the apparatus of FIG. 1, taken substantially along lines 2-2 thereof;

FIG. 3 is an enlarged fragmentary sectional view through the primary electrode of FIG. 2 taken substantially along the lines 3-3 thereof;

FIG. 4 is an outside end view of the primary electrode of the apparatus of FIG. 1, looking in the direction of arrows 4-4 thereof and showing the electrode in a closed or clamping position for holding the end coils of a resistance heating element for welding;

FIG. 5 is a view similar to FIG. 4 but illustrating the electrode in an open position after a welding operation has been completed and the welded components are ejected; and

FIG. 6 is a longitudinal sectional view through a new and improved heating apparatus constructed in accordance with the invention.

Referring now more particularly to the drawings, in FIG. 6 there is illustrated a new and improved electric heating apparatus or heater constructed in accordance with the present invention and referred to generally by the reference numeral 10. The heating structure 10 includes a pair of elongated, generally cylindrical terminal pins 12, the outer projecting ends of which are adapted for connection with a current source for energizing the heater. As best shown in FIG. 3, each terminal pin 12 includes an inner end portion or blunt welding end formed by a plurality of convergent, planar faces 12a tapering inwardly with respect to the longitudinal axis of the pin toward the end and angularly intersecting one another to form a plurality of radially converging ridges IZb adapted for engagement with an outer end convolution or coil 140 at the opposite ends of an elongated, helically coiled, resistance heating element 14 having a plurality of intermediate helical convolutions 14b. The coiled heating element 14 is formed of electrically resistant material which becomes elevated in temperature when electric current is passed through the coil through the terminal pins 12, which pins are welded to the respective end convolutions 14a of the coil, in accordance with the present invention. Many different types of resistance heating wire can be used in forming the heating element or resistance coil I4, and in the past, nickel-chromium alloys have proven highly successful. However, because of the relatively high cost of such nickel-chromium alloys and other materials having the requisite resistance and temperature characteristics, other materials, lower in cost, have been used. Recently, heating wire formed of aluminum-iron alloys or an aluminum-iron composition have been used successfully, except for the fact that welding of these materials is relatively difficult. Because of the elevated temperatures required for welding, the aluminum in the material tends to precipitate out of a strong lattice structure with the iron making the material relatively weak and brittle in the region where excessive heating of the wire has occurred. This results in frequent failures in the vicinity around the weld, which is unsatisfactory. Moreover, when wire of aluminum iron has been used, after the welding has been completed it is necessary to anneal the material before the heater can be drawn or worked and bent to a specific shape required.

The present invention minimizes the amount of heat applied to other adjacent portions of the resistance heating wire and concentrates the heat in the weld area alone so that drawing and bending operations can be conducted after the weld is completed without requiring the annealing of the material, and thus production time is saved and cost is reduced. In the present invention, the area of intense heating for welding is minimized, concentrated, and confined to only the relatively small regions where the weld is actually made, and adjacent portions of the coil and terminal are not heated to a critical temperature as the welding process takes place. Once the elongated terminal pins 12 have been welded to the end coils or resistance heating element 14, the assembled structure is inserted into a heat-resistant, metallic, tubular sheath 16, as shown in FIG. 6. Suitable insulating washers or spacers 18 are mounted on the terminal pins at opposite ends of the sheath 16 to maintain the terminal pins in coaxially centered, insulated alignment within the sheath, and the interior of the sheath is filled with compacted, electrically insulating but heat-conductive packing material 20, such as aluminum or magnesium oxide. After filling of the sheath with insulating material and compacting the material around the coil 14 and terminal pins 12, the outer end portions of the sheath are deformed inwardly to form annular retaining ridges 16a to hold the spacers 18 in place and seal the insulating packing material 20 within the sheath. The assembled heating structure 10, as illustrated in FIG. 6, can then be drawn to reduce the diameter thereof and more tightly compact the insulating material 20 therein around the terminal pins 12 and heating coil 14. Subsequently, the heater may be bent and shaped as desired, without annealing, and the coiled heating element 14 and terminal pins I2 are maintained in spaced insulating relation from the outer tubular sheath 16 by the packing material 20, even though relatively sharp bends and turns are made to form the heating structure to the desired shape.

Welding of the inner ends on the terminal pins 12 to the end convolutions or coils 14a of the helically coiled resistance heating element 14, may be accomplished in accordance with the present invention in apparatus as illustrated in FIGS. 1 through 5 and referred to generally by the reference numeral 22.

The apparatus 22 includes a pair of spaced-apart, primary, coil, clamping electrode assemblies 24, each having a cylindrical bore 26 defined therein and aligned in coaxial alignment with the bore in the other assembly. Each bore is adapted to receive and hold a respective outer end portion of the resistance heating coil 14, including an end coil 14a and one or more intermediate convolutions 14b adjacent thereto.

As best shown in FIG. 3, the electrode assemblies 24 are fabricated of electrically conductive metal, such as copper, and preferably the bores 26 are long enough to accommodate an end convolution 14a and three or four adjacent intermediate convolutions 14b when each coil or turn is tightly pressed against the adjacent coils as shown. The bore 26 is of a diameter slightly smaller than the normal, unstressed, outer diameter of the resistance heating coil (the difierence in diameters is shown in exaggeration in FIGS. I and 3) so that when the end portions of the heating element are inserted into the bores as shown, the coils seated in the bore are compressed radially inwardly and are thus held tightly against any longitudinal displacement, once clamped in place in the electrode assemblies 24. In order to facilitate insertion of the opposite end portions of the heating coil 14 into the bore 26 in the spaced-apart, coil clamping electrode assemblies 24, the inwardly facing portions of the bore 26 are rounded, as at 26a (FIG. 3).

Referring momentarily to FIGS. 2 and 3, each of the primary, coil clamping electrode assemblies 24 includes a fixed jaw member 28 secured in an upright position on a base 30 (FIGS. 4 and 5) and a movable or jaw member 32 is pivotally connected to the fixed portion at 28 at the upper end thereof by a hinge pin 34, joumaled in

hinge members

36 and 38, attached to the upper ends of the

respective jaw members

28 and 30 by cap screws 40. The fixed and

movable jaw members

28 and 30 are fonned with T-shaped grooves or

slots

28a and 30a, respectively, facing one another in order to receive replaceable, clamping

electrodes

42 and 44, respectively, each of which defines half of the surface of the bore 26.

As best shown in FIGS. 2 and 5, the clamping electrode 42 is formed with a recess 26a of semicircular section forming one-half of the surface of cylindrical bore 26, and the clamping electrode 44 is mounted in the movable jaw member 32 and is similarly provided with a facing semicylindrical recess 26b forming the other half of the surface of the bore 26. When the movable and fixed jaw members are closed together as shown in FIG. 4, a perfectly cylindrical bore 26 is formed, and when the members are pivoted apart about the hinge pin 34, as shown in FIG. 5, the bore 26 is split apart or opened up so that the coiled heating element 14 and terminal pins 12 welded thereto are released from compression and can fall downwardly, as shown in FIG. 5, through a suitable discharge slot 300 fonned in the base member 30.

The

replaceable electrodes

42 and 44, which are seated in the T-shaped

grooves

28a and 32a of the jaw members, are retained against longitudinal movement by cap screws 46, and as wear occurs on the outwardly facing edge surfaces of the respective clamping electrodes, the electrodes can be moved outwardly in their respective supporting jaw members to compensate for the wear and prevent a smooth and even outer face.

In accordance with the present invention, each primary clamping electrode assembly 24 has a planar, outer end face 240 formed by the outer end surfaces of the fixed and

movable jaw members

28 and 32 and the outer end surfaces of the

replaceable clamping electrodes

42 and 44 mounted thereon. The outer end faces 240 are spaced apart at a distance which is somewhat less than the normal or unstressed length of the elongated heating coil 14, so that when opposite end portions of the heater coil are inserted in the bore 26, the end coils 14a and intermediate coils 14b adjacent thereto can be compressed tightly together in contact against each other, as shown in FIG. 1 before final clamping pressure is applied by completely closing the movable jaw members 32.

In order to prevent the outer end convolution 14a of the electric heating element 14 from projecting outwardly of the bore beyond the end faces 24a of the respective electrode clamping assemblies 24, each clamping assembly is provided with a movable gate member 48 which is mounted for sliding movement on the outer end face of the fixed jaw member 28 for movement between a bore closing position shown in FIG. 4, and an open position shown in FIG. 5, wherein the forward end portion of the gate is retracted rearwardly of the bore section 26a of the fixed jaw member 28. Each gate member 48 is supported for sliding movement between the open and closed positions shown in FIGS. 4 and 5 by a pair of horizontal, upper and lower, spaced guide members 50, best shown in FIG. 3, which are attached to the fixed jaw member 28 by a plurality of cap screws 52 (FIGS. 4 and 5). The guide members 50 are L-shaped in section and the gate 48 is provided with upper and lower longitudinal edge flanges 48a adjacent the inside surface thereof, which slide within ways formed by the guide members. The inside surface of the gates 48 are thus maintained in tight, sliding contact against the outer end face 24a of their electrode clamping assemblies 24.

The rearward ends of the gates 48 are connected to actuator rods 54 by pins 56, and the actuator rods are moved by suitable motive power means, such as fluid cylinders 58 or solenoids to effect opening or closing of the gate. The fluid cylinders are mounted on brackets 60 affixed to the baseplate 30 by cap screws 62, and are operable to open and close the gates when energized by pressurized fluid supplied to opposite ends thereof through

fluid lines

64a and 64b. The movable jaw members 32 of the respective primary electrode clamping assemblies 24 are movable between a fully open position of FIG. 5 and a tightly closed or clamping position of FIG. 4, respectively, by means of a suitable, overcenter, toggle-locking mechanisms 66 having their outer ends pivotally attached to the support base 30, as M68, and their inner ends attached to the lower end of the jaw members 32, as at 70. Suitable controls and actuating means, such as fluid cylinders or solenoids, may be used to operate the toggle-locking mechanisms 66 in synchronism with the movement of the movable gates 48, if desired, or the components may be operated by hand. However, in most instances where rapid production rates are desired, suitable control means and actuator means are provided to perform the necessary steps in operation of the primary electrode clamping assemblies 24, as described hereinafter.

When the end portions of each heating coil 14 are inserted into the bores 26 of the primary clamping assemblies 24, the end coils are pressed against one another by longitudinal compression of the heater coil, and during this time the bores are cracked open slightly from the closed or clamping position of FIG. 4 to permit easy insertion of the coils, With the coils of each end portion with the bores 26 thus held in longitudinal compression, the movable jaw members 32 are fully closed or clamped and the coils or turns within each bore 26 are compressed radially inwardly and are tightly clamped in position for welding, as shown in FIG. 3. During insertion of the coils and closing of the electrode assemblies 24, as described, the gates 48 are closed to prevent the end turns 14a of the heater coil from projecting outwardly beyond the outer end faces 24a of the respective clamping assemblies, and so that the intermediate convolutions 14b next adjacent the outer end turns 14a can be compressed longitudinally together to the position as shown in FIG. 3. Because the bores 26 are slightly smaller in diameter than the normal unstressed outer diameter of the electric heating coil 14, the turns or convolutions within the bores are compressed radially inwardly and are positively restrained from any movement while the movable jaws 32 are closed and locked in position by the toggle-locking assemblies 66. After the jaw members32 are locked in the closed position, the movable gates are retracted rearwardly to the open position of FIG. 5, to expose the end convolutions 14a for welding with the respective terminal pins l2.

As best shown in FIGS. 1 and 3, adjacent and outboard of the outer end faces 24a of each primary electrode clamping assembly 24, is provided a secondary electrode assembly 72 having a bore 74 coaxially aligned with the bore 26 for receiving a terminal pin 12 for welding. Each secondary electrode assembly 72 includes a fixed electrode 76 and a movable electrode (not shown) pivotally interconnected and arranged generally similar to the arrangement of the primary electrode clamping assemblies 24. The bores 74 are rounded at their outer ends as at 74a in order to smoothly receive the blunt pointed inner ends of the respective terminal pins 12 which are moved longitudinally and inwardly into contact against the end coils 14a of the clamped heater coil 14.

Referring now to FIG. 2, after the primary electrode clamping assemblies 24 have been closed and locked with electrical heating coil 14 clamped therein, terminal pins 12 at opposite ends of the clamped coil are moved axially inwardly (arrow B- in FIG. 3) toward the exposed outer end convolutions l4a until the ridges 12b are forced into point contact with the end coils 14a, as shown in FIG. 3. Because the ridges 12b are formed by the apexes of the intersecting planar surfaces 12a on the inner ends of the terminal pins 12, point contact is made between the ridges and the outer end convolution 14a at a plurality of radially spaced, separate, contact areas A," arranged in circular configuration around the end coils. The circular path on which the contact areas A" are positioned lies inwardly of the mean diameter of the coils of the heating element 14, so that as the terminal pins 12 are moved inwardly (arrow B in FIG. 3) into welding position a plurality of resultant forces F are developed normal to the ridges 1212. Each resultant force F" includes a radial component R" urging the end convolution 14a radially outwardly in opposition to the radially inwardly directed clamping pressure exerted by the clamping

electrode portions

42 and 44 of the primary electrode assemblies 24. In addition, each resultant force F includes a longitudinal component L" tending to more tightly compress the coils together against one another within the bore. Preferably, the planar faces 12a of the terminal pins 12 are sloped at an angle of approximately 50 with respect to the longitudinal axis of the pin and the forces R" and L are roughly equal in value with the forces L" being slightly greater than the forces R." The total amount of contact force or pressure exerted between the ridges 12b and the end coil 14a at the spaced, separate contact areas A is readily controlled by the amount of longitudinal thrust applied to the terminal pin 12 (arrow B) and is regulated for the desired welding contact pressure. Sufficient thrust is applied, depending on the size and type of wire being used to establish good electrical contact between the ridges 12b of the terminal pin 12 and the end convolution 14a of the heater coil 14 but excessive thrust is avoided to prevent the ridges from cutting through and materially deforming the end coil of the heating element. Because of the relatively blunt angle of the surfaces l2a, which form the ridges 1211 on the end of the terminal pin 12, there is little tendency of the terminal pin end to slip too far into the convolutions of the heater coil and cause an opening of the primary clamping electrode assemblies 24.

Because the end portions of the heater coil 14 clamped in the electrode assemblies 24 include several coils rather than only the end coil 14a alone, and because these coils are tightly clamped by the electrode clamping assembly 24 in radially inwardly and longitudinally compressed direct contact with one another, all; not just one, of the coils or turns in the bore 26 helps 0 carry the welding current so that overheating of the wire does not occur during the flow of welding current. In prior arrangements, the welding current flown primarily in a direction longitudinally through the coiled heater element, and because the cross-sectional area available for current flow is thus limited, excessive heating of the coiled heater element during high welding current flow commonly occurred. The welding current in the instant invention flows in paths radially inwardly (or outwardly) through several turns of the coiled element, as indicated by the lines C in FIG. 2, and converges at the regions of point contact A" where maximum heating takes place. The welding current additionally flows longitudinally of the coiled element, from turn to turn, and thus a larger area for current flow is available and reduced heating is achieved. The welding current flow per unit area is extremely high only in the relatively small areas of point contact designated A" between the ridges 12b and the end convolution 14a, and this is precisely where the maximum heating to a high temperature sufficient for welding is required. In short, the intermediate coils or convolutions 12b clamped in the electrode assemblies 24 help to carry the welding current and reduce the current density. In addition, these coils provide for heat dissipation because of the relatively large surface areas thereof exposed to the air. In prior arrangements, high welding currents for relatively long periods were required to raise the end portion of the terminal to a sufficient temperature for welding. This often resulted in overheating of the end coils of the heater coils. In the present invention, however, this problem is overcome by the use of the sharp ridges 12b which concentrates the current in the small contact areas A" on the ridges and it is not necessary to heat the entire end portion of the terminal. Because the welding contact pressure between the terminal pin ridges 12b and the outer end convolution 14a in the areas A" is readily controlled by the thrust applied to the terminal pin 12, the current density and time interval of current flow are more readily controlled, and, accordingly, a shorter period of current flow and a lower current density can be used in applicants invention to effect a better weld. Several suitable current regulators and timing devices are available on the market for this purpose, and a choice suitable for a specific installation is within the skill of the art, once the artisan has the benefits of viewing the present invention.

After a weld has been efiected between the ridges 12b and the end convolution 14a at the radially spaced, separated point contact areas A," a short cooling interval is provided to permit the weld areas to solidify before the primary and secondary

electrode clamping assemblies

24, 72 are opened to release the welded article. When the cooling period has expired, the movable jaw members 32 are opened from the position of FIG. 4 to the position of FIG. 5, and the movable secondary welding electrodes are similarly opened, permitting the welded article comprising a pair of terminal pins 12 and the coiled heating element 14 to fall downwardly from the

open bores

26 and 74 and the cycle or process is then repeated.

In order to move the terminal pins 12 longitudinally through the bores 74 of the respective secondary electrodes 72 and force the ridges 12b into contact against the end convolutions or turns 14a held in the primary electrode clamping assemblies 24 with the desired welding contact pressure, outboard of each secondary electrode clamping assemblies 72 there is provided amovable pin chucking assembly 84 adapted to grasp the outer end portion of a terminal pin 12 and selectively move the pin longitudinally inwardly toward the facing end convolution 14a of the clamped heater coil 14. The movable chucking assemblies 84 are mounted on actuating rods 86 of suitable fluid cylinders 88 and are movable toward and away from the respective pairs of primary and

secondary electrodeclamping assemblies

24 and 72. The fluid cylinders 88 are supplied with pressurized fluid at a selected pressure to effect the desired welding contact pressure at the point contact areas Spaced inwardly of the fluid cylinders 88 are provided storage bins 90 for holding a supply of the terminal pins 12, and each pin has an outlet adjacent the lower end and a movable pin-holding fixture 92 for holding a terminal pin in coaxial alignment with the

bores

74 and 26 until the chucking assembly 84 engages and grasps the outer end portion of the pin. After a pin is grasped by a chucking assembly, the pinholder 92 is moved out of the way and the pin and chucking assembly move inwardly until contact is made with the end coil 14a in the electrode assembly 24. The chucking assemblies support and hold the terminal pins during the welding process and after the weld has cooled, the pins are released and the chucking assemblies move back to the. position shown in FIG. 1, ready for the next operation. Movement of the chucking assemblies 84 and the pinholders 92 is synchronized with the operation of the

clamping assemblies

72 and 24 and the movable end gate 40 so that the process can proceed in automated fashion and produce the welded heater units in rapid succession. Suitable control means (not shown) are provided for effecting the sequentially timed operation of the various operating components of the apparatus 22 to provide for opening and closing of the electrode assemblies, the end gates, and movement of the chucking assemblies for feeding of terminal pins 12 from the storage bins in synchronization therewith.

The apparatus 22 includes hopper and feeder means 94 for holding a supply of the insulating washers I8 and positioning the washers in coaxial relation with the pins 12 for pickup thereon as the terminal pins are moved into welding position by the chucking assemblies 84. As the terminal pins 12 are moved inwardly into welding position, the pins pass through a washer 18 which has been centered in front of the bore 74 of the secondary electrode assembly 72. After each welding cycle is finished and the weld has cooled the chucking assemblies 84 are activated to release he terminal pins and are then moved outwardly to the position of FIG. I in preparation for the next operating cycle. Meantime, another pair of terminal pins 12 from the storage pins 90 are moved into position on the pinholders 92 and a pair of washers 18 are positioned ready for pickup by the terminal pins upon inwardly movement to the welding position.

Operation of the apparatus 22 to effect the method of the invention is briefly described as follows:

With the movable jaw members 32 of he primary electrode clamping assemblies 24 in a partially closed position and the gates 48 in a closed position, an operator inserts opposite end portions of a coiled heating element 14 into the bores 26 and compresses the convolution at each end of the heater coil longitudinally into a tightly compressed condition, as shown in FIG. 3, with the end turns 14a forced tightly against the closed gate members 48. The movable jaw members 32 are closed and locked with the toggle mechanisms 66 holding the coils on the end portions of the heater element in position for welding within the bores 26, as shown in FIG. 3. The end gates 48 are then retracted from the position of FIG. 4 to the position of FIG. 2, exposing the end coils 14a for welding, and the chucking assemblies 84 are activated to grasp the outer ends of the terminal pins 12, which are resting in coaxially centered positions on the pinholders 92. The fluid cylinders 88 are then energized and the terminal pins 12 are moved axially inward into welding position. As this occurs, the pinholders 92 are retracted out of the path of the movable chucking assemblies 84 and the terminal pins 12 pass through the insulating spacer washers 18 which are coaxially centered in pickup position by the feeder mechanisms 94. As the pins 12 thus move inward, the washers 18 are picked up and the ridges 14b of the terminal pins are forced into point contact against the end convolutions 14a of the heater element 14 clamped tightly in the primary welding clamping electrodes assembly 24. The pressure exerted in the contact areas A between the ridges of the pins and the end convolutions 14a of the electric heating element is regulated by control of the pressure of the fluid supplied to the cylinders 88 in order to achieve the desired welding contact pressure. Welding current is supplied to the respective pairs of primary and secondary

electrode clamping assemblies

24 and 72, for a predetermined period of time and a predetermined current value, which is sufficient to raise the temperature of the contacting portions of the pins and coils to the desired value for effecting the weld. After the weld is effected, a brief cooling period is provided for solidification of the weld metal in the contact areas A, and the

electrode assemblies

24 and 72 are opened, and the chucking assemblies 84 are released, permitting the welded assembly to fall downwardly through the openings 30a in the baseplate. Successive cycles of the operation as described may be initiated by a suitable foot pedal actuated by an attendant or operator overseeing the operation or the operation cycle of the process can be automatically controlled.

Because of the unique clamping and holding arrangement of the primary electrode clamping assemblies 24 wherein several turns or convolutions 14b adjacent the respective outer end convolutions 14a are tightly clamped by radially inwardly directed pressure and are longitudinally compressed against one another, and because of the bluntly tapered surface 12a on the terminal pins 12, the weld produced requires the heating of only a relatively small region of point contact areas A" which are radially spaced and separated from one another. An improved, extremely strong weld is obtained, and it is possible to effectively and successfully weld heating elements 14 fabricated of aluminum-iron alloys or mixtures without resulting brittleness in the wire. Moreover, because the weld areas A" are relatively small and concentrated, it is not necessary to heat the entire tip or end portion of the terminals up to the welding temperature, and the time interval needed for effecting the weld and the weld current can be reduced, thereby greatly reducing the possibility of damage by overheating. Moreover, the weld contact pressure is easily controlled by regulation of the pressure supplied to the fluid cylinders 88. The heating apparatus 10, produced in accordance with the present invention, is thus reliable in operation and suitable for many applications. The unique process of the invention makes possible lower production costs, higher production rate, and much better quality and reliability of the heater units so produced.

While there has been illustrated and described a single embodiment of the present invention, it will be apparent that various changes and modifications thereof will occur to those skilled in the art. It is intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An electrical heating structure comprising a resistance heating element having a helical end convolution, a terminal member having one end defined by a plurality of convergent. longitudinally extending, radially spaced, projecting ridges, said ridges being welded to said end convolution at radially spaced points lying on a circle spaced between a mean diameter and an inner diameter of said end convolution.

2. The electrical heating structure of claim 1 wherein said ridges are in point contact with said end convolution, said point contacts being radially spaced on a circle spaced approximately midway between said mean and inner diameter of said end convolution.

3. The electrical heating structure of claim 1 wherein said ridges are angularly disposed relative to said end convolutions at an angle directing approximately equal components of force radially outward thereof and parallel to the axis of generation of said convolutions.

4. An electrical heating structure comprising a resistance heating element formed of iron and aluminum having a helical end convolution, an elongated terminal member having a tapered end defined by a plurality of convergent, longitudinally extending, radially spaced projections, said projections being welded to said end convolution at radially spaced, separate weld areas lying on a circle spaced inside the nominal mean diameter of said end convolution.

5. The electrical heating structure wherein said ridges have longitudinally extending apexes and are V-shaped in cross section, said apexes lying on a conical projection disposed in concentric, tangential relation against said end convolution.

6. The electrical heating structure of claim 5, wherein said conical projection is at an angle of approximately 50 to the axis of generation of said end convolution.

7. The electrical heating structure of claim 4, wherein said weld areas occupy only a fractional portion of the circumference of said circle.

Claims

1. An electrical heating structure comprising a resistance heating element having a helical end convolution, a terminal member having one end defined by a plurality of convergent, longitudinally extending, radially spaced, projecting ridges, said ridges being welded to said end convolution at radially spaced points lying on a circle spaced between a mean diameter and an inner diameter of said end convolution.

6. The electrical heating structure of claim 5, wherein said conical projection is at an angle of approximately 50* to the axis of generation of said end convolution.