The invention relates to a lamp comprising a metal filament which incandesces when an electric current passes therethrough, said metal filament comprising a pair of legs and a coil extending through said legs, each leg having a diameter, a pair of clamps holding respective legs.

More particularly, the invention relates to an improved clamp which prevents collapse of the primary coil in the legs.

Incandescent filaments typically are. made of potassium doped tungsten wire having a diameter on the order of 20-70 um. This is wound on a primary mandrel having a diameter on the order of 80-150 um in order to form a primary coil having a external diameter on the order of 100-300 Fm. This composite is then wound on a secondary mandrel having a diameter on the order of 300-800 um in order to form a secondary coil or barrel. The secondary mandrel is retracted or dissolved, and the primary mandrel is then removed in whole or in part by dissolving.

The coiled coil type filament has a secondary coil or barrel located between a pair of legs which are clamped in lead wires which transmit current through the filament to cause incandescence. In order to prevent collapse of the primary coil in the clamps, the primary mandrel may be retained in the legs. This is accomplished by coating the portions of the primary mandrel extending into the legs with wax and dissolving the central portion. The primary mandrel is preferably molybdenum, which is dissolved in a known acid which does not attack the tungsten.

Where the secondary mandrel is to be dissolved, it is also molybdenum.

For a so-called mandrelless filament, the primary mandrel is removed completely, which is more economical than a selective removal. Without the support provided by a mandrel retained in the legs, however, the primary coil may deform too much in the clamps, causing the tungsten wire to split.

The lead wires have a diameter on the order of 0.5 mm, and according to the prior art are formed over the legs into a teardrop shaped clamp wherein the leg of the filament

is positioned in the base of the teardrop, coaxial with the radius of curvature of the clamp.

In order to achieve the desired compression of the primary coil, this positioning must be precise, because the primary coil is used as the anvil during the forming step. If the leg of the filament is too far from the radius of curvature, it will be compressed too much and the wire will be damaged.

U. S. Patent No. 3,988,630 discloses a lead wire forming apparatus for a mandrelless filament. This patent shows as prior art the steps for forming the teardrop shaped clamp described above using a single pair of forming dies, and further shows improved forming steps using first dies which preform the lead wire to form a hook, and second dies which clamp the hook about the leg. The legs are precisely positioned so that a closely controlled compression of the primary coil is achieved. The high stresses imposed by using the primary coil as an anvil during the initial forming are avoided. However, disadvantageous is the necessity of precisely positioning the legs and undertaking two forming steps for clamping the filament in the leads adds to the time and expense of manufacturing the lamp.

It is an object of the invention to provide a lamp of the type as described in the opening paragraph in which the abovementioned disadvantages are counteracted. This object is attained in that a lamp of the type as described in the opening paragraph is characterized in that each clamp being formed from a lead wire having a first surface and an opposed second surface, said first surface having a flat section between two raised sections, said flat section being recessed below said raised section by a depth less than half the diameter of said legs, each said lead wire being bent so that said raised sections are in mutual abutment and said flat section forms parallel clamping surfaces which engage one of said legs therebetween.

Since the clamping surfaces are parallel, the primary coil in the leg is compressed only to a degree defined by the depth of the recess, regardless of the position of the leg with respect to the center of curvature about which the leg is folded. Typically the recess has a depth of up to 25% of the diameter of the primary coil, whereby the leg is compressed by up to 50%. Thus, where the diameter of the leg (primary coil) is 100 m, the depth of the recess is up to 25 um. These are the values for non-recrystalized filaments; typically the primary coil in the legs is compressed by 20 to 50%. Pre-recrystallized coils are more brittle and are only compressed 10-20%. The lead wires are preferably molybdenum and typically have a diameter of 0.3,0.4,0.5, or 0.6 mm

According to a preferred embodiment, the raised surface at the distal end of the lead is formed by an"S"bend. The offset between the raised section and the flat section may be varied in accordance with the desired distance between the clamping surfaces.

According to another embodiment the distance between the parallel clamping surfaces is controlled by a single raised section, the flat section being recessed by a depth equal to the desired distance between the clamping surfaces.

According to a preferred embodiment, the second surface of the lead wire is provided with a flat section which extends beyond the flat section of the first surface in both directions, thereby lying opposite the raised sections. The flat sections are formed by stamping the end of the lead in dies.

The geometry of the clamp according to the invention offers the advantage of using a mandrelless coil and at the same time assures that the leg will be compressed by a controlled amount every time, regardless of the exact positioning of the leg with respect to the clamping surfaces. It will be understood that the clamp according to the invention is also useful for a filament consisting of a single coil.

Figure 1 is a plan view of one end of a coiled coil type filament; Figure 2 is a plan view of leads clamped directly to a filament; Figure 3 is an end view of the clamps of Figure 2, shown enlarged, according to the prior art; Figure 4A is a plan view showing the first or inside surface of the clamp according to the invention; Figure 4B is a plan view of the second or outside surface of the clamp; Figure 4C is an end view showing the opposed first and second surfaces; Figure 4D is an end view of the clamp as bent when the filament is received; Figure 4E is an end view of the closed clamp.

Figure 5A is a section view of a second embodiment of the clamp; Figure 5B is a section view of a third embodiment of the clamp; Figure 5C is a view of a fourth embodiment of the clamp.

Figure 1 shows a coiled coil type filament 10 consisting of a potassium doped ("Non-Sag") tungsten wire which has been wound around a primary mandrel to form a primary coil 12 extending throughout the length of the filament, and has been subsequently wound around a secondary mandrel to form a secondary coil 14 which is centrally located.

The secondary mandrel has been removed by simply retracting it from the secondary coil, while the primary mandrel-preferably molybdenum-has been subsequently removed by dissolving it in acid.

The central or secondary coil 14 is located between a pair of opposed legs 16 and separated therefrom by so-called K zones 18. For a wire diameter of 50 um, the primary coil is typically wound with a pitch of 340 threads per inch and a diameter of 130 um, and the secondary coil is typically wound with a pitch of 85 threads per inch and a diameter of 450 um.

Figure 2 is a plan view of leads 20 directly clamped to a filament 10, which is schematically shown without a primary coil for simplicity. Each lead 20 is provided with a clamp 22 which is formed around a respective leg 16 to retain it and provide electrical current thereto. The remainder of the lamp includes a base, a bulb, and a halogen filling. The design is also suitable for conventional incandescent lamps having a vacuum or inert gas filling. As used herein, the term"incandescent lamp"includes halogen filled lamps having a filament.

Figure 3 shows the teardrop shaped clamp 21 of the prior art. Since the manufacturing process typically involves holding the filament by the barrel to position the legs in the hook prior to fully closing the clamp, the leg may not be precisely positioned at the center of curvature of the teardrop, and the leg may be overstressed in compression.

Figure 4A is plan view of a first surface 24 of the end of a lead according to the invention prior to forming. The first surface 24 is formed with a flat section 26 which is flanked by two raised sections 28,29 and recessed therebelow. Figure 4B is a plan view of the opposed second surface 30 which has a flat section 32 extending to the distal end 33 of the lead. As shown in the side view of Figure 4C, the flat section 32 extends beyond the opposed flat section 26 so that it lies opposite the raised sections 28,29.

Figure 4D shows the partially formed clamp 22 with abend 23 having a center of curvature between mutually facing sections 27 of the first flat section 26. At this point in time the leg 16 is positioned between these mutually facing sections 26, which form the clamping surfaces. Figure 4E shows the finished clamp with the filament leg 16 in place, the clamping surfaces 27 in parallel, and the raised sections 28,29 abutting. Note that the use of opposed flat surfaces 26,32 is known per se in order to reduce surface stresses during forming. However the use of raised sections 28,29 limits the closure of the gap so that the clamping surfaces 27 remain parallel along the length of the finished clamp. This assures that the primary coil in the legs will not be overstressed, which could cause the wire to split and fail during transport or in use.

Figure 5A depicts an alternative embodiment having a raised sectiom 29 at only one end of the flat section 26, which extends all the way to the distal end 33 of the lead wire.

Here the depth of the flat section below the single raised section is equal to the desired distance between the parallel clamping surfaces, which in turn is determined by the diameter and desired compression of the coil in the leg of the filament.

Figure 5B depicts an embodiment which is similar to Figure 5A insofar as there is a raised section at only one end of the flat section 26 of the first surface. However the opposed flat section 32 of the second surface 30 is also recessed. This essentially provides a manufacturing advantage during the forming.

Figure 5C, like the principle embodiment shown in Figures 4A-4E, depicts two raised sections, one at either end of the flat section 26. However here the raised section 28 at the distal end 33 is made by forming an S bend 34 in the lead wire. The amount of offset in the bend determines the distance between the parallel clamping surfaces 27 formed by the flat section 26. Thus a single stamped wire profile may be formed to accommodate a range of leg diameters.

The foregoing is exemplary and not intended to limit the scope of the claims which follow.