US2034540A - Manufacture of coiled coil or double helical filaments - Google Patents

Description

Patented Mar. 17, 1936 UNITED STATES M A m JFACTURE or 'COILED con. B DOUBLE HELICAL FILAll/[ENTS Colin James 'Smithells, Middlesex, England, assi'gnor to General Electric Company, a corporation of New York No Drawing. Application July 1, 1935,1Seriail No. 29,371. In Great Britain July 18, 1934 4 Claims.

My invention relates to the manufacture of coiled coil or double helical filaments for electric incandescent lamps, that is, filaments in the manufacture of which a straight wire is coiled 5 into a primary helix on a mandrel and this primary helix with its mandrel is then coiled into a secondary helix. It is to be understood that the filaments referred to herein are tungsten.

One of the main problems in the manufacture of such filaments is to prevent distortion of the helices when heated in the finished lamp. It has long been known that this problem can be solved by a stabilizing process, that is, by heating the coiled coil, before the mandrels are removed, to

a temperature at which the stresses set up during Winding are relieved. However, if the heat ing is indefinitely prolonged and is carried out at such a high temperature that complete recrystallization of the metal occurs, the filament becomes brittle and liable to break when handled. This temperature is generally believed to be in the neighborhood of 1500 C.

It appears that this problem might be avoided by heating the filament to a temperature so low that complete recrystallization cannot occur. It

has been proposed already to heat coiled coil filaments to a temperature below that of recrystallization, and in particular to 1300 C., for about an hour.

I have found that this problem can also be avoided by heating the filament to a considerably higher temperature for a much shorter period. In fact by choosing the proper temperature for the furnace, the desired stabilization can be effected, without causing brittleness, by drawing the wire continuously through the furnace at a speed of the order of one meter per minute, and so that the wire remains in the furnace for a period much less than one minute.

It is believed that the effect of such a brief treatment at a relatively high temperature is to produce complete or nearly complete recrystallization in the outer layers of the wire, while leaving the core substantially unchanged. The

strains produced by coiling are concentrated mainly on the outer layers, and the recrystallization of this part will relieve most of the strain. The speed of recrystallization increases with the amount of strain, so that the core will not recrystallize and will provide the necessary strength.

According to the invention, the manufacture of coiled coil filaments in which tungsten wire, first wound on a primary mandrel, is then wound together with its primary mandrel on a continuous length of secondary mandrel, comprises the step of drawing the coil on its mandrels continuously through a furnace, the temperature of the furnace being so high that if the wire were maintained at this temperature for one hour it 5 would recrystallize completely, and the duration of its stay in the furnace being so short that recrystallization is not complete. A temperature between 1500 C. and 1600 C. and a duration of the order of twenty seconds are generally 10 suitable. The extent of recrystallization is, of course, to be judged with reference to stability and brittleness. Recrystallization begins when stability begins to increase and is complete when further maintenance at the same temperature 15 produces no further increase either in stability or in brittleness.

The temperature of the furnace should be as uniform as possible along its length, but perfect uniformity is never attained. It is to be under- 20 stood therefore that the value of 1500 C. to 1600 C. refers to the maximum temperature as determined by the ordinary factory methods, and that in determining the period during which a point on the filament remains within the furnace, any 25 regions where the temperature is less than 1000 C. are to be regarded as outside the furnace.

In a specific example of the process according to the invention, a straight tungsten wire is coiled into a helix on a primary mandrel of molybdenum. 30 The coiling is continuous. This primary coil and mandrel is then coiled on a secondary mandrel, also of molybdenum, uncoiled gaps being left at which the secondary helix can be cut to provide the individual filaments. The structure so formed 35 is then drawn continuously through a molybdenum tube of ten mm. internal diameter and thirty-two cm. long, in a hydrogen-nitrogen atmosphere, the tube being heated to a temperature of 1550 C. The coil on its mandrels is drawn 40 through at a rate of one and one-half meters per minute. The temperature is determined by viewing the tube with an optical pyrometer. The length of the fully heated portion of the furnace is about thirty cm., so that the coil remains in 45 this portion about twelve seconds. The mandrels are then removed by solution in the ordinary manner. The filaments in this particular case are intended for or watt 230 volt lamps. If finer filaments are being made, such as are used 50 for 40 watt 230 volt lamps, the temperature of the furnace may be reduced to 1500 C., the speed of travel being the same.

What I claim as new and desire to secure by Letters Patent of the United States is: 55

1. The method of manufacturing coiled coil filaments for electric incandescent lamps in which tungsten Wire is first coiled on a primary mandrel and then together with its mandrel is coiled on a secondary mandrel, comprising the step of drawing the coiled coil on its mandrels through a furnace at a temperature of approximately 1500 C. to 1600 C. at a speed such that the wire is not completely recrystallized.

2. The method of manufacturing coiled coil filaments for electric incandescent lamps in which tungsten wire is first coiled on a primary mandrel and then together with its mandrel is coiled on a secondary mandrel, comprising the step of drawing the coiled coil on its mandrels through a furnace at a temperature of approximately 1500 C. to 1600 C. at a speed such that a given portion of the wire remains in the furnace for less than one minute.

3. The method of manufacturing coiled coil filaments for electric incandescent lamps in which tungsten wire is first coiled on a primary mandrel and then together with its mandrel is coiled on a secondary mandrel, comprising the step of drawing the coiled coil on its mandrels through a furnace at a temperature of approximately 1500 C. to 1600 C. at a speed such that a given portion of the wire remains in the furnace between five and sixty seconds.

4. The method of manufacturing coiled coil filaments for electric incandescent lamps in which tungsten wire is first coiled on a primary mandrel and then together with its mandrel is coiled on a secondary mandrel, comprising the step of drawing the coiled coil on its mandrels through a furnace at a temperature of approximately 1500 C. to 1600 C. in a reducing atmosphere at a speed such that a given portion of the wire remains in the furnace for less than one minute.

COLIN JAMES SMITHELLS.