US 2454508 A
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Description (OCR text may contain errors)
Nov. 23, 1948. G. G. HERRICK ETAL RESISTOR AND METHOD. 0F MAKING THE SAME l Filed DBG. 13, 1940 y /ZJ Patented Nov. 23,l 1948 BESISTOR AND METHOD OF MAKING THE SAME Y George G. Herrick, Benzinger Township, Elk
County, and Hugh N. Veley, St. Marys, Pa., assignors to Speer Resistor Corporation, St.
m Marys, Pa.. a corporation of Pennsylvania Application December 1s, 1940, serien No. atomo (ci. coi-c3) .2 Claims.
This invention relates to resistors and methods of making the same, and has particular reference to the type of molded resistor commonly used in radio apparatus.
This application is, in part, a continuation of our application Serial No. 307,555, filed December 5, 1939, now Patent No. 2,282,328.
Resistors of the type described above generally comprise conductive portions formed of powdered carbon, clay, asbestos and other materials mixed with a binder, such as a phenolformaldehyde resin, pitch or other similar material. Generally, though not always, these reslstors are provided with an insulating shell formed generally of a type of material similar to that of the conductive portion of the resistor, but lacking conductive constituents such as powdered carbon or metal, or the like. It has been the practice in the past to mold first the resistance Aunit and bake it at. the desired temperaturef to set the binder or otherwise firmly join the ingredients together. Thereafter a coating of an insulating material was molded under heat and pressure around the resistance unit to provide it with a hard shell.
The present invention contemplates the formation of resistors of this general type in a series of operations embodying only a single heating or baking step. In particular, the process contemplates partially forming an insulating shell in a suiliciently compacted fashion to cause it to maintain its shape, filling this shell with the conducting material, compressing the two together with the insertion of leads and finally baking this assembly as a unit. As a result of this operation, the insulating shell andresistance material have more intimate contact whereby the radiation of heat from the finished resistor is greatly facilitated. Thus, for-a given factor of safety, the resistors may be made of smaller size than those of the prior art, or, if of the same size, the factor of safety is considerably increased to enable it to take care of substantial overloads.
The invention further contemplates an improved method of incorporating leads into the resistor to secure products of more uniform resistivity. Resistors of the general type herein referred to have resistances generally diilering quite considerably from their nominal resistances. This is due not only to non-uniformity in the composition of the resistance material, but quite largely due to the varying depth of penetration into the resistance material of the end leads. These resistors are generally of vquite short length, and slight errors in this penetration of the leads will give rise to quite substantial percentage deviations of the actual resistances fro the nominal ones.
A further object of the present invention relates to the location in the resistor of the end leads in such fashion as to secure substantially better uniformity of penetration of the leads into the resistance material. Specifically, this is accomplished by providing the leads with uniform hooked ends, or bent ends provided with shoulders, which, in the assembly operation, accurately determine the penetration of the lead into the resistance material. This advantage of the invention is applicable to resistors which do not have insulating shells -as well as those which do.
A further object of the invention relates to the insulation of the ends of the resistor. This is sometimes, though not always, desirable.
These and other objects of the invention. particularly relating to details, will become apparent from the following description, read in conjunction with the accompanying drawing, in which:
Figures l to 5, inclusive, are diagrammatic sectional views illustrating the steps leading to the formation of a partially finished insulating shell;
Figures 6 and 7 are similar views illustrating the assembly of the resistance material, end insulating material and the leads with this partially formed shelled, and the completion of the molding of the resistor;
Figure 8 is a fragmentary sectional view showing to an enlarged scale the nature of the finished resistor;
Figure 9 is a fragmentary view similar to Figure 8 showing a modied form of the improved resistor; and
Figure 10 is a further fragmentary view similar to Figure 8 showing still another form of the improved resistor.
While variations from the preferred method of formation of a resistor will be obvious, a preferred procedure for the formation of a resistor provided with an insulating shell will now b e described. A large number of insulators are simultaneously formed in a press by the duplication of various operating parts, but the drawing is confined to the parts cooperating for the formation of a single resistor.
A die plate 2 is provided with a large number of cylindrical openings such as 4, each of which serves for the formation and assembly of the parts of a single resistor. Beneath the die 2 are plates 6 and I8 carrying respectively compacting sleeves il land pins i2 aligned in a press with the openings il in the die plate 2. The spacings of these various elements, as Well as others which are later described, are maintained and limited most conveniently by spacing blocks which are placed between them and removed at various stages of the operation by the press operator. Initially, the parts just described are located in the relative positions illustrated in Figure l, with each of the sleeves 8 slightly projecting into the corresponding die plate opening [i and with the top of each pin I2 substantially level with the top of its sleeve, which it closely fits. The operator now pours over the die plate the powdered material A to form the insulating shell, and this is caused to ll up the openings 4, after which the excess may be brushed oif.
There is then placed over the die plate 2 a plate I4 provided with openings I6 of the same size as. and in alignment with, the openings Il. The plate I is then raised to cause its pins I2 to move upwardly to the position illustrated in Figure 2 with the result that the powder A is confined in the annular space between the pin I2 and the walls of the openings 4 and I6. The openings I6 in this case serve to receive the powder which is displaced from the openings 4 by the pin I2.
There are now pressed downwardly from above sleeves I8 carried by a plate, not shown. These sleeves serve to compress the insulating material. This compression is carried out at least to an extent to bring the compressed shell flush with the upper end of the die 2, as illustrated. though the compression may well be carried out to bring the upper end of the shell below the upper surface of the die. Following this operationl and as illustrated in Figure 4, the plate 6 is further raised to provide additional compression of the insulating sleeve by the sleeves 8.
The insulating material will now be compressed to a sufilcient extent to cause it to be self-sustaining. Accordingly, the pin I 2 is withdrawn and then the sleeves 8 and I8 are withdrawn and the plate I4 removed. The result is to leave in the openings Il in the die 2 the compacted insulating shells illustrated in Figure at A'.
A series of lower plungers carried by a plate (not illustrated) are provided with central openings in which are located 'terminal wires B provided with uniform hooked ends C, the free ends of which rest on the tops of the plungers 2li. The die block 2 is assembled with these plungers so that the plungers are in contact with the bottoms of the shells A.
Into the openings in the shells there is then dusted from the surface of the block a small quantity of insulating powder, which maybe of the same composition as the insulating shell. This powder forms a layer on the top of each lower plunger within its shell. Each of the insulating sleeves A is then filled with the conducting powder E, this powder being preferably filled in fiush with the top of the sleeve, whereby its quantity is quite accurately determined.l Over it is then dusted a further small amount of insulating powder F, as illustrated.
A second series of plungers 22, similar to 2li, are carried by a plate (not illustrated) and are provided with eentral holes in which are located lead wires G similar to the lead wires B and provided with uniform hooked ends, indicated at H, the free ends of which abut the bottoms of the plungers 22. These plungers are then assembled with the lower plungers and dies, desirably while they 4 are in a sloping position so that the lead wires do not fall out of the upper openings, and the assembly then placed in a press in which pressure is applied to force both plungers into the openings d, as illustrated in Figure 7. As a result of this operation, the insulating shell is further compressed to its final length and the conducting material is compressed within it. By reason of this simultaneous compression, the two materials are forced into very intimate contact with each other, with the result that in the nnal product heat transfer and dissipation is promoted. During this compression operation, furthermore, the insulating material D and F is compressed to seal the ends of the conductive material. During the compressing operation, the positions of the hooked ends of the leads are defined by the plunger surfaces whlch abut the free ends of the hooks. Accordingly, they extend accurately predetermined distances into the conductive material.
Following this compression, one of the sets of plungers is withdrawn and the other forced further inwardly to eject the resistors. These are then baked in conventional fashion.
One end of the finished resistor is illustrated in Figure 8, wherein it is shown as comprising the shell A", the resistance element E" bonded to the shell, the end resistance coating F" and the lead G having its hooked end H flush with the end of the resistor. The end insulation F" may, of course, be omitted if there is no objection to having the ends of the resistance material exposed.
It will be evident that the described procedure results in the formation of a resistor in a simple fashion with the attainment of uniform results to the end that the variations of resistance from the nominal resistance value may be kept quite low. Thus in cases in use where relatively slight variations may be critical, this molded type of resistor may be used rather than necessitating recourse to the more accurate and far more expensive wire wound types. An obvious variation of the procedure may be adopted if the resistor is to be of the type not having an insulating shell. We do not claim as new the use of a shoulder during the molding to limit the penetration of end leads (this being illustrated in Nickle Patent 1,847,888, dated March 1, 1932), but merely the accomplishment of this end by the bending of thin wire leads.
The result of accurate predetermined penetration of the leads into the resistance material may be secured otherwise than by the use of a hooked end on the wire as heretofore described. Instead, a bend may be provided in the lead wire so that when it is received in an opening in a compressing plunger a predetermined length and shape of wire will project to be embedded in the resistance material. Figure 9 illustrates this product. The insulator is illustrated as comprising an insulating shell J sur'ounding the resistance material K. The lead wire L has a hook or otherwire shaped portion M within the resistance material. The portion M joins the wire at a well defined bend N, which in the process of manufacture, Yessentially the same as that previously described, determines the extent to which the lead wire Amay enter the plunger hole. Thus a predetermined portion of wire of definite shape is embedded in the resistance material. This lead wire arrangement is also adapted for resistors of the uninsulated type but it is not quite as satisfactory for this type as the form previously described because a relatively thin wall will exist outside the bend of the portion M.
The modification illustrated in Figure 10 is a variant of that of Figure 8, which is found to improve the strength of insulators of small size. In this case, the shank O is provided with a bent end P embedded in the resistance material. The arrangement diiers from that of Figure 8 in that the end is slightly deflected to bring'the bend in line with the axis of the shank, thus causing the bend to lie in a more central position in the resistor. As in the case of the modification of Figure 8, the free end Q of the bend determines, during the molding process, the extent of penetration of the lead wire into the body.
What we claim and desire to ter:J Patent is:
l. The method of making a molded resistor comprising compressing, in an opening in a die protect by Letand about a pin extending centrally through said Vopening, loose insulating material to form a cylinder thereof, withdrawing the pin while leaving the cylinder in said opening, closing the lower end of the cylinder by means of pressure applying member carrying a projecting lead wire, lling into the cylinder and about a projecting portion of the lead wire loose resistance material, and applying to both the cylinder and contained resistance material pressure by means of said member and a second opposing member also carrying a projecting lead wire, thereby to form a resistor having the lead wires embedded in the resistance material thereof. Y
2. 'I'he method of making a molded resistor comprising compressing, in an opening in a die and about a pin extending centrally through said opening, loose insulating material to form a cylinder thereof, withdrawing the pin while leaving the cylinder in said opening, closing the lower end of the cylinder by means of a pressure applying member carrying a projecting lead Wire, filling into the cylinder, in succession, a small quantity of loose insulating material, a charge of loose resistance material, and a further small quantity of loose insulating material, and applying to both the cylinder and contained materials pressure by means of a said member and a second opposing member also carrying a projecting lead wire, thereby to form a resistor having insulated ends and having the lead wires embedded in the resistance material thereof.
GEORGE G. HERRICK. HUGH N. VELEY.
REFERENCES CITED The following references are of record in the