US 3102990 A
Description (OCR text may contain errors)
P 1963 K. F. MILLER ETAL 3,102;990
VARIABLE RESISTOR CONTACT Filed Dec. 11', 1961 INVENTORS. KENNETH l-T MILLER THOMAS M. PLQcE, 51?.
BY THE/l? ATTORNEYS. Heme/s, K/EcH, RUSSELL & KERN wear on the surface of the resistancematerial.
life, particularly This invention relates to sliding contacts for variable .resis'tors which are known-as, rheostats, potentiometers,
voltage dividers, and the like. .Thecontact of the inven-" tion is particularly adapted for use with resistors having a fused glass and metal resistance element, which material is ordinarily referred to as a cermet resistance material. Typically -cermet resistance materials are described in US. Patents Nos. 2,950,995 and 2,950,996.
A good sliding contact for a resistance-element such as. a cermet resistance material, should generate little or no electrical noise whenmovingover the surface of the resistance element and, should not produce any significant I A wide range of materials have been used for sliding contacts,
, including carbon, various metals, and a number of alloys such as silver-copper, silver-graphite, gold-copper-silver, and platinum gold-copper. These prior art Contact materials exhibit one ormore of the major deficiencies such ashi'gh electrical noise, short life due to wear of the resistance vmaterial, and smearing of the film of resistance material. to provide a new sliding contact material and method of making same, which contact is unique in composition, provides low electrical noiseand substantially increased elements;
It has been foundthat a superior contact can be pro duced using a plastic mixed with a metalanda lubricant in finely divided form. The lubricant may also be a conductor, such as graphite, and the metal omitted from the mixture. It-has been found that certainjplastics vexhibit lubricating qualities suitable for use'in contacts and, as another alternative, the contact can ,beproduced using a metal mixed with such a plastic.
Accordingly, it is an object of theinventi'on toprovide a molded variable resistor contact for use with a cermet resistance material or the like, comprising a thermosetting plastic and electrical conductor in finely divided form dispersed throughout the set plastic. A further 1 objectis toprovide such a contact incorporating a thermosetting plastic, metal powder, and solid lubricant, with the metal andv lubricant dispersed throughout theset plastic.
I A specific object of the invention is to provide such a contact in which the plastic is a diallyl phthalate resin. A 1
. It is an object of the invention to provide a molded variable resistor contact for use with a cermet resistance material or the like which may incorporate various thermosetting plastics, various metals and/or various lubriduring the serviceable life of the variable: resistor. A further object is toprovide a contact which generates low noise when passed over the surface of a resistance -element. Still another object of the invention is to provide Patented Sept. 3, i963 2 g a contact which may be molded onto its associated contact spring or carrier or may be attached to the spring ,by means of an electrically conductive adhesive.
The invention alsovcomprises novel details of construction and novel combinations and compositions, which will more fully appear in the course of the following description. The drawingmerelYshows and the description merely describes preferred embodiments of the present invention which are given by way ofillustration or example. 'In the drawing:
FIG. 1 is an isometric view of a preferred form of the a potentiometer FIG. 3.
T he structure spring 1-9 which is split at one endto form a plurality It is an object of the present invention a conducting lubricant, or a plastic and a metal. Various of fingers 11, 12, 13. Contacts 14, 15, 16 are molded in place at the ends of the fingers. Of course, one orany number of contacts can be used as desired. Typically, the contact spring may be of beryllium copper, although anyrelatively resilient metal or alloy may be utilized.
A contact itself comprises a molded thermosetting plastic with electrical conductor in finely divided form uniformly dispersed therethrough. Typically, the composition may befa plastic, ametal and a lubricant, or a plastic and plastics, such as epoxy resins, polytetrafluoroethylene,
when used with" cermet resistance V which the contact of the invention is particularly intended,
the composition of the" resistance materialis varied to obtain a variety of resistance values and to vary the I temperature coefficient. of the element.
ents and'the ratios of constituents used in forming the l1" he constitucontact of the invention may be varied to obtain the greatest compatability betweenthe contact and the resistance element, with respect to electrical noise, wear of the contact and resistance material and smearing of material.
It has been found that thepreferred range for the thermosetting plastic in the contact is about l0-70 percent by weight. With compositions utilizing plastic, solid lubricant, and metalpowder, it has'been found that. the prewhich, does not'significa'ntly wear the resistance element 7 i ferred composition range in percent by weight is plastic about .10- and lubricant about 30, with the remainder metal powder.
Some specific examples of compositions in percent by .weight' are set out'bel'ow.
Mixture No. 1
' Silver powder-200 meshfia 51.0
Diallyl phthalate resin (Mesa Plastics Diall 1000) 36.7 Graphite200 mesh 12.3
Mixture No. 2:
of FIG. 1 includes a contact carrier or invention in a rotary potentiometer.
t act carrier.
' times that of the metal contact No. l of this application.
Copper-200 mesh, 60
Epoxy resin (Mesa Plastics Epoxy Resin v 1592) 20 Graphite-200 -rnesh 20 It has been found that the diallyl phthalate resin has superior wear and lubricating properties When used as the binder in for m-ing'a sliding contact and this resin is the preferred plastic for use. in making the contact of the invention. .The diallyl phthalate-silver powder-graphite mixture set out as mixture No. 1 in the above chart is the preferred mixture for the contact, exhibiting the desired mechanical and electrical properties discussed previously' to a superior degree.
An alternative form of. the structure is shown in FIG. 2. Contacts 21, 22, 23 are formed by pressure molding and are then cemented to a contact carrier. 24- using a conductive cement, such as a conductive epoxy resin.
FIGS. 3 and 4 illustrate the use of the contact of the The potentiometer includes a housing 30 and cover 31 with a shaft 32 jourfnaled in bushings 33, 34 carried in the housing and cover,
respectively. A resistance element in the formof a film 35 of cermet resistance material fired to, a ceramic. base a p duced less Wear after 3,574,800 cycles, at which time the test was terminated.
Although exemplary embodiments of the invention essarily departing from the spirit of the invention.
We claim as our invention: 1. A moldedvariable resistor contact for use with a cermet resistance material or the like, consisting of in percent by Weighhdiallylphthalate resin about 37, silver powder about 51, and graphite about 12, with the silver and graphite dispersed throughout the set resin. i
2.'A molded variable resistor contact for use with cermet resistance material or the like, consist-ingof .in percent by Weight, epoxy resin about 15, and copper in finely divided form about 85,.with the copper dispersed throughout the set resin. 1
3. A molded variable resistor Contact for use with a cermet resistance material or the like, consisting of in percent by Weight, diallyl phthalate resin about 40', and acetylene black carbon about 6-0, with the carbon dispersed throughout the set resin.
36 is fitted in the housing 30. A bushing 37 may be provided in the base 36 for receiving the shaft 32. A contact carrier 38, which may be the carrier of FIGS. 1
or 2 is supported on an arm 39 projecting from a collar Of course, the invention is equally suitable with other formsof variable resistors, including linear and helical.
The following is a specific example of one process for making the contact of the invention. Using the composition of mixture No. l, the metal powden'r'esin binder and graphite are milled in a ball mill dry until the materials are thoroughly mixed and reduced to a fine particle size. A quantity of the mixture is then placed in a mold of appropriate shape and formed under pressure and The time, temperature and pressure of molding are dependent upon "the particular plastic used. For the composition of mixture No. 1 using rdiallyl phth alate resin, "a molding-'pressure-of 2,000 psi; at 313? for three minutes was used. i
A second example of a process is given below, using mixture No, Shaving a liquid resin binder. The metal, lubricant and'l-iquid resin with catalyst are thoroughly mixed forming a paste. A quantity of the-paste is then placed in the mold for setting. For the epoxy resin, a
pressure of 2,000 p.s.i. at 340 F. for six minutes was used.
Comparative Wear tests between contacts of the type in diameter and of the composition'described in the afore-' said U.S. Patent No.- 2,950,996 mixture No. 12. Two commercially available contact alloys Were used, Palney #7 from I. M. Ney Co., and Baker alloy 503 from Englehard Industries, ,Inc. The plastic contacts were mixture After 1,000,000 cycles (passage of the contact from one end of the resistance element to the other) both types of the metal contacts produced a substantial and noticeable amount 'of wear on the surface of the films. Alloy contacts previously tested have not exceeded this number of cycles. In contrast,
the molded plastic contact of the present inventionpro- 4-. A molded variable resistor contact for use with a cermet resistance material or the like, consisting of "in percent by weight, tetrafiuoroethylene' polymer plastic 7 about 40, and silver powder-about 60, with the silver dispersed throughout the set plastic. j f
5. A molded variable resistor contact for-use with a cermet resistance material or the like, consisting of in percent by weight, a thermosetting plastic about l0- 70, a solid lubricant in finely divided form up to about 30,v and the remainder metal powder, 'with the lubricant and-metal dispersed throughout the .set plastic. 6. In a potentiometer, the combination of a housing;
a cermet resistance element mounted in said housing;
a movable contact carrier mounted in said housing for traversing said resistance element; 7
and a molded contact fixed to said carrier for contacting said element and consisting of a thermosetting plastic and electrical conductor in finely divided form dispersed throughout the set plastic. 7. In a potentiometer, the combination of: a housing;
a cermet resistance element mounted in said housing;
a movable contact carrier mounted in said housing for 5 traversing said resistance element; and a contact molded about a portion of said car ier for support thereon in contact With said element,
:said contact consisting of a thermosetting plastic, With electrical conductor in finely divided form dispersed therethrough t 8. In a potentiometer, the combination of: a housing; 1 a cermetresistance element mounted in said housing; a movable contact carrier mounted in said housing for traversing said resistance element; 0
and a moldedcontact cemented to said carrier for 1 tacting said element and consisting of athermosetting plastic, metal powder and solidlubricant in finely divided form, with the metal and lubricant dispersed throughout the set plastic.
'10. In a potentiometer, the combination of:
a housing; I
a cermet resistance element mounted in said housing;
a movable contact carrier mounted in said housing for traversing said resistance element;
and a molded contact fixed to said carrier for contacting said element and consisting of in percent by Weight, diallyl phthalate resin about 37, silver powder about 51, and graphite about 12.
:11. A potentiometer comprising:
a resistance element formed of a fused glass having metal particles dispersed therein mounted in said housing;
a movable contact carrier mounted in said housing for traversing said resistance element; 1 and a contact molded about a portion of said carrier for support thereon in contact with said element, said contact consisting of a thermosetting plastic and a metal selected from the group consisting of silver, gold, aluminum and "alloys thereof and copper, with .the metal dispersed throughout the set plastic.
12. The potentiometer defined in claim 11 wherein said molded contact further consists of a lubricant selected from the group consisting of graphite and molybdenum disulfide,
References Cited in the file of this patent UNITED STATES PATENTS 2,740,732 Peck et a1. Apr. 3, 1956 2,790,882 Siegel a Apr. 30, 1957 2,862,088 Mairs Nov. 25, 1958 2,950,996 Place et a1. Aug. 30, 1960 I FOREIGN PATENTS 1,186,513 France Feb. 23, 1959