US2914744A - Encased variable resistance device - Google Patents

Description

Nov. 24, 1959 H. M. SCHENDEL 2,914,744

ENCASED VARIABLE RESISTANCE DEVICE Filed Feb. 27, 1958 INVENTOR. HERBERT M. SCHENDEL BY ,A/

ENCASED VARIABLE RESISTANCE DEVICE Herbert M. Schendel, Santa Ana, Calif., assignor to Beckman Instruments, Inc., a corporation of California Application February 27, 1958, Serial No. 717,907

7 Claims. (Cl. 338164) This invention relates to variable resistance devices and, in particular, to variable resistance devices which are encased and sealed from the atmosphere.

One of the principal limiting factors in the design of precision variable resistance devices such as potentiometers is environmental sensitivity. A typical, non-enclosed, variable resistance device or potentiometer comprises a sliding contact which is adjusted to desired positions on a resistance element by means of a rotatable shaft. Of course, whenever the potentiometer shaft is turned, wear particles are produced from the shaft bearings, the sliding contact and from any other parts that move in conjunction with the shaft. These wear particles find their way between various moving parts in the resistance device, causing a variety of problems. Shaft bearing failure, electrical noise generated by the sliding contact and dielectric failure are typical examples. Moreover, ambient moisture produces galvanic action and other types of chemical corrosion, along with distortion of component insulating materials as a result of absorption of moisture. Wear products, moisture, electrical potential and heat generated from the passage of electrical current create a combination of electro-chemical effects that can and do produce costly deterioration in variable resistance devices.

It is the principal object of present invention to provide a variable resistance device which is relatively insensitive to environmental conditions.

It is a further object of the present invention to provide a variable resistance device which has a greatly increased life expectancy.

It is yet another object of the invention to provide a variable resistance device in which movement of the sliding contact is damped to reduce substantially or eliminate bouncing and peening of the sliding contact.

It is a still further object of the invention to provide a variable resistance device which has reduced electrical noise effects from the sliding contact and improved ability to withstand shock and vibration.

It is another object of the invention to provide a variable resistance device which is capable of dissipating substantially increased amounts of power under varied environmental conditions.

United States Patent According to one aspect of the present invention, these and other objects are realized by disposing a variable resistance device in a liquid-tight case and filling the case with a suitable insulating medium such as oil. Terminal means extending through the case make variable resistance values available exterially of the case.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as constituting the present invention, it is believed that the invention will be better understood from the following description, taken in connection with the accompanying drawing in which the single figure is a View, partially in section, of a variable resistance device constructed in accordance with the present invention.

Referring particularly now to the drawing, there is shown an encased variable resistance device according to the present invention which comprises a variable resistance device 1 disposed within a case 2. Device 1 may suitably be a variable resistance unit such as those variously described and claimed in A. O. Beckman U.S. Patent Nos. 2,454,986 and 2,473,048, both of which are assigned to the assignee of the present invention. It is not necessary, however, that device 1 be a multi-turn variable resistance unit; instead, it may conveniently be a single-turn variable resistor of a type well known to those skilled in the art. From an economic point of view it is advantageous for device 1 to be of a design which is available commercially and, in such event, the conventional lid or cover element should be removed for a purpose which will appear more clearly in the following paragraph.

Variable resistance device 1 comprises a housing 3 of suitable plastic material such as phenol formaldehyde. Disposed along the interior peripheral surface of housing 3 in helical grooves accurately cut therein, is a resistance winding 4 which is constructed of a plurality of turns of wire 5 space wound in helical configuration upon an insulated core 6 of material having relatively high heat conductivity. Conductive contact may be made to individual turns of wire 5 by means of a slidable contact member 7. Contact member 7 is slidably supported upon a rotor member 8 which may be turned through a shaft 9 that is journaled in a bearing 10, the mounting of which will be more fully described hereinafter. tact member 7 is provided with a non-conductive guide element (not shown) which extends into spaces 11 between adjacent turns of helical winding 4 and guides member 7 along winding 4 as rotor 8 is turned. The upper and lower ends of resistance wire 5 are respectively connected to terminals 12 and 13 which extend through and into housing 3 to facilitate affixing the wire ends thereto by means of soldering or welding. Contact member 7 is connected to a terminal 14 by means of a conventional slip ring (not shown).

As has been mentioned heretofore the principal objects of the present invention are realized by disposing variable resistance device 1 within liquid-tight case 2. It is also stated above that variable resistance device 1 can conveniently comprise a variable resistance device or potentiometer available commercially. In such event the conventional cover or lid portion is removedso that the remainder of the device can be attached to a lid member 15 as illustrated in the drawing. Resistance device 1 is attached to lid member 15 by means of screws 16 which threadably engage a split expansion ring 17 that, in turn, fits into a peripheral groove or slot in housing 3 as shown. Shaft 9, which is employed to turn rotor member 8 from the exterior of case 2, is as mentioned above journaled in bearing 10, the outer race of which is disposed in lid member 15 and retained therein with a split expansible retainer ring 17. The shaft and rotor assembly is positioned and retained within lid member 15 by means of a split, dish-shaped spring member 19 that engages a slot 29 in shaft 9. O-rings 21 of a material such as natural or synthetic rubber provide liquidtight seals for rotatable shaft 9.

In order that variable resistance values from device 1 may be available exteriorily of case 2, terminals 12, 13 and 14 are respectively connected to individual terminal means 22 through conductors 23, 24 and 25. The conductors are preferably attached to terminals 12, 13 and 14 by means of solder in order to provide good electrical connection. Terminal means 22 are introduced through case 2 in insulating and liquid-tight fashion by means of hermetic seals 26 of a design well known to those skilled in the art. Conductors 23, 24 and 25 may Conbe protected and insulated from each other and case 2 adjacent terminal means 22 with insulating caps 27. Conductors 23, 24 and 25 are purposely provided with extra length as illustrated in the figure in order that the soldering or otherwise afiixing of conductors 23, 2 and 25 to terminals 12, 13 and lid and terminal means 22 may be accomplished while resistance device 1 is positioned exteriorly of case 2 prior to permanent mounting in a manner which will be more fully described hereinafter. Short-circuits between terminals 12, 113, 14 and case 2, as Well as other accidental short-circuits to the internal surface of case 2, are prevented by positioning a thin sheet 28 of insulating material such as Mylar film around the inner periphery of case 2. Mylar film is a type of saturated polyester prepared from ethylene glycol and terepthalic acid and is sold commercially by E. I. du Pont de Nemours & Co., Inc, Wilmington, Delaware.

For the purpose of providing an expansible volume within case 2 there is provided a bellows 29 which comprises a collapsible member 3% composed of a resilient material such as natural or syntheticrubber. Member 30 is sealed to the adjacent inner surface of case 2 in liquid-tight fashion by means of a washer member 31 which is provided with a threaded boss 32 that extends through case 2 and is mounted thereon by means of a nut 33 and washer 34. The volume 35 enclosed by resilient member 30 communicates with the external atmosphere through a passage as in boss 32 and washer 31. In order to prevent bellows 29 from collapsing when fluid is inserted in case 2 in a manner to be described later, a resilient supporting member 37, preferably composed of natural or synthetic sponge rubber, is positioned within resilient member 30 of bellows 29. Communication of passage 36 with the volume enclosed by resilient member 30 is enhanced by removing a central portion 38 from resilient member 37.

The encased variable resistance device of the invention may be assembled in the following manner. Bellows 29 and terminal means 22 with conductors 23, 24 and 25 attached thereto are respectively mounted in case 2 in fashion which will be obvious to those skilled in the art. Variable resistance device 1 is pre-assembled exteriorly of case 2 and completely tested in conventional manner. Either before or after testing, device it may be mounted upon lid member 15 by means of screws 16 and bearing 10 as previously described. Conductors 23, 24 and 25 'are then respectively attached exteriorly of case 2 to terminals 12, 13 and 14 of resistance device 1. Following this step the assembly of resistance device 1 and lid member 15 is positioned within case 2 which is then roll-sealed into a groove 40 containing an 6 ring 41.

After variable resistance device 1 has been secured within case 2, the entire assembly is placed in a vacuum oven and baked for a suitable length of time to remove moisture from the interior of the case. Access to the interior of case 2 is provided through a passage 42 that is normally sealed by a screw 43, which is threadably engaged with lid member 15, and resilient washer 44, Of course, for the baking procedure screw 43 and washer 44 are removed from lid member 15. After the baking procedure is completed, the interior of case 2 is filled with a suitable liquid such as an oil conventionally used as a dielectric in the transformer industry. Such oils are usually hydrocarbon compounds with a mineral oil base, or silicone oils. It should be noted that the resilient member 37 within bellows 29 prevents the bellows from collapsing when case 2 is filled with oil. After the filling of case 2 is finished, screw 43 and washer 44 are properly positioned to close passage 4-2 and completely seal the assembly.

During operation of the encased variable resistance device of the invention it is desirable to have as much movement as possible of the liquid in case 2 in order that heat generated by passage of current through resistance device 1 may be conducted therefrom to the case 2, in:

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eluding lid member 15, for dissipation into the atmosphere. To provide passage for convection currents of fluid within case 2 and resistance device 1, holes 45 (one of which is shown) may be drilled or formed in housing 3 of device 1. The convection currents of liquid passing through holes 45 not only provide means for carrying heat away from resistance device 1 but also provide means for removing wear particles from the inner surface of helical winding 4 and contact member 7.

While the invention has been described by reference to a particular embodiment thereof, it will be understood by those skilled in the art that numerous changes may be made without departing from the invention. It is therefore intended in the appended claims to cover this and all such equivalent variation of application and structure as are within the true spirit and scope of the foregoing disclosure.

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

l. In combination: a variable resistance device having terminals and a rotatable shaft for adjusting the value of resistance appearing across at least two of said terminals, a liquid-tight case enclosing said variable resistance device and having a lid portion sealed therein, said rotatable shaft being journaled in said lid portion and extending exteriorly thereof to permit rotation of said shaft from outside said case, means for filling said case with an insulating liquid medium, expansible bellows means disposed within said ease to permit expansion and contraction of liquid medium within said case, terminal means sealed in said case adjacent said terminals of said variable resistance device, and means conductively interconnecting respectively said terminals and said terminal means to make variable resistance values available exteriorly of said case.

2. A combination as in claim 1 in which said means for filling said case comprises a removable sealing member disposed in a passage in said lid portion.

3. A combination as in claim 2 in which said means conductively interconnecting respectively said terminals and said terminal means comprises conductors having lengths sufiicient to permit connecting ends of said conductors to said terminals on said variable resistance device prior to assembly of said device within said case.

4. In combination: a variable resistance device including a housing, a resistance element within said housing, a slidable contact adjustable along said resistance element, terminals mounted in said housing and connected respectively to said resistance element and said slidable contact, and a rotatable shaft extending exteriorly of said housing for adjusting said slidable contact to provide variable resistance values across said terminals; a liquid-tight case enclosing said variable resistance device and having a lid portion sealed therein, said rotatable shaft being journaled in said lid portion and extending exteriorly thereof to permit rotation of said shaft from outside said case; means for filling said case with an insulating medium, said variable resistance device being disposed in said case to permit flow of liquid convection currents within said case and through said device; expansible belows means disposed within said case to permit expansion and contraction of liquid medium within said case; terminal means sealed in said case adjacent said terminals of said variable resistance device; and means conductively interconnecting respectively said terminals and said terminal means to make variable resistance values available exteriorly of said case.

5. in combination: a variable resistance device including a housing, a resistance element within said housing, a slidable contact adjustable along said resistance element, terminals mounted in said housing and connected respectively to said resistance element and said slidable contact, and a rotatable shaft extending exteriorly of said housing for adjusting said slidable contact to provide variable resistance values across said terminals; a liquidtight case enclosing said variable resistance device and having a lid portion sealed therein, said rotatable shaft being journaled in said lid portion and extending exteriorly thereof to permit rotation of said shaft from outside said case; means for filling said case with an insulating medium, said housing having passages therein to permit flow of liquid convection currents within said case and through said housing; expansible bellows means disposed within said case to permit expansion and contraction of liquid medium within said case; terminal means sealed in said case adjacent said terminals of said variable resistance device; and means conductively interconnecting respectively said terminals and said terminal means to make variable resistance values available exteriorly of said case.

6. In combination: a variable resistance device including a housing, a resistance element within said housing, a. slidable contact adjustable along said resistance element, terminals mounted in said housing and connected respectively to said resistance element and said slidable contact, and a rotatable shaft extending exteriorly of said housing for adjusting said slidable contact to provide variable resistance values across said terminals; a liquidtight case enclosing said variable resistance device and having a lid portion sealed therein, said rotatable shaft being journaled in said lid portion and extending exteriorly thereof to permit rotation of said shaft from outside said case; means for filling said case with an insulating medium, said variable resistance device being disposed in said case to permit flow of liquid convection currents within said case and through said device; expansible bellows means disposed within said case to permit expansion and contraction of fluid medium within said case, said bellows means including a collapsible member enclosing a resilient supporting member; terminal means sealed in said case adjacent said terminals of said variable resistance device; and means conductively interconnecting respectively said terminals and said terminal means to make variable resistance values available exteriorly of said case.

7. In combination: a variable resistance device including a housing, a resistance element within said housing, a slidable contact adjustable along said resistance element, terminals mounted in said housing and connected respectively to said resistance element and said slidable contact, and a rotatable shaft extending exteriorly of said housing for adjusting said slidable contact to provide variable resistance values across said terminals; a liquidtight case enclosing said variable resistance device and having a lid portion sealed therein, said rotatable shaft being journaled in said lid portion and extending exteriorly thereof to permit rotation of said shaft from outside said case; means for filling said case with an insulating medium, said variable resistance device being disposed in said case to permit flow of liquid convection currents within said case and through said device; expansible bellows means disposed within said case and communicating through a passage in said case with external atmosphere to permit expansion and contraction of fluid medium within said case, said bellows means including a collapsible member enclosing a resilient supporting member having a portion removed therefrom adjacent said passage in said case to enhance communication with external atmosphere; terminal means sealed in said case adjacent said terminals of said variable resistance device; and means conductively interconnecting respectively said terminals and said terminal means to make variable resistance values available exteriorly of said case.

References Cited in the file of this patent UNITED STATES PATENTS 1,238,124 Frickey Aug. 28, 1917 1,389,105 Simmon Aug. 30, 1921 2,122,370 Harrison et al. June 28, 1938 FOREIGN PATENTS 118,613 Great Britain June 26, 1919 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,914,744 November 24, 1959 Herbert M. Schendel It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 4, line 39, for the claim reference numeral "2" read l line 61, for "below" read bellows Signed and sealed this 3rd day of May 1960.,

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents v UNITEDv STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0 2,914,744 November 24, 1959 Herbert M. Schendel It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 39, for the claim reference numeral "2" read l line 61, for "belows" read em bellows Signed and sealed this 3rd day of May 1960.,

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

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