US2664481A - Thermal relay and constant gap spacer - Google Patents

Description

Dec. 29, 1953 H A, PEARL ET AL 2,664,481

THERMAL RELAY AND CONSTANT GAP SPACER Filed NOV. 14, 1951 2 Sheelzs-Shee l f 2 3EME q///////////////////%/f Avrai Dec. 29, 1953 Filed NOV. 14, 1951 H. A. PEARL ET AL THERMAL RELAY AND CONSTANT GAP SPACER 2 Sheets-Sheet 2 Patented Dec. 29, 1953 2,664,481 THERMAL RELAY AND CONSTANT GAP SPACE Iiarry'A. Pearl, Dayton, Ohio, and Merritt Allen Rudner, Camden;r N. J;

Application November 14, 1951, Serial N o. 256,354 19 Claims. (Cl. 20G-122) (Granted` under Title 35, U. S. Code (1952),

sec. 266)` The invention described herein may be manufactured and used by or for the Government for governmental purposes Without payment tous of any royalty thereon.

This invention relates to a thermal contact Ina-king or breaking device capable of maintaininga predetermined xed distance between electrical contacts'whilel-exposed to any variation of ambient temperatures within a wide temperature range, such asv -65 C.- to +4.00 C. by the use or positive and negative thermal coeiiicientof expansion materials. These materials are firmly attached to each otherdn such a manner that the expansion of one material Will eiectivelyl be c-ounterbalanced by the contraction of the other. The switching or relay action is controlled by application of heat to one or-more members of the balanced system in such a way as to cause a variation in the original predetermined xed distance by an amornt suhcient to make the elec. trical contacts touch or separate, thereby opening or closing an electrical circuit. The delay relay otion is governed by the rate of the applied heating or the distribution of the heating coil on the said one or more members.

The object of this invention is to provide a; thermal relay or switch which is capable of vsatisfactory operation over a relatively wide ambient temperature range without being aiected by `ambient temperature variations.

Another object of this invention is to provide a thermal relay or 4switch which contains no moving parts and which, therefore, is reliable and easy to construct and essentially insensitive to the effects of excessive vibration and shock.

Still another object of this invention is to provide a thermal relay or switch which is hermeti# cally sealed so as to avoid the adverse effects of dost and dirt, salt sea atmosphere, humidity, corrosion high altitude,v and which allows the use of a vacuum or inert gas'toincrease contact carrying potentials.

A further' object of tl'iisinventicnv is to obtain thermal relay or switch which may be readily modined so as to be adaptable for various uses, such as an overload make or break relay, underload make or brealcrelay, combination overload and underload make. or break relay, orfa, diere ential type relay, and. whose timenelay is. readib7 adjusted by simple periods ottime delay.

in the accompanying drawings.:

Figure i is a central longitudinalfsection of a device illustrating the basic principles of the subject invention;

mea-ns4 to givelong onshort i Figure 2 is a central longitudinal Section of a modication of the device shown in Figure 1;

Figure 3 is a central longitudinal section of a thermal relay switch, embodying the invention;

Figure 4 is a central longitudinal section of another embodiment of the invention, in which two compound members, insteadof one, are used;

Figure 5 is a central longitudinal section of still another embodiment of the invention utilizing lever action toY provide spacing of the metallic contacts;

Figure 6 is a central longitudinal section of a further embodiment ofl the invention in which the negative thermal coenicent of expansion elements are U-shaped;

Figure 7 is a central longitudinal section of a further embodiment ofthe invention in which the negative thermal coeflicient of expansion elements are S-shaped;

Figure 8a is a ternary diagram for systems comprising lithia, alumina and silica;

Figure 8b is a ternary diagram showing indetail theV crossfhatched portion of Figure 8a and showing areas ofnegative and zero thermal eX- pansion inLithia-Alumina-Silica systems.

Like numerals refer to like parts throughout the several views of the drawings.

Referring to Figure 1, which illustrates the basic principles of the subi'ect invention, A is a compound member, comprising two portions or elements, I and2. Element I is composed of a 'material having a positive temperature coefficient of,V expansion which may be metallic or nonmetallic. Element 2 which is firmly attached to element I, is composed of a material having a negati-ve temperature coeicient of expansion. This negative coeflicient of expansion material may be a ceramic of the lithia-alumina-silica system, suchas spodumene or petalite. Element l is firmly fixed to a holder member having an effectively Referring nov.7 to Figure of the drawing, a portion of acomplete conventional ternary diagram of the system comprising alumina (A1203), silica (SiOz), and lithia (LigO) is shown. The diagram shown in Figure 8b represents that portion of the diagram of Figure Sashown cross-hatched.

The two cross-hatched areas A and B in Figure 8b, are areas., ofy negative or zero thermal eX- pansion in lithia-alumina-silica systems. That is, any material havingthe percentages of lithia, alumina and silica indicatedV ina cross-hatched area is characterized byjeithera negative thermal coeincient of expansion or a zerothermalcoefiicient. The exact areas over which the coeiicent zero thermal coeicient of expansion.

is rero rather than negative are not shown, 'but it is. suiiicient here to say that a material having a Zero thermal coeiiicient may be found at some points Within the cross-hatched areas A and B, the saine being true as to materials having negative thermal coefficients. As an example, points '9, S and E are examples of the negative thermal coenicient materials petalite (Li2OAl2O3-8Si0fi), spodumene (LizO-AlzOs-SiOz), and eucryptite (Li2O-AlzO3-2Si02), respectively. The zero coeriicient of expansion material may also be a ceramic produced by properly combining and 'processing positive and negative coefcient of expansion ceramic materials.

Returning now to Figure 1, elements i and 2 may be oi the same or different lengths and geometric shapes. The lengths of elements i and 2 are chosen so that the length oielement i multiplied by the numerical value of its coefiicient of thermal exi-)ansi on is equal to the length of element 2 multiplied by the numerical value oi' its coefficient of thermal expansion. Thereiore, member A does not eiectivelv change its length even though exposed to wi e variations of ambient temperatures in the range of 65" C. to +iil0 C'. Although holder member 5 is preferably made or" a material having a zero thermal coefficient of espansion. it may also be made of materials so fabricated that its length does not change with variations in ambient temperatur rihe net effect will be to maintain a predetermined fixed gap 5 over the aforementioned temperature range. The geometric shapes of elements l and 2 are selected on the basis or" their thermal condiictivities. For best results, e

elements i and 2' should assume the temperature oi' their environment at the same rate. Since the lengths of elements I and 2 are determined by their thermal coeiiicients of expansion their masses will be varied so that they assume the temperature of their environment at the same rate.

All considerations so far have been limited to maintaining a iiYed predetermined gap 5 under wide variations in temperature. The switching or relav action is controlled by applying heat to element l or element 2 in such manner as to localize the heat to one element as much as possible. Ii the heat is applied to element I which has the positive thermal coetcient of expe nsion, the gap 5 will close but if the heat is applied to element which has a negative coefficient. the gap will open. For relay or switching purposes, holder member ii may be constructed as described in the preceding paragraph. or may be made of materials so fabricated that its change of length is insufncient to open or close the gap, as the case may be, in the event that no thermal energy is applied to the compound member. To sum up, holder member 5 need not be constructed of a single material having a zero thermal. coefr cient of expansion but may be of such construction and composition as to have an effectively zero thermal coencient of expansion. This controlled variation is used to complete or open electrical circuits. The rate at which heat is applied will govern the delayed time action of the relay.

' Member A is not limited to two materials, but may be composed of any convenient number of materials provided the composite materials are so fabricated that its length does not effectively change with ambient temperature over the range specied previously.

' The positions of elements t and 2 are intel'- changeable. in relays, the heat can be applied by electrically energized coils or other suitable devices.

The expansion, contraction materials may be used as conductive members for establishing electrical contacts. When the material is a metal alterations may or may not be necessary depending upon the current carrying capacity. When the material is a non-metal it can be coated with an adherent conductive coating to forni an electrically suitable contact.

Figure 2 discloses the modincaticn of the device described in Figure 1 in which two compound members A and B instead of one, are used. Member A is composed of an element i, made oi' a material having a negative coefficient eX- pansion, nrmly attached to an element 2, made of a material having a positive o eicient oi ex pension. Member E is similarly composed oi an elernenJ 3, having a negative coefficientY of expansion, rrnly attached to the element l which has a positive coelicient ci expansion.

Members A and B, as well as elements i, 2, 3 and i may be of equal or unequal lengths or geometric shapes.

The basic operation of the relay will not be altered if the positions of elements i and 2 are interchanged or the positions or elements E and are interchanged.

Members A and B are each iirmly attached at one end to a base member 6 made of a material having an effectively zero coefficient of expansion so as to maintain a predetermined gap 5 under the temperature range oi 65 C. to +460 C., as shown in Figure 2.

The lengths and geometric shapes of elements i, 2, 3 and il are selected by the use ci the same criteria as specified for Figure 1.

All considerations so iar have been limited to maintaining a fixed predetermined gap 5i under wide variations of temperature. The switching or relay action is controlled by applying heat to members A or B, or both, in such a way as to localize the heat to one or more element-s i, 2, 3 or e. The elements to be heated should be such as to cause an unbalanced expansion which will either close or open gap E. To illustrate: Where elements i and 3 are negative coemcient of expansion materials and elements 53 and fi are positive coeilicient oi expansion materials, heat is applied to element i or element E, or both, the gap i3 will open, while if heat is applied to element 2 or element si, or element 2 and element d .the gap will close. The controlled variation of gap 5 is used to close or open elec trical circuits. The rate at which heat is applied will govern the delay time action oi the relay.

Referring to Figure 3 o the drawing, a practical embodiment of the fundamental principle set forth in connection with Figures l and 2 is shown. Metallic contact i is Welded or otherwise fastened to positive thermal coei'licient of expansion element 4l, which is, in turn, securely fastened to negative coeirlcient o expansion element S, with the entire assembly fastened securely to an effectively zero coefficient or eX- pansion member ii to maintain a constant gap 5 when not energized. Metallic contact lv is Welded or otherwise fastened to member E and mounted collinearly with contact i. Contact leads il and ii are brought out of a hermetically sealed glass enclosure le through a ring seal ii. Glass Wool i2 is packed between the relay assembly 3, d, E, l, i and the glass enclosure it to aid in positioning the assembly and to render the assembly .relativelyA insensitive to shoot: or; vibration Coil heater leads leona iiiare 'eroe-fc out` through ring seal l t and are attache-l heater coil l W 'ch conducts heat when ef gifted to the nositive thermal occhi-cient of e parisien ele "1' el, which capanne-at 1c c minori rateinoting the two metal contacts 'f' together and thereby completing t e circ between lea-ds S and t. When cle-e* member i at a causing contacts 'i tofopen, 't ing the circuit between leads il an 'l may be a resistance metal suchY palladium, platinum, seroicond conductors capable. t comeitia into thermal energy.

. Referring to Figure 4., two metallicl contacts. l and 'l' arevvvelcled or. otherwise Jfastened to positive thermal coehicient of expansion elements i and il which, in turn, are attached to negative thermal coeicient of expansion elements l and 3, with the entire assembly fastened securely to aneiectively zero thermal coefficient of expansion. member e to maintain a constant gap 5 when not energized. Elements i and 3 are shown as having a threaded end portion which can he Screwed into tapped holes in member t. Qther means of mounting may, of course, be us s". Contact leads 8 and il are brought out or" tl hermetieaily sealed glass' enclosure it tiror a ring seal l l. Elements l to t .may be in var ious configurations, other than straight. tali ing compound il may be used to aid in securing the compound membersA A and B to the helft member 5. Glass wool i2 is packed around the relay assembly l through 1. as pointed out pre viously'in connection withvliigure 3. Coil heated leads I3 and is pass through ring seal il ani are attached to heaters l5 and l5 which conduct heat, when. energized, to the positive thermal coeilicient of. expansionmembers g and i which expand at a nrecleterinoei rete cifre lha two lnetalliccontacts 'l and 'l' together and thereby completing the circuit between leads 3 and WhenV deeenergized members 2 and 4 contract ata predetermined rate causing contacts 1 and 'l' to open, thereby breaking the circuit between leads B and By carefully seleotingthematerials of elements i, 2', @and t the relay may be tempera-tre coin- ;censated over the range of from. 65 C. to $400010. to the extent that.v the gap' 5 remains constant regardless of any external .ambientteinf perature Within the compensated range.

Referring to Figure 5, two metallic contacts l and l arewelded or fastened to zero ti .sl coelicient of expansion members lll and' l e which inV turn are attached' to positive' thermal 'coeicient of expansion elements 2 and il'. Elements 2 and d are attached to negative thermal coeioient ofexpansion elements l and 3 with the entire assembly fastened securely to an et fectively zero thermal coeiioient oi expansion member t to maintain zero gap when notr e gizeol. Contact leads 8 and 9 are brought out oi the hermetioally sealed enclosure le through a header type seal ll. Two lever arms, le ansfe, are mounted onY member byv means ofY two pins 2l, 2 l, as shown in Figure' 5; Stakingiooin pound' l'l., may beused to `aid in'securinefcom- Apound rneinbers A and B tozthefholder einher e. A return *spring 2h is usedto' aid innosito and maintaining a constant` gap'. Coil ter leads ltand Mare brought out through seal il andere attached .toheatersl and 6 which conm duct heat, whenzenergized; to the positive thermal cogeiiiciexit'of expansion elements 2 and fi. ments. 2A and 4: then expand at a predetei rate, moving the two lever arms it and ing thetwo metallic contacts 'i and l to mov apart, and. thereby breaking the circuit between leads 8 and 9. When damore-ized, elements 2 and i contract at a predetermined. cousine contacts l' and 1' to close thereby compl circuit between leads 8 and 9. By carefiely selecting the materials of items l, 2, 3, 55 ard l5, the` relaymay be temperature compensated over thev range of from -65 C. to +40D C. to the extent that the gap 5 remains constant regardless of any externalv ambient temperature ivitli inthe compensated range. Moving spring ES belowy the fulcrum. would result in. a normally open relay which would be clos-ed oy ineens of coils on elements l and 3.

previously, except that the portions elements l and 3 ofthe` assembly i through shaped for space-considerations. Portion 4 are screwed into or otherwise attached to, on end of portions l and 3, respectively. assembly I through Il is fastened secu eeotively` zero thermal coeicient of expansion member 6, as clearly shown in Figure 6.

The embodiment of Figure 7 is similar to that of Figure 6, diiering from the one shown in Fig-- ure 6 only in that the conguration of the elements l and 3 are S-shapei rather than U- shaped, and in that the leads are brought out from a common seal Il, in the manner shown in the embodiment of Figure 5. rEhe operation is identical toA that'of Figure 6, previousi cle scribed.

We claim:

l. A Contact making'and breaking' device corna- ,nrising a compound member terials some of which have a positi coehicient of expansion and ot yers of a nesative thermal coefficient o so fabrcatefithat the length o` member does not change with w ambient temperature, a holder in? rely.

a positive thermal coefcient ci exnan secondV element made of a material navi ative thermal coeihciento expansion, and'second elementsV being fixedly atta onevenother. at one-end thereof, a holster ber having an eiectively Zero thermal ot expansion, the other end of one of ments being connected to one end of said member. andtheother end of the other oi accessi elements being mounted adjacent the other end of said holder member and so positioned with respect thereto as to form a gap therebetween which is independent of ambient temperature, and means for supplying thermal energy to one element only of said compound member to cause the latter to change in length, whereby said gap spacing is varied in response to said means for supplying thermal energy while remaining insensitive to variations in ambient temperature.

3. A contact making and breaking device coinprising a compound member composed of a rst portion having a positive thermal coefncient of expansion, a second portion having a negative thermal coeilicient or" expansion, said nrst and second elements being iixedly attached to one another at one end thereof, so that the length of said compound member is independent of ambient temperature, a holder member having an eiectively aero thermal coefficient of expansion, said compound member having the other end or" said rst portion attached to one end of said lholder member and the other end of said second portion so positioned with respect to the other end of said holder member as to form a, gap which remains iixed regardless ci ambient temperature, and means for applying heat to said iirst portion only, thereby causing said compound member to expand and close said gap.

4. A contact making and breaking device comu prising a compound member composed of a rst portion having a positive thermal coeiiicient of expansion, a second portion having a negative thermal coeiicient of expansion, said 'first and second elements being xedly attached to one another at one end thereof, so that the length of said compound member is independent of ambient temperature. a holder member having an effectively zero thermal coefficient of expansion, said compound member having the other end ot said rst portion attached to one end of said holder member and the other end of said second portion so positioned with respect to the other end of said holder mcmber as to form a. gap which remains fixed regardless of ambient temperature, and means for applying heat to said second portion only, thereby causing said compound member to contract and open said gap.

5. A contact making and breaking device comprising a compound member composed of a irst portion having a positive thermal coeiicient of expansion, a second portion having a neffative thermal coeiiicient oi expansion, said rst and second elements being :xedly attached to one another at one end thereof, so that the length of said compound member is independent of ambient temperature, a holder member having an eiiectively zero thermal coenicient of expansion, said compound member having the other end of said second portion attached to one end of said holder member and the other end oi said rst portion so positioned with resnect to the other end of said holder member to form a gap which remains fixed regardless of ambient temperature7 and means for applyi g heat to said iirst portion only thereby causing said compound member to expand and close said gap. g

5. A, Contact making and breaking device comprising a compound member composed ci a rst portion having a positive thermal coehcient of expansion, a second portion having a negative thermal coeilicient of expansion, said first and second elements being fixedly attached to one another at one end thereof, so that the length of said compound member is independent of ambient temperature, a holder member having an eiectively zero thermal coeicient of expansion, said compound member having the other end of said second portion attached to one end of said holder member and the other end oi said nrst portion so positioned with respect to the other end of said holder member as to form a gap vwhich remains fixed regardless of ambient temperature, and means for applying heat to said second portion only thereby causing said compound member to contract and open said gap.

'7. A contact making and breaking device having a hermetically sealed enclosure comprising a compound memberwhose length is independent of ambient temperatures and consisting of a rst element made of Va material having a positive thermal coencient of expansion and a second element made of a material having a negative thermal coefcient of expansion, said. first and second elements being xedly attached to one another at one end thereof, a holder member hav-I ing an eectively zero thermal coeri -t ci er;- pansion, the other end of one of said elements be'ng connected to one end of said holder member and the other end of the other of said elements being mounted adjacent the other end of said holder member and so positioned With respect thereto as to form a gap therebetween which is independent of ambient temperature, a iirst metallic contact attached to said other end of the other of said elements and a second metallic contact attached to said. other end of said holder member, a pair of contact leads, one connected to each o said metallic contacts and passing eX- ternally of said contact making and breaking device through a seal in said enclosure, and means for supplying thermal energy to one element only of said compound member, said means comprising a coil surrounding said one element and coil heater leads attached to the coil and passing externally of said Contact making and breaking device through a seal in said enclosure. whereby said gap spacing is varied in response to said means for supplying thermal energy while remaining insensitive to variations in ambient temperature.

S. A Contact making and breaking device cornprisng rst and second compound members Whose lengths are independent of ambient temperature and each consisting of a iirst element having a positive thermal coecient oi? expansion and a. second element having a negative thermal coerlicient of expansion, said rst and second elements of each compound member being iixedly and collinearly attached to one another at one end thereof, a holder member having an eectively zero thermal cceiiicient of expansion, said iirst compound member being attached to one end of said holder member and said second compound member being attached to the other end of said holder member in such manner as to form a gap between the unattached ends ci said compound members which is independent ci ambient tei perature, and means for supplying thermal energy to one element only of each o said compound members, causing the latter to change in length, whereby the gap spacing is varied in response to said means for supplying thermal energy while remaining insensitive to variations in ambient temperature.

9. A contact making and breaking device having a hermetically sealed enclosure comprising first and second compound members Whose lengths are independent of ambient temperature and each consisting of a first element having a positivev thermal coeicient of expansion and a second element having a negative thermal coeiiicient of expansion, said first Yand second elements of each compound member being nxedly and collinearly attached to one another at one end thereof, a holder member' having an effectively zero t errnal coefficient of expansion, said rst compound inember being attached to one end oi said holder niember and said second compound rneinber being attached to the other end of sani holder rnernber in such manner to forni a gap between the unattached ends oi said compound nienibens which is independent oi ambient temperature, a metallic contact attached to each of said unattached ends oi said compound members, a pair of Contact leads, one connected to each of metallic contacts vand passing externally of Contact making and breaking device through a seal in said enclosure, and nieans for supplying thermal energy to one element only of each oi said compound members, said means comprising a pair of coils, one surrounding each of said one elements of said compound members, said coils being connected in parallel, and coil heater leads attached to said coils and passing externally of said Contact making and breaking device through a seal in said enclosure and causing the latter to change in length, whereby the gap spacing is varied in response do said means for supplying thermal energy While remaining insensitive to variations in aznbient temperature.

1G. A contact making and breaking device as recite-i in claim 9 in which said second elements are U-shapedll. A contact making and breaking device as recited in claim 9 in which said second elements are e-shapecl.

l2. A contact making and breaking device having a hermetically sealed enclosure comprising' first and second compound members whose lengths are independent of ambient temperature and each consisting oi a nrst element having a positive thermal coeilicient of expansion and a second element g a negative thermal coeiiioi expansion, said First and second elements of each compound inernber being nxedly and coln linearly attached to one another at one end the eci, a holder member having an effectively zero thermal coelhcient of expansion, said rst compound ineinber being attached to one end of said holder member and said second compound member oeing attached to the other end or said holds member, first and second lever members pivotally mounted on said holder il einber and having one end oi Said rst lever nie, ber engaging the unattached end ci said iii-st compound member and a corresponding end of second lever member engaging the unattached enel of said second compound rnernber, the other ends of said lever members adapted to lie adjacent one another so as to form a gap therebetween, a spring having one end connected to sai-:l rst lever mernber and the other end connected to said second lever member to aid in positioning and maintaining constant said gap, a of tallic contacts, one fastened to each of other ends of said lever member a pair oi contact lt Lds, one connected to each of said metallic contacts and passing externally of said Contact making and lorealrirrT device through a seal n said sealed enclosure, said gap being insensitive to changes in ambient ternperature, means for supplying thermal energy to one element only of each of said compound members g the latter to change in length and actuate said lever members against the bias iii) 10 of said'sprng, whereby the gap spacing is varied in response to said means ior supplying thermal energy While remaining insensitive to variations in ambient temperature.

13. A contact making and breaking device recited in claim 12 in which said supplying thermal energy comprises tivo connected coils, each coil surrounding sai,r element of the respective compound i and coil heater leads connected to and passing externally of said contact making and breaking device through said seal.

le. A thermal relay comprising a composite member in the form oi a loop having a gea therein, said loop consisting of sections inade oi materials yhaving positive, negative and zero coeiiicients ci thermal expansion, with each junction between sections having coefficients ci op" posite sign including the axis passing through the gap faces ci said member and with the lengths of the sections having positive i tive coerhcients proportionei relative to sain,I coeiiicients so that an overall cha i the teniperature oi said member produces movement of said faces, means relative movement of said faces coro ing inea-ns for applying heat er-:clusively to one of the said materials having csitive and negative coefficients, a pair of electrical contacts, and means mechanically coupling said contacts said faces for actuation thereby.

l5. A thermal comprising a ber made of a material having a thermal expansion of zero, a pair oi disposed contacts, means for oppcsitelv supporting contacts on said base mernber, said supper ineens comprising serially arranged. sections ea made of one of tvvo materials loving therir coefficients of expansion of opposite sign, vvi the axis passing through said contacts al passing through at least a part of each .i and through each junction between sections ci opposite sign, the lengths of said sections een proportioned relative to said coeiiicients an overall change in the temperatur oi relay produces no relative movement of contacts, and means for producing relative movement of said contacts comprising for applying heat exclusively to one of two materials.

l5. A thermal relay comprising a :i be' made of a material having a coefflcier thermal expansion of zero, a pair ci op disposed contacts one or which is said base member, means for sirop other oi said contacts on said base said means comprising an attaching a consisting or" a pair of serially eine made of materials having coenicients expansion ci opposite signs, a: through said contacts also passing through least a portion of each of said` sections and the lengths or said sections being proportioned rel" tive to said coefficients so that an overall che in the temperature oi said relay produces relative motion or said contacts, for producing relative motion of said contacts core; prisng means for applying heat to one said sections.

17, A thermal relay comprising a hase ber made of a material having a coefficient thermal expansion of zero, a pair oi oL disposed contacts, similar means t said contacts to said base ing means comprising a pair oi iii ' loaned sections made of materials having coecients of thermal expansion of opposite signs, the axis passing through said contacts also passing through at least a portion of each of said sections, the lengths of the sections of both attaching means being proportioned relative to said coecients so that an overall change in the temperature of said relay produces no relative motion of said contacts, and means for producing relative motion of said contacts comprising means for applying heat to those sections having coefficients of like sign.

18. Apparatus as claimed in claim 17 in which the section adjacent said base member in each attaching means is U-shaped.

19. Apparatus as claimed in claim 17 in which 12 the section adjacent said base member in each attaching means is S-shaped.

HARRY A. PEARL. MERRITT ALLEN RUDNER.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 10 2,172,666 Michel Sept. 12, 1939 2,262,205 Schachtner Nov. 1l, 1941 2,574,869 Green Nov. 13, 1951 FOREIGN PATENTS 15 Number Country Date 583,436 Great Britain Dec. 18, 1946

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