US3890067A

US3890067A - Rubbing seal system for a rotary combustion engine

Info

Publication number: US3890067A
Application number: US391375A
Authority: US
Inventors: Vemulapalli D N Rao; Yeshwant P Telang
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1973-08-24
Filing date: 1973-08-24
Publication date: 1975-06-17
Anticipated expiration: 1992-06-17

Abstract

A sealing assembly is disclosed for use in providing a dynamic seal between a rotor and the surrounding housing of a rotary internal combustion engine. An apex seal of considerable height is arranged to fit within a first receptacle in at least one location along the periphery of the rotor; the apex seal has a transverse opening separating the body of the seal into first and second support portions. A second receptacle is defined about the ends of the first receptacle for receiving a corner seal adapted to engage the side housing. The seals are comprised of a material having volatile impurities. Resilient expanded silicone sponge material is utilized to either or both urge the apex seals and corner seals to their respective engaging surfaces; mechanical springs may be embedded in the sponge material for assisting the uniform nature of the resilient force of said sponge material. In one or both of said apex and corner seals, grooves are defined for receiving an embedment comprised of a friction smearable fluxing agent in the form of polyarylsulfone.

Description

United States Patent Rao et a1.

1 1 RUBBING SEAL SYSTEM FOR A ROTARY COMBUSTION ENGINE {75] Inventors: Vemulapalli D. N. Rao, Woodhaven;

Yeshwant P. Telang, Grosse lle. both of Mich.

[73] Assignee: Ford Motor Company, Dcarborn,

Mich,

[221 Filed: Aug. 24. 1973 [21] Appl. N01: 391,375

[51] Int. C1....F01c19/02;F04c 27/00; FOlc 21/00 [58] Field of Search 418/113, 120-123.

156] References Cited UNITED STATES PATENTS 409,800 8/1889 Owen 418/113 2,268,868 l/1942 Given H 277/227 3,102,518 9/1963 Anderson 418/120 3,130,964 4/1964 Johnson 1 i 1 1 i 267/152 3,161,350 12/1964 Lorcher 1 1 1 418/122 3,194,488 7/1965 Fuhrmann.... 418/113 3,196,849 7/1965 Paschke 418/121 3,268,157 8/1966 Frcnzcl 418/122 3,289,649 12/1966 Lamm 418/178 3,756,754 9/1973 Sakamaki 418/113 3,761,207 9/1973 Seidl 418/121 Zia. [/8

1 June 17,1975

FOREIGN PATENTS OR APPLICATIONS 1.116.951 11/1961 Germany 418/122 Primary Examiner-John J. Vrablik Attorney, Agem, or I-irm-Joseph W. Malleck; Keith L, Zerschling [57] ABSTRACT A sealing assembly is disclosed for use in providing a dynamic seal between a rotor and the surrounding housing of a rotary internal combustion engine. An apex seal of considerable height is arranged to fit within a first receptacle in at least one location along the periphery of the rotor; the apex seal has a transverse opening separating the body of the seal into first and second support portions. A second receptacle is defined about the ends of the first receptacle for receiving a corner seal adapted to engage the side housing. The seals are comprised of a material having volatile impurities. Resilient expanded silicone sponge material is utilized to either or both urge the apex seals and corner seals to their respective engaging surfaces; mechanical springs may be embedded in the sponge material for assisting the uniform nature of the resilient force of said sponge material. In one or both of said apex and corner seals, grooves are defined for receiving an embedment comprised of a friction smearable fluxing agent in the form of polyarylsulfonei 19 Claims, 12 Drawing Figures PATENTEDJUN 17 I975 SHEET F'lG.9

F'IGLIZ RUBBING SEAL SYSTEM FOR A ROTARY COMBUSTION ENGINE BACKGROUND OF THE INVENTION A highly efficient gas-tight seal between the apex seals and rotor housing ofa typical rotary internal combustion engine is difficult to obtain. The problem is principally due to the typical shape of the rotor housing (epitrochoid) and the offset journalling of the rotor providing for planetary movement. As a result, variable dynamic loads are imposed upon the apex seal during a complete revolution, the peak loading being extremely high relative to loading in other regions of the path. Gas pressure from the combustion chamber has been utilized to urge the apex seal radially outwardly to accommodate the varying contour of the rotor housing and the varying inertial loads applied to the apex seal. This has only been partly successful since shifting of the seal within its receptacle is required. A more uniform form of loading is desirable.

The composition, of which the apex and corner seals are constructed, present another unique problem in that most compositions incur a cyclical variation of their coefficient of friction during normal operation. For example, those seals which have been formed from commercial graphite exhibit a dry rubbing friction vari ation which is explained in part by a moisture-free dry environment at temperatures above 400450F. Initially the low friction of the graphite is attributed to occluded impurities which may be natural or synthesized in the graphitic material. These impurities allow the graphitic material to glide easily under an applied tangcntial stress such as experienced during the rubbing application of the seals thereby permitting the crystallites to obtain a favorable orientation resulting in low friction.

As the higher temperatures of operation are reached, the impurities evaporate eventually resulting in a return to a high coefficient of friction. When the surface layer is worn off, the coefficient of friction again drops due to the exposure of another incremental layer of graphitic material containing again the impurities. The duration of the cycle of variance in the coefficient of friction depends upon the operating temperature achieved.

SUMMARY OF THE INVENTION A primary object of this invention is to provide a simple and reliable sealing assembly for use in providing a gas-tight dynamic seal in a typical rotary internal combustion engine; the assembly should be easy to manufacture and assemble, yet capable of efficient operation under more severe operating conditions at higher speeds than seal assemblies now in use.

Another object of the invention is to provide a material design for the sealing structure which has improved dry lubrication qualities and overcomes the incidents of cyclic variation of friction during dynamic sealing.

Another object of the invention is to provide a sealing assembly which achieves a more uniform loading condition for the seal elements constituting the assembly, the uniform loading particularly optimizing the loading at high speeds thereby resulting in improved sealing efficiency and extended life for the seals.

Particular features pursuant to the above objects comprise the use of polyarylsulfones (such as Astrel or Bakelite) as a seal embedment or application to a contacting surface. A metal oxide-fluoride material may also be used if unusually high temperatures are encountered. The seal body material normally contains volatile impurities which would evaporate under temperature of frictional operation experienced by the seal structure; the embedment or application is arranged to flux the surface and prevent premature volatization of such impurities except under a controlled condition, thereby resulting in a decrease in the cyclic variation of friction. Another feature is the use ofa resilient silicone sponge material interposed between the rotor and seals (including the apex and corner seals) in a manner to urge at least one of the seals into dynamic engagement with the housing. The resilient material thereby maintains a more uniform loading under the inertial force variations of the rotor.

SUMMARY OF THE DRAWINGS FIG. I is an enlarged sectional view of a portion of a typical rotary internal combustion engine, illustrating the sealing assembly ofthis invention in a preferred embodiment;

FIG. 2 is a view similar to FIG. 1 illustrating a different arrangement of elastic material;

FIGS. 3-5 represent alternative perspective views in a schematic manner of embedments utilized for the apex or corner seals;

FIG. 6 is an enlarged schematic illustration of the microstructure of material utilized for the seals of this invention but without the use of embedments; a layer of graphite crystallites are oriented for easy glide and low friction under shear stress against a mating surface, such as the housing:

FIG. 7 is a schematic view similar to that in FIG. 6 showing the rearrangement of the microstructure as a result of the use of fluxing embedments;

FIG. 8 is a fragmentary end view of the sealing assembly showing the corner seal in operation about the apex seal;

FIG. 9 shows an alternative embodiment illustrating a mechanical assist spring which is effective to apply biasing force not only in a direction radially outwardly but in a side thrust direction.

FIG. 10 is an enlarged schematic illustration of another alternative sealing assembly wherein the resilient material is utilized to impose both a force in a radially outward direction as well as in a sideways direction to simultaneously urge the apex and corner seals into proper engagement;

FIG. 11 is a view similar to that in FIG, 10 showing the use of an assist spring; and

FIG. 12 is a view similar to FIGS. 10 and I1 illustrating the forces that are exerted in the embodiments.

DETAILED DESCRIPTION Turning now to the figures, particularly FIG. 1, there is shown a preferred embodiment which is effective to achieve the dual objects of providing a superior dry lubricating characteristic for the sealing assembly and at the same time optimize seal loading at high engine speeds. The sealing assembly (comprised of elements A through F) is adapted to provide a dynamic seal between a rotor 19 and a housing 10, the housing having a typical epitrochoidally shaped end wall lla presented by a rotor housing portion 11.

Side wall portions

12 and 13 present flat inwardly facing side walls and I30 respectively. The sealing assembly elements include receptacle A (formed as a slot extending across one of th apices of the triangular-shaped rotor l9). receptacles ll defined at the ends of each of the receptacles A. apex seal means C is disposed in each of the receptacles A and corner seal means D is disposed in each of the receptacles B. Resilient sponge material E is provided in the spaces between the rotor and apex or corner seal means to provide a uniform loading characteristic. Embedments F are provided in at least the apex seal means for promoting a smearing or fluxing condition to enhance the dry lubrication of the seal material.

ln more particularity. receptacle A is defined as a slot 25 (FIG. 8) having a width 40 (FIG. 8) and has a longitudinal extent substantially commensurate with the width l8 of rotor 1'). Slot 25 has a bottom wall 250 against which the resilient material E bears for exerting a radial loading force against the apex seal means. The receptacle 8 is formed as a round recess 26 at each side of the rotor 19; each recess is arranged to surround the ends of each receptacle A. The inner most portion of each receptacle B provides an alignment surface 26a for maintaining the corner seal parallel with respect to the

side wall surfaces

13a and 12a.

Each of the apex seal means comprise a slug or body of material 14 having a generally elongated extent ef fective to fit within the slot 25 and having opposite ends 140 and 14b in close proximity to the

walls

12a and 13a. preferably in light engagement; a crown portion [4c of the body is adapted to engage the inner epitro choid wall lla of the rotor housing. The body 14 has a substantial height 80 (FIG. 8); a transverse opening 81 separates the body into supporting

portions

82 and 22. Body 14 may have recesses 15 and 36 to define steps distinct from the bottom 17 thereof.

The material of body 14 is comprised of a form of graphite and should have a high hardness at high elevated temperatures; the minimum hardness of the wear body should be at least R.- 30 at about 400F. Most commercial graphite, as well as standard graphite contain a high content of volatile impurities. Standard graphite and commercial graphites exhibit the incidents of cyclical friction in dry rubbing seal applications due to the moisture-free dry environment at tem peratures above 400450F. The low friction of the graphite is attributed to the impurities between the graphite crystallite layers. Significantly. water. as one of these impurities, facilitates easy glide under an ap plied tangential stress such as the rubbing of the seal against the rotor housing. Crystallites are permitted to attain a favorable orientation resulting in low friction. At high temperatures, however. the evaporation of the impurities still occurs. eventually resulting in the increase of friction. When the surface layer is worn off, friction drops again due to the availability of the impurities in the freshly exposed layers. The duration of the cycle depends upon the attained temperature and the cycle decreases rapidly with an increasing attained temperature. For special graphites. synthesized with hygroscopic salts, the water of hydration in the salts is released at higher temperatures facilitating easy glide. But even in these graphites the evaporation and loss of impurities is quite rapid.

The corner seals 20 and 21 are shaped to fit about the ends of the apex seal and to the depth of the receptacle B. Each of the corner seals have an extension (20 and 210 respectively) which project into the opening 81 of the apex seal and provide a means for urging the corner seal into engagement with the ends of the apex seal. To

4 rigidify each of the corner seals. a nose (3i and 33 re spectfully) is provided on the corner seals and adapted to lit within openings (32 and 34 respectively) in the rotor housing at the back of receptacles B. S Dry-lubrication The embedment F comprises one or more strips or deposits ofmatcrial (see FIGS. 35) within recesses 42 in the crown portion of the body M. The embedments are separated and are comprised of a material which is H) designed to distribute across the major portion of the crown l4a under frictional forces to pack the pores of the body 14 and prevent non-uniform loss of volatiles (dry lubricants). The embedments must be arranged to replinish the outermost exposed layers of the embedl5 ments as they are frictionally worn away or smeared during successive wear cycles of the body. The embedments are comprised of a polyarysulfone fluxing material which decomposes at or above 550F. Polyarysulfones, such as Astrel or Bakelite P-l300, may further contain additives of up to 40% graphite and 20% boron nitride. A metal or metal oxide'fluoride (such as copper-fluoride with 9% sodium fluoride and l 1% lithium fluoride) may be used if the expected engine operating temperature is particularly high and should be applied to the housing for achieving comparable results. The friction smearable material may alternatively be applied as a slurry to the crown surface of the apex seal or even to the rotor housing; the latter will afford some degree of reducing the cyclic coefficient of friction but will not be as great as by the use of an embedment.

The embedments may be defined in various configurations to meet the requirement of adequate location for uniform smearing or fluxing. The illustration in FIG. 4 represents approximately the embedments used in FIG. 1; rectangular strips 41 are embedded in recesses 42. However, embedments 43 may be arranged on a bias as that shown in FIG. 3, or they may be provided with a V-shaped cross section 44 as that shown in FIG. 5 (the widest part of the V being at the outermost exposed surface of the embedment).

As shown in FIG. 6. an apex seal composition is shown having no fluxing embedments. The loads expe rienced by the seal will result from centrifugal forces and the friction between the seal and the rotor housing. The interface temperature increases with increasing engine speed due to the increased centrifugal forces and the increased surface speed. Conventional graphite and metal impregnated graphite seals loose lubricity; chatter begins as soon as the surface temperature exceeds 500F. thereby resulting in very high seal wear and leakage. Even though the crystallites are favorably orientated initially, because of the gliding action provided by the first impurities, the impurities begin to volatize and certain of the favorably oriented crystallites will be removed by wear. A randomly oriented crystallite will be substituted thereby increasing friction.

To remedy this, and as shown in FIG. 7., embedment 41 provides a ready supply of fluxing polymer at the rubbing interface between the apex seal and the rotor housing 11. The polymer will, of course, smear 0r flux across the surface of both the rotor housing and the apex seal; quantities of the polymer will be squeezed into the porosity of the main body of the apex seal. thereby sealing off and preserving the impurities in the polymer which act as dry lubricants. Furthermore. the

favorably oriented crystallites adjacent the interface LII will be maintained and not torn loose during rubbing action; this insures that the randomly oriented crystallites will be subsurface.

Uniform Loading Although the concept of utilizing a resilient means E in a single chamber is comprehended within this invention, as shown in FIG. 9, opening 81 defining chamber 30 may also contain resilient means. Chamber 29 (between the bottom 25a of the slot 25 and the apex seal) receives sponge material to serve as a primary resilient means for the seals. The resilient means is comprised of an elastometer or expanded silicone sponge material such as RTV silicone rubber produced by GE or Dow Corning which cure at room temperature. Such silicone sponges consist of polydimethyl siloxane and polymethyl phenyl siloxanes. Each of these silicone sponges are capable of retaining elasticity at temperatures up to 500F for over 2,000 hours of use. The sponge material exerts a uniform force both radially outwardly and sideways. Thus a force is exerted against extensions 21a and 20a to retain the corner seals tight against the ends of the apex seal; a sideways force is also exerted against the corner seals to in turn engage the side housings. To supplement the force of the sponge material,

mechanical springs

23 and 24, of convoluted configuration, are utilized. Spring 24 acts between the base of receptacle A and the auxiliary apex seal; spring 23 acts between the opening in the apex seal and extensions 20a and 210. Spring 24 acts as the primary pressurizing spring for the seals and spring 23 acts as a pressure relief.

The embodiment of FIG. 2 illustrates the same seal assembly elements as in FIG. 1. except that the uniform loading is provided by a thin coating 60 of elastomer sealant between the engaging surfaces of the corner and apex seal means. There is no sponge material in

chambers

29 and 30. Spring 24 performs as the primary resilient means to obtain seal engagement with the housing.

In FIGS. and 11, embodiments are illustrated which further expand the use of the silicone sponge 71 to act as biasing force for side seals 70 which are annularly arranged on the sides of the rotor 72. The sponge material surrounds an extension 73 of the corner seal and projects between the end of the apex seal 75 and the slot 76 in the corner seal 78. Thus, a greater bulk adhesive sealant is used between the corner and apex seals. Thus chatter of the apex seal will not affect the peformance of the corner seal. FIG. It) differs from 11 in that the primary apex seal spring 77 is not present. In FIG. 12, the reaction forces (designated by arrows), generating a seal load on the apex and corner seals, are illustrated.

FIG. 9 represents another alternative embodiment whereby the embedments 61 extend transversely across the surface of the graphite apex seal 62 to promote a smear distribution 66. The sponge material 63 contains a single spring 64 which is arranged to exert a force not only radially outwardly directly against the bottom of the apex seal 62, but also has end portions 640 which exert a side thrust against comer seals 65 to augument the resiliency of the sponge material.

The advantages of the above sealing assembly as shown in various embodiments, comprise among others, (a) simplified assembly due to one step curing of the sponge or sealant, (b) permits use of conventional materials for the housing, such as cast iron or alumi num in the as-cast condition. (c) higher engine compression ratios can be achieved due to better sealing a higher speeds and (d) high temperature metal-fluorides (850-900F) as the sponge material opens up the possibility of an air cooled rotary engine.

5 We claim as our invention:

1. A sealing element carried by a rotor in a rotor housing of a rotary internal combustion engine comprising:

a. a wear-resistant body comprised of a composition which will maintain high hardness at elevated temperatures and contains volatile impurities,

b. a plurality of separated embedments disposed in said body and adjacent to at least one surface of said body which is adapted for frictional engage.- ment with said rotor housing, the embedments being comprised of a material effective to be distributed across major portions of said one surface under the influence of frictional forces and effective, when so distributed to pack the pores of said body to prevent non-uniform loss of said impurities, said embedments being arranged so that the distributed material can be replenished during successive wear cycles of the body.

2. The assembly as in claim 1, in which the rotor and side housings are comprised of as-cast materials selected from the group consisting of aluminum and cast iron, said apex and corner seal means being effective to interengage the as-cast surface of said housings.

3. A sealing element as in claim 1, in which said body is comprised of graphite.

4. A sealing element as in claim 1, in which the body consists ofa material having a hardness value of at least R. 30 at 400F, said embedment consisting of a material having a decomposition temperature of at least 550F.

5. A sealing element as in claim 4, in which the embedment consists of a polyarlsulfone having a decomposition temperature at least 600F.

6. A sealing element as in claim 5, in which the polyarlsulfone has filler material selected from the group consisting of fine graphite powder and boron nitride.

7. The sealing element as in claim 4, which said embedment material consists of an admixture of metal oxide and fluoride having a decomposition temperature in the range of 850900F.

8. The sealing element as in claim 7, in which said ad mixture consists of 80% copper oxide, 9% sodium fluoride, and 11% lithium fluoride.

9. A sealing assembly for use between a rotor and the surrounding rotor housing and side housings of a rotary internal combustion engine, the assembly comprising:

a. a slotted receptacle in at least one location of said rotor,

b. apex seal means disposed in said slotted receptacle effective to primarily interengage at an area with said rotor housing, and

c. high temperature mechanical resilient means biasing said apex seal means to effect said engagement under uniform loading across said interengaging area, said resilient means consisting of expanded silicones which maintain elasticity up to at least 500F.

10. A sealing assembly for use in providing a gas-tight dynamic seal between a rotor and a rotor housing of a rotary internal combustion engine, the housing having side and end wall portions. the assembly comprising:

a. a receptacle in at least one location of said rotor periphery,

b. an apex seal and a corner seal disposed in said receptacle. said corner seal being effective to move relative to the rotor along a line passing away from said receptacle toward the housing end wall. said apex seal having a central transverse opening extending internally through said seal. and

c. resilient means disposed in the space defined be tween said receptacle bottom and apex seal and in the opening. said resilient means cooperating to uniformly urge both said apex seal radially outwardly and urge the corner seal along said line for effecting interengagement of the corner seal with the end wall.

H. The assembly as in claim it), in which the resilient means is comprised of silicone expanded sponge material having a temperature stability at least to 500F.

12. The assembly as in claim 10. in which the apex seal and corner seal are both comprised of a material selected from the group consisting of graphite having volatile impurities. and high temperature metallic al loys having volatile dry lubricant impurities distributed throughout 13. The assembly as in claim 10. in which at least one of said spaces containing said resilient means has a mechanical spring embedded therein to assist the resilient force acting upon said seals.

M. The assembly as in claim 10, in which said rotor has another receptacle surrounding the end of said first 8 receptacle. said corner seal being disposed in said another receptacle adapted to sea] with the housing side portion as well as with the ends of said apex seal means, said corner seal having an extension acting as a bearing between said resilient means and apex seal.

15. The assembly as in claim 10, in which said apex seal has at least one groove in that portion of the surface adapted to engage the end wall portion. and a polymer-based embedment in said one groove effective to cyclically flux into the pores of said outer apex seal in response to wear for cyclically decreasing the friction coefficient thereof.

16. The assembly as in claim 15, in which the polymer base flux contains at least one filler material selected from the group consisting of fine graphite powder. and boron nitride powder.

17. The assembly as in claim 15, in which said one groove extends substantially on a bias to the elongated extent of said seal.

18. The assembly as in claim 15, in which the apex seal is defined as an elongated strip extending substantially transversely across the width of said rotor, and said one groove extending substantially parallel to the elongated extent of said apex seal.

19. The assembly as in claim 18, in which said groove has substantially a V-shaped cross section, the widest part of said V being disposed at the exposed surface of said groove.

Claims

1. A sealing element carried by a rotor in a rotor housing of a rotary internal combustion engine comprising: a. a wear-resistant body comprised of a composition which will maintain high hardness at elevated temperatures and contains volatile impurities, b. a plurality of separated embedments dispoSed in said body and adjacent to at least one surface of said body which is adapted for frictional engagement with said rotor housing, the embedments being comprised of a material effective to be distributed across major portions of said one surface under the influence of frictional forces and effective, when so distributed to pack the pores of said body to prevent nonuniform loss of said impurities, said embedments being arranged so that the distributed material can be replenished during successive wear cycles of the body.

4. A sealing element as in claim 1, in which the body consists of a material having a hardness value of at least Rc 30 at 400*F, said embedment consisting of a material having a decomposition temperature of at least 550*F.

5. A sealing element as in claim 4, in which the embedment consists of a polyarlsulfone having a decomposition temperature at least 600*F.

7. The sealing element as in claim 4, which said embedment material consists of an admixture of metal oxide and fluoride having a decomposition temperature in the range of 850*-900*F.

8. The sealing element as in claim 7, in which said admixture consists of 80% copper oxide, 9% sodium fluoride, and 11% lithium fluoride.

9. A sealing assembly for use between a rotor and the surrounding rotor housing and side housings of a rotary internal combustion engine, the assembly comprising: a. a slotted receptacle in at least one location of said rotor, b. apex seal means disposed in said slotted receptacle effective to primarily interengage at an area with said rotor housing, and c. high temperature mechanical resilient means biasing said apex seal means to effect said engagement under uniform loading across said interengaging area, said resilient means consisting of expanded silicones which maintain elasticity up to at least 500*F.

10. A sealing assembly for use in providing a gas-tight dynamic seal between a rotor and a rotor housing of a rotary internal combustion engine, the housing having side and end wall portions, the assembly comprising: a. a receptacle in at least one location of said rotor periphery, b. an apex seal and a corner seal disposed in said receptacle, said corner seal being effective to move relative to the rotor along a line passing away from said receptacle toward the housing end wall, said apex seal having a central transverse opening extending internally through said seal, and c. resilient means disposed in the space defined between said receptacle bottom and apex seal and in the opening, said resilient means cooperating to uniformly urge both said apex seal radially outwardly and urge the corner seal along said line for effecting interengagement of the corner seal with the end wall.

11. The assembly as in claim 10, in which the resilient means is comprised of silicone expanded sponge material having a temperature stability at least to 500*F.

12. The assembly as in claim 10, in which the apex seal and corner seal are both comprised of a material selected from the group consisting of graphite having volatile impurities, and high temperature metallic alloys having volatile dry lubricant impurities distributed throughout.

13. The assembly as in claim 10, in which at least one of said spaces containing said resilient means has a mechanical spring embedded therein to assist the resilient force acting upon saId seals.

14. The assembly as in claim 10, in which said rotor has another receptacle surrounding the end of said first receptacle, said corner seal being disposed in said another receptacle adapted to seal with the housing side portion as well as with the ends of said apex seal means, said corner seal having an extension acting as a bearing between said resilient means and apex seal.

15. The assembly as in claim 10, in which said apex seal has at least one groove in that portion of the surface adapted to engage the end wall portion, and a polymer-based embedment in said one groove effective to cyclically flux into the pores of said outer apex seal in response to wear for cyclically decreasing the friction coefficient thereof.

16. The assembly as in claim 15, in which the polymer base flux contains at least one filler material selected from the group consisting of fine graphite powder, and boron nitride powder.