Patent Publication Number: US-6212770-B1

Title: Method of forming a roller assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit from U.S. Provisional Applications Ser. No. 60/076,184 filed on Feb. 27, 1998, Ser. No. 60/076,106 filed on Feb. 27, 1998, and 60/076,107 filed on Feb. 27, 1998. 
     This application relates to copending U.S. applications: 
     Ser. No. 60/076,184 entitled “Stator Structure for a Track Opening of a Rotary Machine”, by Keith T. Honda et al.; 
     U.S. Ser. No. 09/259,497 issued as U.S. Pat. No. 6,086,326 entitled “Stator Structure for a Track Opening of a Rotary Machine”, by Keith T. Honda et al.; 
     U.S. Ser. No. 60/076,106 entitled “Stator Assembly for a Rotary Machine”, by Keith T. Honda et al.; 
     U.S. Ser. No. 60/076,107 entitled “Roller Assembly for Guiding an Actuating Ring”, by Keith T. Honda et al.; 
     U.S. Ser. No. 09/261,606 issued as U.S. Pat. No. 6,106,227 entitled “Roller Assembly for Guiding an Actuating Ring”, by Keith T. Honda et al.; 
     U.S. Ser. No. 09/258,621 entitled “Method of Forming a Track”, by Keith T. Honda et al; 
     and 
     U.S. Ser. No. 09/261,603 issued as U.S. Pat. No. 6,092,987 entitled “Stator Assembly for a Rotary Machine”, by Keith T. Honda et al. 
    
    
     TECHNICAL FIELD 
     This invention relates to a stator assembly for an axial flow rotary machine and more particularly to structure for actuating a device for selectively flowing air from a working medium flowpath of the machine. While the invention was conceived in the field of axial flow, gas turbine engines for bleeding air at a point between two compressor rotor assemblies, it has application to other devices in such engines that employ an actuating ring. 
     BACKGROUND OF THE INVENTION 
     Axial flow gas turbine engines include a compression section, a combustion section and a turbine section. A flowpath for medium gases extends through these sections of the engine. During operation, the gases are pressurized in the compression section and fuel is added in the combustion section. The fuel is burned to add energy to the pressurized gases. The hot, pressurized gases are expanded through the turbine section to provide the work of compression and hot, high pressure gases for subsequent use. 
     U.S. Pat. No. 4,827,713 issued to Paterson et al entitled “Stator Valve Assembly for a Rotary Machine” is an example of such a gas turbine engine. In Paterson, the compression section of the engine is provided with two independent mechanical compressors. During transient operating conditions, one compressor can provide more flow than can be accommodated by the second compressor. Accordingly, the compression section is provided with a plurality of passages extending about the working medium flowpath to allow a portion of the air to escape form the compression section. In particular, the engine has a housing extending circumferentially about the axis of the engine. The housing has a plurality of openings. Each opening is covered or uncovered by an axially translating valve ring having circumferentially extending seal surfaces axially which move to engage resilient seal members. The valve ring is moved from an opened to a closed position by actuating means and includes guide roller assemblies in one ring and guide slots in the other ring. 
     The engine has circumferentially extending resilient seal members which extend axially. In the closed position, the seal members extend between a valve ring and the seal surface. The valve ring is urged by simple actuating means from an open position to a closed position to axially compress the resilient seal members on either side of the openings to provide a gas tight seal. 
     The valve ring is movable between an opened and a closed position and being guided by a slot and guide pin combination for urging the valve ring axially against resilient sealing members, the slots being contoured to provide a mechanical advantage as the axial compressive force is applied to the resilient sealing members. 
     The valve ring is movable between an open position and a closed position. The ring is guided by a slot and pin (cam follower) configuration, the slot having a preselected contour as the ring compresses the resilient seal members such that an inclined plane effect is provided during compression. In one embodiment, a feature is a bushing which engages the cam follower and the adjacent stator structure as the valve ring is moved to the open position. 
     As will be realized, the loss of working medium gases through the openings under steady state conditions when the compressors are operating at their design point will cause a decrease in the efficiency of the engine. Accordingly, it is desirable to insure that flow does not occur through these openings under conditions which do not require the diversion of flow from the flowpath. 
     Another problem is wear on the surface of the track opening and fracture failures of the pin which holds the hardface roller assembly in engagement with the track opening. A particular concern of the prior art design shown in FIG. 10 is the potential failure of the roller assembly occasioned by the shank breaking or by the thrust element at the head of the pin coming free and allowing the rotatable element of the roller to fall into the engine and degrade the performance of the engine. For example, the roller assembly has a thrust element which is bonded or mechanically attached to the pin. Experience has shown that this thrust element has the capability to become separated from the pin. 
     The above art notwithstanding, scientists and engineers working under the direction of Applicant&#39;s assignee have sought to decrease leakage through such seals by improving the sealing effectiveness of the seals and decreasing the effect that distortion resulting from operative load has on seal components. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method for forming a roller assembly having a pin includes forming a race on the head of the pin by forming an outwardly facing surface on the pin and an axially facing surface on the pin to bound the bottom and the side of the race and forming at least a portion of the third side of the race by pressing a collar against the pin. 
     In accordance with one embodiment of the present invention, the method includes trapping the collar between the head of the pin and a support and pressing the collar against the head of the pin and the support by creating a tensile force in the pin. 
     In accordance with one detailed embodiment of the present method, the method includes spacing the interior of the collar away from the pin adjacent the support and pressing the interior of the collar against the pin adjacent the area of the race and resisting bending forces from the external forces in the race region of the pin with a turning moment exerted by the support through the collar at a location in proximity to the race. 
     A primary feature of the present method is forming the race for a rotatable element on a roller assembly by pressing a trapped collar against one side of the pin. Another feature of the present method is forming the race with a thrust element which is one piece with the remainder of the pin such that the thrust element is not bonded nor mechanically attached to the pin. Another feature is using a collar for shifting the turning moment exerted by a support on the pin from the support to a location closer to the race. 
     A primary advantage of the present is the durability of a machine having a roller assembly which results from trapping the collar of the roller assembly with the remainder of the roller assembly and avoiding the use of a detachable thrust element which might separate and enter the machine. Another advantage is the durability of the roller assembly which results from decreasing bending stresses in the pin of the roller assembly by transferring loads from the support to the roller assembly to a location closer to the race region of the roller assembly. Still another advantage is the durability of the roller assembly which results from disposing a self lubricating coating between the rotatable element of the roller assembly and the remainder of the roller assembly, whether the material is bonded to a portion of the structure forming the race, bonded to the rotatable element or disposed as an independent structure between the rotatable element such that the material is in faying contact with the rotatable element and structure forming the race. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of a gas turbine engine with the exterior of the engine broken away to show a portion of the interior of the engine. 
     FIG. 2 is an enlarged cross-sectional view of a portion of the engine shown in FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of a portion of the engine shown in FIG. 2 with portions of the engine broken away for clarity. 
     FIG. 4 a  is a front view of a portion of the stator assembly shown in FIG.  2  and FIG.  3 . 
     FIG. 4 b  is a partial perspective view of the housing shown in FIG. 3 showing liner, the roller assembly and two adjacent slots formed in the housing. 
     FIG. 5 is an enlarged side elevation view of a portion of the engine shown in FIG. 3 showing in detail the relationship of the elements of the roller assembly. 
     FIG. 6 is an alternate embodiment of the roller assembly shown in FIG.  5 . 
     FIGS. 7 and 7 a , are partial perspective view of the structure shown in FIG. 4 b  in exploded fashion shown in the relationship of the liner and rivets to the housing. 
     FIG. 8 is a plan view taken along the lines  8   a — 8   a  of FIG.  3 . 
     FIG. 9 is a bottom view of the structure shown in FIG.  8 . 
     FIG. 10 is a view of a prior art roller assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a side elevation view of an axial flow, gas turbine engine  10  of the turbofan type having an axis A. The exterior of the engine is broken away to show a portion of the interior of the engine. 
     The engine  10  has an annular compression section  12 , a combustion section  14  and a turbine section  16 , which are disposed about the axis A. A primary flowpath  18  for working medium gasses extends circumferentially about the axis of the engine and rearwardly through the sections of the engine. A secondary flowpath  20  for working medium gases, commonly called a bypass flowpath, is radially outwardly from the first flowpath and extends rearwardly through the outermost portion of the compression section of the engine. 
     The compressor section includes a fan  22 , a first compressor section  24  and a second compressor  26  spaced rearwardly from the first compressor. The first compressor is commonly called the low pressure compressor and the second compressor is commonly called the high pressure compressor. These compressors are designed to operate at different speeds. 
     The engine has a stator assembly  27  having a compressor bleed system  28 . The compressor bleed system includes a plurality of passages  30 , an annular chamber  32  and a second plurality of passages (not shown) which place the primary flowpath  18  for working medium gases in flow communication with the secondary flowpath  20  through the chamber  32 . 
     FIG. 2 is an enlarged view of a portion of the engine shown in FIG. 1 showing in further detail the compressor bleed system  28 . An inner case  36  has a flowpath wall  38  which outwardly bounds the primary flowpath  18 . A housing assembly  40  has a housing  42 . The housing extends circumferentially to bound a portion of the annular chamber  32 . Each passage  30  extends from the primary flowpath  18  to the chamber  32  through the inner flowpath wall and through the housing. 
     The housing  42  is of a one-piece construction. The housing has a pair of seal surfaces  44 , 46  extending circumferentially about the housing. The seal surfaces are oriented in the axial direction and face upstream. The seal surfaces are spaced axially defining a seal region  48  therebetween. An outwardly facing, cylindrical surface  49  extends axially between the seal surfaces. The seal surfaces are located such that each passage through the one-piece ring has an opening  50  in flow communication with the seal region. 
     The housing has a plurality of axially extending flanges  52  as shown in FIG. 4 a . Each local flange has an edge  53 , outwardly facing surface  54 , an inwardly facing surface  55  and a plurality of track openings, as represented by the track opening  56 , which adapts the housing to receive a plurality of roller assemblies  76 . 
     The compressor bleed system includes a valve  62  having a valve ring  64 . The valve ring has a sleeve  66  outwardly of the passageways  30  in the housing. The sleeve is concentrically disposed with respect to the cylindrical surface  49  of the housing. The valve is movable to a first, open position (shown in FIG. 3) and to a second, closed position as shown in FIG.  2 . The valve ring has an inwardly facing surface  70  which is adapted, if required under operative conditions, to slidably engage the corresponding outwardly facing cylindrical surface  49  on the housing. The valve ring has two axially facing seal members  72 , 74  which are oriented in the downstream direction and which are spaced axially by a distance which permits each to engage an associated seal surface  44 , 46  on the housing. The compressible seal members may be made of any material that is compatible with the environment and which resiliently deforms upon the application of pressure. One particular compressible seal member is disclosed in U.S. patent application entitled “Valve Seal Assembly”, (EH-10146), assigned to the assignee of the present invention, the subject matter of which is incorporated herein by reference. 
     FIG. 3 is an enlarged cross-sectional view of a portion of the engine shown in FIG. 2 with portions of the engine broken away for clarity. FIG. 3 shows in detail the roller assembly  58  and a liner  76  for the track opening  56  in the flange. The roller assembly  58  includes a pin  78 , a bearing surface  80  on a rotatable element or roller  82  which engages the pin and a collar  84  disposed between the pin and one of the supports  86  of the valve ring  70 . The pin extends through holes  88 , 92  in the valve ring which are slightly (thousandths of an inch) larger than the pin. A fastener, such as nut  94 , engages the pin to exert a force on the pin causing the pin to clamp the collar against the support. 
     FIG. 3 also shows the liner  76  for the roller assembly  58 . The liner is disposed in the local flange  52  of the housing in the first track opening  56 . Elastomeric material  96  is disposed between the walls of the liner assembly and in some applications may extend between the bottom of the liner and the housing. Fasteners, as represented by the rivets  98 , extend between the liner and the flange of the housing to attach the liner to the housing. 
     FIG. 4 a  shows a front view of a housing  42  having the passages  30  (not shown) for bleed air from the flowpath  18  of the engine. As shown in FIG. 4 a , the housing has seven local flanges  52 , each having one of the first plurality of openings for the liner. In addition, the housing has a flange  52   a  which adapts the housing to engage a bellcrank assembly for driving the valve ring between the first and second positions. The bellcrank assembly may be of the type shown in U.S. Pat. No. 4,827,713, issued to Patterson, or might be of the type shown in U.S. Pat. No. 6,076,423 entitled “Bell Crank Mechanism”, filed on even date herewith and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. 
     FIG. 4 b  is a partial perspective view of the housing  42  shown in FIG.  3  and FIG. 4 a  showing the liner  76  and the roller assembly  58 . Two adjacent track openings are formed in one of the local flanges of the housing. There are two types of track openings  56 , 57 . Each of the first plurality of track openings  56  receives the liner. The liner has a first rail wall  102 , a second rail wall  104  and an end wall  106  extending circumferentially and axially between the walls  102 , 104 . As can be seen, the track opening extends over a portion of the housing having a shelf  108 . A space S between the shelf and the liner receives the elastomeric material shown in FIG.  2 . The liner has a base  112  having a hole extending radially therethrough as represented by the hole  174  shown in FIG.  7 . The second type of track opening  57  is designed to receive a damping roller assembly of the type shown in FIG.  6 . The second track opening does not have a liner. 
     FIG. 5 is an enlarged side elevation view of a portion of the engine shown in FIG. 3 showing in detail the relationship of the elements of the roller assembly  58 . The roller assembly  58  is disposed about an axis Ap and has a raceway, commonly referred to as a race, which receives the rotatable element, roller  82 . The race might simply be the adjacent elements having surfaces which bound the opening that receives the rotatable element, and may include a thin layer of material on the surfaces or a thin layer of material in faying contact with the adjacent elements, which are represented by the race  114 . The material may be disposed in the assembly as a unit or sprayed on the adjacent surfaces. Alternatively, the thin layer of material might be directly applied to the rotatable element. The material used with the race may be formed of, for example, a proprietary Teflon fiberglass material disposed in a polyamide resin such as a self lubricating proprietary composition available from the Kamatics Corporation as K-Therm T-87 Material. The Kamatics Corporation is a Kaman Company, P. 0 . Box 3, Bloomfield, Conn. 06002. 
     The pin  78  of the roller assembly  58  forms or bounds a portion of the race  114  which receives the rotatable element  82 . The pin has a first end  116  and a second end  118 . The pin has a head  122  at the first end which extends circumferentially about the axis Ap to define the innermost portion of the race for the rotatable element. A thrust member  124  extends radially outward from the head. The thrust member is formed as one piece with the head. The thrust member has an axial facing surface  126  which forms or bounds one side of the race. A cylindrical shank  128  extends axially from the head and is disposed about the axis Ap. The shank has a diameter Ds which is smaller than the diameter of the head Dh. A fillet radius R extends between the head and the shank. The pin includes a clamping surface  130  which extends radially between the shank and the bottom of the race. The clamping surface faces in an axial direction toward the second end  118  of the pin for exerting a clamping force on the collar  84 . 
     The collar  84  has a first end  132  disposed adjacent the head  122  of the pin  78 . A second end  134  adapts the collar to engage the support  86 . A first radial surface  136  at the first end extends radially outwardly and engages the clamping surface  130  of the head  122  of the pin. The first radial surface extends beyond the clamping surface to form or bound the second side of the race. 
     A second radial surface  138  of the collar  84  is spaced axially from the first radial surface by a distance Lr. The second radial surface extends circumferentially about the collar to form a clamping surface for the collar. The clamping surface adapts the collar to engage the support  86  of the ring. 
     The collar  84  has an internal axial surface  139  which extends from the first radial surface  136  to the second radial surface  138 . The internal axial surface further has a first region  140  adjacent the head which engages the shank  128  with a compressive force. The collar has a second region  141  spaced radially outwardly from the shank  128  to leave a gap G between the collar and the shank which extends to the support  86 . The second region is spaced radially outwardly of the first region which extends from the second region to the clamping surface  130 . 
     FIG. 6 is an alternate embodiment  176  of the roller assembly  58  shown in FIG.  5 . This embodiment is used as a damper roller assembly. Each of these damper roller assemblies is disposed in one of the plurality of second track openings  57  to provide damping as the ring moves from the open to the closed position. In this embodiment, the damper roller assembly has a roller  177  using rubber material  180  molded into a circumferentially extending steel member  182 . One embodiment of the member is a steel rim having a circumferentially extending bottom  183   a , a pair of radially extending sides  183   b , and an axially extending lip  183   c . The rubber is a Viton Rubber material impregnated with Kevlar fibers and is available from Jonal Laboratories Inc., Meriden, Conn. The rubber material is molded onto the steel rim (AMS 5630) which allows greater rim engagement and bond strength. 
     The race is formed as is the race shown in the embodiment of FIG. 5 with K-Therm T-87 Material from the Kamatics Corporation. The remainder of the assembly  176  is very similar to the hard roller assembly shown in FIG. 5 utilizing a pin  178 , a collar  184  and a head  222 . It is a one piece construction with a thrust member  224  extending radially outwardly from the pin. As used with respect to the roller assemblies in FIG.  5  and FIG. 6, the term “one piece” means that the thrust element is formed as a unitary structure with the pin and is not joined, mechanically attached or bonded to the pin. In addition, the damper roller assembly has a shank  228  on the pin which extends through the support of the ring which is spaced radially slightly from the support to provide a gap between the support and the shank. 
     FIGS. 7 and 7 a  are a partial perspective view of the structure shown in FIG. 4 in exploded fashion showing the relationship of the liner  76  and rivets  98  to the local flange  52  of the housing  42 . The housing has pairs of holes  43  for receiving the rivets. The rivets have a spring constant in the installed condition which is less than the spring constant of the housing and are able to flex in the installed condition should there be differential thermal growth between the housing and the liner. 
     The track openings  56 , 57  extend axially and circumferentially in the flange  52  and adapt the flange to receive the roller assemblies  82 , 174 . The first openings  56  extend through the flange from the outer surface  54  to the inner surface  55  of the flange at least over a portion of the track opening. The opening extends axially and circumferentially over a portion of the remainder of the housing to form the shelf  108  shown in FIG.  4 . The shelf has an outer surface  109  which faces in the radially outward direction and which adapts the housing to receive the elastomeric material  96  or may be simply spaced slightly from the liner  76  in the installed condition. 
     Each of the first openings  56  is bounded a first side  142  and, by a second side  144  spaced circumferentially from the first side and a third side  146  which extends circumferentially between the first side and the second side. In the embodiment shown, the first and second sides may be formed such that they are relatively parallel being formed by radiuses having a common center. However, these sides are not necessarily made so precisely. This permits for easy machining in the repair or modification of a structure having worn surfaces in the first track openings. A method of modifying the slot is to remove worn material from both surfaces until the opening is large enough to accommodate a liner  76 . Thus, the method of modification has the advantage of not requiring precise machining of the first track openings once the holes  43  for the fasteners are precisely located. 
     The liner  76  has a first end  152 , a second end  154  and an opening or track  156  extending from the first end to the second end. The liner includes the first rail wall  102  extending from the first end between the first end and the second end. The first rail wall does not necessarily extend to the second end but may terminate at that location in embodiments which do not have an end wall. The first rail wall  102  has a constant radius for a region which extends over one half the distance between the first end and the second end. The second rail wall  104  is spaced laterally from the first rail wall in the generally circumferential direction. The second rail wall extends parallel to the first rail wall from the first end between the first end and the second end. The second rail wall has a constant radius for a region which is parallel to the first rail wall. The second rail wall may terminate at the second end as may the first rail wall. Alternatively, and as shown, an end wall  106  extends laterally from the first rail wall to the second rail wall. 
     A plate  157  extends laterally with respect to the rail walls. The plate includes the base  112  having a thickness Tb. The plate extends to the first rail wall  102  and the second rail wall  104 . The plate might be continuous for the region between these two walls or may be formed of a series of local reinforcing braces which extend between the walls with one or more holes or openings between the braces as represented by the opening  174 . The plate also engages the end wall. The combination of the plate, and the end wall  106  supporting the first rail wall and the second rail wall enables machining of the liner out of a hard material such as steel while maintaining reproducibility between liners. Reproducibility avoids difficulties associated with installing the liner and insuring the new rail walls follow the track of the roller assembly where the liner is used to modify an existing housing formed of relatively soft material, such as aluminum alloy, which has a worn track opening. 
     A first flange  158  extends laterally from the base  110  adjacent at least one of the walls, such as rail wall  102 . The first flange has a radial thickness Tfa and at least one hole  162  which adapts the base  110  to receive a fastener. In the embodiment shown, the flange has two holes to receive rivets. The first flange adapts the liner to engage the circumferentially extending stator structure in the engine such as the housing  42 . The flange has an outer surface  164  which engages in the inner surface  55  of the housing in a faying relationship, as shown with reference to FIG.  7  and FIG.  8 . The second flange  166  extends laterally from the base adjacent one of said walls having a cylindrical portion. The second flange has a thickness Tfb. The thickness Tfa and Tfb are larger than the thickness Tb to provide a larger bearing area to the rivets and to reduce the stiffness of the liner under operative conditions by reason of the decreased thickness Tb of the base in comparison of the thickness Tfa and Tfb. The second flange adapts the liner to engage the housing, has holes  162   a , and has an outer surface  172  which is in faying relationship in the installed condition with the inner surface of the local flange. In addition, the end of the liner has an opening, such as the opening  174 , extending through the base which interrupts the continuity of the base, decreases its spring rate and provides for the drainage of liquid should the liner accumulate such liquid by reason of its location at the top of the engine. 
     The rail wall which the roller assembly engages has a contour which is adapted to conform closely to contour of the roller assembly in the installed condition at the second, closed position as shown in FIG.  2 . For example, the inner wall  104  at the closed position extends substantially parallel to the outer wall or surface of the rotatable element such that there is line contact between the cylindrical wall and the rotatable element. The location of the closed position is normally within the width of the track as measured between the walls and the location at which the walls are parallel as shown in FIG.  8 . 
     As shown in FIG. 9, a portion of the plate  157  of the liner  76  extends over the outer surface of the shelf  108  and the liner flanges  158 , 166  engage the inner surface  55  of the local flange. As a result, the construction could experience the loss of three rivets  98 ; and as long as one rivet retains the liner in place, the liner would not fall free from the housing by reason of the flanges and the base being trapped in opposite radial directions by the shelf  108  and the inner surface  55  of the housing. In such a case, the single rivet prevents the liner from backing out of its trapped engagement between the outer surface of the shelf and the inner surface of the flange. 
     During operation of the gas turbine engine shown in FIG. 1, working medium gases are flowed along the primary and secondary flowpaths  18 , 20 . As the gases pass through the first and second compressor  24 , 26 , and as the operating speeds of the individual compressors are adjusted during transient conditions, it may be desirable to divert some of the flow from the primary flowpath  18  to the secondary flowpath  20 . The valve ring  64  is moved from the closed position shown in FIG. 2 to the open position shown in FIG.  3 . The valve ring is guided positively in a spiral motion by the roller assemblies  58 . Each roller assembly moves in an associated track opening  56  in the housing  42 . The liner of the opening is formed of a harder material (steel) than the adjacent housing (aluminum alloy AMS 4218 or AMS 4235 material depending on temperature). The liner provides a more durable structure as the hard, inelastic roller of the roller assembly (steel) moves along the line of spiral motion in the slot. The liner might be provided with a wear-resistant coating, such as an aluminum bronze plasma spray coating or a molydisulfide coating, to provide anti-gallant properties to the liner. 
     At least a portion of the rail walls  102 , 104  of the liner is cylindrical. The cylindrical shape approximates the ideal spiral or helical surfaces followed by the roller  82 , as it moves from each contact point to the next. This simplifies manufacture of the liner, by permitting use of a two axis cutting machine to form the rail walls. A more complex four or five axis cutting machine could form rail walls having the ideal shape. However, cylindrical walls facilitate consistently reproducing the contour of the tracks and correctly inspecting the finished product. 
     As the roller follows the cylindrical track, the roller has point contact at each location that it presses against the wall. As a result, the roller  82  follows a line of contact until the roller reaches the closed position of the housing  42  at which location the cylindrical surface of the roller conforms closely to the cylindrical surface of the liner. This insures in the highly loaded closed position that the roller assembly engages the cylindrical surface of the second rail wall  104  of the liner with a line of vertical contact with less pressure than were it to engage the wall with point contact as it does in the lightly loaded condition as it moves between the open and closed positions. In other embodiments, the roller might have a crown which reduces the line of contact in the closed position. This has the advantage of more precisely locating where the external force Fe acts on the roller and on the track wall. 
     Differences in the thermal expansion occur between the liner and the housing. The liner  76  is configured to have a spring rate in tension which is sufficiently low in series combination with the spring rate of the fasteners such that it is less than the spring rate of the remaining portion of the valve ring. In this embodiment, the spring rate is significantly less. In addition, the housing  42  has the local flanges  52  having a relatively low spring rate compared to the rest of the housing  42 . Each local flange supports a liner from the housing like fingers extending from a palm. This provides the advantage of allowing the housing  42  to accommodate changes in temperature by growing significantly more in the radial direction than does the liner. A significant advantage is that it permits use of less dense but higher thermal expansion material, such as the aluminum alloy mentioned earlier, for the relatively massive housing. 
     The lower spring rate in the liner  76  results in part from reducing the thickness of the plate  157  which extends between the walls  102 ,  104 ,  106 , removing material from the plate, such as through the drain hole  113  or additional holes or cut outs between the track wall, and providing local flanges  158 , 166  to the plate to attach the plate to the housing  42 . Thus, even though the coefficient of thermal expansion of the housing is approximately twice the coefficient of thermal expansion of the liner and even though the hard material of the liner has a higher modulus of elasticity E, the liner  76  for the track grows and contracts with the housing, accommodating changes in diameter of the housing as the housing experiences different temperatures under engine operative conditions. This avoids over-stressing the housing, the liner and the fasteners for the liner. 
     Vibrations in the ring  64 , vibrations in the liner  76  and vibrations in the roller assembly  58  all provide the potential for increased wear of, and stresses in, the roller assembly and the liner. The vibrations also produce stresses in these components. Anti-gallant material on the interior of the liner and damping media for vibrations act to reduce these stresses and the wear that results from the vibrations. One satisfactory elastomeric material is silicone rubber and is available from the General Electric Company, Waterford, N.Y. 12188, as RTV 159 Silicone Rubber. Any rubber sealant or elastomeric potting material would provide damping. The elastomeric material damps vibrations which are transmitted from the ring to the liner and damps vibrations in the liner which result from engine operation and from the roller assembly vibrating as the roller assembly follows the track. These result from the roller assembly moving radially in and out or in any direction, although the elastomeric material in this embodiment is more effective for damping vibrations in the circumferential and axial directions. In addition, the elastomeric material damps vibrations that occur as the roller assembly tracks the liner with impact loads as the vibrating roller assembly moves from the open to the closed position. 
     Damper roller assemblies  176  are also provided at each local flange as in the past. The newly designed damper assemblies, similar to the cam follower roller assemblies, have an elastomeric roller. The damper roller assemblies are disposed about the circumference of the housing adjacent between the hard roller assemblies at each local flange. These roller assemblies dampen vibrations in the valve ring that are transmitted to the roller assembly and vibrations in the housing that are transmitted to the liner assembly even though the elastomeric material disposed between the liner and the housing acts to dampen those vibrations. Thus as the roller assembly move to the open position, the resilient damping roller assemblies  176  are compressed providing a means to damp vibrations and block the roller assembly from wearing the liner. 
     As the valve ring moves between the open and closed positions, the radially facing surfaces  68 , 70  on the valve ring and on the housing  50 , 54  act to center the valve ring about the housing. Further centering is provided by the roller assembly which provides a spline type connection, as where a plurality of roller assemblies are spaced around the circumference of the valve ring and the housing, and, the roller assembly is attached to one ring and engages the other housing. 
     As shown in FIG. 2 by the reaction forces F 1 , F 2  and F 3 , the seal compression loads in the closed position are all reacted in the housing area of the static structure. This includes F 1  which is the roller assembly load on the support structure (and Fe, which is the reactant load on the roller assembly and is shown in FIG. 5) in reaction to the seal loads, and F 2  and F 3  which are seal loads on the support structure. This has the advantage of reducing any turning moments which result from the coupling of the forces and reduces relaxation of the seal compression during operative conditions as compared to structures in which such loads are more widely separated through use of a several piece construction for reacting the loads. 
     The following discussion of the reaction of the roller assembly to the external force Fe assumes statically determinate structures and rigid members to illustrate the conceptual operation of the design and the advantages of the design. Under some operating conditions, the structure may become indeterminate and flex but the advantages will remain and are based on the following understanding of its operation. 
     Torquing the nut which engages the pin of the roller assembly causes a tensile force Tf in the pin. The pin exerts a clamping force on the collar through the clamping face or surface  130  of the pin  78 . The clamping force provides a preload and places the collar  84  in compression. The clamping force presses the clamping surface  138  of the collar against the support  86  around the entire circumference of the collar with a predetermined force Fn. The present construction has advantages whether the predetermined force is at the correct level or at less than the correct level which may change the interaction between the collar and the support. 
     The roller transmits the external reaction force Fe to the head of the pin. The collar resists the external force Fe on the pin with the frictional force Ff developed between the clamping surface of the collar and the support. This frictional force Ff results from the coefficient of friction at the clamping surface multiplied by the normal force Fn at the clamping surface on the collar against the support. The resisting force is represented by the resultant force Fr (opposing the external force Fe) and acts at the first region  140  of the internal axial surface of the collar which is pressed on the shank of the pin. The fit is 0.0002-0.0012 inches tight on a diameter, but is not important to achieve the primary advantages of the invention. The fit could be loose if adequate preload/clamping could be maintained to provide a friction force between the collar and pin that is larger than the force Fe. For example, the collar might be loose having a conical interface with the pin to resist the force without friction. This would result in the same bending stresses in the pin. In other alternate constructions, the first region of the internal surface might be spaced slightly from the shank of the pin but inwardly of the second region. In such a construction, the benefit of the present invention is realized as the pin deflects slightly to engage the internal surface at the first region such as might occur where the clamping force is lost or was never applied because of insufficient tightening of the nut. 
     The force Fr acts in close proximity to the force Fe by engaging the pin at the first region. This is significant because the resisting force Fr balances the external force Fe but creates a force couple Fe-Fr. The force couple Fe-Fr has a moment arm La and exerts a bending moment on the pin and bending stresses in the pin. The moment Fe-Fr and the stresses are proportional to the length of the moment arm La, which is shortened by having Fr act in close proximity to Fe. The frictional force Ff with the force in the pin Fc creates a moment acting on the collar equal to the moment of the external force couple Fe-Fr. The moment of Ff-Fc is resisted by the force couple Fs-Fp acting on the collar from the support and the pin. Thus, the moment of Fs-Fp equals the moment of Fe-Fr and the couple Fs-Fp causes internal bending stresses in the pin that are proportional to the couple Fe-Fr. 
     A particular advantage of the present invention is the magnitude of the moment Fe-Fr and the resulting bending stresses in the pin. The moment of Fe-Fr is smaller than prior art constructions because the moment arm La is smaller than those constructions in which the collar has a press fit along the entire length of the shank (which moves the resultant force Fr away from the head of the pin and increases the moment) or prior art constructions having a shoulder on the pin which exerts a clamping force directly on the support, such as is shown in FIG.  10 . In this last example, the moment arm Ls for the force Fr has a length which is much greater than La and greatly increases the moment the couple Fe-Fr which ultimately acts at the surface of the support and causes bending stresses in the pin. 
     The couple Fe-Fr is maintained by maintaining the gap G at a value that prevents contact between the collar and the shank with bending deflections. The load path changes if contact occurs. Making the gap G larger than the minimum necessary to prevent contact at the second region decreases clamping area at the support for the same outside diameter of the collar at the support. The frictional force required to keep the assembly from sliding is at least equal and opposite to the force Fe. It is a function of clamping force (preload) and is independent of area. However, maintaining clamping area at the support results in less wear for supports formed of softer than steel, but lighter weight aluminum structures. 
     The roller assembly of the present invention also has advantages in constructions where the external force Fe exceeds the frictional force Ff. This may occur because of an anomaly in operating conditions or because the preload Fn is too small because of undertorquing. 
     In such cases, the external force Fe will cause bending of the shank and the collar may slide or tilt slightly. Again, the moment arm Lr is increased by spacing the collar away from the shank which decreases the amount of normal force Fs of the couple Fs-Fp and the frictional force required to balance the moment required to resist the moment caused by the external force. Several advantages result from this construction. Reducing Fs reduces the pressure on the surface and reduces galling at the surface of the support which results from micro adjustments of the collar in response to the external bending moment. 
     A significant advantage for durability of the pin is the absence of a stress riser (change in diameter of the pin) at the location of the maximum bending moment in the shank. The maximum bending moment in the shank takes place where the internal bending moment in the shank combined with Ff-Fc resists the external bending moment. This takes place at the location of the support, spaced by the distance Lc from the clamping face of the pin. This avoids having the clamping face of the pin at the support and its associated stress riser connected with the increase in diameter to form the clamping face. 
     Two other advantages result from spacing the clamping surface of the pin from the support with the collar: the first is removing the decreased surface area at the clamping face of the pin from bearing on the soft surface of the support. which results from the fillet radius R introduced between the shank and the clamping face on the pin to decrease the stress concentration by avoiding a sharp change in cross section of the pin. The reduced surface area increases the potential of yielding or wearing of the support, which could cause a loss of clamping. In the present construction, the clamping face of the pin with the fillet radius engages the collar. Although the pin has reduced area which increases the pressure exerted by the normal force (which galls or yields the soft surface of the aluminum alloy support in the prior art) it does not have that effect on the hard surface of the steel collar. In addition, this reduced area is not reduced significantly, because the fillet radius can be small due to a higher stress concentration allowed by the reduced bending moment at this location). The area is not important for galling on the collar and the collar is still capable of maintaining the clamping requirement. 
     As will be realized, the roller assembly might be rigidly mounted to the housing and the liner mounted on the valve ring. These are equivalent structures to the structures disclosed and claimed herein. Finally, one or both of the compressible seal members might be mounted on the housing rather than being retained by the valve ring as shown in FIG.  2 . 
     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in the form and detail thereof may be made without departing from the spirit and scope of the claimed invention.