Abstract:
A method of manufacturing a carbon-carbon brake disc uses a restraint fixture ( 12 ) that includes a preform retention region configured to limit contracting forces applied against a preform ( 10 ) in the preform retention region when the restraint fixture ( 12 ) thermally contracts. In one embodiment, the restraint fixture ( 12 ) comprises a band ( 12 ) having a first surface defining the preform retention region and a first expansion portion ( 26, 28, 29 ) adapted to deform upon application of a force to the band ( 12 ).

Description:
This application is a Divisional of co-pending application Ser. No. 10/942,258, filed on Sep. 16, 2004 now abandoned, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. §120 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a fixture for holding a preform during a heating process, and, more specifically, toward a restraint fixture for holding a preform that expands during a heating process and limits the force applied to the preform when the restraint fixture cools. 
     BACKGROUND OF THE INVENTION 
     Brake discs for aircraft or automobiles require materials having high heat resistance and long wear. Asbestos has been used in these applications, due to its heat resistance properties. Asbestos-based friction components have limited applicability under severe use conditions because the polymeric resins used to bind the asbestos fibers together decompose at elevated temperatures. The use of carbon-carbon brake components has therefore become more common. 
     Among the types of substrates used to make carbon-carbon parts are discontinuous carbon fiber moldings, nonwoven airlaid carbon substrates, woven carbon fiber substrates, and braided carbon fiber substrates. Carbon-carbon parts can also be made by using fibers that are precursors to carbon fibers to build a preform and then converting the fiber to carbon fiber. The substrates are typically stacked on top of each other to a desired thickness, and then the stacked substrates may be needle-punched together to join or consolidate the substrates to each other by intermingling fibers between the layers of substrates to form the preform. Other methods of manufacturing a preform or impregnating the preform with pitch may also be used. 
     An alternate method of consolidating the layers of a substrate is to use a pitch or resin binder and press the materials under heat and pressure to form a preform. The preform is then typically batch carbonized to reduce its non-carbon content. The carbonized preform may then be die cut or machined to a desired shape for further densification. 
     The preform may be further densified by a chemical vapor deposition (CVD) process, e.g. with hydrocarbon gasses, by resin infiltration or by pitch infiltration and then carbonized yet again. These densification processes may be repeated until the desired density is attained. The preform may then be heat-treated to reorient the carbon atoms, which modifies the thermo-mechanical properties of the preform, machined if necessary, and the non friction surfaces treated with an anti-oxidant to form the finished carbon-carbon part. 
     It has been found that, for preforms containing pitch, if the preform is carbonized without undergoing a stabilization step, a significant amount of the liquid pitch runs out of the preform during carbonization. This limits the density of the resulting carbon-carbon preform. In an effort to address this problem, the present inventors have attempted to carbonize performs while they are contained in a restraint fixture. The use of such a restraint fixture substantially addresses the problem of run out, but may cause damage to the finished perform. This is because, while the preform and the restraint fixture both expand when heated during the carbonization process, the restraint fixture contracts to a greater extent that the preform as it cools and thus squeezes and sometimes damages the preform. It would therefore be desirable to provide a restraint fixture for quickly and efficiently producing preforms that reduces the need for a stabilization step and avoids pitch run out during carbonization. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention comprises a restraint fixture including a band having a central portion having first and second ends and a first finger extending from the first end at a first angle and a second finger extending from the second end at a second angle. The center portion is curved so that its first end lies adjacent to its second end, and the first finger is connected to the second finger. 
     Another aspect of the invention comprises a restraint fixture that includes a band having a central portion having first and second ends, a first finger extending from the first end and a second finger extending from the second end. The center portion is curved so that the first end lies adjacent to the second end and the first finger is connected to the second finger. The band has a first configuration wherein the first finger extends from the first end at a first angle and the second finger extends from the second end at a second angle, and a second configuration wherein the first finger extends from the first end at a third angle different than the first angle and the second finger extends from the second end at a fourth angle different from the second angle. 
     A further aspect of the invention comprises a restraint fixture that includes a band having a first surface defining a preform retention region and a first expansion portion adapted to deform upon application of a force to the band first surface. 
     Another aspect of the invention comprises a restraint fixture that includes a band having a first surface defining a preform retention region and an arrangement for limiting a force exerted by the band on a preform in the preform retention region during a thermal contraction of the band. 
     An additional aspect of the invention comprises a restraint fixture that includes a band having first edge and a second edge and an inner surface defining a preform retention region, wherein the band has an inner diameter at the first edge greater than the inner diameter at the second edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and features of the present invention will be better understood after a reading of the following detailed description together with the drawings wherein: 
         FIG. 1  is a perspective view of a restraint fixture according to an embodiment of the present invention holding a preform before a heating process has been carried out on the preform; 
         FIG. 2  is a perspective view of the restraint fixture and preform of  FIG. 1  after a heating process has been carried out on the preform; 
         FIG. 3  is a perspective view of a variation of the restraint fixture of  FIG. 1  holding a plurality of preforms separated by separation plates; 
         FIG. 4  illustrates the arrangement of the preforms and separation plates of  FIG. 3 ; 
         FIG. 5  is a top plan view of a carbon-carbon variation of the restraint fixture of  FIG. 1 ; 
         FIG. 6  is a top plan view of a restraint fixture according to a second embodiment of the invention; 
         FIG. 7  is a top plan view of a restraint fixture according to a third embodiment of the invention; 
         FIG. 8  is a top plan view of a restraint fixture according to a fourth embodiment of the invention; 
         FIG. 9  is a top plan view of a restraint fixture according to a fifth embodiment of the invention; 
         FIG. 10  is a top plan view of a restraint fixture according to a sixth embodiment of the invention; 
         FIG. 11  is a top plan view of a restraint fixture according to a seventh embodiment of the invention; 
         FIG. 12  is a detail view of region XII of  FIG. 11 ; 
         FIG. 13  is a top plan view of a restraint fixture according to an eighth embodiment of the invention; 
         FIG. 14  is a side elevational view of a restraint fixture according to a ninth embodiment of the invention; and 
         FIG. 15  is a top plan view of the restraint fixture of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein the showings are for the purpose of illustrating preferred embodiments of the invention only and not for the purpose of limiting same,  FIG. 1  shows a preform  10  surrounded by a generally circular restraint fixture  12  comprising a steel band  14  having an inner surface  16 , an outer surface  18 , a first end  20  and a second end  22 . Band  14  includes a first or top edge  11  lying in a first plane and a second or bottom edge  13  spaced from the first edge and lying in a second plane generally parallel to the first plane. Restraint fixture  12  includes a generally circular central body portion  24  defining a closed circular region having a first end  21  and a second end  23 , a first finger  26  formed by bending band  14  at central body first end  21  at an angle of about 90 degrees to the band body portion  24 , and a second finger  28  formed by bending band  14  at central body portion second end  23  at about a 90 degree angle to band body  24 . Central body portion  24  is curved so that central body portion first end  21  lies adjacent to or contacts central body portion second end  23  and so that the inner, planar surface  16  of first finger  26  is aligned with and may contact the inner planar surface  16  of second finger  28 . These planar contact surfaces of first finger  26  and of second finger  28  may be referred to herein as “first and second planar contact surfaces.” Each of first finger  26  and second finger  28  may include an opening  30  for receiving a fastener, such as, for example, a screw  32 , for joining the first and second fingers  26 ,  28  together; however other ways of connecting the first and second fingers  26 ,  28  are envisioned. As explained below, it is preferable that opening  30  is formed near the first and second ends  18 ,  20  of band  14  leaving portions of the first and second fingers between openings  30  and preform  10  in contact with one another but unattached. An inner wall  34  may also be provided for engaging an inner surface  36  of preform  10  as illustrated in  FIG. 1 . Inner wall  34  is preferably used with all embodiments of the present invention; for clarity, it is only depicted in  FIG. 1 . 
     In use, restraint fixture  12  is placed around preform  10 , and the first finger  26  and second finger  28  are fastened together with fastener  32 . When a standard carbonization process is carried out on the restraint fixture  12  and preform  10 , at a temperature of about 750° C., for example, both the preform  10  and the restraint fixture  12  thermally expand. However, once the preform is carbonized, the new coefficient of thermal expansion will not allow the preform to shrink back to its original size when it cools. The restraint fixture  12 , however, will shrink as it cools, and as it shrinks, it applies force to the circumference of preform  10 . This squeezing may damage the preform. However, as will be appreciated from  FIG. 2 , when the restraint fixture  12  of this embodiment of the present invention shrinks, fingers  26 ,  28  bend away from one another and form a gap  29  between central body portion first end  21  and central body portion second end  32 . In this second configuration, the angle between each finger and the central body portion  24  increases, and the force applied against the preform  10  is thus limited. Fingers  26 ,  28  thereby form an expansion region that expands, or more precisely, limits the contraction of restraint fixture  12  as the restraint fixture  12  cools. Therefore, restraint fixture  12  is deformed as it contracts, does not contract to its original size, and limits the force applied against preform  10  during cooling. 
     Band  14  may be formed from a variety of thicknesses of steel. The present inventors have found that steel sheet as thin as 18 gauge or as thick as one half inch can be used to form band  14 . When thinner material is used, the distance between opening  30  and preform  10 , in other words, the length of first finger  26  and second finger  28 , may be relatively small because band  14  and fingers  26 ,  28  will readily deform. When a thicker band  14  is used, such as one formed from quarter inch steel, the fingers  26 ,  28  must be longer so that the pressure of band  14  contracting around preform  10  during cooling will be sufficient to deform the first and second fingers  26 ,  28  without damaging the preform  10 . Beneficially, while thinner bands generally can only be used once, thicker bands are reusable and may be used, for example, up to about 100 times before the repeated heating and cooling cycles render them unsuitable for further use. Alternately, band  14  can be formed from a carbon-carbon material that is both strong and will withstand a greater number of heating and cooling cycles before failing. A carbon-carbon band  37  is illustrated  FIG. 5 . A benefit of carbon-carbon band  37  is that carbon-carbon can be used at temperatures higher than those at which metals can be used. 
     As illustrated in  FIGS. 3 and 4 , a taller fixture  12 ′ can also be used to hold multiple preforms  10  stacked vertically with steel separation plates  38  therebetween. The plates  38  separating the preforms may have inner diameters smaller than the inner diameters of the preforms and outer diameters larger than the outer diameters of the preforms as illustrated in  FIG. 4 . 
     A second embodiment of the present invention is illustrated in  FIG. 6  wherein a restraint fixture formed from first and second generally U-shaped steel members  40 ,  42  is illustrated. First and second steel members  40 ,  42  are joined together at first and second fingers  44 ,  46  at their terminal ends. This embodiment provides benefits similar to those provided by the first embodiment, but allows for the material of the first and second steel members  40 ,  42  to deform at two locations, between each pair of fingers  44 ,  46 , to limit stress on preform  10  during thermal contraction. 
     A third embodiment of the invention is illustrated in  FIG. 7  wherein a restraint fixture  50  is formed from a continuous band of material  52  having a crimped portion  54  that is formed either when the band  52  is made or at the time the band  52  is placed around preform  10 . Crimped portion  54  deforms or opens when restraint fixture  50  cools to limit the stress on preform  10 . A band  50  having multiple crimped portions  54  could also be used. 
     A fourth embodiment of the invention is illustrated in  FIG. 8  wherein band  14  is shown with first finger  26  and second finger  28  secured with a clip or loop or band member  60  that will stretch or break when the contraction of the band  14  around the preform  10  begins to pull first finger  26  away from second finger  28  as the band cools. The band member  60  is weaker than the first and second fingers and thus breaks or deforms before the first and second fingers  26 ,  28  bend. Band member  60  may be made from a material that is relatively thin and weak which fails under a given amount of stress or may be relatively strong but which weakens when exposed to the heat of the carbonization process to help ensure that band  60  only breaks once the restraint fixture  12  has served its purpose. 
     A fifth embodiment of the invention is illustrated in  FIG. 9  wherein a restraint fixture  70  is shown having first and second unconnected ends  72 ,  74  which overlay one another in a sliding fashion so that an inner surface of first end  72  overlies an outer surface of second end  74 . Band  74  is made from spring steel or other resilient material that so that first end  72  slides over second end  74  as preform  10  expands during heating and maintains a relatively constant force on the outside diameter of the preform. When the preform and restraint fixture cool, a relatively constant force is maintained on the exterior of the preform, but the restraint fixture does not contract to its original configuration and thus does not damage the preform. 
     A sixth embodiment of the invention is illustrated in  FIG. 10  wherein a restraint fixture  80  comprising a steel band  82  with a cut  84  is disclosed. A projection  86  is provided at one end of band  82  which is received in a recess  88  at the other end of the band. The shapes of the projection  86  and the recess  88  should be complimentary, but may differ from those shown in  FIG. 10 . Cut  84  allows band  82  to open as the band cools and contracts to limit the forces applied against the preform, while projection  86  and recess  88  limit pitch run out before the pitch cures. 
     A seventh embodiment of the invention is illustrated in  FIGS. 11  and  12  wherein a restraint fixture  90  is shown that is similar to the restraint fixture of the fifth embodiment of  FIG. 9 , but wherein each of the first and second overlapping ends  92 ,  94  includes ratchet teeth  96  (shown in detail in  FIG. 12 ). The overlapping ends  92 ,  94  move little if at all relative to one another as the restraint fixture  90  expands. However, as the restraint fixture  90  cools and begins to shrink around a more-slowly shrinking preform, the ratchet teeth  96  allow the overlapping ends  92 ,  94  to slide relative to each other in one direction and prevent the interior of the restraint fixture from becoming too small. 
     An eighth embodiment of the invention is illustrated in  FIG. 13  wherein a restraint fixture  100  comprising a carbon-carbon band  102  having a cut  104  is shown encircling a preform  10 . A band of carbon-carbon fibers  106  is used to secure band  102  about preform  10  which fibers  106  may break as the restraint fixture expands or which may have a negative coefficient of thermal expansion and thus expand as the restraint fixture cools, reducing the amount of force applied against the preform. 
     A ninth embodiment of the invention is illustrated in  FIGS. 14 and 15  wherein a restraint fixture  110  is illustrated comprising an uninterrupted band  112  having a sloped inner wall  114 . In this embodiment, the band contracts as it cools, but sloped inner wall  114  causes cured preform  10  to slide axially (upwardly in  FIG. 14 ) and out of restraint fixture  110  as band  112  cools to minimize damage to the preform  10 . 
     The invention has been described in terms of several embodiments, however, other modifications and additions will become apparent to those skilled in the art upon a reading of this disclosure and such modifications and additions are intended to be included within the scope of this patent.