Patent Publication Number: US-2015083533-A1

Title: Customized friction for brakes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of PCT International Application Serial No. PCT/US2012/35743 filed on Apr. 30, 2012, the contents of which are incorporated herein by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     Exemplary embodiments generally relate to braking systems, such as those used to slow and/or stop an elevator car or counterweight, for example in an over speed condition. More particularly, the embodiments can relate to an elevator safety brake system having a composite friction surface. 
     A typical safety braking system is attached to an elevator car and comprises a pair of wedge-shaped brake shoes having substantially flat or grooved frictional surfaces. These frictional surfaces are ordinarily positioned on opposite sides of the stem portion of a T-shaped guide rail supported on an elevator hoistway wall. These wedge-shaped brake shoes are activated by a governor mechanism which forces the wedge-shaped brake shoes along an adjacent guide shoe assembly which in turn forces the frictional surfaces of the brake shoes to make contact with the guide rail to slow or stop the car. 
     As very tall buildings are built, high speed and high load elevators have become necessary to service the numerous floors in such buildings. The safety braking requirements of such elevators have become increasingly demanding. It has been determined that conventional cast iron is not preferable as a consistent friction material at high speeds and loads required by such modern elevator systems because of the excessive heat generation during sliding. Consequently, conventional cast iron may have issues such as excessive wear and instability of coefficient of friction caused by the high frictional heating. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment of the invention, a brake element is provided including a friction material. The friction material includes a polymer-based ceramic matrix composite material having a plurality of fibers. The plurality of fibers is arranged at an angle to a braking direction. 
     Alternatively, in this or other aspects of the invention, the brake element has a coefficient of friction greater than 0.3 when the plurality of fibers are at an angle of about 90 degrees. 
     Alternatively, in this or other aspects of the invention the brake element is an insert on a brake shoe. 
     Alternatively, in this or other aspects of the invention, the brake element is used in combination with one or more other brake elements. 
     Alternatively, in this or other aspects of the invention, one of the brake elements has a length or width different from another brake element. 
     Alternatively, in this or other aspects of the invention, the one or more brake elements have a uniform and equal thickness. 
     Alternatively, in this or other aspects of the invention, the friction material is mechanically attached to a surface of the brake shoe. 
     Alternatively, in this or other aspects of the invention, the friction material is chemically attached to a surface of the brake shoe. 
     Alternatively, in this or other aspects of the invention, the polymer-based ceramic matrix composite material includes a matrix phase of a silicon carbide phase, a silicon oxycarbide phase, and/or a carbon phase. 
     Alternatively, in this or other aspects of the invention, the polymer-based ceramic matrix composite material includes a reinforcement phase of a silicon carbide and/or a carbon. 
     Alternatively, in this or other aspects of the invention, the brake element is part of an elevator safety brake system for stopping an elevator car or counterweight. 
     According to an alternate embodiment of the invention, a method is provided for making a brake element including providing a friction material. The friction material include a polymer based ceramic matrix composite material having a plurality of fibers. The plurality of fibers are arranged at an angle to a braking direction. 
     Alternatively, in this or other aspects of the invention, the polymer-based ceramic matrix composite material includes a matrix phase of a silicon carbide phase, a silicon oxycarbide phase, and/or a carbon phase. 
     Alternatively, in this or other aspects of the invention the friction material is an insert on the first surface. 
     Alternatively, in this or other aspects of the invention, the polymer-based ceramic matrix composite material includes a reinforcement phase of a silicon carbide and/or a carbon. 
     Alternatively, in this or other aspects of the invention, the plurality of fibers is arranged by selecting an angle such that the friction material provides a desired coefficient of friction. 
     Alternatively, in this or other aspects of the invention, the angle is 90 degrees. 
     Alternatively, in this or other aspects of the invention, the desired coefficient of friction is greater than or equal to 0.3. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of embodiments of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a machine room-less elevator system in a hoistway; 
         FIG. 2  is a side view of an elevator brake shoe according to an exemplary embodiment of the invention; and 
         FIGS. 3   a - 3   c  are front views of exemplary elevator brake shoes according to exemplary embodiments of the invention. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a simplified schematic diagram of an elevator safety brake system  10  is illustrated. The brake system  10  comprises a pair of actuators  20  which are attached to an elevator car  12  on opposing sides of a guide rail  14  supported in an elevator hoistway (not shown). Each actuator  20  includes a wedge-shaped guide shoe  26  which is movable within a housing  22  in a direction generally perpendicular to the guide rail  14 . The guide shoe  26  is biased in the direction of the guide rail  14  by a coil spring  24 . The surface  28  of the guide shoe  26  facing the guide rail  14  is inclined. The actuator  20  additionally includes a wedge-shaped brake shoe  30  having a similar inclined surface  32  complementary to the inclined surface  28  of the guide shoe  26 . The brake shoe  30  is also provided with a surface  34  opposite the inclined surface  32 , and facing the guide rail  14 . The brake shoe  30  is located between the guide shoe  26  and the guide rail  14 . A brake pad  36  having a high friction material is attached to the surface  34  of the brake shoe  30  facing the guide rail  14 . A plurality of rollers  40  is positioned between the inclined surface  28  of the guide shoe  26  and the complementary inclined surface  32  of the brake shoe  30 . The rollers  40  provide a low friction contact between the adjacent inclined surfaces  28 ,  32  of the guide shoe  26  and brake shoe  30  respectively. The guide shoe  26 , biased by spring  24 , applies a normal force F2 in the direction of the guide rail  14  on the brake shoes  30  through the rollers  40 . 
     In an emergency situation wherein the application of the brake system  10  is desired, a force F1 in the direction of the elevator car is applied to the bottom of the brake shoes  30 , causing them to move towards the elevator car  12 . In one embodiment, this force may be applied by a rope, cable, or mechanical linkage connected to a governor (not shown). The inclined surfaces  28 ,  32  of the guide shoes  26  and the brake shoes  30  cause the brake shoes  30  to move in the direction of the rail until the brake pad  36  contacts the surface of the guide rail  14 . As those skilled in the art will appreciate, the brake pad  36  is applied to the rail  14  with a normal force F2 supplied by the spring  24 . The amount of braking force applied to the guide rail  14  by the brake shoe  30  is substantially and directly proportional to the friction coefficient between the high friction material used in the brake pad  36  and the material of the rail  14 . As braking occurs, heat generated in the brake pad  36  deleteriously affects the friction coefficient between the brake pad  36  and the rail  14 . Under excessive heat, a substantial reduction in hardness as well as deformation of the high friction material may occur, ultimately leading to brake failure. In previous applications, the brake pad  36  used in the brake system  10  to provide a friction surface has been formed from gray cast iron. Gray cast iron, while suitable for low speed, low load conditions, cannot operate as a consistent friction material at high speed and load conditions. In other applications, higher cost materials are used as inserts for elevator systems having a greater load and traveling at a greater speed. The above described elevator safety brake system  10  is exemplary and other safety brake systems are considered within the scope of the invention. In addition, the safety brake system  10  could additionally or alternatively be used on the counterweight of the elevator system. 
     Referring now to  FIG. 2 , an exemplary brake shoe  30  is illustrated. In one embodiment, at least a portion of a brake pad or friction material insert  36  attached to the surface  34  of the brake shoe  30  facing the rail  14  ( FIG. 1 ) includes a ceramic matrix composite (CMC). An exemplary CMC includes a polymer-based carbon-ceramic composite friction material, such as the Polymer-to-Ceramic Composite (PTCC)™ material manufactured by Starfire Systems for example. The coefficient of friction of a CMC is generally in the range of between about 0.15 and about 0.5. The friction properties of the CMC may be altered by adjusting the formulation of the matrix. In one embodiment the matrix phase of the CMC includes silicon carbide (SiC), silicon oxycarbide (SiOC), and/or carbon (C) based phases and/or the reinforcement phase of the CMC includes silicon carbide (SiC) and/or carbon (C). 
     The coefficient of friction of the CMC material is also affected by the orientation of the fibers, such as carbon fibers for example, within the matrix. Experimental results indicated that a non-linear relationship exists between the angle of the fiber orientation and the direction of sliding. Multiple friction mechanisms may exist, including adhesion, plastic deformation, plowing, and/or cutting, when the friction material insert  36  is in sliding engagement with rail  14 . The direction of the fibers will control how many of these sliding mechanisms occur, and the magnitude of these mechanisms. For example, when the fibers are parallel to the sliding direction, indicated by the direction of arrow A, only some of these friction mechanisms apply and the coefficient of friction is minimized. However, when the fibers are positioned perpendicular to the sliding direction, as illustrated in  FIG. 2 , more friction mechanisms exist and the coefficient of friction of the CMC is maximized An intermediate orientation (e.g. fibers arranged 30° to the sliding direction) has the same friction mechanisms as the perpendicular orientation, but provides a lower coefficient of friction than the perpendicular orientation (and greater than the parallel orientation). Therefore, if a larger coefficient of friction is desired, such as equal to or greater than 0.3 for example, the fibers should be arranged at an angle (up to perpendicular) to the sliding direction. In one embodiment, the orientation of the fibers in the matrix is controlled throughout the whole thickness of the CMC and not just at the surface that contacts the rail  14  to maintain a generally constant coefficient of friction and to ensure consistent wear. 
     The thickness of the friction material insert  36  can be constant and should be large enough to prevent deformation during operation of the safety brake system  10 . In one embodiment, the thickness of the friction material insert  36  is greater than or equal to about 0.25 inches. Referring now to  FIG. 3   a , the illustrated friction material insert  36  may consist of a single CMC insert  38 . Alternately, as shown in  FIGS. 3   b - 3   c , the friction material insert  36  may consist of any number of CMC inserts  38 . The plurality of CMC inserts  38  attached to the surface  34  of the brake shoe  30  may be identical, or alternately, each CMC insert may have a different length, width and thickness. The dimensions of each CMC insert  38  may be determined from the calculated contact pressure, contact area, contact shear stress and friction force. All CMC inserts  38  attached to a brake shoe  30  may have a uniform thickness to promote equal wear across the plurality of inserts. In one embodiment, the CMC inserts  38  cover the entire rail facing surface  34  of the brake shoe  30 . Alternatively, the CMC inserts  38  may cover only a portion of the surface  34  of the brake shoes  30  facing the rail  14 . The amount of the surface  34  that the CMC inserts  38  need to cover is dependent on the contact pressure between the CMC inserts  38  and the rail  14  and also the dissipated energy in the system. In one embodiment, the contact pressure is estimated as a function of the loss of speed and the trip speed of the elevator. 
     The CMC inserts  38  may be attached to the brake shoe  30  using known mechanical or chemical methods. For example, the inserts  38  may be attached to the brake shoe directly via a mechanical fastener  50  or alternatively may be attached to the brake shoe  30  by a braze material interface (not shown). Braze material which may be used in the present invention includes a low melting alloy containing  3  or more metals in powder or foil form. The braze alloy may be selected based on its ability to wet the CMC inserts  38  and the brake shoe  30  and its ability to withstand the operational conditions of the brake. Alternately, chemical methods such as adhesion may be used to attach the CMC inserts  38  and the brake shoe  30 . The adhesive may be a heat resistant rubber like material, such as heat resistant silicone for example. Because the CMC inserts are attached to the brake shoe  30  in a manner similar to conventional high friction materials, the CMC inserts  38  may be used in retrofit and modernization applications with pre-existing brake shoes  30 . 
     By using CMC inserts  38  on the surface  34  of the brake shoe  30  facing the rail  14 , the durability and performance of the safety system  10  is improved. CMC inserts have a high temperature stability and stable friction performance over a wide temperature range, making them better than conventional high friction materials for elevator applications having speeds of up to about 20 m/s. The CMC inserts can be customized to have a stable coefficient of friction by selecting proper materials and processing methods that additionally allow for quick dissipation of generated heat. By improving the efficiency of the safety brake system  10 , the brake shoes  30  can be reduced in size compared to current systems. In addition, by using CMC inserts  38 , the manufacturing and replacement costs of the safety brake system  10  are reduced. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.