Abstract:
An elevator safety brake for stopping an elevator car is provided with a brake shoe having a molybdenum alloy friction surface for contacting an elevator guide rail surface to provide a stopping force. The molybdenum alloy contains 99.4 weight percent molybdenum, 0.5 weight percent titanium and 0.1 weight percent zirconium. The friction surface of the brake formed from the alloy exhibits a consistent high friction and low wear suitably accommodating high speed, high load elevators installed in very tall buildings.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a safety braking system for slowing or stopping a vertically moving object, such as an elevator car, in an over speed condition. More particularly, the present invention relates to an elevator safety brake system for slowing or stopping an elevator car having a molybdenum alloy friction surface. 
     2. Description of the Prior Art 
     A typical safety braking system is attached to an elevator car and comprises a pair of wedge shaped brake shoes having substantially flat frictional surfaces. The flat 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. 
     In a typical safety braking system, the wedges may be loaded with up to approximately 56,000 lb (250,000 N) normal force by applying approximately 8000 psi over a 7 in 2  surface (55,000 kPa×0.0045 m 2 )). Using cast iron frictional surfaces having a nominal coefficient of friction with respect to the guide rail at approximately 6 m/s of approximately 0.15, the 56,000 lb (250,000 N) force acting upon a wedge creates a frictional force of approximately (11,200 lb (50,000 N) on the frictional surface of the wedge. In a conventional elevator cab design using cast iron frictional surfaces, there are four frictional surfaces which generate a total potential stopping force of approximately 45,000 lb (200,000 N). 
     As very tall buildings are built, high speed, high load elevators (typically 4 to 8 ml/s but up to 12.5 m/s) have become necessary to service the numerous floors in such buildings. Such elevators have a load rating of up to about 16,000 kg. The safety breaking requirements of such elevators have become increasingly demanding. It has been determined that conventional gray cast iron cannot operate as a consistent friction material at high speeds and loads required by such modern elevator systems due to breaking failures caused by excessive wear and a reduced coefficient of friction caused by high frictional heating. Accordingly, there is a need for elevator safety brake shoes made with alternative friction materials which provide low wear and consistent high friction to accommodate the high speeds and loads of elevators installed in very tall buildings. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an elevator safety brake for stopping an elevator car. 
     It is another object of the present invention to provide a reliable elevator safety brake having a consistent high coefficient of friction and low wear for use in high speed, high load elevators. 
     These objects are accomplished, at least in part, by an elevator safety brake having a brake shoe formed from a base and a friction surface attached to the base for contacting an elevator guide rail surface. At least a portion of the friction surface comprises an alloy material formed from approximately 99.4 weight percent molybdenum, 0.5 weight percent titanium and 0.1 weight percent zirconium. The safety brake is provided with an actuator for pressing the friction material of the brake shoe against the guide rail surface to stop the elevator car. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings, not drawn to scale, include: 
     FIG. 1, which is a simple schematic illustration of an elevator safety brake system with two friction wedges positioned on opposite sides of a guide rail; 
     FIG. 2, which is a simple schematic illustration of an elevator safety brake having a molybdenum alloy friction plate applied to the rail facing surface of the shoe base; 
     FIG. 3, which is a simple schematic illustration of the embodiment of FIG. 2 further including a cross hatch pattern machined therein; 
     FIG. 4, which is a simple schematic illustration of an alternative embodiment of the present invention showing a plurality of molybdenum alloy friction tiles attached to the rail facing surface of the shoe base; 
     FIG. 5, which is a simple cross-sectional view of the embodiment illustrated in FIG. 4, taken along the line  5 — 5  to illustrate the attachment of the friction tiles via a compliant material; 
     FIG. 6, which is a simple schematic illustration in which the molybdenum alloy friction material is attached to the base of the brake shoe in the form of a plurality of pins; 
     FIG. 7, which is a simple cutaway schematic illustration in which the molybdenum alloy friction material is attached to the base of the brake shoe in the form of a plurality of buttons held by a plate; and 
     FIG. 8, which is a simple isometric view of a button. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 provides a simplified schematic illustration of a known elevator safety brake system upon which the present invention may be used. The brake system  10  comprises a pair of actuators  12  which are attached to an elevator car  14  and positioned in an opposing relationship about a guide rail  16  supported in an elevator hoistway (not shown). The actuators  12  are formed, in part, by a wedge shaped guide shoe  18  which is movable within housing  20  in a direction which is generally perpendicular to the guide rail  16 . The guide shoe  18  is biased towards the guide rail  16  by spring  22 . The guide shoe  18  has an inclined support surface  24 . A wedge shaped brake shoe  25  having base  26  is provided with an inclined guiding surface  28  which is complimentary to the inclined support surface  24  of the guide shoe  18 . The brake shoe  25  is also provided with a rail facing surface  30 . The brake shoe  25  is positioned between the guide shoe  18  and the rail  16 . A brake pad  32  having a high friction material is attached to the rail facing surface  30  of the brake shoe base  26 . A roller cage assembly containing a plurality of rollers  34  is positioned between the inclined support surface  24  of the guide shoe  18  and the complimentary inclined guide shoe facing surface  28  of the brake shoe  25 . The rollers  34  provide a low friction contact between the complimentary inclined adjacent surfaces  24  and  28  of the guide shoe  18  and the brake shoe  25 , respectively. The guide shoe  18 , biased by spring  22 , applies normal force F N  in the direction of the rail  16  on brake shoe  25  through rollers  34 . 
     In an emergency situation wherein the application of the brake system  10  is desired, a force F A  in the direction of the elevator car  14  is applied to the base  26  of the wedge shaped brake shoes  25  which causes the shoes  25  to move towards the elevator car  14 . Ordinarily, force F A  is supplied by a rope, cable or mechanical linkage connected to a governor (not shown). The inclined complimentary surfaces  24  and  28  of the guide shoe  18  and the brake shoe base  26 , respectively, cause the brake shoe  25  to move towards the rail  16  until contact between the pad  32  and the rail  16  is made. As those skilled in the art will appreciate, the pad  32  is applied to the rail  16  with normal force F N  supplied by the spring  22 . The amount of braking force developed by normal force F N  is substantially and directly proportional to the friction coefficient μ k  between the high friction material used in the brake pad  32  and the rail material  16 . As braking occurs, heat tends to become accumulated in the brake pad  32  which can deleteriously alter the friction coefficient μ k  between the pad material and rail material. If the heat becomes high enough for a given material, a substantial reduction in the hardness, as well as deformation or fusion of the high friction material may occur, which in turn may cause brake failure. 
     In the prior art, the brake pad  32  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 view of the short comings of gray cast iron in such applications, it has been found that the gray cast iron material used as the high friction material in pads  32  may be replaced with a molybdenum alloy material. A brake pad having a molybdenum alloy material according to the present invention, which will be described more fully below, is capable of operation under the conditions required for an elevator operating at contract speeds of up to 10 meters per second with a load rating of up to 16000 Kg. It has been further found that the pads made in accordance with the present invention have significant mechanical toughness, thermal shock resistance, negligible wear rates on rail steel and appreciable coefficient of friction on rail steel. 
     A 30 mm diameter plate formed from TZM molybdenum alloy No. 364 conforming to ASTM standard B 387-90 containing approximately 99.4 weight percent molybdenum, 0.5 weight percent titanium and 0.1 weight percent zirconium was attached to a steel substrate to form a 30 mm diameter test tile and the edges of the molybdenum alloy material were provided with a chamfer. The test tile was loaded with a normal force of 11,000 N against a rotating 2 meter diameter disk under conditions which were selected to simulate an emergency stop on a typical steel hoistway guide rail surface under high load and high velocity conditions. A frictional force of nearly 6,000 N was generated which indicates that the material had a nominal coefficient of friction with the rail steel of about 0.40 which is about 1.5 times that of typical gray cast iron grade 30. The TZM molybdenum alloy tile showed very little wear, about  1  percent of the wear exhibited by the typical gray cast iron grade 30. The rail damage caused by the molybdenum alloy was equivalent to the damage caused by grade 30 cast iron. The material performed adequately under all rail conditions simulated (clean rail, rusted rail, oiled rail, wet rail and roughened rail). 
     As illustrated in FIG. 2., the friction surface of the brake may be in the form of a plate  34  which is attached to the base  26 . As illustrated in FIG. 3, a cross hatch pattern  36  may be machined therein to provide a topography to the friction surface. Referring to FIGS. 4 and 5, alternatively, the friction surface may be formed from a plurality of tiles  38  of the TZM alloy material. The tiles  38  may be attached to the base  26  directly via mechanical fasteners (not shown) or alternatively, the tiles may be attached to the base  26  via a compliant material interface  40  which enables resilient movement of the alloy friction material relative to the base  26  and allows a greater number of tiles to make contact with the rail surface in the event that the rail facing surface of the brake shoe base  26  is not perfectly true with the rail  16 . Compliant material which may be used in the present invention includes heat resistant rubber like material such as heat resistant silcone. Like the single plate  34 , one or more of the tiles  40  may be provided with a cross hatch pattern machined therein (not shown). 
     Referring to FIG. 6, the friction surface may also be formed from a plurality of pins  42  of the alloy material which are positioned so as to be generally transverse to the direction of relative movement between the friction surface and the guide rail surface. In most cases, these pins  42  are oriented horizontally in a elevator system, however, they need not be. The pins  42  may be attached to the base  26  via a compliant material interface in the manner shown for attachment of the tiles  38 . The compliant material interface enables resilient movement of the pins  42  relative to the base  26 . 
     Referring to FIGS. 7 and 8, in still yet another embodiment, the friction surface may be formed from a plurality of buttons  46  of the alloy material having a head  48  and a stem  50 . Each of the heads  48  of the plurality of buttons  46  are held adjacent to the base  26  by a fastening plate  52  having a plurality of openings  54  wherein the plurality of stems  50  project therethrough. 
     As will be understood from the foregoing description, according to the present invention, several embodiments of a safety brake system for stopping an elevator have been described. The molybdenum alloy employed therein provides a high coefficient of friction which is advantageous in that lower normal forces and smaller, lighter springs and safeties can be employed. It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications may be made thereto by persons skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof. Hence, the present invention is deemed limited only by the appended claims and the reasonable interpretation thereof.