CA2141720C

CA2141720C - Wheel tread conditioning brake shoe

Info

Publication number: CA2141720C
Application number: CA 2141720
Authority: CA
Inventors: Bruce W. Shute; Joseph F. Formolo
Original assignee: Westinghouse Air Brake Co
Current assignee: Westinghouse Air Brake Co
Priority date: 1995-01-10
Filing date: 1995-02-02
Publication date: 1999-08-31
Anticipated expiration: 2015-02-02
Also published as: CA2141720A1

Abstract

A brake shoe for engagement with the tread of a railway car wheel having a tread conditioning agent incorporated in the composition friction brake shoe material of the brake shoe block for altering the adhesion between the wheel and rail on which the railway car is guidably supported. The tread conditioning agent is a material that is dissimilar from the composition friction material of the brake shoe block, being chosen in accordance with a lubricating quality or an abrasive quality depending upon whether the brake shoe use is intended to reduce or increase wheel/rail adhesion. In either case, the chosen conditioning material has a wear rate that exceeds that of the composition friction material so that the degree of conditioning is controlled by the wear rate of the composition friction material and the percentage of brake surface of the brake shoe block occupied by the conditioning material.

Description

~ 2141720 WHEEL TREAD CONDITIONING BRAKE SHOE
Background of the Invention The present invention is related to brake shoes for railway cars and particularly to an improved railway car brake shoe that is adapted to influence the adhesion between the railway car wheel and the rail on which the wheel runs.
Railroad locomotives and cars have historically provided braking effort through wheel tread braking. Brake shoe engagement with the wheel tread produces friction that dissipates the energy of momentum in the form of heat. In order for such braking to be effective, wheel rotation is required. The adhesion due to the friction between the wheel tread and rail tends to maintain such wheel rotation, as the wheel is braked.
Excessive wheel wear and brake shoe changeouts have been found to be cyclically high during the winter season One theory attributes this, at least in part, to the more pronounced cleaning effect that the brake shoe has on the wheel as a result of the higher moisture contact during wintertime. In that such cleaning action tends to increase the contact friction or adhesion between the wheel and rail, increased contact stresses are created in the wheel, such contact stresses being particularly high during the guiding and steering action of the wheel set, particularly when encountering track curvature.
The combination of braking effort through friction between the brake shoe and wheel tread surface and the 214172~
adhesion between the wheel and rail translates into thermal and contact stresses of the wheel, which often result in wheel damage. High thermal and contact stresses are known to contribute to minute failure of the wheel tread due to the condition known as spalling.
Another related problem encountered by some railroads where high levels of retardation are required is wheel sliding, which results in wheel tread flat spots. Such flat spots have a deleterious effect on equipment, lading, and passengers due to the high vibration and noise produced by the flat spots. Repairing such a damaged wheel requires removal of the wheel and subsequent machining of the wheel tread to remove the flat spot.
Summar of the Invention y The object of the present invention, therefore, is to provide a friction composition brake shoe capable of altering the adhesion between the wheel of a railway car and the track rail without adversely affecting the wheel braking force;
Another object of the invention is to incorporate a conditioning agent in the brake shoe composition friction material to decrease the wheel/rail adhesion;
Another obj ect of the invention is to incorporate a conditioning agent in the brake shoe composition friction material to increase the wheel/rail adhesion;
Another object of the invention is control the rate of application of a brake shoe conditioninq agent to a railway 21~1720 car wheel tread.
Briefly, these objectives are achieved in a brake shoe comprising a brake block having a braking surface adapted to engage the tread of a railroad car wheel, the brake block being a molded friction composition material to which is added a different material that conditions the railroad car wheel tread in such sense as to affect the adhesion level between the car wheel and rail in response to braking engagement between the brake shoe and wheel tread.
Brief DescriPtion of the Drawinqs The foregoing objects and other objects and advantages of the present il~vention will be understood from the following more detailed explanation of the invention when taken in conjunction with the accompanying drawings in which:
Fig. 1 is a plan view showing the braking surface of a brake shoe block in accordance with one embodiment of the invention;
Fig. 2 is a section view taken along the lines 2-2 of 2 0 Fig . l;
Fig. 3 is a plan view showing the braking surface of brake shoe block in accordance with another embodiment of the invention; and Fig. 4 is a section view taken along the lines 4-4 of 2 ~ Fig . 3 .
Descri~tion and O~eration Referring now to Figs. 1 and 2, there is shown a brake 21~1720 shoe 10 comprlsing a backing plate 12 having a key bridge 14 for affixing the brake shoe to a bake head (not shown) by conventional means such as a brake shoe key (also not shown). Affixed to kacking plate 12 is a molded brake shoe block 16, which is preferably segmented so as to be separated along a centerline of the krake shoe bridge. The molded brake shoe block is formed from a composition friction material, such as is typically employed in railroad type brake shoes, and is preferably affixed to the backing plate by being bonded thereto during the process of molding the friction brake block.
Each segment of brake block 16 is formed with a recess 20 that is preferably cylindrical in shape and in which recess is received an insert 22 having a shape corres~Qnding generally to recess 20. Insert 22 is a material dissimilar from the composition friction material of brake block 16 and in this embodiment of the invention constitutes a lubricant, such as molybdenum disulfide.
Being normally available in powder form, the molybdenum disulfide is envisioned as being combined with a suitable resin, preferably in the proportion of about 95 percent to 5 percent, and hardened into a preformed insert 22 that corresponds to the shape of recess 20. In one aspect of the invention, insert 22 may be incorporated with brake block 16 during the aforementioned molding process, such that recess 20 is actually formed by and thus conforms to the shape of the insert, which is accordingly secured 21~1~20 therein. Other means of securing the insert 22, such as a press fit with a molded-in recess 20 for example, may also be viable.
In addition, such other methods of incorporating the molybdenum disulfide in brake block 16 include that of blending the powder-like lubricant with the composition friction material prior to molding, whereby a substantially homogenous dispersion of the molybdenum disulfide throughout the composition friction material is realized following molding of the brake block 16; and pelletizing the molybdenum disulfide and interspersing such pellets in the composition friction material prior to molding into brake block 16, such that a distinctly non-homogenous distribution of the lubricating material is realized when the molding process is complete.
In all cases, the rate of wear of the lubricating material is greater than that of the composition friction material, so that the latter controls the rate of application of the lubricating material to the wheel tread.
In addition, the total surface area of the lubricating inserts 22 is generally in the range of 5 to 15 percent of the surface area of brake shoe block 16, being chosen in accordance with their respective wear rates so that when brake shoe 10 is brought into braking engagement with the tread of a railroad car wheel, a light film of lubricant is deposited on the wheel tread. In thus applying such a desired film of lubricant, the friction level between the . ` 21~172Q
.
wheel tread and rail can be reduced sufficiently to allow for a partial slip condition to exist, particularly on track curvature, thereby lowering wheel/rail contact stresses. By controlling the rate of application of the lubricant, reduced wheel stress can be achieved, while still providing wheel/rail adhesion levels as reguired to support such brake force as necessary to achieve a desired rate of retardation.
Referring now to Figs. 3 and 4, there is shown a brake shoe 10 having a backing plate 12, a key bridge 14 and a molded composition friction brake block 16.
Each segment of brake block 16 is formed with a recess 21 that is preferably rectangular in shape and in which recess is received an insert 23 having a shape corresponding generally to recess 21. Insert 22 is a material dissimilar from the composition friction material of brake block 16 and in this embodiment of the invention constitutes an abrasive, such as cast iron. Of course other suitable abrasives are also within the scope of the invention. In one aspect of the invention, as shown in Fig. 3 and 4, the cast iron insert is incorporated with brake block 16 during the process of molding the composition friction brake block, such that recess 21 conforms to the shape of the insert, which is accordingly secured therein.
In addition, such other methods of incorporating the cast iron with the friction composition brake block 16 21~17 2D
include homogenous dispersion of a suitable abrasive and non-homogenous blending of the abrasive and friction composition material prior to molding thereof.
In all such cases, the rate of wear of the abrasive material i6 greater than that of the composition friction material. Rectangular inserts 23 traverse a major portion of the width of brake block 16, so that in response to engagement of brake block 16 with the wheel tread during a brake application, the wheel tread is abraded. Cast iron has been found to satisfactorily produce a desired micro level of wheel tread abrasion when having a surface area that is generally in the range of 5-25 percent of the surface area of the brake shoe block. Such conditioning of the wheel tread with successive brake applications establishes and maintains an optimum level of adhesion to prevent break-away of rolling contact between the wheel and rails. In this manner, not only is wheel rotation maintained to assure that vehicle retardation can be achieved through on-tread braking, but such wheel rotation decreases the occasion of slid flat spots on the wheel tread .
In accordance with both of the foregoing aspects of the invention, the wheel tread conditioning provides significantly increased wheel life and decreased maintenance costs, a reduced vibration environment for the rail car equipment, lading and passengers, and reduced noise levels.

Claims

1. A brake shoe adapted to engage the tread of a railroad car wheel via which the railroad car is guidably supported on a railed track, said brake shoe comprising:
a) a backing plate; and b) a brake lining affixed to said backing plate and having a braking surface engageable with said wheel tread, said brake lining comprising:
i) a first and a second segment disposed on opposite sides of a midpoint of a key bridge of said backing plate;
ii) a first material in the form of a friction composition suitable for braking said railroad car wheel, said first material formed on each of said first and said second segments and having at least one recess formed in said braking surface of said first material; and iii) a second material dissimilar from said first material, said second material disposed as an at least one discrete insert in said first material, said second material conditioning said railroad car wheel tread so as to affect adhesion between said wheel tread and said railed track, said second material having a wear rate dissimilar to that of said first material.

2. A brake shoe, according to claim 1, wherein said second material is a lubricant, said railroad car wheel tread being conditioned thereby to reduce the adhesion between said wheel tread and said railed track.

3. A brake shoe, according to claim 2, wherein said lubricant is molybdenum disulfide.

4. A brake shoe, according to claim 1, wherein said second material is an abrasive, said car wheel tread being conditioned thereby to increase the adhesion between said wheel tread and said railed track.

5. A brake shoe, according to claim 4, wherein said abrasive is cast iron.

6. A brake shoe, according to claim 1, wherein said second material is a solidified mixture of about 95%
molybdenum disulfide and about 5% resin.

7. A brake shoe, according to claim 6, wherein said solidified mixture corresponds in shape to said at least one recess.

8. A brake shoe, according to claim 1, wherein said at least one discrete insert in said first material is cylindrical in shape.

9. A brake shoe, according to claim 8, wherein the total area of said at least one discrete insert in said first material is in the range of between about 5% and about 15% of the total area of said braking surface.

10. A brake shoe, according to claim 8, wherein said cylindrically shaped said at least one discrete insert is molybdenum disulfide.

11. A brake shoe, according to claim 1, wherein said at least one discrete insert in said first material is rectangular in shape.

12. A brake shoe, according to claim 11, wherein the total area of said at least one discrete insert in said first material is in the range of between about 5% and about 25% of the total area of said braking surface.

13. A brake shoe, according to claim 11, wherein said rectangular shaped said at least one discrete insert is cast iron.

14. A brake shoe, according to claim 1, wherein said at least one discrete insert in said first material is preformed and said at least one recess of said first material having a shape conforming thereto so as to provide an interference fit therewith.

15. A brake shoe, according to claim 1, wherein said at least one discrete insert in said first material is two.

16. A brake shoe, according to claim 15, wherein said two inserts are disposed as a first insert in a first segment of said first material and a second insert disposed in said second segment of said first material.