Abstract:
A brake drum is disclosed which is configured for providing an external visual indication of braking surface diameter safe operating limits while installed on a vehicle and includes one or more indentations in its outer surface overlaying the area defined by the interior braking surface where the depth of an indentation is at least co-level with a prescribed safe operating diameter of the braking surface.

Description:
BACKGROUND 
     1. Field 
     The present invention relates generally to brake drums, and more particularly, to brake drums for heavy-duty vehicles having an external visual indication of whether braking surface diameter is within safe operating limits. 
     2. Description of the Problem and Related Art 
     A brake drum&#39;s maximum allowable braking surface diameter is the largest diameter that a brake drum&#39;s braking surface can be machined, or worn to, and still maintain satisfactory characteristics for safe and expected braking performance. It is a violation of Department of Transportation regulations to use a brake drum whose braking surface diameter exceeds the maximum allowable diameter. At this time, there is not an accurate method of determining whether a brake drum&#39;s braking surface diameter has worn to, or exceeded, the maximum allowable diameter while the brake drum is installed on a vehicle. Accordingly, the entire wheel must be removed from the vehicle and the brake drum surface diameter must be measured. It is both time consuming and expensive—not to mention safety issues involved with removing and reinstalling wheel end equipment—to remove a brake drum from an axle to determine whether or not a brake drum has worn to the point that the braking surface diameter exceeds the maximum allowable diameter. This can lead to brake drums being discarded and replaced sooner than needed, or it can lead to brake drums remaining in service after the braking surface diameter exceeds the maximum allowable diameter. 
     With reference to  FIGS. 7 and 7A  of the drawings, a prior art technique to permit visual inspection of brake surface diameter has used a notch  30  machined circumferentially around the braking surface at the lip of the drum for wear indication. This method, however, may provide misread inspections because of uneven wear of the brake linings and drum. A step is worn into the lining near the lip of the drum. This uneven wear leads to inaccurate evaluations of both lining wear and brake drum wear, leading to either early removal of a good brake drum, or worse, non-removal of a bad brake drum. 
     SUMMARY 
     The present disclosure is directed to a brake drum configured for accurate, visual indication of braking surface adequate thickness without removal from the vehicle on which it is installed. 
     For purposes of summarizing the invention, certain aspects, advantages, and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any one particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
     An exemplary brake drum includes one or more indentations in its outer surface overlaying the area defined by the interior braking surface where the depth of an indentation is at least co-level with a prescribed safe operating diameter of the braking surface. 
     This and other embodiments will also become readily apparent to those skilled in the art from the following detailed description of the embodiments having reference to the attached figures, the invention not being limited to any particular embodiment(s) disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
         FIG. 1  is an inboard perspective view of an exemplary brake drum with wear indicator in braking surface outer wall in an unworn condition; 
         FIG. 2  is inboard perspective view of an exemplary brake drum with wear indicator notches in braking surface outer wall at maximum braking surface diameter; 
         FIG. 3  is an outboard perspective view of an exemplary brake drum with wear indicator notches defined in the outer wall where the braking surface is in an unworn condition; 
         FIG. 4  is an outboard perspective view of a brake drum with wear indicator notches defined in the drum&#39;s outer wall where the braking surface is at maximum braking surface diameter; 
         FIG. 5  is a sectional view of a brake drum with wear indicator notch defined in the outer wall where the braking surface is in an unworn condition; 
         FIG. 6  is a sectional view of a brake drum with wear indicator notch in braking surface outer wall at maximum braking surface diameter; 
         FIGS. 7 , &amp;  7 A is a section view of a prior art brake drum with a circumferential notch showing potential uneven wear pattern of lining that can cause an inaccurate indication of braking surface wear; 
         FIG. 8  is a section view of a brake drum with a wear indicator notch defined in the outer surface of the drum at a depth greater that the level of the maximum braking surface diameter and the braking surface is in an unworn condition; and 
         FIG. 9  is a section view of the brake drum of  FIG. 8  where the braking surface is worn. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1 through 9  of the drawings. The elements of the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Throughout the drawings, like numerals are used for like and corresponding parts of the various drawings. 
     This invention may be provided in other specific forms and embodiments without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all aspects as illustrative only and not restrictive in any manner. The following claims rather than the foregoing description indicate the scope of the invention. 
     Furthermore, reference in the specification to “an embodiment,” “one embodiment,” “various embodiments,” or any variant thereof means that a particular feature or aspect of the invention described in conjunction with the particular embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment,” “in another embodiment,” or variations thereof in various places throughout the specification are not necessarily all referring to its respective embodiment. Finally, as used in this description, terms such as “inboard” and “outboard” are to be understood as they relate to a vehicle on which the brake drum is mounted. 
     An exemplary brake drum  10  comprises a brake surface  15  made from cast iron or other friction and wear resistant material, a mounting plate  14  at the brake drum&#39;s outboard end with mounting holes for mounting the drum to the hub of a vehicle axle, a wraparound, or shoulder portion  12 , and one or more squealer bands  13  circumscribing the brake drum. A brake drum  10  is typically used with a brake shoe  20 . In operation, the shoe  20  is selectively applied to the braking surface  15  of the brake drum  10  through actuation of the brake. Brake drum  10  converts kinetic energy and potential energy of the vehicle into heat energy by providing a friction surface for the brake shoe  20  thereby transmitting brake torque to the wheels and tires of the vehicle. 
     A notch  11 , or indentation, is defined in the outer surface of the drum  10 , preferably near the shoulder  13  area. The indentation  11  is formed with a depth d in relation to the outer surface of the drum  10  that terminates at or below within the maximum allowable diameter  22  of the braking surface  15 . The indentation  11  extends inboard from the shoulder  13  so that at least a portion of the area of the indentation  11  overlaps the area of the braking surface that makes contact with the brake shoe  20 . As the braking surface  15  is worn away through use, and the maximum diameter is reached, the area covering the indentation  11  is worn away, revealing an opening  16  that may be visually perceived in inspection. Since the indentation  11  is located on the shoulder  13  of the brake drum  10 , it may be inspected while the drum  10  is still mounted on the vehicle. Thus, if upon visual inspection, the interior of the drum  10  cannot be seen, the brake drum is within the maximum diameter. On the other hand, if the interior of the brake drum can be seen through the opening  16 , the braking surface is worn beyond the maximum allowable diameter and the drum  10  needs to be replaced. It is contemplated that a plurality of indentations  11  may be defined in the drum  10  as described above at different locations about the circumference of the drum surface, that are generally accessible while the drum is installed on the vehicle, to increase the possible inspection locations. 
     As an alternative, a deeper indentation  11 ′ may be manufactured as described above except to a depth d 1  coinciding with a braking surface diameter less than the maximum allowable braking surface  15  diameter. As such, when the braking surface has sufficiently worn away such that the interior is visually perceivable, it indicates that the braking surface  15  has worn close to, but has not yet reached, the maximum allowable diameter of the braking surface  15  of the brake drum  10 . This provides an additional margin of safety. In addition, the deeper indentation  11 ′ may be used in conjunction with the shallower indentation  11 . 
     Through the use of the aforementioned features, the inventive brake drum provides a number of advantages. Because braking surface diameter may be easily visually inspected, a determination can be made by shop personnel, the driver, or law enforcement personnel. The brake drum will not have to be removed from the axle and measured to determine whether the braking surface has reached the maximum allowable diameter. This reduces vehicle down time, labor, and maintenance costs attributable to inspection. Lastly, use of the windows  16  reduces likelihood of a brake drum remaining in service after the braking surface diameter has reached or exceeded the maximum allowable diameter. 
     As described above and shown in the associated drawings, the present invention comprises a heavy duty brake drum having a brake lining wear indicator. While particular embodiments of the invention have been described, it will be understood, however, that the invention is not limited thereto, since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is, therefore, contemplated by the appended claims to cover any such modifications that incorporate those features or those improvements that embody the spirit and scope of the present invention.