AUTOMATIC BRAKE ADJUSTING MECHANISM

Abstract: The invention relates to an automatic adjusting mechan­ ism for a wedge actuated vehicle brake which includes an extendible plunger assembly (25) having an adjusting bolt (54) threaded internally of a sleeve (50) having one end received within a plunger (48) and includes means for piloting the adjusting sleeve (50) internally of the plunger (48).

Description: AUTOMATIC BRAKE ADJUSTING MECHANISM Background of the Invention The present invention relates to automotive vehicle brakes and more specifically., to an automatic brake adjusting mechanism for a wedge actuated brake for heavy duty service on trucks and coaches. In wedge actuated brakes the brake shoes are moved outwardly into engagement with the friction surface of a surrounding brake drum by forcing a wedge between the adjacent ends of a pair of axially aligned plungers disposed between the ends of a pair of brake shoes. The wedge is mounted in such a manner as to permit it to float slightly with its position between the plungers being partly determined by reaction forces exerted on the wedge by the plungers. Examples of such brake mechanisms are illustrated in United States Patent 3;037,584 issued June 5, 1962, to F. T. Cox et al and United States Patent 3,599,762 issued August 17, 1971, to F. T. Cox et al. Such brake mechanisms are manufactured in sizes ranging from 15" to 42" in diameter with the larger sizes used primarily for off-highway vehicles. These brakes are subject to tremendous loads due to the size of the vehicles and the loads they carry. This results in relatively rapid wear of the brake shoe linings which may be further compounded by frequent stopping, particularly on other than level terrain as encountered in off-highway and intra-city service. It is therefore customary to equip such brakes with a mechanism that will automatically adjust the brake actuator to accommodate for brake lining wear and to maintain the travel of the actuator plungers and their associated brake shoes relatively constant throughout the normal life of the brake shoe linings. An example of one such automatic adjusting mechanism is described in United States Patent 3,068,964 issued December 18, 1962, to. W. J. Williams et al. In such adjusting mechanisms, one of the plungers is comprised of an assembly of parts in which the length of the plunger assembly is automatically increased or extended to compensate for a reduction in the brake shoe lining thickness as the friction lining wears during normal service. Improvements to that type of automatic adjusting mechanism are dislcosed in United States Patent 3,570,632 issued March 16, 1971, Patent 3,797,613 issued March 19, 1974, and Patent 3,822,003 issued July 2, 1974. Although such automatic adjusting mechanism provides satisfactory and reliable service, it has been determined that such service and reliability may be even further enhanced by preventing substantial axial misalignment of the component parts of the adjusting plunger assembly. Summary of the Invention - The present invention provides an improved self-adjusting mechanism for a wedge type brake actuator which includes an extendible plunger assembly having an adjusting bolt threaded internally of a sleeve having one end slidably and rotatably received within a plunger mounted for rectilinear movement in a- cylindrical bore provided to the brake actuator housing and means are provided to pilot the adjusting sleeve internally of the plunger to prevent substantial axial misalignment of the adjusting bolt and sleeve relative to the plunger. In the preferred embodiment, a first pair of axially spaced, cylindrical bearing surfaces are provided to the sleeve and are radially and slidably received by a second pair of axially spaced, cylindrical bearing surfaces provided internally of the plunger. The advantages offered by the present invention will become apparent from the following description of the embodiment shown in the accompanying drawings. Description of the Drawings In the drawings, wherein like reference numerals refer to like parts: Figure 1 is a fragmentary side elevation showing the upper part of a brake assembly incorporating the present invention; Figure 2 is a section taken along line 2-2 of Figure 1; and Figure 3 is an enlarged view, partly in section, of the axially extendible plunger assembly of Figure 2. Description of the Preferred Embodiment With reference to the drawings, Figure 1 shows a brake assembly comprising a brake drum 10, a pair of brake shoes 12 and 14 and a wedge type brake actuator 20. The brake drum is mounted by conventional means on a ground engaging vehicle wheel (not shown). The actuator 20 is secured by bolts to a support usually referred to as a spider 17 fixed to the vehicle axle beam or housing (not shown). Brake shoes 12 and 14 are supported for movement relative to the spider 17. As shewn by Figure 1, the adjacent ends of the webs 18 and 19 of brake shoes 12 and 14 are urged into engagement with the opposite ends of the actuator 20 by means of a return spring 13. In the preferred embodiment, the opposite ends of the brake shoe webs 18 and 19 are similarly urged into engagement with the opposite ends of an actuator by means of still another return spring as disclosed in United States Patent 3,037,584. The brake shoes 12 and 15 are respectively provided with friction linings 15 and 16 adapted to engage the internal friction surface 11 of brake drum 10 upon outward movement of the brake shoes. Figure 2 illustrates details of the wedge type actuator 20 which is comprised of a housing 21 formed with opposed similar coaxial cylindrical end bores 22 and 24 which respectively slidably mount a plunger assembly 25 and a plunger 26 for rectilinear movement therein. One side of housing 21 is provided with an opening 28 coaxial with a hollow boss 29 internally threaded at 30 to mount a tubular support 31 of a fluid motor (not shown) having a reciprocating rod 32 carrying a wedge 34 disposed between rollers 35 and 36 located for rolling movement within the grooved inner ends 38 and 39, respectively, of the plunger assembly 25 and the plunger 26. Rollers 35 and 36 are preferably loosely mounted on the rod 32 by a carrier 40 and, when rod 32 is moved further into the housing 21 to actuate the brake, the rollers 35 and 36 respectively displace the plunger assembly 25 and the plunger 26 outwardly relative to the housing bores 22 and 24 against the force of the return spring 13 to move the linings of the brake shoes 12 and 14 into frictional contact with the internal surface 11 of the brake drum. When the brake is disengaged, the return spring 13, acting through brake shoe webs 18 and 19, moves the plunger assembly 25 and the plunger 26 into the housing bores 22 and 24 as the wedge 34 is retracted with the operative elements of the actuator 20 returning to their respective positions as shown in Figure 2. When the brake is disengaged, the plunger assembly 25 and plunger 26 are each seated against a shoulder provided at the inner end of the housing bores 22 and 24. Plunger 26 has a side slot 41 coacting with the inner end of a cap screw 42 which prevents rotation of the plunger 26 in bore 24. A flexible seal unit 44 is pressed fit to a counter bore at the outer end of bore 24 to prevent the entry of dirt and moisture between the plunger 26 and bore 24. The outer end of plunger 26 is recessed or grooved at 45 to slidably receive the end of the brake shoe web 19. Plunger assembly 25 is comprised of several elements which are movable relative to each other for the purpose of adjusting or extending the length of the plunger assembly to thereby maintain the desired running clearance between the brake shoe friction linings and the brake drum and optimum operation of the brake by assuring relatively constant travel of the wedge 34, and the plunger assembly and the plunger 26 within the actuator housing 21. Plunger assembly 25 is thus comprised of an outer member or plunger 48 slidable in housing bore 22 and having an internal cylindrical bore 49 slidably rotatably mounting an inner sleeve 50 sometimes referred to as an adjusting nut. The inner sleeve 50 is formed with an internally threaded bore .51 which receives the threaded shank 52 of an adjusting bolt 54. The outer end of the adjusting bolt 54 comprises an enlarged flange 58 having a toothed periphery 59 to provide a star wheel. A brake shoe retainer 60 having a slot 61, is mounted to the adjusting bolt flange 58 and resiliently held in place by a U-shaped arms 62. The arcuate end of the brake shoe web 18 extends through the slot 61 of retainer 60 and bears against the axially outer end face of the adjusting bolt 54. The slot 61 in the retainer 60 prevents lateral misalignment of the end of brake shoe web 18 and the adjusting bolt 54 while the resilient U-shaped arms 6.2 each having a detent between adjacent teeth in the toothed periphery 59 of the adjusting bolt flange 58 permit the flange 58 and adjusting bolt 54 to be rotated relative to the retainer 60 and the brake shoe web 18 thereby allowing for manual adjustment by rotating the flange 58 to rotate the adjusting bolt 54 internally of the sleeve 50 until the desired running clearance is obtained. A bellows type expanding flexible seal 64 having its outer end enclosed within a metal case 65 seated in a counter bore provided around the housing bore 22 and its inner periphery snug against the adjusting bolt between the flange 58 and the threaded portion of the shank 52 prevents dirt, moisture and other materials from entering the housing bore 22. The outer periphery of sleeve 50 is formed with a plurality of helical teeth 68. A pawl assembly 70 is provided internally of a bore 71 through actuator housing 21. The bore 71 is substantially normal to the axis of the cylindrical bore 22 and the inner end of bore 71 intersects cylindrical bore 22. The pawl assembly is comprised of a pawl 74, a compression spring 75 and a hollow cap screw 76 threaded to the outer end of the bore 71. The pawl 74 extends through a side slot 78 provided to plunger 48. The inner end of the pawl 74 is provided with a plurality of buttress teeth 79 which are biased into engagement with the helical teeth 68 provided to the external surface of sleeve 50. The pawl 74 interacts with the side slot 78 of plunger 48 to prevent rotation of the plunger 48 in housing bore 22. The external surface of the sleeve 50 is also provided with a pair of axially spaced, cylindrical bearing surfaces 80 and 81 with each bearing surface being located adjacent one of the axially spaced ends of the helical teeth 68. The interior of the plunger 48 is provided with a second pair of axially spaced cylindrical bearing surfaces adapted to slidably receive and radially support the bearing surfaces 80 and 81. The outer end of the bore 49 of plunger 48 is machined to slidably receive and provide a radial bearing surface for the bearing surface 80 of sleeve 50. The axially inner end of the bore 49 of plunger 48 is machined to provide a cylindrical bearing surface 86 of smaller diameter which slidably receives and radially supports the bearing surface 81 provided to the sleeve 50. The cylindrical bearing surfaces 80, 81 are thus respectively radially and slidably received by the axially outer end of the bore 49 of plunger 48 and the cylindrical bearing surface 86 of plunger 48. In the preferred embodiument, the sleeve bearing surfaces 80 and 81 are about 43mm in axial length and are machined to a diameter of about 30.4mm and 28.2mien, respectively. The plunger bearing surface 86 is about 3.6mm in axial length and the surfaces 49 and 86 are respectively machined to provide a radial clearance of about .1 to .15mm, thereby permitting a thin film of lubricant between the bearing surfaces. During normal operation of the brake, the plunger assembly 25 including the sleeve 50 is displaced axially outwardly in housing bore 22 by the wedge 34 and roller 35. The pawl 74 in bore 71 is in a fixed axial location relative to housing bore 22 If the axially outward movement of the sleeve 50 is greater than the corresponding axial length of the helical teeth 68, the pawl 74 will compress spring 75 and the buttress teeth 79 on the pawl 74 will skip to an adjacent set of helical teeth 68. Then, when the brake is released, the buttress teeth 79 will rotate the helical teeth 68 and sleeve 50 to extend the adjusting bolt 54 which is threaded internally of the sleeve 50 and thereby extend the effective length of the plunger assembly 25 to maintain the desired clearance between the friction linings on the brake shoes and the friction surface of the brake drum. During return movement of the plunger assembly 25, the sleeve 50 is free to rotate on the rotary bearing surfaces 80 and 81 internally of the plunger 48 while the retainer 60 prevents rotation of the adjusting bolt 54. Detents in the resilient legs 62 of the retainer 60 are seated between adjacent teeth in the adjusting bolt flange 58 and the end of the brake shoe web 18 extends through the retainer slot 61 to prevent rotation of the retainer 60. The axially spaced, cylindrical bearing surfaces 80 and 81 provide radial support to the sleeve 50 internally of the plunger at all times during actuation of the brake and prevent substantial axail misalignment of the adjusting bolt and sleeve relative to the plunger. This insures against misalignment of the sleeve internally of the plunger which could damage the helical teeth 68 provided to the exterior the sleeve 50 or prevent automatic adjustment of the plunger assembly 25 under extreme service conditions. The invention may also be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing description is therefote to be considered as illustrative and not restrictive, the scope of the invention being defined by the appended claims and all changes which come within the.meaning and range of equivalency of the claims are therefore to be intended to be embraced thereby.

Claims: What is claimed and desired to be secured by letters patent is: 1. In a wedge actuated brake assembly having a movable brake shoe with friction lining adapted to engage a surrounding brake drum, a self-adjusting brake actuating mechanism comprising a housing having a cylindrical bore therein, a plunger assembly including an adjusting bolt threaded internally of a sleeve having one end slidably and rotatably received within a plunger mounted for rectilinear movement in said cylindrical bore, said sleeve having a plurality of helical teeth on its external surface, a toothed pawl carried by said housing in fixed axial location relative to said cylindrical bore, spring means urging said toothed pawl radially into engagement with said helical teeth, said plunger assembly being movable by said wedge to move said brake shoe toward said drum and being axially extendible by rotation of said sleeve relative to said adjusting bolt to accommodate wear of the friction lining of said brake shoe, and means piloting said sleeve internally of said plunger to prevent substantial axial misalignment of said adjusting bolt and sleeve relative to said plunger. 2. The wedge actuated brake assembly defined by claim 1 wherein said means piloting said sleeve internally of said plunger includes an annular bearing surface adjacent said one end of said sleeve. 3. The wedge actuated brake assembly defined by claim 1 wherein said means piloting said sleeve internally of said plunger includes a pair of axially spaced, cylindrical bearing surfaces on said sleeve with one of said bearing surfaces adjacent said one end of said sleeve. 4. In an automatic adjusting mechanism for a wedge actuated brake assembly of the type which includes an axially extendible plunger assembly having an adjusting bolt threaded internally of a sleeve having one end slidably and rotatably received within a plunger mounted for rectilinear movement in a cylindrical bore, the improvement comprising a first pair of axially spaced cylindrical bearing surfaces extending circumferentially around said sleeve and second pair of axial spaced cylindrical bearing surfaces internally of plunger, said second pair of axially spaced cylindrical bearing surfaces being adapted to slidably receive said first pair of cylindrical bearing surfaces to pilot said sleeve and maintain substantial axial alignment of said adjusting bolt and said sleeve internally of said plunger.

Inventors: CREWSON, GARY EDWARD
Applicants: ROCKWELL INTERNATIONAL CORPORATION
Application year: 1983
Main classification: F16D65
IPC codes: F16D65