DRUM BRAKE ADJUSTERS

Abstract: A self-contained drum brake adjuster mechanism utilises an escapement drive arrangement (90,96) to provide adjustment. A bi-metal spring (78), used in the normal adjustment operation, contracts at higher brake temperatures to prevent over-adjustment of the brake when the brake drum (l2) is expanded excessively due to heat generated by heavy brake applications.

Description: This invention relates to adjuster mechanisms for use in drum brakes. More particularly, the invention is concerned with a mechanism of the expandable-strut type using a screw-nut adjusting arrangement, with all the actuating mechanism being self-contained as a part of the expandable strut. Such adjuster mechanisms in conformity with the invention are particularly useful in leading-trailing drum brake arrangements, but their application is not limited to such arrangements. A typical drum brake adjuster in common use in production motor vehicles uses an expandable strut with an adjuster screw-nut or star wheel rotated thereon to move on threads to increase the effective strut length to adjust the brake shoes as needed. The screw-nut or star wheel is actuated by a pawl formed as a part of a pawl lever. The pawl lever is usually pivoted on the web of one of the brake shoes or some other brake member such as a wheel cylinder housing, a brake shoe anchor or a backing plate. The pawl lever is commonly spring-loaded for operation. One example of such an adjuster is shown in US-A-3 963 l00. This expandable strut adjuster has an operating arm for mechanical operation pivoted to one of the shoes. The strut extends between the shoes and includes a ratchet formed on the outer periphery of a nut threaded on a portion of the adjustable-length strut. A pawl lever has one arm carrying a pawl which co-operates with a ratchet, a second arm engaging one end of the strut, and biasing means urging the second arm into permanent engagement with the end of the strut to urge it axially into engagement with the operating arm. The brake assembly in which the adjuster is installed has a parking brake lever or operating arm pivoted on the web of the other shoe, and therefore at the opposite end of the expandable strut from the pawl lever. This arrangement requires the assembly of several parts on a brake shoe, as well as mirror-image parts for left-hand and right-hand drum brakes. Adjusters of the type shown in the said US-A-3 963 l00 have sometimes been provided with a thermal expansion spring element comprising two leaves joined at a hinge-like section, the two leaves having aligned openings through which the threaded part of the adjuster is received so that the leaves are positioned between the tubular part and the adjuster nut. When this element is exposed to high brake temperatures, the spring element normally expands so that the leaves move apart, so increasing the space between the tubular part and the adjuster nut and overcoming the force of the pawl lever spring in so doing, and thereby causing the strut to increase in effective length and act as if the brakes have not been expanded as much as is actually the case. This action therefore prevents the adjuster mechanism from adjusting for hot brake drum expansion by thermally expanding the length of the adjustable strut under these conditions. Also, the location of the thermo-sensitive element in the area of the strut is such that if the brake is applied heavily, and rapid and heavy braking action is taking place, the brake heat may build up so fast that the thermo-sensitive element does not have time to be responsive to the heat build-up, and the brake may adjust irrespective of whether adjustment is in fact required. As or immediately after the brake is released, the thermo-sensitive element will become responsive to the high temperature and will then expand so as to further expand the strut. This can cause the brake shoes to drag, since the expansion of the strut is in effect a brake-actuation type of movement. This can in turn cause more brake lining wear than would otherwise be the case, and can also generate brake heat which may even be sufficient to maintain the thermo-sensitive element in the expanded position so that the brakes continue to drag. The arrangement can therefore defeat the very purpose for which it is intended. In the structures embodying the invention, the thermo-sensitive element or elements are not thermally loaded in a direction which will force any additional drag on the brake shoes whenever the high temperatures resulting from excessive brake heat are sensed. Even if the elements do not immediately sense the heat because of a fast rise in brake temperature due to fast and heavy brake actuation, the elements will not cause the shoes to be further expanded in addition to effecting brake adjustment. Therefore the elements will not permit the brake system to get into a condition which is degenerative because of the action of the thermo-sensitive element under the adverse conditions described above. Expandable strut-adjusting mechanisms of the type used in duo-servo brakes are typically shown in US-A-3 050 l56 and US-A-3 l03 992, these adjusters also being actuated by pawls which are pivoted on a brake shoe web. US-A-2 570 398 discloses a leading-trailing shoe adjusting mechanism using a star wheel to expand a strut. When adjustment is required, a pawl mounted on a brake shoe web is rotated against a pawl spring to advance the pawl to the next tooth on the star wheel. When the brake is released, the pawl spring moves the pawl in the other direction and rotates the wheel through an angle corresponding to the one tooth. In the mechanism disclosed in the said patent, the pawl is part of an arm which comprises a bi-metal blade. If the brake becomes hot from prolonged application, the bi-metal blade will bend away from the wheel and the pawl will not contact a new tooth even if the brake clearance is greater than normal. By the present invention there is provided a drum brake adjuster having the features specified in claim l hereof. Preferred embodiments of drum brake adjuster mechanisms in accordance with the present invention include a self-contained expansion strut in which the left-hand and right-hand parts are the same. This therefore permits adjusters to be manufactured for the brakes for the left-hand side and the right-hand side of a vehicle without any changes in construction or numbers of parts, for example. The present mechanisms utilise an escapement drive which may readily provide fine or close adjustment. The mechanisms may include a spring which is sensitive to the higher temperatures occurring in the drum brake with hard braking, and operates to disable the adjuster while such higher temperatures exist. This prevents the adjuster mechanism from over-adjusting as a result of heat expansion of the brake drum increasing the distance required for the shoes to move to engage the drum and therefore resulting in a signal being sent to the adjuster mechanism indicating that adjustment is required when in fact no adjustment may be needed after the brake has cooled off. If adjustment is accomplished while the brake is extremely hot, the amount of adjustment may be sufficiently great to leave the brakes locked up after the drum cools and contracts, because it then contracts to an extent such that it engages the brake shoes in braking relation, even with the brakes in the released condition. In one embodiment of a drum brake adjuster in accordance with the present invention, the escapement mechanism used is of the anchor or crown-wheel type. In another embodiment, the escapement mechanism is of the sliding-frame type. Either mechanism uses the escapement pallets as drive members which drive the escape-wheel. The escape-wheel is formed as part of the adjusting nut. Use of either embodiment permits the adjuster mechanism to be tailored to the particular brakes involved by replacing the escapement mechanism providing one adjustment level with an escapement mechanism providing a different adjustment level. By way of example, it is generally accepted that a travel of 0.76 mm (0.030 inch) is necessary for a normal effort stroke or apply stroke by the wheel cylinder. In the typical wheel cylinder with opposed pistons each driving a plunger connected to one of the brake shoes, this would be assumed to be 0.38 mm (0.0l5 inch) per piston. In any event, a strut in conformity with the present invention will accommodate the whole of the travel of the pistons without any need to take account of different individual piston travels. Any additional distance over the basic 0.76 mm (0.030 inch) apply stroke would be the added gap to provide the established desired clearance. Fine or close adjustment to a level of 0.38 mm (0.0l5 inch) centre line clearance is easily accomplished with double-acting escapement mechanisms in conformity with the invention as disclosed herein. In the drawings: Figure l is an elevational view, having parts broken away and in section, of a drum brake assembly including a first embodiment of a drum brake adjuster in accordance with the present invention, the drum brake assembly being shown in the brake-released position; Figure 2 is an enlarged view of a portion of Figure l, likewise with parts broken away and in section, but illustrating the brake assembly of Figure l in the brake-apply position; Figure 3 is a fragmentary cross-sectional view, having parts broken away, of a part of the brake adjuster mechanism of Figure 2, in the direction of arrows 3--3 of that Figure; Figure 4 is a perspective view, with parts broken away and in section, of escapement mechanism used in the brake adjuster mechanism of Figures l and 2, in the direction of arrows 4--4 of Figure 2; Figure 5 is a view similar to Figure l, with parts broken away and in section, but showing a second embodiment of a drum brake adjuster in accordance with the present invention, the drum brake assembly being shown in the brake-released position; Figure 6 is a fragmentary cross-sectional view, in the direction of arrows 6--6 of Figure l, illustrating a part of escapement mechanism in conformity with the invention; Figure 7 is an elevational view, having parts broken away and in section, of the escapement mechanism in the direction of arrows 7--7 of Figure 6; Figure 8 is a perspective view, with parts broken away and in section, of a major portion of the brake adjuster mechanism of Figure 5; Figure 9 is another view similar to Figure l, with parts broken away, showing another embodiment of the drum brake adjuster in accordance with the invention, the drum brake assembly being shown in the brake-release position; Figure l0 is a fragmentary cross-sectional view in the direction of arrows l0--l0 of Figure 9, and illustrating the brake adjuster mechanism of that Figure; Figure ll is a fragmentary cross-sectional view in the direction of arrows ll--ll of Figure 9 and illustrating a portion of the brake adjuster mechanism of that Figure; and Figure l2 is an enlarged view of a portion of the brake adjuster mechanism shown in Figure l0 but showing the mechanism with the strut expanded to provide adjustment action. The drum brake assembly l0 illustrated in Figures l, 2, 5 and 9 includes a drum l2 to be braked, a wheel cylinder l4, and brake shoes l6 and l8 mounted on a backing plate 20. The wheel cylinder l4 is of the typical opposed-piston type, with the pistons therein driving opposed plungers 22 and 24. The plunger 22 engages the shoe end 26 of the brake shoe l6, and the plunger 24 engages the shoe end 28 of the brake shoe l8. The drum brake assembly illustrated is preferably of the leading-trailing type in which the shoe l6 is the leading shoe and the shoe l8 is the trailing shoe. The ends of the shoes opposite the shoe ends 26 and 28 are anchored to the backing plate in a manner well known in the art and not shown in the drawings. A strut 30, l30 or 330 engages and extends between the brake shoes l6 and l8. The strut 30 is used to mechanically apply the brakes and includes an adjuster mechanism for adjusting the released position of the brake shoes, established by the minimum effective length of the strut. A retraction spring 32 has one spring end 34 secured to the shoe l6 and the other spring end 36 secured to the shoe l8. A parking brake actuating lever 38 is pivotally secured at 40 to the shoe web 42 of the shoe l8 and is also positioned in engageable relation with one end of the brake adjuster mechanism 30, l30 or 330. As is well known in the art, clockwise movement of the lever 38 about its pivot 40, as viewed in Figures l and 2, will result in mechanical force being exerted through the strut 30, l30 or 330 to expand the shoes l6 and l8 until the shoe linings 44 and 46 engage the friction braking surface 48 provided on the interior of the brake drum l2. The strut 30 of Figures l and 2 has a brake adjuster mechanism 50 formed as a part thereof. The strut assembly includes axially aligned strut sections 52 and 54. The strut section 52 has one end 56 which is notched in the usual manner and fits within a notch in the shoe web 58 of the brake shoe l6, as is well known in the art, so as to engage that shoe in force-transmitting relation. The other end of the strut section 52 is formed as a tube portion 60 provided with a recess 62 so that the tube open end 64 extends towards the brake shoe l8. A shoulder 66 is formed on the tube portion 60 where the strut section end 56 joins the tube portion. The strut section 54 has an end 68 formed similarly to the end 56 of the strut section 52, and also has an externally threaded portion 70 which extends towards the brake shoe l6 and is received in the recess 62 of the tube portion 60 for reciprocal movement. The strut section end 68 engages the brake shoe web 42 of the shoe l8 and also engages the parking brake lever 38, as is well known in the art. A sleeve 72 is coaxially received over the strut portion 52. The sleeve 72 has one end 74 through which the strut section 52 extends. The sleeve end 74 is normally in abutting relation with the shoe web 58, and is slotted so that it is held against rotation by receipt of a portion of the shoe web 58 within the slot. The sleeve end 74 provides an annular spring seat 76 on the inner side thereof, the spring seat surrounding a portion of the strut section 52. A compression spring 78 is received within the sleeve 72 about the strut section 52, and has one end abutting the shoulder 66 and the other end abutting the spring seat 76. The spring 78 urges the strut section 54 towards the shoe l8 and the sleeve 72 towards the shoe l6. An adjuster nut 80 is threaded on the externally threaded portion 70 of the strut section 54 so that one nut side 82 is axially engageable with the tube open end 64. The position of the adjuster nut 80 on the externally threaded portion 70 of the strut section 54, co-operating with the tube open end 64 of the strut section 52, thus establishes the minimum effective length of the strut 30. The portion of the escapement mechanism for rotating the adjuster nut 80 to obtain brake adjustment is supported on the sleeve 72 by a frame 84 which has an arm 86 extending over the outer periphery of the adjuster nut 80. The escapement verge (pivot pin) 88 is secured to the arm 86 so that its axis is in perpendicular skew relation to the common axis of the strut sections 52 and 54 and the sleeve 72. The verge 88 pivotally supports a U-shaped member 90 which is the escapement drive means of the escapement mechanism. The member 90 has arms 92 and 94 which straddle the adjuster nut 80. The adjuster nut 80 has face teeth 96 formed on the side 82 thereof, and face teeth 98 formed on the other side thereof. The adjuster nut 80, with its face teeth 96 and 98, forms an escape-wheel which is the escapement driven member of the escapement mechanism. The arms 92 and 94 straddle the adjuster nut (escape-wheel) 80, and respectively have pallets l00 and l02 formed on the ends thereof. The pallets l00 and l02 are positioned in co-operation with the teeth 96 and 98 to engage those teeth in alternate driving relation. A temperature-sensitive bi-metal spring l04 may be mounted on the U-shaped member 90. The spring l04 may also be generally U-shaped so that its substantially parallel legs l06 and l08 are received within the U-shaped member 90 and engage the arms 92 and 94. The spring l04 does not normally exert a pressure on the arms 92 and 94 which will move them apart. However, with an increase in brake operating temperature when the brake has been applied heavily, so that the brake drum l2 has expanded to the extent that it is no longer desirable to adjust the brake shoes in accordance with the distance between the brake shoe linings and the drum friction surface 48, the heat within the brake will cause the spring l04 to expand its legs l06 and l08 apart, so moving the arms 92 and 94 outwardly and axially away from the adjuster nut 80 so that the pallets l00 and l02 cannot engage the teeth 96 and 98 in driving relation irrespective of the tendency of the adjusting mechanism to cause them to do so. This will render the adjuster escapement mechanism ineffective to adjust the brake as long as the brake temperature is at or above the predetermined brake temperature beyond which it is not desired to make such adjustments. This will prevent over-adjustment of the brake under such high heat conditions. Instead of, or in addition to, the bi-metal spring l04, the spring 78 may be a bi-metal spring also sensitive to such brake temperatures so that the spring 72 will contract at the predetermined temperature beyond which it is not desired to provide brake adjustment. As will be described below in relation to the operation of the adjuster mechanism, this will likewise prevent over-adjustment of the brake in response to that condition. In the brake-released condition illustrated in Figure l, the brake shoes l6 and l8 are retracted so that their respective brake linings 44 and 46 are spaced radially inwardly to the desired extent from the friction-braking surface 48 of the drum l2, and they are held in this position by the retraction spring 32. The brake shoe l6, in abutting engagement with the sleeve 72 and also with the strut section 52, has moved the sleeve 72 against the compressive force of the spring 78 so that the verge 88 is near or beyond the plane of the side of the adjuster nut 80 on which the teeth 98 are formed. In moving to this position, the U-shaped escapement drive member 90 is pivoted on the verge 88 and about the point of engagement of the pallet l02 with one of the teeth 98, thereby raising the pallet l00 over one of the teeth 96. The mechanism is therefore in the position shown in Figure l. The tube open end 64 of the strut section 52 is in abutting engagement with the side 82 of the adjuster nut 80, and the effective minimum length of strut 30 is therefore established. This effective minimum length establishes the spacing between the brake shoes l6 and l8 and therefore the amount of clearance between the brake linings 44 and 46 in relation to the brake drum friction braking surface 48. When the brake is actuated by brake pressure transmitted to the wheel cylinder l4, the plungers 22 and 24 are moved outwardly of the wheel cylinder to move the brake shoes l6 and l8 into engagement with the friction-braking surface 48 of the drum l2. This occurs against the force of the retraction spring 32. The spring 78 will continue to urge the strut section 52 rightwardly towards the shoe l8, and therefore the strut section end 56 will be maintained in position while the shoe l6 moves leftwardly as viewed in Figures l and 2. Since the spring 78 also acts against the sleeve 72, it will move that sleeve in following relation to the shoe l6 so as to maintain the sleeve end 74 in abutting engagement with the shoe web 58. This axial movement of the sleeve 72 will carry with it the support frame 84 and therefore the verge 88. The verge 88 will be moving towards the plane of the side 82 of the adjuster nut 80, and this will cause a slight pivoting or rocking action of the U-shaped escapement drive member 90 about the verge 88. If the leftward movement of the sleeve 72, as viewed in Figures l and 2, is insufficient to require adjustment of the strut 30, the pallets l00 and l02 will not exert sufficient force on the teeth 96 and 98 of the adjuster nut to drive the nut rotationally. However, if the amount of shoe actuation required to expand the shoes into contact with the brake drum is sufficient to require adjustment, the escapement drive member 90 will be moved by the support frame 84 and the verge 88 in a direction away from the brake shoe l8 to such an extent that the pallet l00, by escapement action, will cause the adjuster nut 80 to be driven rotationally in the direction indicated by the arrow ll0. This will cause the adjuster nut 80 to be moved axially by the threaded action of the nut on the threads of the externally threaded portion 70 of the strut, such axial movement of the nut being leftwardly as viewed in Figures l and 2. This will slightly increase the effective minimum length of the strut 30. Upon brake release, the retracting spring 32 will pull the brake shoes l6 and l8 back towards the position shown in Figure l, so moving the sleeve 72 rightwardly relative to the strut section 54 and the adjuster nut 80 as viewed in those Figures and again compressing the spring 78 in so doing. This rightward movement of the sleeve 72 will also cause the frame 84 and the verge 88 to move rightwardly back to the position shown in Figure l. This movement will cause the pallet l02 to move the adjuster nut (escape-wheel) 80 further in the direction of the arrow ll0, so completing the double-acting adjustment of the strut 30 by means of the escapement mechanism. If the heat generated by actuation of the brake assembly l0 is sufficient to raise the temperature in the area of the spring 78 to the predetermined temperature above which it is not desired to provide adjustment, and if the spring 78 is a bi-metal spring which will contract at such higher temperatures, the contraction of the spring 78 under such conditions will result in less or even no leftward movement of the sleeve 72 during brake actuation since the amount of expansion of the spring 78 would therefore be decreased or even reduced to zero. Therefore there would be no leftward movement of the spring 72 and there would be no adjustment actuation of the escapement mechanism. If the bi-metal spring l04 is used, either independently of the bi-metal construction of the spring 78 or concurrently therewith, the expansion of the spring l04 to remove the pallets l00 and l02 from engagement with the teeth 96 and 98 of the adjuster wheel 80 will also render the adjustment mechanism inoperative to adjust the brake when the temperature is sufficient to cause the bi-metal spring l04 to so expand. The embodiment shown in Figures 5 to 8 of the drawings is a modification of the embodiment shown in Figures l to 4. Since many of the brake elements are the same or similar, similar reference numerals have been used where appropriate. The strut and adjuster mechanism forming a part of the strut are somewhat different from the corresponding parts of the earlier-described embodiment. Otherwise the brake construction is the same as that shown in Figures l to 4, such that the description thereof will not be repeated. The strut assembly l30 of Figures 5 to 8 has a brake adjuster mechanism l50 formed as a part thereof. The strut assembly has like or similar axially aligned strut sections 52 and 54 constructed and arranged as shown and described in relation to Figures l to 4. A somewhat modified sleeve l72 is coaxially received over the strut portion 52. The sleeve modification is in the area of the support frame and the escapement drive means. The other portions of the sleeve are constructed in the same manner as for the sleeve 72 of Figure l. The spring 78 acting on the sleeve l72 and on the strut section 52 is a similar spring to that shown in Figure l, and in the same way as described above the spring 78 may be a bi-metal spring which contracts at elevated temperatures and prevents adjustment when the brake is overheated. A modified adjuster nut l80 is threaded on the externally threaded portion 70 of the strut section 54 so that the nut side l82 is axially engageable with the tube open end 64. Because the nut l80 is to be rotated in the opposite direction to the direction of rotation of the nut 80 to expand the strut assembly, the threads on the strut section 54 threaded portion 70 are of the opposite hand to the corresponding threads in the arrangement of Figures l to 4. The modified escapement mechanism for rotating the adjuster nut l80 to obtain brake adjustment is supported on the sleeve l72 by a support frame l84. This frame includes an arm l86 extending over the outer periphery of the adjuster nut l80. The arm l86 is somewhat J-shaped, with the first portion being radially beyond the periphery of the nut l80. The arm l86 terminates in another arm l92 which is positioned radially inwardly relative to the first portion of the arm l86, and may also be slightly circumferentially spaced therefrom, as illustrated. The frame l84 includes another arm l94 which extends across the adjuster nut l80. The adjuster nut is provided with teeth l96, illustated as being formed as small blocks extending radially outwardly from the main body of the adjuster nut l80. The teeth l96 are circumferentially spaced around the outer periphery of the main body of the adjuster nut l80 so that the arm l94 may extend between the teeth as illustrated in the drawings and the arm l92 may, upon sufficient axial movement in the leftward direction as viewed in the drawings, also extend between two adjacent teeth l96. The free ends of the arms l92 and l94 are bevelled to respectively provide escapement pallets 200 and 202. Each tooth l96 is provided with four tooth faces which are in surface-engageable and driving relation with either the pallet 200 or the pallet 202 at various times. More particularly, referring to a single tooth l96, the tooth has faces 204 and 206 which generally face towards the side l82 of the adjuster nut but are in planes at an angle to the major plane of the adjuster nut l80. The tooth also has faces 208 and 2l0 which generally face towards the other side of the adjuster nut and are similarly angularly positioned. Therefore, as viewed in the drawings the teeth l96 appear somewhat like cubes with the four teeth faces being so arranged that the face 206 is at times engageable by the pallet 202 and the face 2l0 is at times engageable by the pallet 200. The relative axial spacing of the pallets 200 and 202 is such that when the arm l92 is moved leftwardly until the pallet 200 engages a toothed face 2l0, the arm l94 is moved sufficiently leftwardly to be clear of the rotating path of the teeth l96. Therefore the escapement drive movement of the escape-wheel adjuster nut l80 by means of pallet 200 and one of the teeth will not be obstructed by the location of the arm l94. Similarly, when the arms l92 and l94 are moved rightwardly as viewed in the drawings such that the pallet 202 acts on the tooth face 206, the arm l92 will be moved so as to be clear of the rotating path of the teeth l96 to accommodate movement of the adjuster nut l80 by the escapement drive mechanism. When the brake is actuated, the operation is generally similar to that of the first embodiment described above. The sleeve l72 will move leftwardly relative to the strut sections 52 and 54 under the force of the spring 78, carrying with it the escapement drive mechanism formed by the frame l84. If the leftward axial movement is sufficient, the pallet 200 will engage a tooth face 2l0 and, due to the bevelled arrangement thereof, will then move the tooth and therefore the adjuster nut l80 rotationally in the direction of the arrow 2l2. This will, by threaded action of the threaded portion 70, cause the adjuster nut l80 to be moved slightly leftwardly, so providing some adjustment to the mechanism. As the brake is released, the sleeve l72 will move rightwardly to return to the position shown in Figure 5, so that the pallet 202 will engage a toothed face 206 and will move a tooth l96 and therefore the adjuster nut l80 slightly further in the direction of the arrow 2l2, thereby accommodating a slight additional adjustment of the mechanism. If there is insufficient movement of the sleeve l72 in the leftward direction to cause the pallet 200 to engage the tooth face 2l0 and then, by further leftward movement, to move the tooth, no adjustment of the mechanism will be required or accomplished. Upon subsequent brake release, the sleeve l72 and the escapement drive mechanism, including the pallet 202, will merely return to the position shown without engaging a tooth in drive relation. Therefore the adjuster mechanism l50 will not make an adjustment which is not required. As described above, when the spring 78 is a bi-metal spring which contracts at elevated brake temperatures occurring when it is not desirable to adjust the mechanism, the sleeve l72 will not be moved leftwardly sufficiently to cause adjustment. The embodiment shown in Figures 9 to l2 of the drawings is a modification of the embodiment shown in Figures 5 to 8. Since many of the brake elements are the same or similar, like reference numerals have been used where appropriate. The strut and adjuster mechanism forming a part of the strut are somewhat different from the corresponding parts of the earlier-described embodiment of Figures l to 4. In other respects the brake construction is the same as that shown in Figures l to 4, such that the description thereof will not be repeated. The strut assembly 330 of Figures 9 to l2 has a brake adjuster mechanism 350 formed as a part thereof. The strut assembly has axially aligned strut portions 352 and 354 constructed and arranged in a manner similar to those shown and described in relation to Figures l to 8 above. A modified sleeve 372 is coaxially recieved over the strut portion 352. The sleeve modification is in the area of the body, at one end, and the escapement drive means at the other end. The body 373 of the sleeve 372 is formed to be clipped over the strut portion 352, as is better seen in Figure ll. The sleeve end 374 is slotted to receive a flat end 375 of the strut portion 352 about which the sleeve body 373 is clipped. A spring 378, acting on the sleeve 372 and on the end 376 of the strut part 377, is a bi-metal spring which contracts at elevated temperatures so that it exerts no spring force between the sleeve end 374 and the strut abutment 376 and prevents adjustment when the brake is overheated. A further modified adjuster nut 380, constructed much like the nut 80 of Figures l to 4, is threaded on the externally threaded portion 370 of the strut section 354 so that the nut side 382 is axially engageable with the tube open end 364. The nut 380 is to be rotated in the opposite direction of rotation to that of the nut 80 for the purpose of expanding the strut assembly. Therefore the threads on the strut section threaded portion 370 are of the opposite hand to the similar threads in the arrangement of Figures l to 4. The modified escapement mechanism for rotating the adjuster nut 380 to obtain brake adjustment is formed as an integral stamped part of the sleeve 372 on the sleeve end 384. This includes an arm 386 extending over the outer periphery of the adjuster nut 380. The arm 386 has another arm 392 which is struck so as to be positioned radially inwardly relative to the body of the arm 386. The arm 386 includes another arm 394 which is also struck so as to be positioned radially inwardly relative to the body of the arm 386. The arms 392 and 394 are therefore on opposite sides of the adjuster nut 380. The adjuster nut 380 is provided with side face teeth 396 and 398 respectively aligned for driving engagement with teeth 400 and 402 formed on the arms 392 and 394. The teeth 400 and 402 respectively provide escapement pallets for escapement driving action. Use of one tooth 400, 402 on each arm 392, 394 will give adequately fine adjustment for most brakes. However, if finer adjustments are required, the tooth 400 may be made as two or even three teeth, as may the tooth 402. When the brake is actuated, the operation is generally similar to that of the second embodiment described above. Specifically, the sleeve 372 will move leftwardly relative to the strut portions 352 and 354 under the force of the spring 378, as indicated by an arrow 404 in Figure l2, and carry with it the escapement drive mechanism formed by the sleeve end 384. If the leftward axial movement is sufficient, the pallet or tooth 402 will engage a tooth 398, and, due to the bevelled arrangement thereof, will then move the tooth and therefore the adjuster nut 380 rotationally in the direction of the arrow 4l2 of Figure 9. This will, by threaded action of the threaded portion 370, cause the adjuster nut 380 to be moved slightly leftwardly, so providing some adjustment to the mechanism. As the brake is released, the sleeve 372 will move rightwardly to return to the position shown in Figure 9, so that the tooth or pallet 400 will engage a tooth 396 and will move the adjuster nut 380 slightly further in the direction of the arrow 4l2, thereby accommodating a slight additional adjustment of the mechanism. If there is insufficient movement of the sleeve 372 in the leftward direction to cause the tooth or pallet 402 to engage a tooth 398 and then, by further leftward movement, to move the tooth, no adjustment of the mechanism will be required or accomplished. Upon brake release, the sleeve 372 and the escapement drive mechanism, including the pallet or tooth 402, will merely return to the position shown in Figure 9 without engaging a tooth in drive relation. Therefore the adjuster mechanism will not make an adjustment which is not required. As described above, the spring 378 is a bi-metal spring which contracts at elevated brake temperatures occurring when it is not desirable to adjust the mechanism. The sleeve 372 will then not be moved leftwardly, as viewed in Figure l2, sufficiently to cause adjustment. The invention is thus embodied in a compact escapement drive arrangement which may provide for brake adjustment in either left-hand or right-hand brake assemblies of a vehicle, and which may prevent adjustment when the brake becomes overheated. Any of several types of escapement mechanism may be used, provided that they are sensitive to axial movement of an element such as the sleeve 72, l72 or 372.

Claims: 1. A drum brake adjuster characterised in that an expandable strut (30) is adapted to extend between first and second brake shoes (l6 and l8) of a drum brake (l0) to be actuated and released and adjusted, and the expandable strut comprises: a first member (52), a second member (54) telescopically received in the first member (52), and a third member (72) telescopically receiving the first member (52) and a portion of the second member (54), all in coaxial relation, the first (52) and third (72) members being adapted to engage the first brake shoe (l6) and the second member being adapted to engage the second brake shoe (l8), and spring means (78) acting on the first (52) and third members (72) to continually urge the first member (52) towards the second member (54) and the third member (72) away from the first member (52); an adjusting element (80) on the second member (54) normally engaging the first member (52) in abutting relation to limit the minimum length of the strut (30) and movable on the second member (54) in a direction towards the first member (52) to increase the effective minimum length of the strut (30) when sufficient brake shoe wear and actuation necessitate brake adjustment, the adjusting element (80) including an escapement driven member (96) which is incrementally actuable to provide such adjustment; and escapement drive means (90) on the third member (72) engageable, in a brake actuation and release cycle in which brake adjustment is required, in driving relation with the escapement driven member (96) at least once during such a brake actuation and release cycle, the escapement drive means (90) when so engaging the escapement driven member (96) driving the adjusting element (80) to move the same on the second member (54) in a direction axially towards the first member (52) so that when the brake is fully released the first member (52) abuttingly engages the adjusting element (80) at an increased strut minimum-length position to establish an adjusted position of the brake shoes (l6 and l8). 2. A drum brake adjuster according to claim l, characterised in that the spring means (78) includes a temperature-sensitive spring which at a predetermined brake temperature contracts sufficiently to render the spring means (78) unable to move the third member (72) axially relative to the adjusting element (80) and the second member (54) through a sufficient distance to cause the escapement drive means (90) to drive the escapement driven member (96), thereby rendering the adjuster ineffective to change the brake adjustment as long as the brake temperature is at or above the predetermined brake temperature and preventing over-adjustment of the brake under that condition. 3. A drum brake adjuster according to claim l or 2, characterised in that the adjusting element (80) is threadedly mounted on the second member (54) and on rotary movement relative to the second member (54) is movable axially by threaded action in the direction towards the first member (52), the escapemeht drive means (90) is engageable in driving relation with the escapement driven member (96) during the brake actuation portion of a brake actuation and release cycle in which brake adjustment is required, the escapement drive means (90) when so engaging the escapement driven member (96) drives the adjusting element (80) rotationally to move the same by threaded action on the second member (54) in a direction axially towards the first member (52), and the escapement drive means (90) is further engageable with the escapement driven member (96) in driving relation during the brake release portion of such a cycle, the escapement drive means (90) when so engaging the escapement driven member (96) driving the adjusting element (80) rotationally to move the same by threaded action further on the second member (54) in a direction axially towards the first member (52). 4. A drum brake adjuster according to claim 3, characterised in that the escapement driven member (96) is a wheel-like nut (80) having face teeth (96,98) on each side thereof and defining an escape-wheel, the escapement drive means (90) being a generally U-shaped member having arms (92 and 94) straddling the escape-wheel (80) and having a pallet (l00,l02) on each of the arms (92 and 94), the U-shaped member having a verge (88) secured to the third member (72) with the axis thereof being perpendicularly skew to the common axis of the first (52) and second (54) and third (72) members, the U-shaped member being arcuately movable about the said verge (88) with the pallets (l00,l02) engaging the teeth (96,98) in alternate driving relation as the third member (72) is moved relative to the second member (54) and the escapement driven member (96) in one axial direction upon brake application and in the opposite axial direction upon brake release, the pallets (l00,l02) acting on the teeth (96,98) to move the escapement driven member (96) arcuately on the second member (54) when the movements of the third member (72) are sufficient to cause first one and then the other of the pallets (l00,l02) to advance to an adjacent tooth (96,98) from the one with which it is aligned for driving engagement while in the brake-released condition. 5. A drum brake adjuster according to claim 4, characterised in that the U-shaped member has a temperature-sensitive spring (l04) mounted thereon and engaging the arms (92,94) so as to be able to exert a spring force tending to spread the arms (92,94) apart, the temperature-sensitive spring (l04) expanding with an increase in brake temperature and at a predetermined brake temperature expanding sufficiently to move the arms (92,94) outwardly in a direction axially away from the escape-wheel (80) so that the pallets (l00,l02) cannot engage the teeth (96,98) in driving relation irrespective of the amount of axial movement of the third member (72), thereby rendering the adjuster ineffective to adjust the brake as long as the brake temperature is at or above the predetermined brake temperature and thus preventing over-adjustment of the brake under that condition. 6. A drum brake adjuster according to claim l, for use in a drum brake assembly having means (l4) for expanding the first and second shoes (l6 and l8) to engage a brake drum (l2) in braking relation and means (32) for retracting the shoes (l6 and l8) for disengagement from the brake drum (l2) upon release and returning the shoes (l6 and l8) to an adjusted position determined by the brake adjuster (50), characterised in that: the first member (52) of the expandable strut (30) comprises a first strut section formed at one end with a tube portion (60) having an open end (64) and having the other end (56) of the first strut section adapted to engage the first shoe (l6), and the second member (54) of the expandable strut comprises a second strut section formed at one end with an externally threaded portion (70) extending into the said tube portion (60) and having the other end (68) of the second strut section adapted to engage the second shoe (l8); the adjusting element (80) comprises an adjuster nut threaded on to the threaded portion (70) of the second strut section and having one side engageable with the open end (64) of the tube portion (60) of the first strut section and co-operating with the strut sections to define the effective length of the strut (30) and therefore the position of the shoes (l6 and l8) relative to the brake drum (l2) when in the brake-released condition, the adjuster nut (80) having the escapement driven member (96) formed thereon; the third member (72) comprises a sleeve having a spring seat (76) formed on one end (74) thereof with the other end (56) of the first strut section received through the spring seat (76) so that the tube portion (60) of the first strut section is received within the sleeve (72) and extends outwardly therefrom in a direction towards the adjuster nut (80), the sleeve (72) further having a support frame (84) extending over the adjuster nut (80), with the escapement drive means (90) positioned on the support frame (84) to engage the escapement driven member (96) of the adjuster nut (80) in escapement-driving relation; and the spring means (78) is received in the sleeve (72) and acts on the spring seat (76) and the first strut section to continually urge the tube portion (60) of the first strut section in a direction outwardly of the sleeve (72) and against the adjuster nut (80) and to continually urge the sleeve (72) in a direction axially away from the adjuster nut (80); the spring means (78) upon brake actuation moving the sleeve (72) and the support frame (84) and the escapement drive means (90) axially relative to the adjuster nut (80) and upon sufficient of such axial movement causing the escapement drive means (90) to act on the escapement driven member (96) to cause the adjuster nut (80) to rotate arcuately to increase the effective length of the strut (30) and thereby adjust the brake. 7. A drum brake adjuster according to claim 6, characterised in that the escapement driven member (96) formed on the adjuster nut (80) comprises a double-acting escapement driven member, the escapement drive means (90) formed on the said other end of the sleeve (72) comprises a double-acting escapement drive means positioned to engage the double-acting escapement driven member (96) in double-acting escapement driving relation, and upon brake release the sleeve (72) and the support frame (84) and the escapement drive means (90) are moved axially against the force of the spring (78) by brake-retracting movements of the brake shoes (l6 and l8) and by such movement cause the escapement drive means (90) to again act on the escapement driven member (96) to cause the adjuster nut (80) to further rotate arcuately to increase the effective length of the strut (30) and further adjust the brake. 8. A drum brake adjuster according to claim 6 or 7, characterised in that the sleeve (372) has a body portion (373) between one end and the other end of the sleeve (372), the body portion (373) being formed as a clip which in cross-section extends circumferentially more than l80° and less than 360° about the tube portion of the first strut section (352) and is made of a spring material so as to be capable of being clipped over the tube portion by lateral movement of the sleeve (372) relative to the tube portion and to be removed therefrom in a similar manner. 9. A drum brake adjuster according to claim 8, characterised in that the body portion (373) of the sleeve (372) is arcuate in cross-section throughout more than l80° and less than 360° so as to be removable from and replaceable on the tube portion of the first strut section (352) without the need for removal of the strut (30) from a drum brake in which it is installed. l0. A drum brake adjuster according to claim l, characterised in that when the brake is fully released the first member (52) is moved to axially engage the adjusting element (80) at the increased strut minimum-length position, and temperature-responsive means (78) is responsive, at and above a predetermined brake temperature above which it is not desirable to adjust the brake because of possible over-adjustment under such temperature conditions in the brake, to prevent the escapement drive means (90) from driving the escapement driven member (96) during a brake actuation and release cycle, thereby rendering the adjuster ineffective to change the brake adjustment as long as the brake temperature is at or above the predetermined brake temperature. 11. A drum brake adjuster according to claim l0, characterised in that the temperature-responsive means (78) is a bi-metal spring constituting the spring means.

Inventors: SHELLHAUSE, RONALD L.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1986
Main classification: F16D65
IPC codes: F16D65