Patent Application
20130240308
The subject invention relates to an automatic slack adjuster that includes an adjustment assembly that is installed from one direction into a blind-bore formed within a slack adjuster housing.
An automatic slack adjuster is used to adjust a brake assembly as brake linings wear. The automatic slack adjuster adjusts the amount of slack, or “free play”, in the brake to ensure that an associated air brake chamber can produce enough actuation force. With too little slack, the brake may drag and overheat, and if there is too much slack, the brake may not be capable of generating enough braking effort to stop the vehicle.
Packaging automatic slack adjusters on axles and suspensions can be difficult in many situations due to lack of available space. In one known configuration, the slack adjuster includes a worm gear that is coupled to a camshaft. A worm shaft is in driving engagement with the worm gear. An adjustment assembly couples the worm shaft to a push rod of a brake chamber. The adjustment assembly is configured to automatically adjust slack as brake linings wear by rotating the worm shaft, which in turn rotates the worm gear and camshaft. The slack adjuster is also configured with a manual adjuster that allows for selective manual adjustment of the slack, which is useful when new brake linings are installed for example.
The worm shaft, worm gear, and adjustment assembly are mounted within a housing. The housing has a lever coupled to the push rod of the brake chamber. The worm shaft in this configuration is positioned to extend in a vertical direction relative to the worm gear, i.e. generally parallel to the lever. A thru-bore having two open ends is formed in the housing to receive the worm shaft and adjustment assembly. The thru-bore is machined from two different directions and adjustment components are inserted through both open ends for installation purposes. Having two open ends provides flexibility for installation of manual adjuster components.
One disadvantage with this known configuration is that the vertical orientation of the worm shaft increases the overall width of the slack adjuster. This makes it difficult to package the slack adjuster in all possible packaging locations. Another disadvantage is that the two open ends of the thru-bore provide potential leak paths for contaminants that may be sprayed onto the slack adjuster by rotating tires.
Other known configurations use a worm gear that is orientated to extend in a horizontal direction relative to the worm gear, i.e. the worm shaft extends transversely relative to the lever. These configurations also include a thru-bore with two open ends. The thru-bore is machined from two different directions and adjustment components are inserted through both open ends for installation purposes. While the horizontal orientation of the worm shaft improves packaging, there is still potential for significant leakage of contaminants into the thru-bore, which can result in premature wear and failure of adjustment components.
A slack adjuster provides automatic slack adjustment during normal braking operations by adjusting a camshaft via an adjustment assembly. In one example, the slack adjuster comprises a worm gear configured to be coupled to the camshaft, a worm shaft in engagement with the worm gear, and a housing having a lever configured for connection to a brake chamber output rod. The housing includes an opening that receives the worm gear and a blind bore that receives the worm shaft. An adjustment assembly is installed within the blind bore and is coupled to the worm shaft. The adjustment assembly is configured to automatically rotate the worm shaft when brake slack exceeds a predetermined limit.
In one example, the blind bore extends in a horizontal direction that is transverse to the lever.
In one example, the blind bore is machined from a single direction. The worm shaft and adjustment assembly are then inserted through the only open end of the blind bore. An end cover and seal cooperate to provide a sealed interface once the worm shaft and adjustment assembly are installed.
In one example, the adjustment assembly comprises a control disc that is fixed for rotation with the worm shaft and an actuator disc that is coupled to a push rod. The push rod is configured to be coupled to the lever via a coupling member.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
As shown in
The brake shoes 16 are coupled to a brake spider 24 via anchor pins 26. The brake spider 24 is configured to be mounted to an axle housing or non-rotating wheel end component. A return spring 28a facilitates returning the brake shoes 16 to the non-applied position upon completion of a braking request. Shoe retaining springs 28b couple the brake shoes together adjacent the anchor pins 26.
As shown in
When the brake pedal is released, air pressure is exhausted from the brake chamber 30 and a spring or other return mechanism is configured to return the actuating rod 32 and lever 34 to a retracted, brake-off position. This also causes the brake shoes 16 to return to a non-applied position. In this non-applied position, there is a clearance space S, i.e. slack, between brake friction linings 36 on the brake shoes 16 and the engagement surface 22 on the brake drum 18. Over time, the thickness of the brake friction linings 36 decreases or wears, and this in turn increases the slack. If slack becomes excessive, it may not be possible to provide a sufficient braking force to the drum 18, i.e. the actuating rod 32 may not be able to rotate the lever 34 far enough to exert a sufficient braking force against the drum 18.
To ensure that the actuation lever 34 is rotated to produce a desired amount of brake force, the lever 34 is coupled to a coupling member 38, which can be adjusted throughout the life of the brake shoes to ensure that the cam 20 is properly positioned in relation to the brake shoes to provide the desired braking force. An automatic slack adjuster mechanism 40 is used to automatically readjust the coupling member as the brake linings wear.
When an automatic slack adjuster mechanism 40 is installed, a brake chamber stroke measurement is set, which corresponds to the desired or correct shoe-to-drum clearance. As the brake linings 36 wear, this clearance, i.e. slack, increases and the brake chamber actuating rod 32 must travel farther to apply the brakes. Once this clearance or “slack” is increased beyond a predetermined limit, the slack adjuster is configured to automatically adjust during the return stroke to maintain the correct shoe-to-drum clearance. If the air brake chamber push rod stroke is within acceptable limits during operation, no adjustment occurs.
In the example shown in
The slack adjuster mechanism 40 includes a worm shaft 52, which has a second set of worm teeth 54 that are directly engaged with the first set of worm teeth 50. The worm shaft 52 is positioned within the housing 44 such that the worm shaft 52 is extending in a generally horizontal direction relative to the lever 34. In other words, the worm shaft 52 extends transversely relative to the lever 34, as opposed to extending generally parallel to the lever 34.
The slack adjuster mechanism 40 further includes an actuator disc 60, a control disc 62, a bushing 64, and an end cover 66. The housing 44 includes a blind-bore or blind-hole 70, i.e. a hole having only one open end, which receives these components, as well as the worm shaft 52, as a sub-assembly. The blind-hole 70 is solely formed by a machining operation from one side of the housing 44 such that the hole 70 does not extend entirely through the housing 44 as shown in
The worm shaft 52 includes a gear portion 72 that includes the second set of worm teeth 54, a reduced neck portion 74 that supports the bushing 64, and a splined shaft portion 76 that is coupled to the control disc 62 via a mating splined bore 78 formed within the control disc 62. As shown in
The control disc 62 includes a first end face 84 that faces the bushing 64 and a second end face 86 that faces the end cover 66. The control disc 62 includes a plurality of control teeth 88 on the second end face 86. A resilient member 90, such as a coil spring for example, has one spring end in engagement with the first end face 84 and a second spring end in engagement with the bushing 64. The resilient member 90 loads the control teeth 88 into engagement with actuator teeth 92 on the actuator disc 60. As such, the actuator 60 and control 62 discs are always in contact with each other. In one rotational direction, i.e. a brake apply direction, the actuator teeth 92 skip past the control teeth 88. In an opposite rotational direction, i.e. a brake return direction, the actuator teeth 92 drive against the control teeth 88, causing the control disc 62 to rotate, and hence the worm shaft 52 to rotate.
The actuator teeth 92 are formed on a first end 94 of the actuator disc 60, which faces the control disc 62. A second end 96 of the actuator disc 60 faces the end cover 66. The actuator disc 60 has an outer circumference that is close to the diameter of the blind-hole 70 such that the actuator disc 60 sits within the blind-hole in a light press-fit or slightly loose fit. A snap ring 98 is installed within the blind-hole to hold the actuator disc 60 at a desired axial location.
The end cover 66 includes a hub portion 100 at one end and a knob portion 102 that extends outwardly of the housing 44 at an opposite end. The hub portion 100 includes an outer surface 104 that has a slightly smaller diameter than an inner diameter of a center bore 106 of the actuator disc 60. This allows the hub portion 100 to be inserted into the center bore 106 without contacting the actuator disc 60. The hub portion 100 includes a blind-bore 108 that has a splined inner circumferential surface 110. An end face 112 of the hub portion 100 abuts against the control disc 62.
A seal 114 holds the end cover 66 within the bore 70. An optional snap ring 116 (
The knob portion 102 extends outwardly of the housing 44. In one example, the knob portion 102 is configured to provide a grasping surface and/or a tool engagement surface 124 to provide a manual adjustment feature. This will be discussed in greater detail below.
The housing 44 (
The slack adjuster mechanism 40 operates in the following manner. A brake request is initiated, for example, by pressing a brake pedal. In the brake apply direction, if there is significant lining wear, i.e. lining wear has exceed a predetermined limit, the push rod 132 is lifted and rotates the actuator disc 60 such that the actuator teeth 92 skip past the control teeth 88 at least one increment, i.e. at least one tooth increment. The adjustment is made on the brake return stroke as the actuator teeth 92 and control teeth 88 are in driving engagement upon opposite rotation, i.e. the actuator 60 and control 62 discs are locked together on the return stroke. This causes the worm shaft 52 to provide incremental adjustment by rotating the worm gear 42, which in turn rotates the camshaft 14 to adjust the position of the cam 20.
The slack adjustment mechanism 40 can also be manually adjusted as needed. In a normal, non-adjustment position (
To manually adjust the worm gear, the end cover 66 is pressed axially inward in a direction toward the worm shaft 52. This movement results in two simultaneous actions. First, the control disc 62 is disengaged from the actuator disc 60. Second, the splined inner circumferential surface 110 of the end cover 66 is slid into engagement with the splined shaft portion 76 of the worm shaft 52, as shown in
This manual adjustment configuration is advantageous for several reasons. The end cover 66 serves to seal the open end of the blind-hole 70 to prevent contaminants from adversely affecting the internal components. Also, a single tool can be used to provide the adjustment. Finally, the end cover does not interface with other components during non-manual adjustment operations. As such, the torque required to adjust the slack is low. Also, there is no wear or damage to adjuster components during manual adjustment because the end cover has disengaged the automatic adjustment system, i.e. the control disc 62 is disengaged from the actuator disc 60. Application Ser. No. ______, directed to manual adjustment, is filed on an even date herewith and is assigned to the assignee of the present application.
Another advantage with the subject slack adjuster mechanism 40 is provided by the blind-hole 70. By assembling the internal slack adjuster components as a sub-assembly that is inserted into the blind-hole 70, overall packaging size is reduced. The compact sub-assembly of internal components, i.e. actuator disc 60, control disc 62, spring 90, bushing 64, and end cover 66, allow for a very short worm shaft 52 and a narrower housing 44. Further, by using a blind-hole as opposed to having a hole open at both ends, a potential leak path is eliminated. This is critical due to the location of the slack adjuster in relation to the vehicle tires which spray water and road debris onto the slack adjuster. Finally, the blind-hole configuration offers a design where all machining can be accomplished from one direction. All the internal components are loaded in the same direction, which simplifies assembly and shortens the time for manufacturing purposes.
The cam operated brake is shown merely as an example, it should be understood that other brake configurations could also be used with the subject slack adjuster. Further, although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
