Adjuster strut

Details Adjuster strut Adjuster strut

2002-05-09

2004-02-17

Lavinder, Jack (Department: 3683)

Brakes

Wheel

Transversely movable

C188S1960BA

Reexamination Certificate

active

06691838

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an adjuster strut for use in drum brake assemblies to compensate for wear in friction lining.
BACKGROUND OF THE INVENTION
Adjuster struts are employed in drum brake assemblies in order to progressively adjust the separation between pairs of brake shoes as brake friction lining wears. A simplified example of a brake assembly that employs an adjuster strut is shown in
FIG. 1
, and this shows a drum brake assembly
1
. The assembly
1
includes a pair of brake shoes
3
of T-shaped cross-section, an abutment
4
disposed between one pair of facing ends of the brake shoes
3
and about which the brake shoes can pivot, a hydraulic actuator
5
that applies an actuating force during brake actuation against the other pair of facing ends of the brake shoes
3
, to move the brake shoes radially outwardly and into braking engagement with the internal braking surface of the drum
2
, and an adjuster strut
6
. The adjuster strut
6
is disposed generally horizontally in engagement with the inwardly depending web of one (or each) of the T-shaped brake shoes
3
and the web of the parking brake lever and in absence of an actuating force being applied to the brake shoes
3
by the hydraulic actuator
5
, the respective webs engage either end of the strut
6
under the biasing influence of a return spring
7
and locate the shoes radially. Thus, the length of the strut
6
determines the radial spacing of the brake shoes
3
when no braking force is being applied. Through use, the friction lining of the brake shoes
3
wears over time and the adjuster strut
6
is lengthwise adjustable to increase the separation of the brake shoes and thus compensate for lining wear. It is the manner in which the adjuster strut is lengthened that the present invention has principal concern.
Various adjustable strut arrangements are known. Australian Patent No. 525459 in the name of The Bendix Corporation, discloses one form of an adjustable strut that employs a pawl and ratchet arrangement. This arrangement is complex and thus is relatively expensive to manufacture. Additionally, the relatively high number of parts that constitute the strut increases the potential for failure, as well as the cost of manufacture and necessity for regular maintenance.
A further strut arrangement is shown in European Publication No. 0262014, in the name of Bendix France and this strut suffers the same drawbacks associated with the above described known strut. This and the earlier known forms of adjuster strut are indicative of known struts in general, which typically are complex in construction.
European patent application EP 936 375 discloses a further strut arrangement that includes a rotatable wheel having internal and external parts. The respective parts are coaxial and are relatively rotatable in one direction of rotation and mutually rotatable in the opposite direction, by way of a clutch arrangement disposed between the parts. By this arrangement, the internal part can shift axially on a threaded shank to move the rotatable wheel, and by that movement, compensate for lining wear. This arrangement suffers from the drawbacks associated with the earlier discussed prior art in its complexity and cost to manufacture and its potential for failure and requirement for maintenance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an adjuster strut which is of reduced complexity compared to known struts.
According to the present invention there is provided an adjuster strut for use in a drum brake assembly to shift the brake shoes of the assembly radially outwardly to as to compensate for wear in brake friction lining, said strut being of adjustable elongate extent and having axially extended and retracted conditions, said strut including engagement means disposed at opposite ends thereof for engaging opposed braking members of the brake assembly, a rotatable member which is rotatable about the lengthwise axis of said strut, an axially movable member which is movable axially upon forward rotation of said rotatable member to lengthen the elongate extent of said strut, in said retracted condition, and biasing means for biasing said strut toward said axially extended condition upon radial expansion of the brake shoes of said brake assembly during brake actuation, said rotatable member having an axially disposed face and a cam surface formed on said face, said cam surface defining a plurality of teeth formed in an annular ring, said strut including an elongate arm which is mounted for resilient pivoting movement and which includes a free end portion biased resiliently into engagement with said cam surface at least during axial extension of said strut, said arm being disposed at an oblique angle relative to said axially disposed face, wherein in use, said arm resiliently pivots during axial extension of said strut to said axially extended condition and causes said rotatable member to rotate in said forward direction, said biasing means being arranged to resist rotation of said rotatable member in a reverse direction opposite to said forward direction upon said strut moving from said axially extended condition to said axially retracted condition during radial contraction of said brake shoes, so that the elongate extent of said strut in said axially extended condition is not reduced as said strut moves to said axially retracted condition.
For the purposes of this specification, the expression “braking members” is to be understood as including the opposed brake shoes of a brake assembly and if provided, the parking brake lever of the assembly.
In one arrangement, upon separation of braking members during brake actuation, the rotatable and axially movable members are shifted by the biasing means to an axially extended position, and the arm resiliently pivots from a rest or home position in a forward direction and by that pivoting movement, the free end of the arm rotates the rotatable member in the forward direction to relatively extend the axially movable member. Upon return movement of the braking members the rotatable and axially movable members retract axially from the axially extended position, while the arm returns to the rest position which causes the free end of the arm to shift relative to the cam surface in the reverse direction. Upon sufficient shifting movement (upon sufficient lining wear), the free end moves past one tooth of the cam surface to a position adjacent a circumferentially spaced tooth. In this arrangement, each time the arm is positioned adjacent a new tooth, it is positioned to catch or engage that new tooth and to rotate the rotatable member forward by that engagement during radial expansion of the brake shoes.
In most arrangements, the arm can be arranged to operate either in tension or compression. In the above arrangement the arm typically would undergo tensile loading when the axially movable member is shifted axially outwardly to extend the length of the strut and that tensile loading would cause the arm to pivot and rotate the rotatable member in the forward direction. However, it equally is permissible that the arm be arranged for compression loading to rotate the rotatable member and in that arrangement, rotation of the rotatable member occurs during return axial movement of the axially movable member. In this latter respect, the arm may pivot and thus the free end portion thereof may shift relative to the cam surface during outward axial movement of the axially movable member, without rotating the rotatable member in the forward direction. However, if that shifting movement is sufficient, the free end portion may engage a tooth of the cam surface, such that upon return axial movement of the axially movable member, with the arm in compression, the arm may rotate the rotatable member in the forward direction and thus adjustably lengthen the strut as required.
The arrangement of the arm to act either in tension or compression is considered to be a matter of design choice. It is therefore to be appreciated that where refer

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