GB2046379A - Improvements in drum brake automatic adjusters - Google Patents

Abstract

The invention provides means whereby a drum brake automatic adjuster only operates from time to time, rendering the adjuster less sensitive to sudden changes, as occur due to thermal expansion or mechanical deflection of the drum. An eccentric cam wheel (14), placed alongside the adjuster nut (13) on a rod (12) moves the pawl (27) in and out radially relative to the nut, only allowing it to engage the nut periodically. The wheel has ratchet teeth, like the nut, and is also rotated by the pawl - nut (13) is threaded on rod (12) whilst wheel (14) is rotatably mounted on but not threaded to the rod (12). A tag (28) prevents the adjuster operating at all as the linings become worn out. <IMAGE>

Description

SPECIFICATION Improvements in drum brake adjusters This invention relates to vehicle drum brake automatic adjusters, to compensate for wear of the brake linings.

Such adjusters often include, as the basis of the mechanism, a screw threaded rod on which a nut is mounted, the nut being rotated along the rod as adjustment becomes required. The rod and nut may form part of a strut which holds the brake shoes apart.

A problem with brakes having such adjusters is that thermal expansion of the drum is usually sensed as being a requirement for adjustment, i.e. as if it were lining wear. When the drum later cools, and contracts it can therefore happen that the shoes and linings cannot retract out of contact with the drum, and the brake is locked. It is not too dangerous if a single application of the brake is effective to expand the drum: the adjuster normally takes up only a little of the slack per application. It is repeated actuations with the drum hot, such as occur during descent of a long winding slope, that most readily lead to lockingon when the drum cools. Similarly, mechanical deflections of the drum and other brake components can be sensed as being a requirement for adjustment, so that repeated heavy brake applications can lead to locking on.This problem does not arise if the brake is provided with a very large running clearance, but that would severely constrain the design of the brake actuator means.

The problem might be alleviated by arranging the adjuster to have a more refined action, in that it takes up even less of the slack per application than is conventionally the case. This could at least in theory, be achieved by having a finer screw thread. However, adjusters have to be robust and reliable under a wide variety of operating conditions, and such refinements would make the adjuster, if reliable, very expensive.

The invention is aimed at solving the above problem cheaply, and without loss of reliability.

In accordance with the invention, there is provided a drum brake adjuster comprising an adjustable length strut having a threaded rod, a complementari threaded nut on the rod, the nut being provided with ratchet teeth, a pawl which, in response to a requirement for adjustment, is movable, when engaged with the teeth, to move the teeth and turn the nut, and a cam means for holding the pawl out of engagement and for bringing it into engagement with the teeth for alternating, predetermined periods.

A drum brake incorporating an adjuster according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure l is an elevation of the drum brake, Figure 2 shows a partly sectioned plan, on a larger scale, of the adjuster shown in Figure 1.

The brake shown in Figure 1 is generally similar to that disclosed in the Specification of our British patent number 1460847, in particular with reference to Figure 4 thereof, to which reference is made for a better understanding of the following description.

Referring to Figure 1, two brake shoes 1,2 are adapted to be urged apart into rubbing contact with a drum (not shown) by means of a hydraulic expander 3, or alternatively by means of a mechanical lever 4 and strut 5. The lever 4 is pivotally connected to one of the shoes 2 by a pivot 6. The lever 4 is adapted to be pulled by a cable (not shown) secured to its end 7.

The shoes are urged together, away from the drum, by shoe return springs, only one of which 8 is illustrated. When unactuated, the shoes rest against an abutment 9 and against shoulders 10, 11 on the strut 5 (Figure 2).

The strut 5 is of variable length, and comprises a threaded rod 12, a nut 13, a cam wheel 14 and a cylinder 15. The nut 13 is threaded complementarily to the rod 12, and is movable in a helical path, following the thread, along the rod. The wheel 14 has a plain cylindrical hole 16, which is of such a diameter as to allow the wheel to pass freely along the rod 12, and to rotate relative to the rod, without following the thread. The cylinder 15 is dimensioned to fit over the rod 12.

The rod 12 and the cylinder 15 are formed with slotted or bifurcated ends, tongues 17, 18 19 and 20 of which lie alongside the shoes 1, 2. The shoes themselves are located in the brake, and the engagement with the tongues ensures that neither the rod 12 nor cylinder 15 can rotate.

Clockwise movement of the lever 4 causes the strut 5 and with it the shoe 1 to move to the left, and the consequent reaction at the pivot 6 causes the other shoe 2 to move to the right. Thus, both shoes are applied to the drum by movement of the lever 4, the strut 5 transmitting brake actuation forces between the shoes.

A pawl lever 21 is pivoted to the shoe 1, on a hollow pivot pin 22, a link 23 passing through the pivot pin 22, and through a hole 24 in the shoe 1,and acting as an anchorforthe return spring 8. One arm of the lever 21 has an abutment 25 which engages the tongue 17 of the rod 12. A spring 26 biases the pawl lever 21 clockwise, maintaining the abutment 25 against the tongue. The spring 26 also acts to bias the lever inwards, i.e. into the plane of Figure 1. The spring 26 is arranged to provide insufficient force to cause the shoulder 10 to move clear of the shoe 1; the force due to the return springs keeps the shoulder 10, 11 engaged with the shoes, when the brake is unactuated.

At the other arm of the lever 21 is a pawl 17 engaging axial ratchet teeth on both the nut 13 and the wheel 14. The teeth on the nut 13 are pitched on a circle concentric with the axis of the thread.

When the hydraulic expander 3 is actuated, the shoes 1,2 are moved apart to apply the brake. The strut 5 now transmits none of the actuation forces and is biased by the spring 26, which rotates the pawl lever 21 clockwise and moves the strut 5 to the right, causing the shoulder 10 to move clear of the shoe 1. The rotation of the pawl lever 21 gives rise to downwards movement of the pawl 27, which, if great enough, as described below, is effective to turn the nut 13.

The wheel 14 has a circular circumference, which is eccentric with respect to the hole 16 in the wheel 14. Thus the wheel 14 has a radius, with respect to the hole 16 and hence to the axis of the thread, which varies around the circumference. The smallest radius of the wheel is arranged to be substantially equal to the radius of the nut 13.

The pawl 27 is axially long enough radially to face the circumferences of both the nut 13 and the wheel 14. As the wheel 14 rotates, the pawl 27 is caused to move radially in and out, so that it only contacts the circumference of the nut 13, and hence engages the teeth of the nut, when the smallest radius of the wheel 14 is in line with the paw 27 i.e. in the condition illustrated in Figure 2.

The wheel 14 is caused to rotate by the action of the pawl 27, which drags the wheel around as it moves (tangentially) downwards. The pawl is biased radially towards the wheel and nut by the spring 26.

A certain amount of frictional resistance to rotation of both the nut 13 and the wheel 14 occurs as a result of the force which is axially compressing the strut 5 due to the action of the spring 26. The nut 13 transmits this force from the threaded rod 12 to the wheel 14: the wheel then transmits it to the cylinder 15. It is arranged that the thread friction is high enough so that rotation of the wheel is not transmitted (by frictional contact between the wheel and the nut) to the nut. Furthermore, the wheel 14 itself has sufficient frictional resistance to rotation so that it is not rotated in the reverse sense when the pawl 27 returns (upwardly as seen in Figure 1) upon release of the brake. Thus the pawl 27 will "click-over" the ratchet teeth on the wheel 14, if the requirement for adjustment is such that movement of the pawl is far enough.Once per revolution of the wheel 14, the pawl 27 comes radially close enough to the nut 13 to click over the ratchet teeth on the nut, as well. Thus, if the pawl 27 movement is great enough, this movement also turns the nut 13, and adjusts it along the threaded rod, to compensate for lining wear.

It will be appreciated from the foregoing description that the adjuster operates only from time to time, alternating with periods when it cannot operate. During most positions of the wheel 14, the pawl 27 is held clear of the nut 13, and cannot rotate it.

The effect is that adjustment take-up is much slower, making the adjuster much less sensitive to sudden changes in pawl movement, as may occur due to thermal expansion of the drum. The thread however need be no finer than the threads which have been found adequately reliable in known adjusters.

The nut 13 and the wheel 14 move axially right wardiy along the thread during adjustment, as the linings become worn. The pawl 27 remains with the shoe 1 and so must be long enough in the axial direction, to overlie the teeth of the nut and wheel throughout adjustment.

When the brake is becoming excessively worn, however, it is an advantage if further adjustment then ceases, to avoid the possibility of the metal platform of the shoe being adjusted into contact with the drum, when the lining is quite worn away. A tag 28 on the pawl provides this benefit in a cheap and convenient fashion, since when the tag engages the wheel 14, no part of the pawl can engage the teeth on the nut 13, at any circumferential position of the wheel so no further adjustment takes place. The driver of the vehicle notices this (by the increasing actuator travel) and thus is alerted to examine the brakes.

The above-described adjuster has the advantage that since it only operates periodically, the effect is as if the screw thread were very fine, though the thread can actually be of a more conventional pitch, and hence robust and adequately free from seizure problems. Repeated brake actuations with the drum expanded, due either to thermal expansion or mechanical deflection, cause only occasional operations of the adjuster, and are not sufficient to make it over-adjust and to lock-on.

The arrangement of the teeth of the nut and wheel is such that it is simple to bring the pawl into and out of engagement with the teeth. Furthermore, the adjuster is easy to construct and occupies little more space than known adjusters.

The eccentric arrangement of the wheel makes for ease of construction, as circular components are relatively easy to form, compared for instance with a lobed (and hence variable-radius) circumference.

Claims (7)

1. A drum brake adjuster comprising an adjustable length strut having a threaded rod, a complementarily threaded nut on the rod, the nut being provided with ratchet teeth, a pawl which, in response to a requirement for adjustment, is movable, when engaged with the teeth, to move the teeth and turn the nut, and a cam means for holding the pawl out of engagement and for bringing it into engagement with the teeth for alternating predetermined periods.

2. An adjuster as claimed in claim 1, wherein the teeth are axially formed and are pitched on a circle concentric with the axis of the thread, in which the movement of the pawl to turn the teeth is tangential to the nut, and in which the pawl is resiliently biased radially to engage the nut.

3. An adjuster as claimed in claim 1 or 2, wherein the said cam means comprises a wheel rotatably mounted on the threaded rod, the wheel not being threaded to the rod, wherein the circumference of the wheel is of varying radius with respect to the thread axis, and wherein the pawl directly engages the circumference of the wheel so that as the wheel rotates it moves the pawl radially in and out with respect to the teeth on the nut.

4. An adjuster as claimed in claim 3, wherein the wheel is circular, and is mounted eccentrically on the rod, the smallest radius of the wheel being substantially equal to the radius of the nut.

5. An adjuster as claimed in claim 4, wherein the wheel comprises teeth formed on its circumference, the teeth on the wheel being engaged by the pawl.

6. An adjuster as claimed in claim 5, which includes means, effective only when the linings are almost worn out, for maintaining the pawl out of engagement with the nut.

7. An adjuster for a vehicle drum brake, constructed and adapted to operate substantially as described with reference to, and as illustrated in, the accompanying drawings.