It is an object of the present invention to provide an electrically operated drum brake which has a rapid response.

Thus according to the present invention there is provided an electrically operated drum brake comprising a brake shoe displaceable into contact with an associated rotatable brake drum by an electrical actuator to apply the brake, the shoe reacting against a support abutment carrying a load sensing means which actions discontinuance of operation of the electrical actuator in a brake disengaging sense when the load applied to the support abutment by the shoe falls to a predetermined level.

Such a drum brake can be set-up to achieve a minimum shoe to drum clearance (or even slight shoe and drum contact) when the brake is released thus ensuring a quick brake response when the brake is actuated which is particularly important when the brake, for example, forms part of an electrically controlled anti-skid or traction control system.

Conveniently the electrical actuator may comprise an electric motor operated screw jack which increases or decreases its effective length dependent on the direction of current through the motor to apply or release the associated shoe.

The load sensing means may comprise a load cell positioned on the support abutment which gives a voltage output into an associated control circuit dependent on the shoe load applied to the abutment. The control circuit is preferably temperature sensitive to avoid possible over adjustment due to drum expansion.

Alternatively, the load sensing means may comprise an arrangement in which, when the brake is applied, the shoe displaces a support member carried by the abutment against the action of a spring means to close a switch in a brake release circuit and in which during brake release the switch opens when the force applied to the support member by the shoe which is closing the switch drops below the opposing force applied to the support member by the spring means.

Conveniently, the spring means may comprise a belleville spring which surrounds a stem on the support member which operates directly on the switch to close the switch.

By appropriate positioning of the load sensitive means the present invention is applicable to both leading/trailing shoe drum brakes and duo-servo drum brakes.

Figure 1 shows a side view of part of a drum brake in accordance with the present invention.

Figure 2 shows a fragmentary view in the direction of arrow A of figure 1 ; Figure 3 shows a control circuit including a drum brake in accordance with the present invention, and Figure 4 shows an alternative form of load sensing.

Referring to figure 1 this shows a drum brake 10 in which a pair of brake shoes 11 and 12 are supported on a backplate 13 in the normal manner and have their ends 1 la and 12a reacting against a backplate supported abutment 13 a. Between the other ends 1 lb and 12b of the brake shoes is provided an electric actuator 14 in the form of a motor having a casing 15 secured to backplate 12 which encircles a winding 16 and an armature 17 which rotates an inner tubular member 18. Inner member 18 is screw-threaded onto a central projection 15 a of casing 15 so that rotation of inner member 18 increases or decreases the effective length of actuator 14, depending on the direction of rotation of armature 17, to apply or release the brake.

A bearing cap 18a is provided between inner member 18 and the contacting end 1 lb of shoe 11 to accommodate the rotation of inner member 18 by armature 17.

Brake shoe return springs 26 act between shoes 11 and 12 in the conventional manner and a hand brake operating lever 27 pivotted on shoe 12 at 28 is actuated by a cable 29. Lever 27 includes an off-set cam 27a which bears on casing 15 and which displaces actuator 14 to move shoe 11 when lever 27 is rotated by cable 29 to apply the hand brake.

In accordance with the present invention a load sensing means 40, for example in the form of a load cell, is positioned between end 1 la of shoe 11 and abutment 13a. Load cell 40 is connected to a control circuit shown diagrammatically in figure 3 in which a microprocessor 41 receives the voltage output from load cell 40 and produces output signals to control solenoid-operated switches 42 and 43 as will be explained below.

Contacts 42a and 42b of switch 42 have two positions and are used to reverse the direction of the current applied to motor 14 by battery 44. This switch is controlled from microprocessor 41 via line 41 a in response to signals from a transducer in the form of a rotary potentiometer 45 associated with brake pedal 46 which is depressed by the vehicle operator against a spring 47 to give the brake pedal the conventional pedal feel. Current is supplied to motor 14 in one direction via lines 48 and 49 when pedal 46 is depressed to increase the effective length of the motor to apply the brake. Similarly when the pedal 46 is being released current is supplied to motor 14 in the other direction via lines 50 and 51.

Line 50 also includes switch 43 which is opened by microprocessor 41 via line 41b when the signal from load cell 40 falls below a predetermined load level. Thus as soon as the load applied to abutment 13a by shoe 11 drops below the predetermined load level further release of the brake is stopped by the opening of switch 43 which is in the brake release circuit.

Suitable choice of the load level at which switch 43 opens therefore enables the retraction of shoe 11 (and hence shoe 12) to be controlled so that a very small shoe clearance is obtained thus enabling the brake to have a quick response when actuated. This is a particularly important feature wherein the brake forms part of, for example, an electronically controlled anti-skid or traction control system where quick response is essential.

As an alternative to arranging micro processor 41 to open a switch in the brake release circuit when the load applied to abutment 13 a falls to a predetermined level, invention may be realised by arranging that a timer is started when the predetermined load level is reached which allows the motor to continue retraction for a predetermined time period thus again controlling the shoe to brake drum clearance of the brake.

In a still further alternative arrangement when the predetermined load is reached the retraction of the brake shoe may be limited by counting a predetermined number of turns of the retracting motor after the load level has been reached thus again controlling the shoe to drum clearance.

The various alternative arrangements described above are preferably temperature sensitive to avoid possible over adjustment due to drum expansion.

Figure 4 shows an alternative load sensing arrangement in which the load cell 40 is replaced by an arrangement comprising a shoe support member 60 having a stem 61 which directly operates switch 43 in the brake retraction circuit. Shoe support member 60 is supported from abutment 13a and is moved towards this abutment against the action of a spring means in the form of a belleville washer 62. As will be appreciated, when the brake is applied belleville washer 62 is squashed flat and switch 43 is closed by stem 61. During brake release washer 62 remains squashed until the force supplied to shoe support member 60 by washer 62 is greater than the force supplied to support member 60 by shoe 11 whereupon support member 60 moves away from abutment 13a allowing switch 43 to open.

Thus the load level at which switch 43 opens can be directly tuned by use of an appropriate spring characteristic in washer 62. As will be appreciated other forms of spring means could be used in place of belleville washer 62.

Although the present invention has been described above in relation to a leading/trailing shoe drum brake it is equally applicable to a duo-servo drum brake which has no fixed abutment 13a between the shoes and in which the braking forces are communicated back to the right hand end of actuator 14 (as viewed in Figure 1). In such an arrangement the load sensing means 40 is preferably located between the right hand and of actuator 14 and the end 12b of shoe 12 in order to experience the total braking force although it may be located between shoes 11 and 12 if desired.

Also, the amount of retraction of the shoes could also be made variable dependent on the level of braking force applied since high levels of braking force will result in more deflection of the components of the brake and hence require more retraction.