1. A drum brake assembly for a heavy commercial vehicle, the drum brake assembly comprising: a brake drum; first and second brake shoes to be moved towards a braking surface of the brake drum during a braking operation; a cam shaft for transmitting braking force from an actuator to apply the brake; a first tappet and a second tappet for transmitting braking force from the cam shaft to the first and second brake shoes; a wear adjuster mechanism for adjusting the first and second tappets to address wear of the brake shoes and the braking surface of the brake drum, the wear adjuster mechanism comprising a cross shaft for transmitting rotational force from the first tappet to the second tappet; and a sensor for detecting rotation of the cross shaft about a longitudinal axis of the cross shaft in order to monitor wear of the brake shoes and the braking surface of the brake drum.
2. A drum brake assembly according to claim 1, wherein the sensor is configured to continuously detect rotation of the cross shaft, such that wear of the brake shoes and the braking surface is continuously monitored.
3. The drum brake assembly according to claim 2, wherein the sensor is mounted on the cam shaft, preferably wherein the sensor is configured to detect angular displacement of the cam shaft about the longitudinal axis of the cam shaft.
4. The drum brake assembly according to claim 2 or claim 3, further comprising a rotation indicator for indicating rotation of the cross shaft to the sensor, wherein the rotation indicator includes a first portion configured for movement with the cross shaft, and a second portion engaged with the first portion such that movement of the first portion causes proportional movement of the second portion, wherein the sensor is configured to detect movement of the second portion.
5. The drum brake assembly according to claim 4, wherein the first portion comprises a threaded portion of the cross shaft, preferably wherein the threaded portion is integral to the cross shaft, or wherein the threaded portion comprises an externally threaded sleeve mounted to the cross shaft for rotation with the cross shaft.
6. The drum brake assembly according to claim 5, wherein the second portion comprises a gear with teeth corresponding to the thread of the first portion, such that rotation of the threaded portion with the cross shaft causes rotation of the gear.
7. The drum brake assembly according to claim 6, wherein the gear is mounted on the cam shaft; preferably wherein, when the sensor is mounted on the cam shaft, the gear is mounted on the cam shaft via the sensor.
8. The drum brake assembly according to claim 5, wherein the second portion comprises a collar mounted on the cross shaft, and an arm, wherein the collar comprises an internal thread for engaging the threaded portion and a radial projection configured to engage the arm such that rotation of the cross shaft causes axial movement of the collar with respect to the cross shaft, and pivoting of the arm about a first end of the arm, and wherein the sensor is configured to detect pivoting of the arm.
9. The drum brake assembly according to claim 8, wherein the first end of the arm is mounted on the cam shaft; preferably wherein, when the sensor is mounted on the cam shaft, the first end of the arm is mounted on the cam shaft via the sensor.
10. The drum brake assembly according to claim 1 or claim 2, further comprising a housing, wherein the sensor is mounted to the housing.
11. The drum brake assembly according to any preceding claim, wherein the sensor is an encoder, or wherein the sensor is a potentiometer.
12. The drum brake assembly according to any preceding claim, further comprising a control system for monitoring wear of the brake shoes and the braking surface, wherein the control system is configured to calculate wear from cumulative motion of the cross shaft during multiple braking operations, preferably wherein the control system comprises an indicator for indicating the state of wear of the brake shoes and/or the braking surface.
13. A method of monitoring wear of the brake shoes and the braking surface of the drum brake assembly of claim 12, the method comprising the steps of: detecting motion of the cross shaft via the sensor; and using the control system to calculate wear from cumulative motion of the cross shaft; preferably further comprising the step of providing an indication to an operator of the state of wear of the brake shoes and/or the braking surface.
14. A drum brake assembly for a heavy commercial vehicle, the drum brake assembly comprising: a brake drum; first and second brake shoes to be moved towards a braking surface of the brake drum during a braking operation; a cam shaft for transmitting braking force from an actuator to apply the brake; a first tappet and a second tappet for transmitting braking force from the cam shaft to the first and second brake shoes; a wear adjuster mechanism for adjusting the first and second tappets to address wear of the brake shoes and the braking surface of the brake drum; and a sensor configured to detect angular displacement of the cam shaft about the longitudinal axis of the cam shaft.
15. The drum brake assembly according to claim 14, wherein the sensor is mounted on the cam shaft.
