Auto-adjusting device with liner wear detection

Abstract

An auto-adjusting device for a drum brake, a drum brake and a liner wear detection system are disclosed. The self-adjusting 60 device comprises a first part 64 and a second part 68 linearly displaceab

Description

The invention relates to an auto-adjusting device for a drum brake, comprising a first part and a second part which is linearly displaceable with respect to said first part, and a spacing mechanism which adjusts the distance between first and second part to a compensating distance to compensate for liner wear. It also relates to a drum brake and a liner wear detection system. For drum brakes, auto-adjusting devices which compensate for the wear of brake liners are known. Mechanical adjusters are among the earliest and most common types of auto-adjusting devices used in drum brakes. They typically rely on a ratcheting mechanism or a screw-type adjuster to maintain the correct clearance between the brake shoe and the drum as the liner wears down. Also known, for instance, are self-energizing adjusters which leverage the force generated during braking to facilitate the adjustment process or hydraulic adjusters utilizing the brake fluid pressure to make automatic adjustments. If the wear of the liners is advanced too much or the respective liner is completely worn, conducting a safe braking manoeuvre is no longer possible, which is a severe safety risk. DE 199 15 007 C1 discloses a drum brake comprising an automatic adjustment device for compensating for the wear of friction linings and a drum. The adjustment device comprises a signalling device which supplies a signal dependent on an adjustment travel of the adjustment device. The signalling device comprises a switch which is activated once a certain wear threshold is reached. A disadvantage of this solution is that only when wear has reached a maximum allowed threshold and the switch is activated, a warning can be issued. The object of the invention is to provide an improved auto-adjustment device with integrated liner wear detection. According to the invention, this object is solved by an auto-adjusting device according to claim 1. The auto-adjusting device for a drum brake comprises a first part and a second part which is linearly displaceable with respect to the first part, and a spacing mechanism which adjusts the distance between first and second parts to a compensating distance to compensate for liner wear, whereby the auto-adjusting device comprises a liner wear indicator which is built to gradually measure the compensating distance. Preferred embodiments are subject of the dependent claims. The invention is based on the consideration that in modern drum brakes auto adjusters are provided maintaining the constant distance between the drum and the lining material. Also known are liner wear indicators which signal a threshold wear and are built as a switch which becomes active one the threshold wear state is reached. It would be, however, advantageous to know gradually how much the lining is worn on the assembly. Applicant has found hat information of the current wear state is accessible by integrating a gradual liner wear indicator with an auto-adjuster. If the compensating distance of the auto-adjuster is measured, information on the wear state can be deduced from this measured compensating distance. In this way, an intelligent auto-adjuster is realized which not only compensates for wear but also determines the wear status gradually. Preferably the liner wear indicator comprises a strain element connected to the first part and second part and a strain gauge attached to the strain element. As with wear first and second pars move apart from each other, the strain element tends to assume a straighter shape. The decrease of strain in the strain element can be detected by the strain gauge. The signal of the strain gauge represents the liner wear state. In an alternative preferred embodiment, the liner wear indicator is built to measure the compensating distance with a sensor emitting waves by an emitter and receiving reflected waves reflected from a reflecting element. The time of flight of the emitted waves between their emission and their return to the sensor after reflection represents the compensating distance and therefore the liner wear state. Preferably, the sensor is arranged on the first part of the auto-adjusting device or a first element connected to the first part and the reflecting element is arranged on the second part of the auto-adjusting device or a second element connected to the second part. Advantageously, the first and second elements are built as sleeves. The sensor can be built to emit sound waves or electromagnetic waves. In a first preferred variant, the sensor an ultrasonic sensor. In a second preferred variant, the sensor is a LIDAR sensor. Both sensors have their advantages and disadvantages. A LIDAR sensor generally is more accurate and has more range compared to ultrasonic sensors and its data transmission rate is usually faster. A LIDAR sensor has lesser noise compared to an ultrasonic sensor but does not work through transparent objects. An ultrasonic sensor is generally cheaper but is affected by soft objects. The reflecting element is preferably built a plate or surface with good reflection properties for the waves emitted by the sensor. The first part of the auto-adjusting device preferably at a first end comprises a trunnion for engagement with a leading shoe of a drum brake, whereby the second part at a second end comprises a cross strut for engagement with a trailing shoe of the drum brake. In a preferred embodiment, a clicker is arranged at the said first part. The auto-adjusting devices preferably comprises a ratchet wheel encompassing the first part at least partly (in axial direction), and comprises a pawl connected to the clicker which is engaged with the ratchet wheel. The invention also relates to a drum brake which comprises a liner wear indicator described above. The invention further relates to a liner wear detection system, comprising an auto-adjusting device described above and a measuring unit with the liner wear indicator of the auto-adjusting device connected to a signal input side of the measuring unit. The output signal generated by the measuring unit is advantageously communicated to the driver by means of gradual warning indicator or a percentage information on the dashboard. The output signal can also be transmitted to a control unit of the brake system (ABS, ASR, brake-by-wire). A strong change in the signal during such a period indicates particularly strong, short-term brake wear and indicates high and indicates a high load on the brake. Overloading of individual brakes can be avoided by taking appropriate countermeasures. The advantages of the invention are especially as follows. The liner wear indicator integrated into the auto-adjusting device / auto-adjuster allows to provide to the driver / owner on the dashboard the status of lining material in the brake assembly without any physical inspection on the car or taking the car to the garage centre. A preferred embodiment is described below in connection with a drawing. In this drawing, FIG. 1 shows a drum brake with an auto-adjuster; FIG. 2 shows an auto-adjusting device in a preferred embodiment in state of no liner wear; FIG. 3 shows the auto-adjusting device of FIG. 2 in a state of liner wear; FIG: 4 shows an auto-adjusting device in a further preferred embodiment in state of no liner wear; FIG. 5 shows the auto-adjusting device of FIG. 4 in a state of liner wear; FIG. 6 shows an auto-adjusting device in a further preferred embodiment in state of no liner wear; FIG. 7 shows the auto-adjusting device of FIG. 6 in a state of liner wear; FIG. 8 very schematically shows a liner wear indicator system. Same parts are labelled with identical reference numerals in all figures. A drum brake 2 shown in FIG. 1 comprises a back plate 6, a leading shoe 10 and a trailing shoe 14 with a hand brake lever 24 as well as a wheel cylinder 18. The drum brake 2 comprises an upper spring 28 and a lower spring 32 as well as an abutment 36 and an abutment rivet 40. An optional cable guide plate 44 is provided in drum brake 2. The drum brake 2 further comprises a liner inspection plug 48, a shoe hold down clip 52 and a shoe hold down pin 56. Integrated into drum brake 2 is a auto-adjusting device 60 or auto-adjuster, which compensates for liner wear and which is described in detail below. In FIG. 2, an auto-adjusting device 60 which is part of drum brake 2 of FIG. 1 is shown in a first preferred embodiment in a state with no liner wear. The auto-adjusting device 60 comprises a first part 64 and a second part 68, whereby the second part 68 is linearly displaceable with respect to first part 64. To this end, first part 64 comprises a sleeve 72 which receives a cylinder part 76 of second part 68. First part 64 comprises a trunnion 80 for engagement in the leading show 10 of a drum brake 2. Second part 68 comprises a cross strut 85 for engagement with the trailing shoe 14 of a drum brake 2. In first part 64 a clicker 88 is arranged. A pawl 92 connected to clicker 88 or built integrally with clicker 88 engages with a ratchet wheel 94 which at least partially in axial direction of auto-adjusting device 60 encompasses first part 64. When the auto-adjusting device 60 is assembled on the drum brake 2, the clicker 88 is in a compressed condition I in a preloaded condition. After application of the brake pedal, the shoes 10, 14 are moved out by wheel cylinder 18. This releases the compression of clicker 88, which further pushes the trunnion 80 towards a web of leading show 10. If the movement of shoes 10 ,14 is still larger, clicker 88 stretches towards a straighter shape to release the compression. The pawl 92 integrated with clicker 88 pushes the ratchet wheel 94 up during release of compression. This results in an increment of the next teeth of ratchet wheel 94. The rotary movement of ratchet wheel 94 is converted into a linear movement of cross strut resulting in compensation of excessive gap between Shoes 10, 14 and back plate 6. Integrated with the auto-adjusting device 60 is a liner wear indicator 100 which in the preferred embodiment shown comprises a strain element 104 and a strain gauge 108 attached to strain element 104 for measuring strain of strain element 104. The strain element 104 is connected at a first end to first part 64 and at a second end to second part 68. This is achieved by a sleeve 112 attached to second part 68. It in this way extends clicker 88 towards second part 68. Clicker 88 is thus extended till the fork end with cross strut 84 of second part 68 which engages the web of trailing shoe 14. The strain gauge 108 which monitors the strain acting on strain element 104 which can be built as a bendable / compressible plate. As the brake liner starts wearing the auto-adjusting device 60 pushes the fork ended side towards the drum. Due to this movement, the strain element 104 with strain gauge 108 will extends in a tendency to become flat, thus, producing a difference in strain value from the initial setting value. In FIG. 2, a distance between an upper side of fork part of cross strut 84 and an upper side of strain element 104 is indicated by letter “D” (“upper” refers to the figure and not to the mounting state of the auto-adjusting device 60). In FIG. 3, the auto-adjusting device 60 is shown in a state with liner wear. The length L between first part 64 and second 68 corresponding to the compensated distance has increased as cylinder part 76 has moved to the right in FIG. 3. The distance D has decreased and the strain element 104 has bent towards a straighter shape. This change of strain of strain element 104 is measured by strain gauge 108 and the signal of strain gauge 108 can be mapped to a wear state of liners of drum brake 2. An auto-adjusting device 60 in a second preferred embodiment is shown in FIG. 4. The auto-adjusting device 60 differs from auto-adjusting device of FIGs. 2 and 3 in the design of liner wear indicator 100. Indicated in FIG. 4 is a contact area 120 of trunnion 80 with leading shoe 10 and a contact area 124 of cross strut 84 with trailing shoe 14. A first sleeve 128 is attached to first part 64 and a second sleeve 132 is attached to second part 68. Attached to first sleeve 128 is an ultrasonic sensor 140 and attached to second sleeve 132 is a reflection plate 144 for ultrasonic waves. As the brake liner starts wearing the auto adjuster mechanism pushes the fork ended side towards the drum. The ultrasonic sensor 140 is used to measure the distance d (see FIG. 5) compensated by the auto adjuster at any given time. Since ultrasonic sensor 140 uses sound as a medium to measure distance, sound from sensor is deflected back from the reflection plate 144 or detection plate to measure the distance d. To combat the issue of positional variations in length (which are not relevant to pad wear), preferably the distance to new vs. worn condition is taken as 100% and then split into 10 to denote wear in terms of increment of 10%. The output signal of the ultrasonic sensor 140 represents the liner wear. In FIG. 5, the auto-adjustment device 60 of FIG. 4 is shown in a state with liner wear. In FIG. 6, an auto-adjusting device 60 in still a further preferred embodiment is shown. The auto-adjusting device 60 differs from the auto-adjusting device of FIGs. 4 and 5 in that the liner wear indicator 100 comprises a LIDAR sensor 150 and a reflection plate 154 or detection plate. As the brake liner starts wearing the auto adjuster mechanism will push the fork ended side towards drum 2. The Laser / LIDAR sensor 150 is used to measure the distance d (see FIG. 7) compensated by the auto-adjusting device 60 at any given time. The LIDAR sensor 150 is attached to first sleeve 128 and a detection plate or reflection plate 154 is attached to a second sleeve 132 right before the cross strut 84 which is connected to the web of trailing shoe 14. Since LIDAR uses light / laser as a medium to measure distance, a laser beam emitted from the LIDAR sensor 150 is deflected back from the reflection plate 154 to measure the distance d. Also here, to combat the issue of positional variations in length (which are not relevant to pad wear), preferably the distance to new vs. worn condition is taken as 100% and then split into 10 to denote wear in terms of increment of 10%. In FIG. 7, the auto-adjustment device 60 of FIG. 6 is shown in a state with liner wear. A liner wear detection system 160 is shown in FIG. 8. It comprises an adjusting-device 60 with liner wear indicator 100 as well as a measuring unit 164. The sensor 140, 150 or gauge 108 is connected to an input side of measuring unit 164 by a signal connection 168. The measuring unit 164 processes the sensor or gauge sensor data and produces an output signal 172 representing the liner wear state. The output signal 172 can, for instance, be optionally further processed and routed to the dashboard. In this way, the driver obtains a clear indication of the wear state from the no-wear condition of the liner until the condition in which the liner is not functioning properly anymore and liner replacement becomes necessary. List of reference numerals 2 drum brake 6 back plate 10 leading shoe 14 trailing shoe 18 wheel cylinder 24 hand brake lever 28 upper spring 32 lower spring 36 abutment 40 abutment rivet 44 cable plate guide 48 liner inspection plug 52 shoe hold down clip 56 shoe hold down pin 60 auto-adjusting device 64 first part 68 second part 72 sleeve 76 cylinder part 80 trunnion 84 cross strut 88 clicker 92 pawl 94 ratchet wheel 100 liner wear indicator 104 strain element 108 strain gauge 112 sleeve 120 contact area 124 contact area 128 first sleeve 132 second sleeve 140 ultrasonic sensor 144 reflection plate 150 LIDAR sensor 5 154 reflection plate 160 liner wear detection system 164 measuring unit 168 signal connection 172 output signal 10 D distance L length d distance

Claims

1. Auto-adjusting device (60) for a drum brake (2), comprising a first part (64) and a second part (68) which is linearly displaceable with respect to said first part (64), and a spacing mechanism which adjusts the distance between first (64) and second part (68) to a compensating distance (d) to compensate for liner wear, characterized in thatsaid auto-adjusting device (60) comprises a liner wear indicator (100) which is built to gradually measure said compensating distance (d).

2. Auto-adjusting device (60) according to claim 1, whereby said liner wear indicator (100) comprises a strain element (104) connected to said first part (64) and said second part (68), and a strain gauge (108) attached to said strain element (104).

3. Auto-adjusting device (60) according to claim 1, whereby said liner wear indicator (100) is built to measure said compensating distance (d) with a sensor (140, 150) emitting waves by an emitter and receiving reflected waves reflected from a reflecting element (144, 154).

4. Auto-adjusting device (60) according to claim 3, whereby said sensor (140, 150) is arranged on said first part (64) or a first element connected to said first part (64) and whereby said reflecting element (144, 154) is arranged on said second part (68) or a second element connected to said second part (68).

5. Auto-adjusting device (60) according to claim 4, whereby said first and second elements are built as sleeves (128, 132).

6. Auto-adjusting device (60) according to claim 4 or 5, whereby said sensor is built as an ultrasonic sensor (140).

7. Auto-adjusting device (60) according to claim 4 or 5, hereby said sensor is built as a LIDAR sensor (150).

8. Auto-adjusting device (60) according to one of the claims 1 to 7, whereby said first part (64) at a first end comprises a trunnion (80) for engagement with a leading shoe (10) of a drum brake (2), and whereby said second part (68) at a second end comprises a cross strut (84) for engagement with a trailing shoe (14) of said drum brake (2).

9. Auto-adjusting device (60) according to one of the claims 1 to 8, whereby a clicker (88) is arranged at said first part (64).

10. Auto-adjusting device (60), comprising a ratchet wheel (94) encompassing said first part (64) at least partly, and comprising a pawl (92) connected to said clicker (88) which is engaged with said ratchet wheel (94).

11. Drum brake (2), comprising an auto-adjusting device (60) according to one of the claims 1 to 10.

12. Liner wear detection system (160), comprising an auto-adjusting device (60) according to one of the claims 1 to 10 and a measuring unit (164) with said liner wear indicator (100) of said auto-adjusting device (60) connected to a signal input side of said measuring unit (164).

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