Abstract:
A co-axial wear adjuster and wear sensor for a brake caliper includes a sleeve having an external tooth form engageable with a toothed adjustment wheel of a brake tappet. The sleeve rotates with the tappet and is threaded to engage a relatively non-rotatable, but axially movable, bolt. A wear sensor is partly within the sleeve and has a plunger operably engaged with the bolt. The bolt may be tubular to carry a screw-threaded head whereby a position of the plunger may be set.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to United Kingdom Patent Application No. GB 0608956.9 filed on May 5, 2006.  
       BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to a wear adjuster of a disc brake, particularly to an air actuated a disc brake.  
         [0003]     Disc brakes, as are well known, include a rotor and a brake caliper which straddles the rotor and can be actuated to apply a braking force to the rotor via opposed brake pads. In commercial vehicles, air actuation is almost universal, and an air actuator is provided at each vehicle wheel to actuate the brake caliper mechanically via a lever.  
         [0004]     It is not generally practicable to arrange the actuating mechanism at an outboard side of the rotor, and thus the relevant components are always located at an inboard side. Particularly with vehicles having smaller wheels, such inboard mounting is rather space consuming so that design freedom of suspension and steering components may be compromised. Furthermore, the provision of additional features, such as wear sensors, may be problematic.  
         [0005]     In one conventional design, the actuating mechanism includes an inboard tappet operable to directly apply an inboard brake pad, an automatic wear adjuster at one side of the tappet, a manual wear adjuster at the other side of the tappet, and a wear sensor having an electrical output indicative of brake pad and rotor wear. In addition to physically locating these components in the brake caliper, each component must also be sealed against moisture and dirt to avoid corrosion and/or seizure.  
         [0006]     EP-A-1596092, in the name of the present applicant, proposes a rotary wear indicator mounted on a manual wear adjuster mechanism that has a visual output for giving an indication of pad wear to a mechanic. The wear indicator is removed for de-adjustment of the adjuster mechanism (for example, to replace worn brake pads) and must be re-set upon replacement.  
         [0007]     What is required is a wear adjuster/wear indicator which is adapted for replacement or adjustment of brake pads without requiring re-setting or calibrating.  
       SUMMARY OF THE INVENTION  
       [0008]     According to the invention, there is provided an assembly of a wear adjuster and a wear indicator of a brake caliper. The wear adjuster has an axis of rotation and includes a sleeve rotatable about the axis of rotation. The wear indicator includes a body at one end of the sleeve and a plunger movable relative to the body on the axis of rotation. The plunger is located within the sleeve.  
         [0009]     The wear adjuster and the wear indicator are coaxial, thus minimizing the space occupied and permitting direct abutment between the wear adjuster and the wear indicator. In one embodiment, the assembly further includes a bolt screw-threaded in the sleeve at the opposite end to the body. The bolt is adapted to be restrained against rotation and has a head engaged by the plunger.  
         [0010]     In such an arrangement, rotation of the sleeve as a consequence of incremental wear adjustment causes axial movement of the bolt due to the screw-threaded connection, and hence relative movement of the plunger on the axis. A wear sensor may, for example, be a linear transducer having an output indicative of a relative extension of the plunger. The output is preferably electrical and adapted to be directly proportional to movement of the incremental wear adjuster. In one example, the wear sensor includes a cylindrical body from which the plunger protrudes and which is inserted at least partially in the sleeve.  
         [0011]     In one embodiment, the bolt is tubular and internally screw-threaded to receive a separate head protruding on a plunger side for contact with the plunger. In such an arrangement, the initial relative position of the plunger may be set by screwing the head relative to the bolt. Thereafter, no further adjustment of the head is required in the event that the wear sensor is removed to permit rotation of the adjuster sleeve, for example, for de-adjustment of the usual tappet.  
         [0012]     The sleeve can be turned by an internal discontinuity that is engageable by a turning tool inserted via the mouth of the sleeve. Alternatively, a slot at the mouth may be provided.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Other features of the invention will be apparent from the following description of a preferred embodiment illustrated by way of example only in the accompanying drawings in which:  
         [0014]      FIG. 1  illustrates in schematic plan a conventional disc brake installation of a commercial vehicle; and  
         [0015]      FIG. 2  is a schematic axial section through a combined adjuster/wear sensor according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     A conventional air actuated brake caliper installation is illustrated schematically in  FIG. 1 . A brake rotor  11  has an axis of revolution  12 . Bridging the brake rotor  11  is a brake caliper body  13  (dotted outline) including a fixed part mounted on a vehicle axle/stub axle and a sliding (bridge) part adapted to transfer braking forces from an inboard side to an outboard side of the brake rotor  11 .  
         [0017]     Brake pads  14  and  15  are arranged on either side of the brake rotor  11 . The outboard brake pad  14  is actuated by an outboard member  16   a  of the sliding part. The inboard brake pad  15  is actuated via a tappet assembly  17  slidable in the sliding part and reacts against an inboard member  16   b  of the sliding part.  
         [0018]     A brake actuator  19  acts via a conventional operating lever (not shown) to urge the tappet assembly  17  and the inboard member  16   b  (as represented by an arrow  20 ) apart, thus directly applying the inboard brake pad  15  to the brake rotor  11 , and indirectly applying the outboard brake pad  14  to the brake rotor  11  via the outboard member  16   a.    
         [0019]     The tappet assembly  17  includes a cylindrical tappet  21  internally threaded and slidable, but not relatively rotatable, in the caliper body  13 . The tappet assembly  17  includes an adjuster wheel  22  screw threaded to the cylindrical tappet  21 . The adjuster wheel  22  is externally toothed as illustrated, and relative rotation thereof results in an increase or reduction in an axial length of the tappet assembly  17 .  
         [0020]     The brake caliper includes several additional components on the inboard side. A wear adjuster includes a body  31  mounted in a caliper sliding part and a relatively rotatable toothed output wheel  32  in mesh with the adjuster wheel  22 . The toothed output wheel  32  moves unidirectionally in operation (typically by virtue of a wrap spring or roller clutch), and the wear adjuster is actuated by an arm of the operating lever (not shown), which causes the separation indicated by an arrow  20 .  
         [0021]     Accordingly, the adjuster wheel  22  can be rotated by the toothed output wheel  32  to increase a length of the tappet assembly  17  to compensate for wear of the brake pads  14  and  15  and the brake rotor  11 . Such an arrangement is well known and need not be further described here.  
         [0022]     A manual adjuster is provided on an opposite side of the tappet assembly  17  that includes a shaft  33  which incorporates a de-adjuster wheel  34  in mesh with the adjuster wheel  22 . The shaft  33  extends axially on the inboard side and terminates in a head  35  adapted to receive a turning tool. Any suitable head form may be provided (internal or external) such that application of the turning tool, such as a socket spanner, can rotate the de-adjuster wheel  34  to permit a reduction in length of the tappet assembly  17  for fitting of replacement brake pads. The wear adjuster may need to be deactivated to permit de-adjustment, and the de-adjuster may be used in reverse for setting an initial pad/rotor clearance after pad replacement.  
         [0023]     Also mounted in the sliding part of the caliper body  13  is an electronic wear sensor including a fixed body  36  and a resiliently biased plunger  37 . The plunger  37  bears against a cranked arm  39  which is screw-threaded to the shaft  33 . As the brake pads wear, the adjuster wheel  22 , and consequently the manual de-adjuster wheel  34 , is rotated. As a result, the cranked arm  39  moves in the direction of the axis of revolution  12  and hence alters the extension of the plunger  37  with respect to the fixed body  36 , thus giving a changing electrical output via leads  38 . The wear sensor may, for example, include a linear potentiometer giving an output voltage proportional to extension of the plunger  37 .  
         [0024]     The pitch of the screw thread between the cranked arm  39  and the shaft  33  may be different from that of the tappet assembly  17  to give a desired output signal from the wear sensor. Such an arrangement is known and need not be further described here.  
         [0025]      FIG. 2  illustrates an embodiment of the invention. A combined wear adjuster/wear sensor  41  includes a toothed adjuster wheel  42  for mesh with a tappet adjuster wheel  22 . The toothed adjuster wheel  42  is equivalent to the de-adjuster wheel  34  of  FIG. 1 .  
         [0026]     A tubular body  43  (also referred to as a sleeve) is fast within the toothed adjuster wheel  42  and extends to the inboard side. The tubular body  43  can rotate with the toothed adjuster wheel  42  within a support defined by a caliper bridge  45 .  
         [0027]     A cylindrical extension  46  of the tubular body  43  projects towards an outboard side and has a screw threaded interior to receive a screw-threaded sleeve  47 . An outboard end of the sleeve  47  has a projection  48  for non-rotational, but axially slidable, engagement with respect to the brake housing  49  via a keyway  60 . The sleeve  47  is also screw threaded on the inside to receive a setting bolt  50  having a bolt head  51  at the inboard side. The assembled screw threaded sleeve  47  and the setting bolt  50  form a contact member  62 . A cylindrical wear sensor  52  (or wear indicator) enters within the tubular body  43  and is in axial abutment with an outer face  55  of the caliper bridge  45 . The wear sensor  52  is retained by a bracket  56  and a set screw  57 . O-rings  58  provide a seal between the wear sensor  52  and the caliper bridge  45  to prevent ingress of moisture. The wear sensor  52  has a spring biased plunger  54  in contact with the bolt head  51 , as illustrated.  
         [0028]     The tubular body  43  has an internal hex  53  which is axially spaced from a body of the wear sensor  52 . In use, the setting bolt  50  is located at a suitable projection from the sleeve  47  according to a length of the tappet assembly  17 . The initial setting step plays no part in the function of the combined adjuster/wear indicator, and thereafter the setting bolt  50  and the sleeve  47  may be considered as a unitary contact member  62 . In some circumstances, the setting bolt  50  may be omitted so that the plunger  54  bears directly on a suitably shaped end of the sleeve  47 , which in that circumstance would form the contact member  62  alone.  
         [0029]     In use, and with reference to  FIG. 1 , progressive wear of the brake pads  14  and  15  will cause relative rotation of the adjuster wheel  22  by the toothed output wheel  32 . As a consequence of tooth meshing, the toothed adjuster wheel  42  will rotate. The sleeve  47  is however fixed against rotation with respect to the brake housing  49 , and accordingly will move relatively to the right (as viewed) with respect to the tubular body  43 . As a consequence, the plunger  54  will be pushed inwardly of the body of the wear sensor  52 , thus changing the output thereof.  
         [0030]     The plunger  54  moves into the wear sensor  52  and as it does, the output from the wear sensor  52  changes to continuously indicate an amount of wear. By utilizing continuous indication, the wear sensor  52  produces a signal indicating the amount of wear rather than a condition for replacement (i.e., binary indication). This may be achieved, for example, with a variable resistor actuated by the plunger  54 .  
         [0031]     As in the prior art, the sleeve  47  and the tubular body  43  may move at a different relative rate to the cylindrical tappet  21  and the adjuster wheel  22  to provide a desirable output range of the wear sensor  52 , which is a direct indication of wear of the brake pads and the rotor. An exact linear relationship is not, however, a requirement of the invention provided that the effective stroke of the plunger can be related to the wearing thickness of the brake pads and the rotor.  
         [0032]     Output from the wear sensor  52  is via a twisted wire pair, but other arrangements are possible. For example power, ground and signal wires may be provided.  
         [0033]     When the brake pads are fully worn, and due for replacement, the wear sensor  52  is removed by releasing the set screw  57 , and a hex key is applied to the internal hex  53  to wind the cylindrical tappet  21  inwards with respect to the adjuster wheel  22 , at the same time screwing the sleeve  47  to the right with respect to the brake housing  49 . After turning to the desired degree which permits pad replacement, the internal hex  53  may be wound in the opposite direction to set initial pad/rotor clearance. Thereafter, the wear sensor  52  is inserted up to the outer face  55 , and the plunger  54  will extend under resilient loading to the bolt head  51  (at maximum extension).  
         [0034]     The internal hex  53  may be provided at a different axial location, or may be substituted by some other suitable discontinuity (for example, a slot in the end of the tubular body  43 ), which is adapted to receive a turning tool.  
         [0035]     Removal and replacement of the wear sensor  52  does not change the relative position of the plunger  54  with regard to brake pad thickness. Accordingly, the plunger  54  will adopt an appropriate extension without further setting measures, and thus a mechanic merely removes the wear sensor  52  to rotate the tubular body  43  and then replaces the wear sensor  52 . Non-replacement of the wear sensor  52  may be sensed by the plunger  54  being at maximum extension, and accordingly starting of the vehicle engine may be inhibited.  
         [0036]     The plunger  54  is in one example continuously in contact with the sleeve  47  during operation of the assembly during operation. “Operation” is defined as the situation in which the wear sensor  52  is installed on the brake, and as such is capable of indicating the brake wear.  
         [0037]     The foregoing description is only exemplary of the principles of the invention. Many modifications and variations are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than using the example embodiments which have been specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.