Abstract:
A drum brake includes an automatic adjusting apparatus for compensating for the wear of friction linings and of a drum. To detect the wear of the drum brake, the adjusting apparatus includes a signalling device, which supplies a signal dependent upon an adjusting travel of the adjusting apparatus. The adjusting travel is generated by a variable length adjusting strut, which includes a pressure sleeve and a threaded rod, which is accommodated in the latter and is screwable by means of a pinion out of the pressure sleeve.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to an automatic drum brake adjusting apparatus for compensating for friction lining and drum wear, comprising a signalling device, which supplies a signal dependent upon an adjusting travel. The invention further relates to a drum brake having such an adjusting apparatus.  
           [0002]    EP 0 460 378 A2 describes a brake actuating apparatus having a brake adjusting device, which is used to maintain an approximately constant clearance between the brake shoes and a brake drum as well as to monitor brake lining wear. In this case, a first signal transmitter, which produces a signal corresponding to the travel during negotiation of the clearance, and a second signal transmitter, which produces a signal corresponding to the travel during adjustment, are provided. The signals may be electrically evaluated. The provision of two signal transmitters on the adjusting device is cost and labour intensive.  
           [0003]    A drum brake having an automatic adjusting apparatus is known from DE 25 08 614 C2. The adjusting apparatus comprises a strut, which is variable in length and is disposed between a brake shoe and an actuating lever and held by spring action in abutment with the brake shoe. A ratchet lever is pivotally supported on the brake shoe and engages into a ratchet wheel rotatably supported on the strut. As the friction lining provided on the brake shoe wears, the ratchet lever turns the ratchet wheel and hence adjusts the length of the adjustable strut.  
           [0004]    In EP 0 140 241 it is proposed to equip such a drum brake with a wear warning conductor, which is fastened in a plug made of elastic, electrically insulating plastics material to one end of a brake shoe. The wear warning conductor is laid in a loop around the plug in such a way that, during braking, it comes into contact with the brake drum when the friction lining is extensively but not completely worn away. Similar wear warning systems are known from U.S. Pat. No. 1,902,759, U.S. Pat. No. 2,146,357, U.S. Pat. No. 2,217,176 and DE 709 251. The aim of these known apparatuses is to determine when friction linings of the drum brakes of the vehicle need to be replaced or brake shoes need to be adjusted.  
           [0005]    DE 709 251 and U.S. Pat. No. 2,182,554 disclose apparatuses, by means of which it is possible to identify whether all of the brakes are identically adjusted and hence may also be applied uniformly and simultaneously. DE 709 251, in this case, on the basis of the previously described principle of a sliding contact between a wear warning conductor and the brake drum provides that each drum brake is equipped with such a wear warning conductor. With each of the latter a lamp is associated and the simultaneous illumination of the lamps additionally indicates to the driver that all of the wear warning conductors are simultaneously coming to lie against the drum brake, i.e. that all brakes are simultaneously applied. U.S. Pat. No. 2,182,554 proposes to determine by means of a temperature-dependent resistor on each drum brake whether the drum brakes are heating up uniformly. In the event of non-uniform heating, non-uniform application of the brakes is inferred.  
           [0006]    The function of brake systems is generally to decelerate a vehicle, keep its speed constant when travelling downhill and immobilize it for parking. During such braking operations the brakes at least of each individual axle are usually subject to uniform stress.  
           [0007]    However, new brake systems have been assigned additional functions. For example, to prevent locking of the wheels (antilock control) and slipping of the wheels of an axle provided with a differential gear (traction control), individual wheels of the vehicle are to be braked. Similarly, to stabilize the driving state (vehicle dynamics control), individual wheels of the vehicle have to be braked. Thus, during such braking operations individual brakes of the brake system are subject to widely differing degrees of stress and hence to differing degrees of wear. The difference in the degree of wear of the brakes is accentuated by the fact that in modern brake systems, such as e.g. an electrohydraulic brake, extremely high and, at the same time, widely fluctuating braking forces may be summoned up. The wear of the friction linings may therefore vary considerably.  
           [0008]    Furthermore, in modem brake systems new, particularly lightweight materials are being used, which are however susceptible to wear and relatively expensive. For example, light metal alloys and/or light metal composites are being used for brake drums. The surface of such materials may easily be damaged under the action of metal friction (metal on metal), making early detection of worn friction linings particularly essential.  
         SUMMARY OF THE INVENTION  
         [0009]    This invention is to provide an automatic drum brake adjusting apparatus, which makes accurate identification of friction lining wear possible in a simple manner.  
           [0010]    To achieve this, the invention provides an automatic drum brake adjusting apparatus of the type described initially, in which a variable-length adjusting strut is used, which comprises a sleeve and a threaded rod, which is accommodated in the latter and is screwable out of the sleeve by means of a pinion. Such an adjusting apparatus has, with regard to the signal detection according to the invention, the advantage that by means of a mechanism comprising a threaded rod and a sleeve the required adjusting travel is sensitively adjustable so that an accurate signal is generated.  
           [0011]    The signal generated by the signalling device is advantageously brought to the attention of a vehicle driver visually by means of a warning lamp and/or acoustically by means of a warning sound. This is possible in a particularly simple manner and may, for example, alert the vehicle driver to the fact that the drum brakes require servicing. It is however also possible to process the signal in a control unit of the brake system (ABS, TCS, brake-by-wire) and, in this case, take into account a change of the signal during predetermined periods of time. An extreme change of the signal during such a period indicates particularly high brake wear within a short time and high stressing of the brake. By virtue of appropriate countersteering an overstressing of individual brakes may be avoided.  
           [0012]    According to a particularly simple form of construction of the adjusting apparatus according to the invention, the threaded rod after a predetermined adjusting travel actuates a switch, which immediately triggers an (optical and/or acoustic) warning signal.  
           [0013]    To achieve an arrangement of the switch, which is space saving and protected from environmental influences, the switch is preferably disposed in the sleeve.  
           [0014]    Since a drum brake—as a result of being provided immediately adjacent to the wheels—is exposed to high loads resulting from impacts, water and dust, the signalling device should be of a particularly rugged design. This may be achieved, for example, by means of a switch with a purely mechanical switching contact.  
           [0015]    According to another refinement, the switch takes the form of an electromagnetic, inductive or capacitive switch. Thus, on the one hand, an influencing of the adjusting apparatus by the switch is ruled out and, on the other hand, the time characteristic of the adjusted travel may be determined in a particularly advantageous manner. A friction lining with a high degree of wear within a short time indicates, for example, overheating of the drum brake.  
           [0016]    According to one form of construction a drum brake, in which the adjusting apparatus according to the invention is used, is developed in such a way that at least one leading brake shoe is provided with an additional signalling device for indicating wear of the friction lining. It is then possible to obtain two redundant signals, which with an extremely high degree of certainty indicate a wear of the drum brake. The signal of the signalling device on the adjusting apparatus is dependent upon the friction lining and drum wear, while the signal of the additional signalling device is dependent exclusively upon the friction lining wear. From the difference of the two signals conclusions may therefore be drawn about the wear of the drum alone.  
           [0017]    In a refinement, the drum brake is developed in such a way that a conductor leading to the signalling device extends inside the resetting springs, which reset the brake shoes. Such a conductor may moreover extend through an opening in an anchor plate of the drum brake, which opening is disposed axially symmetrically relative to another opening, which is provided for penetration of a hand brake cable, wherein a component of a plug-in connector is preferably fastened in the other opening. The drum brake may moreover be developed in that the conductor extends through an opening in a dome of the anchor plate, wherein a plug-in connector is preferably fastened by means of a tubular rivet in the opening. In this manner, the attachment of the conductors is of a particularly secure and durable design.  
           [0018]    The operating ability of the signalling devices is checked in a particularly advantageous manner in that a conductor leading to the signalling device is closed-circuit-protected.  
           [0019]    The additional signalling device is fastened to a brake shoe preferably by way of a lock washer, spreader arms or a lock nut. Thus, it is extremely easy to assemble and disassemble.  
           [0020]    Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a front view of a form of construction of a drum brake according to the present invention.  
         [0022]    [0022]FIG. 2 is a view taken along the section II-II in FIG. 1.  
         [0023]    [0023]FIG. 3 is a view taken along the section III-III in FIG. 1.  
         [0024]    [0024]FIG. 4 is a partial sectional view of a first form of construction of an adjusting apparatus according to the invention in a position such as is adopted with new friction linings.  
         [0025]    [0025]FIG. 5 is a view of the adjusting apparatus in FIG. 4 in a position such as is adopted with worn friction linings.  
         [0026]    [0026]FIG. 6 is a partial sectional view of a second form of construction of an adjusting apparatus according to the invention with a proximity switch.  
         [0027]    [0027]FIG. 7 is a first form of construction of the proximity switch of FIG. 6 to an enlarged scale.  
         [0028]    [0028]FIG. 8 is a second form of construction of the proximity switch of FIG. 6 to an enlarged scale.  
         [0029]    [0029]FIG. 9 is a third form of construction of the proximity switch of FIG. 6 to an enlarged scale.  
         [0030]    [0030]FIG. 10 is a diagram of an adjusting apparatus according to the invention.  
         [0031]    [0031]FIG. 11 is a first embodiment of an additional signalling device in a section through a brake shoe.  
         [0032]    [0032]FIG. 12 is a second form of construction of an additional signalling device in a section through a brake shoe.  
         [0033]    [0033]FIG. 13 is a third form of construction of an additional signalling device in a section through a brake shoe. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Referring now to the drawings, FIG. 1 shows a drum brake  10  having two brake shoes, namely a left, leading brake shoe  12  and a right, trailing brake shoe  14 , which are fastened to an anchor plate  16  by means of holding-down devices  18  and  20  respectively. The brake shoes  12  and  14  are supported by their, in the drawing, bottom ends  22  and  24  against a dome  26 , which projects out of the drawing plane away from the anchor plate  16 . A hydraulic wheel brake cylinder  32  is disposed in a known manner between the top ends  28  and  30  of the two brake shoes  12  and  14 . The two brake shoes  12  and  14  are radially surrounded by an, in section, circular brake drum  34 . Situated in the wheel brake cylinder  32  are two pistons (not shown) which, when hydraulic fluid is supplied, spread the brake shoes  12  and  14  apart from one another and press them against the inside of the brake drum  34 .  
         [0035]    Besides the hydraulic actuation, which is provided for service brake operations, the drum brake  10  may also be used as a parking brake. To this end, a hand brake lever  36  (partially illustrated by dashes) is provided, which is mounted rotatably in a bearing  38  in the top end region of the right brake shoe  14 .  
         [0036]    For adjusting the brake shoes  12  and  14  in the event of friction lining wear an adjusting apparatus  40  is provided, which comprises an adjusting strut  41 , which extends in the top region of the drum brake  10  between the two brake shoes  12  and  14 . The adjusting strut  41  comprises a pressure sleeve  42 , a pressure rod  44  accommodated in the latter and an adjusting pinion  46  screwed onto the pressure rod. Underneath the bearing  38  the hand brake lever  36  is coupled by the adjusting strut  41  to the left brake shoe  12 . The free end of the hand brake lever  36  is connected to a hand brake cable  48 . When the latter is pulled to the left, the hand brake lever  36  swivels about the bearing  38  and, in so doing, via the adjusting strut  41  presses the brake shoes  12  and  14  against the drum  34 . The braking torque is introduced via the dome  26  into the anchor plate  16  and from the latter into the vehicle axle.  
         [0037]    Disposed in the top and bottom end region of the brake shoes  12  and  14  are brake shoe retracting springs  52  and  50 , which bias the brake shoes away from the brake drum so that the brake shoes  12  and  14 , after release of the brake, return into their inoperative position. In the inoperative position, the spring forces of the retracting springs  50  and  52  are taken up by the adjusting strut  41  as well as by the dome  26 .  
         [0038]    A resetting spring  54  wound spirally around the hand brake cable  48  assists the swivelling of the hand brake lever  36  back into the inoperative position until the latter abuts a stop  56  on a radially inwardly extending web plate  58  of the right brake shoe  14 .  
         [0039]    Fastened to the outside of each of the brake shoes  12  and  14  is a friction lining  60  and  62 , which upon actuation of the brake is moved towards the drum  34  and pressed against the inside of the latter. The brake shoes  12  and  14  in this case swivel about the supporting surfaces of the dome  26 . The adjusting strut  41  is therefore relieved of the retraction force of the top brake shoe retracting spring  52  and the distance between the supporting surfaces of the adjusting strut  41  against the web plate  58  of the right brake shoe  14  (or the stop face on the hand brake lever  36 ) and a web plate  64  of the left brake shoe  12  increases.  
         [0040]    A two-armed adjusting lever  66 , which is likewise part of the adjusting apparatus  40 , is pivotally supported on the web plate  64  by means of a clamping sleeve  68 , which serves as a bearing and is pressed into the web plate. The adjusting lever  66  is supported by a shorter left lever arm  70  against the pressure rod  44 , while a right, longer lever arm  72  engages into a saw tooth system  74  of the adjusting pinion  46 . An adjusting spring  75  is disposed between the adjusting lever  66  and the bottom region of the web plate  64  and pulls the adjusting lever  66  downwards. The adjusting spring  75  therefore constantly exerts on the adjusting lever  66  a torque acting in clockwise direction around the clamping sleeve  68 .  
         [0041]    When the distance between the supporting surfaces of the adjusting strut  41  against the web plates  58  and  64  increases, the adjusting lever  66  may swivel about the clamping sleeve  68 . The right lever arm  72  in this case rotates the adjusting pinion  46 , which screws the pressure rod  44  out of the pressure sleeve  42 . The pressure rod  44  is therefore fixed in a position of further withdrawal relative to the pressure sleeve  42 . After release of the brake and resetting of the brake shoes  12  and  14 , the adjusting lever  66  is also swivelled back into the original position. The adjusting lever  66  in this case slides in accordance with the extent of the previously executed swivelling motion back over the saw tooth system  74  of the adjusting pinion  46  and latches in behind a tooth of the saw tooth system  74 . A full cycle of an adjusting motion at the adjusting strut  41  is thereby completed.  
         [0042]    [0042]FIGS. 4 and 5 show an enlarged view of the adjusting strut  41 . The adjusting strut  41  comprises the pressure sleeve  42  having a tubular portion  76 , into which the pressure rod  44  projects with a part of a threaded rod  78 . The adjusting pinion  46  is screwed on the threaded rod  78  until it comes into abutment with the pressure sleeve  42 . Clamping forces, which act upon the ends of the adjusting strut  41 , are therefore transmitted via the tubular portion  76  of the pressure sleeve  42  to the adjusting pinion  46  and from the latter to the threaded rod  78  of the pressure rod  44 .  
         [0043]    Disposed on the base in the tubular portion  76  of the pressure sleeve  42  is a switch  79  comprising an insulating sleeve  80 , in which extends an electrically conducting contact pin  82  biased by means of a compression spring  84 . An electric line  86  connects the contact pin  82  to an indicating apparatus (not shown), which may produce a warning sound or a warning light, or to a circuit arrangement  88 , which is described below and illustrated in FIG. 10.  
         [0044]    The compression spring  84  may be made of electrically insulating material but may alternatively take the form of a metal wire spring if additional insulation is provided between the contact pin  82  and the base of the tubular portion  76 . Alternatively, an insulating disk may be inserted between the compression spring  84  and the contact pin  82 , or a head  92  of the contact pin  82  is made of insulating material.  
         [0045]    [0045]FIG. 4 shows the state with new friction linings  60  and  62 . The contact pin  82  under the action of the compression spring  84  rests against the end face  90  of the threaded rod  78  disposed in the tubular portion  76 . An electric monitoring current flows through the line  86 , the contact pin  82 , the end face  90 , the pressure rod  44 , the brake shoes  12  and  14  as well as the anchor plate  16 .  
         [0046]    As a result of the previously described rotation of the adjusting pinion  46  the pressure rod  44  is moved out of the pressure sleeve  42  to generate an adjusting travel. With increasing wear of the friction linings  60  and  62  the end face  90  of the pressure rod  44  is moved further and further out of the pressure sleeve  42  until the contact pin  82  has negotiated a distance X and the head  92  rests against the base of the insulating sleeve  80 .  
         [0047]    Upon yet further wear of the friction linings  60  and  62  the end face  90  of the pressure rod  44  distances itself from the contact pin  82  (FIG. 5) and the monitoring current is interrupted.  
         [0048]    [0048]FIG. 10 shows the circuit arrangement  88  which, upon interruption of the monitoring current, actuates an active warning lamp. The monitoring current flows from a voltage source with a  12  V output voltage, e.g. a vehicle battery, via a series resistor RI, which has a resistance of e.g. 330 ohms. The switch  79 , which is shown in a closed state in FIG. 10, connects the circuit with low resistance to a ground terminal. An interruption of the switch  79  (shown by dashes) has the effect that the current is now conducted via a resistor R 2 , which has a resistance of e.g. 510 ohms, and an active warning lamp L to ground. The signal triggered by the wear of the friction linings  60  and  62  therefore leads to illumination of the warning lamp L.  
         [0049]    The circuit arrangement may also be designed in such a way that just a single interruption of the monitoring current leads to constant illumination of the warning lamp.  
         [0050]    Via the length of the contact pin  82  the travel X of the switch  79  and hence the admissible wear of the friction linings  60  and  62  as well as of the drum  34  is definable. The sleeve  80  may, for such purpose, be pressed to a predetermined depth into the pressure sleeve  42 .  
         [0051]    As contact pin  82  and compression spring  84  in the embodiment described above it is possible to provide a brush sufficiently known from the field of electric motor construction as well as generator construction, a spring-guided sliding contact made of graphite, e.g. natural graphite, metal graphite or electrographite, which effects the transfer of current to a collector or to a slipring.  
         [0052]    In further forms of construction the switch  79  takes the form of a proximity switch. FIGS.  6  to  9  show three examples thereof. FIG. 7 shows an inductive, FIG. 8 a capacitive and FIG. 9 a magnetic force-actuated sampling of the adjusting travel.  
         [0053]    In such forms of construction it is further provided that an additional signalling device  94  is disposed on one of the friction linings of the drum brake. As is shown in FIG. 1, the additional signalling device  94  is let into the friction lining  60  in the bottom region of the leading brake shoe  12 .  
         [0054]    FIGS.  11  to  13  show details of forms of construction of such an additional signalling device  94 . The signalling device  94  is inserted into a cavity of a rivet shank  96 , such as is provided for a lining rivet, and comprises a wire loop  98  embedded in a plastics material. In a known manner, the wire loop  98  projects into a recess  100  of the friction lining  60  so that, in the event of wear of the friction lining  60 , the wire loop  98  is cut through and so a signal may be supplied to a circuit arrangement  88 .  
         [0055]    The additional signalling device  94  may be fastened to the brake shoe  12  in various ways. The additional signalling device  94  illustrated in FIG. 11 is pressed into the cavity of the rivet shank  96 , flanged and additionally fastened by means of a lock washer  102 . The additional signalling device  94  illustrated in FIG. 12 is inserted directly into the rivet shank  96  and comprises a plurality of spreader arms  104 , which after penetrating the rivet shank  96  spread apart from one another and prevent the signalling device  94  from moving out. The additional signalling device  94  illustrated in FIG. 13 carries a nut  108  on a threaded shank  106 . The warning device  94 , having been passed through the rivet shank  96 , is secured by means of the nut  108 .  
         [0056]    The electric conductors and/or lines leading to the wire loop  98  and accordingly also the electric line  86  of the switch  79  lead to plug-in connectors  110  and  112  which, as is shown in FIGS.  1  to  3 , are fastened in a tubular rivet  114  of the dome  26  and in an opening  116  in the anchor plate  16 , which opening is symmetrical relative to an opening  117  for fastening the hand brake cable  48  to the anchor plate  16 . The lines further extend through the top and bottom brake shoe retracting springs  50  and  52  as well as through an opening  118  of the web plate  58 .  
         [0057]    As the illustrated forms of construction of the signalling device  79  may be used practically without modifications in existing standard brakes, the adjusting apparatus  40  is also suitable for the retrofitting of already supplied drum brakes.  
         [0058]    In accordance with the provisions of the patent statues, the principle and mode of operation of this invention have been described and illustrated in its preferred embodiments. However, it must be understood that the invention may be practiced otherwise than as specifically explained and illustrated without departing from the scope or spirit of the attached claims.