Technical Field

This invention relates to a method for indicating wear of brake linings or brake pads at braking by means of a brake mechanism, which has two brake applying members and a synchronizing shaft rotatively connecting these two members for accomplishing synchronizing of a wear compensating ro¬ tation at both members. It also relates to a wear indicator for carrying out this method. Background of the Invention

Brake linings or brake pads are gradually worn at bra¬ king. Normally, a slack adjuster built into the brake mechanism compensates for the increasing slack or distance between the brake linings or brake pads and a brake drum or brake disc resulting from the lining or pad wear. However, the brake linings or pads have to be replaced before they are completely worn out, because otherwise the brake effect can be lost and the drum or disc be permanently damaged. It is therefore usual to provide the brake lining or brake pad with some type of wear indicator, giving a signal when the lining or pad is nearly or completely worn out .

Existing wear indicators are often unreliable and ex¬ pensive and operate under extremely harsh conditions due to the fact that they are arranged in the lining or pad. Further, it may be desirable to supply signals indica¬ tive for the wear rates at the different wheels to a cen¬ tral vehicle computer, which modifies the individual brake pressures at the wheels for obtaining an equalizing of the wears . The object of the invention is therefore to obtain an improved method and device for indicating wear of brake linings or brake pads at a brake mechanism of the above

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mentioned type.

The Invention

This is according to the invention accomplished by a method, which is characterized in that the angular movement of the synchronizing shaft is measured by electric or electromagnetic means and in that an increasing angular movement at consecutive brake applications is recorded as indicative for a brake lining or brake pad wear.

This means that instead of sensing the actual wear of the brake lining or brake pad the rotation of the synchro¬ nizing shaft is taken as an indication of the brake lining or pad wear.

A synchronizing shaft of the kind referred to and uti¬ lized according to the invention is typically used in a so called wedge brake unit for a drum brake in heavy road ve¬ hicles, such as trucks and buses. A corresponding design may, however, equally well be used in disc brakes, and the invention is also applicable to such disc brakes.

At its ends the synchronizing shaft is provided with gear wheels each cooperating with external helical teeth on a rotatable part of each brake applying member. The syn¬ chronizing shaft has the purpose of transmitting a slack adjusting movement from one brake applying member, where a slack adjuster is arranged, to the other. The invention also relates to a wear indicator for carrying out the above method, characterized in that means on the synchronizing shaft are arranged to cooperate with stationary means in the brake mechanism to provide an electric counting signal indicative for angular movement of the shaft and thus for the brake lining or brake pad wear. In a preferred embodiment the synchronizing shaft is provided with a magnet for cooperation with a stationary electronic switch, preferably a hall switch, arranged in the vicinity thereof in the brake mechanism. Other means for obtaining the desired sensor function

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between the shaft and the stationary member are also possi¬ ble.

In order to differentiate between the two rotational directions of the synchronizing shaft, so that pulses ema- nating from reverse rotation of the shaft can be deducted, the shaft is provided with two axially and angularly dis¬ placed magnets for cooperation with a hall switch each.

In a practical embodiment the hall switches are ar¬ ranged on a circuit board together with a processor, a me- mory, and a back-up battery.

When the wear indicator is used in a wedge brake unit with a wedge for pressing the brake applying members apart at brake application, the wedge is provided with a magnet for cooperation with a further hall switch on the circuit board in order to set the arrangement to zero. The Drawing

The invention will be described in further detail be¬ low under reference to the accompanying drawing, in which a brake mechanism incorporating the invention is shown in a side view, partly in section.

Detailed Description of a Preferred Embodiment The drawing illustrates a brake mechanism in the form of a wedge brake unit for a drum brake, preferably for use on a heavy road vehicle, such as a truck or a bus. Such a brake unit and its use are well known in the art, and only a brief description of the brake arrangement is given without reference to any drawing.

A wedge brake unit of the type shown in the drawing can be mounted on a fixed part at the wheel to be braked. The force-transmitting push rods of the unit are connected to brake shoes provided with brake linings. The brake shoes with its brake linings are pushed against a rotating brake drum at braking.

A wedge brake unit as shown in the drawing has a hou- sing 1. A left tappet 2 and a right tappet 3 are guided for

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axial movements only in the housing 1. In the area between these two tappets 2 and 3 a wedge 4 is movable substanti¬ ally perpendicularly to the common axis of the two tappets 2 and 3. As is well known in the art, the wedge 4 is con- nected to a piston of a fluid actuated brake cylinder (not shown) . Force transmitting rollers 5 and 6 are arranged between mutually parallel surfaces on the wedge 4 and the respective tappets 2 and 3. In the shown case the wedge 4 is additionally guided by a guide roller 7 arranged between mutually parallel surfaces, which are perpendicular to the common axis of the tappets 2 and 3, on the wedge 4 and in the housing 1, respectively.

A tappet nut 8 and 9, respectively, is arranged on each of the non-rotatable tappets 2 and 3. Each tappet nut 8 and 9 is rotatably arranged on its respective tappet 2 and 3. Tappet screws 10 and 11 with external threads engage internal threads in the respective tappet nuts 8 and 9. The thread directions of the threads at the left hand side (tappet nut 8 and tappet screw 10) are opposite to those at the right hand side (tappet nut 9 and tappet screw 11) for reasons described below. The same is true also for the directions of helical teeth provided on the periphery of the tappet nuts 8 and 9.

Each tappet screw 10 and 11 is provided with means at its free end for non-rotatable connection with a brake shoe (not shown) ; the tappet screws are arranged to press the brake shoes apart for braking engagement with a brake drum. The non-rotatable arrangement of the tappet screws 10 and 11 means that they will move axially, when the correspon- ding tappet nuts 8 and 9 are rotated.

The tappet 2 or 3 , the tappet nut 8 or 9, and the tappet screw 10 or 11 forms a unit, which may be referred to as a brake applying member.

A slack adjuster - to be briefly described only - is arranged in the housing 1 for cooperation with the right

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tappet nut 9. The slack adjuster includes the following co¬ axial elements: a toothed clutch 12, a powerful clutch spring 13, a worm gear 14 engaging the helical teeth of the right tappet nut 9, and a spring 15. The function of a slack adjuster of this general type is known in the art and will not be described here, as it is not essential for a proper understanding of the invention. It is only necessary to say that if the slack or distance between the brake shoe connected to the tappet screw 11 or rather the brake lining provided thereon and the brake drum with which the brake shoe cooperates increases over a set value - depending on wear of the brake lining - the slack adjuster 12-15 will rotate the tappet nut 9, so that the tappet screw 11 is pushed out a distance for restoring the above mentioned slack to the desired value.

This procedure is repeated, until the brake lining on the brake shoe connected to the tappet screw 11 is worn out and has to be replaced. A manual retraction of the tappet screw 11 - over the tappet nut 9 and worm screw 14 - is possible by means of a tool grip 16 on the axle of the slack adjuster 12-15.

A synchronizing shaft 17, rotatably journalled in the housing 1, is at its respective ends provided with helical gear wheels 18 and 19 engaging the tappet nuts 8 and 9. The function of this synchronizing shaft - hereinafter collec¬ tively numbered 17-19 - is to transmit the rotational move¬ ments of the right tappet nut 9 to the left tappet nut 8 and thus to accomplish the same slack adjusting at the brake shoe connected to the left tappet screw 10 as at the one connected to the right tappet screw 11. The synchroni¬ zing shaft 17-19 will also rotate back and forth at each brake application and brake release without any slack ad¬ justing, when the tappet nuts 8 and 9 only move axially due to the permanent engagement between the tappet nuts 8 and 9 and the gear wheels 18 and 19.

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The arrangement so far described is state of the art. The invention relates to the indication of the actual wear of the brake linings on the brake shoes connected to the tappet screws 10. It appears from the description above that a measure of this wear is the axial movement of each of these tappet screws 10 and 11 corresponding to the rota¬ tional movement of the tappet nuts 8 and 9 and thus the ro¬ tation of the synchronizing shaft 17-19. By means of the arrangement described below the rotation of the synchroni- zing shaft 17-19 is used to indicate the brake lining wear. In the vicinity of the synchronizing shaft 17-19 a circuit board 20 is attached to a cover 21, mounted on the housing 1. On its side facing the shaft 17-19 the circuit board 20 is provided with two hall switches 22, whereas the shaft 17-19 is equipped with two corresponding, angularly displaced magnets 23. Each pair of hall switch 22 and mag¬ net 23, arranged close to each other, constitutes a sensor for the rotation of the shaft 17-19. By the provision of two such pairs, where there is an angular displacement between the two magnets 23 on the shaft 17-19, it is possi¬ ble to assess the rotational direction of the shaft 17-19. Although the term "hall switch" is used, it should be noted that any suitable electronic switch can be employed. As an alternative, the sensor type may be different: A coil may be used instead of the hall switch and may possi¬ bly cooperate with a cog wheel instead of the magnets. Also sensors of capacitive or resistive type may be used.

Every time a magnet 23 passes its corresponding hall switch 22, a pulse will be communicated to a processor 24, which counts the pulses and converts the signal to a desi¬ red form, analogous or digital. Also arranged on the cir¬ cuit board 20 is a memory 25, which stores the counting.

On the opposite side of the circuit board 20 batteries 26 for current supply at failure of the ordinary current supply may be provided.

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In order to set the arrangement to zero a further mag¬ net 27 is arranged on the wedge 4. At the first brake application after the mounting of new brake linings the magnet 27 will get close to a further hall switch 28, which delivers a pulse for setting the memory 25 to zero. Please note that the arrangement has been shown as being placed below the brake mechanism for the sake of clarity; in rea¬ lity it is placed at the side of it, so that the magnet 27 passes close to its hall switch 28. By means of a special locking function in the processor 24 the above mentioned setting ^' to zero may not occur until the synchronizing shaft 17-19 has been rotated backwards past a zero point at the mounting of new brake linings .

As an alternative the arrangement can be set to zero by means of a device, which actuates the hall switch 28 mechanically, or another switch, that provides a signal to the processor 24.

A still further magnet 29 may be provided on the upper part of the wedge 4 for the purpose of warning for an excessive stroke of the wedge.

The synchronizing shaft 17-19 is imparted a rotational movement from two sources, namely on one hand the rotation stemming from the slack adjuster 12-15 for compensating for the brake lining wear, on the other hand the rotation back and forth at each brake application and release. Due to the provision of the two angularly displaced magnets 22 on the shaft 17-19, the arrangement is able to neutralize the influence of the latter source.

The arrangement is primarily used for supplying sig- nals indicative for the wear rates at the different wheels to a central vehicle computer, which modifies the wheel brake pressures for equalizing the wear at the different wheels. A secondary use may be to supply an indication of the wear to the vehicle driver. Although the description of the invention has been

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focussed on a so called wedge brake unit for a drum brake, it should be noted that the idea can equally well be utili¬ zed at other brake arrangements. Especially, it may be uti¬ lized at a disc brake arrangement provided with a synchro¬ nizing shaft.

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