Brake

A self-adjusting brake having a brake drum, primary and secondary brake shoes, a brake cylinder and a stationary brake shoe support for floatingly mounting the brake shoes. The latter are arranged interiorly of the brake drum and coact with it, the ends of the brake shoes being curved and pressed against the brake cylinder and the brake shoe support by means of return springs which also serve to lift the brake shoes off the brake drum when the brake is in non-actuated condition. Each brake shoe carries a brake lining and has a web which has associated with it an inclined surface that coacts with a respective pin, the inclination of the surfaces being such that the amount of play between the brake lining and the brake drum, when the brake is in non-actuated condition, remains substantially constant during displacement of the brake shoe as a result of wear of the brake lining. The return springs are so configured and arranged that the line of action of the resultant spring force acting on the primary brake shoe passes through the instantaneous axis of the primary brake shoe and the resultant spring force acting on the secondary brake shoe passes through the instantaneous axis of the secondary brake shoe. The inclined surfaces are arranged in that half of each respective brake shoe which abuts the brake cylinder, with each inclined surface being tangential to the instantaneous axis of the respective brake shoe.

BACKGROUND OF THE INVENTION 
The present invention relates to a self-adjusting brake, particularly a 
drum-type brake equipped with internal brake shoes which are 
self-adjusting to compensate for wear of the brake lining. 
More particularly, the present invention relates to a self-adjusting brake 
having a brake drum, two brake shoes, a brake cylinder and a stationary 
brake shoe support for floatingly mounting the brake shoes. The latter are 
arranged interiorly of the brake drum and coact with it, the ends of the 
brake shoes being curved and pressed against the brake cylinder and the 
brake shoe support by means of return springs which also serve to lift the 
brake shoes off the brake drum when the brake is in non-actuated 
condition. One of the two brake shoes is the primary brake shoe and the 
other is the secondary brake shoe, these designations being taken with 
respect to the direction of rotation of a wheel with which the brake is 
associated. The primary brake shoe is the one the leading edge of which, 
during normal rotation of the drum, contacts the brake cylinder, whereas 
the secondary brake shoe contacts the brake cylinder with its trailing 
edge. Each brake shoe has a T-shaped cross section constituted by a flange 
forming the top of the T and a web which forms the stem of the T, the 
flange of each brake shoe carrying a brake lining, with each web having 
associated with it means which form an inclined surface coacting with a 
respective stationary pin, the inclination of the inclined surfaces being 
such that the amount of the play between the brake lining carried by the 
brake shoes and the brake drum, when the brake is in non-actuated 
condition, remains substantially constant during displacement of the brake 
shoes resulting from wear of the brake lining. 
Various efforts have been made to develop self-adjusting brakes of the 
above type while reducing the very high costs incident to such brakes, 
such as interlocks, detents, wedges and friction members. In one such 
self-adjusting brake, as shown, for example, in German Patent Application 
No. 1,077,077 published March 3rd, 1960, the inclined surfaces are 
arranged approximately in the middle of the brake shoe, at the outer edge 
of the web, and are constituted by the material of which the web itself is 
made. The precise inclination of these surfaces required for trouble-free 
operation must of necessity be determined experimentally. This, however, 
is not wholly satisfactory because frequently a long series of tests must 
be carried out until a practical result is obtained. The brake arrangement 
has a total of four return springs, there being two for each brake shoe. 
Both of the return springs associated with each brake shoe are inclined 
with respect to the longitudinal axis of the associated brake cylinder, 
one of them being connected at its two ends to the ends of the two brake 
shoes which abut the brake cylinder in question whereas the other of the 
two springs has one connected to the brake cylinder and the other end to 
the respective brake shoe. The configuration and effective lines of action 
of the springs must be such that when the brake is in its non-actuated 
condition, all of the forces exerted by the springs are brought into a 
stable equilibrium so as to avoid any forces or moments which would tend 
to move the brake shoes out of the concentric position. It is not readily 
apparent how this is or can be done, or what the direction of the 
effective forces is. 
It is, therefore, the primary object of the present invention to provide a 
self-adjusting brake which overcomes the above drawbacks, namely, a brake 
which incorporates a self-adjusting device and which can be built in such 
a way as to obtain the desired result. 
SUMMARY OF THE INVENTION 
With the above objects in view, the present invention resides in a 
self-adjusting brake of the above type, in which return spring means are 
provided which are so configured and arranged that the line of action of 
the resultant spring force acting on the primary brake shoe passes through 
the instantaneous axis of the primary brake and the line of action of the 
resultant spring force acting on the secondary brake shoe passes through 
the instantaneous axis of the secondary brake shoe, the location of these 
axes being defined more particularly below. Moreover, the inclined 
surfaces are arranged in that half of each respective brake shoe which 
abuts the brake cylinder, with each inclined surface being tangential to 
the instantaneous axis of the respective brake shoe. 
In accordance with a preferred embodiment of the present invention, the 
return spring force is provided by three springs, one of which is parallel 
to the longitudinal axis of the brake shoe support and has its ends 
connected, respectively, to the two ends of the two brake shoes which abut 
the brake shoe support. The second spring has one end connected to the 
primary brake shoe and its other end to a stationary point of the brake, 
while the third spring has one end connected to the secondary brake shoe 
and its other end to a stationary point of the brake. The second and third 
springs each form a relatively large angle with the longitudinal axis of 
the brake shoe support.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawing, the same shows a brake according to the 
present invention, which includes a brake drum 1 associated with a wheel 
(not shown), within which drum are arranged two brake shoes 2 and 3, the 
same being suspended on a so-called floating mount in that the curved 
lower ends of the brake shoes bear against a stationary brake shoe support 
11 while the curved upper ends of the brake shoes bear against a brake 
cylinder 12 which is located in the region of the upper end of the brake 
arrangement. Thus, when fluid under pressure is supplied to the brake 
cylinder, the brake shoes will be urged into frictional engagement with 
the inner cylindrical surface of the brake drum 1. 
Assuming the normal direction of rotation of the wheel carrying the drum to 
be that which is indicated by the arrow, the brake shoe 2 constitutes the 
primary braking element whereas the brake shoe 3 constitutes the secondary 
braking element. That is to say, with the wheel with which the brake drum 
1 is associated, rotating as shown by the arrow, the primary brake shoe, 
as explained above, is the one the leading edge of which contacts the 
brake cylinder 12. 
As is conventional in the art, each brake shoe has a T-shaped cross 
section, the shoes 2 and 3 having so-called cross pieces or webs 4, 5, 
respectively, as well as curved flanges carrying the usual wear-resistant 
brake lining which comes into actual contact with the inner cylindrical 
surface of the brake drum 1. 
When the brake pressure is released, the brake shoes 2 and 3 are returned 
to the non-actuated or rest position by means of return springs 6, 7 and 
8. The distance which, with the brake in non-actuated condition, the brake 
shoes are spaced from the inner surface of the brake drum is the amount of 
play which, during initial assembly of the brake, can be adjusted, for 
example, by means of a set screw (not shown) forming part of the 
stationary brake shoe support 11. 
Due to progressive wear of the brake linings, the brake shoes 2 and 3 will 
gradually turn down, this movement taking place about their respective 
instantaneous axes M.sub.p and M.sub.s. The locations of these axes can 
readily be determined by those skilled in the art (e.g. German Journal 
"Automobiltechnische Zeitschrift" 1954, pages 197-202). The axes M.sub.p, 
M.sub.s are formed in the illustrated embodiment, by the intersection of 
the extended longitudinal axis a of the stationary brake shoe support 11 
with respective lines b.sub.p, b.sub.s, each of which passes through the 
geometric center 0 of the brake arrangement and is itself at right angles 
to a respective line c.sub.p, c.sub.s, that passes through point 0 and the 
theoretical maximum pressure point P.sub.p, P.sub.s, of the respective 
brake shoe. But for such an arrangement, the amount of play, or the size 
of the gap between the brake shoe and the inner surface of the brake drum 
when the brake shoes are in the non-actuated position, would continuously 
increase as the brake linings become increasingly worn; this, in turn, 
would mean that as the linings become more and more worn, the amount of 
travel through which the brake shoes would have to move in order to bring 
about a braking action would increase. To prevent this increased travel, 
the webs 4, 5, of the brake shoes are provided with inclined surfaces 9 
and 10, respectively, which, in the illustrated embodiment, are in the 
form of elongated openings formed in the webs themselves. These openings 
have protruding into them stationary abutment pins or the like 14 and 15 
which are mounted on the brake casing or cover plate 13, the arrangement 
of the parts being such that when the brake pressure is released, the 
inclined surfaces 9, 10, will come to abut against the pins 14, 15, 
respectively, under the influence of the return springs. The gap that is 
formed between the surfaces 9, 10 and the abutment pins 14, 15, when a 
brake force has been applied represents the play of the brake, which, as 
mentioned above, is selected during the assembly of the brake components. 
According to the present invention, the inclined surfaces 9, 10 are 
arranged in the half of each brake shoe which coacts with the brake 
cylinder 12, with the inclined surface 9 of the primary brake shoe 2 being 
oriented tangentially with respect to the respective instantaneous axis 
M.sub.p and the inclined surface 10 of the secondary brake shoe 3 being 
oriented tangentially with respect to the respective instantaneous axis 
M.sub.s. Moreover, the return springs 6, 7 and 8 are so configured and 
arranged that the resultant line of action of the return spring force 
acting on the primary brake shoe 2 passes through the associated 
instantaneous axis M.sub.p and the resultant line of action of the return 
spring force acting on the secondary brake shoe 3 passes through the 
associated instantaneous axis M.sub.s. As a result of this, the size of 
the gap selected during the assembly of the brake components will remain 
substantially the same, with a high degree of accuracy, despite 
progressive wear of the brake linings. The closer the inclined surfaces 9, 
10, are to the brake cylinder 12, the more closely will the originally 
selected gap size be maintained. Moreover, the spring force that has to be 
exerted by the return springs is reduced. In theory, the optimum result 
would be attained if the inclined surfaces were circular arcs having at 
their center the instantaneous axis of the respective brake shoe, but in 
practice, a high degree of constancy is maintained simply by letting the 
inclined surface extend substantially tangentially with respect to these 
axes. 
Thanks to the above-described configuration and arrangement of the return 
springs, the brake shoes will, when the brake is in its non-actuated 
condition, always lie firmly against their two abutments, namely, the 
stationary brake shoe support 11 and the abutment pins 14, 15, 
respectively. This is so because none of the return springs produce 
components which would tend to move the brake shoes up or down. If this 
were not so, it would, for all practical purposes, not be possible to 
maintain the amount of play constant because then the brake shoes could 
drift up or down, depending on the direction of the spring force 
component. In the first case, the selected amount of play would be 
increased, and in the second case, it would be decreased. 
In the illustrated embodiment, the return spring means includes the three 
springs 6, 7 and 8 referred to above. The first spring 8 extends parallel 
to the longitudinal axis of the stationary brake shoe support 11 and has 
its ends connected to those ends of the two brake shoes 2 and 3 at which 
these brake shoes abut the stationary brake shoe support 11. The second 
spring 6 has one end conncted to the primary brake shoe 2 and its other 
end connected to a stationary point on the brake housing or cover 13. 
Similarly, the third spring 7, has one end connected to the secondary 
brake shoe 3 and its other end anchored to a stationary point of the brake 
housing or cover 13. The amount of travel of the return springs, whose 
spring tension remains practically constant, depends on the amount of play 
of the brake. 
The above-described brake adjustment device will readily function even 
during heat build-up in the brake, which is something that not every 
self-adjusting brake device can do. As is known in the art, when the brake 
heats up, the diameter of the brake drum increases, and many conventional 
brake adjusting devices, after carrying out their intended function of 
compensating for the changed dimension resulting from the increased 
temperature, will, after the brake has cooled down, and with the brake in 
its non-actuated position, cause the brake shoes to come into contact with 
the brake drum, and sometimes even cause the brake shoes to lock the 
wheel. The self-adjusting compensating device according to the present 
invention, however, will maintain the predetermined amount of play even 
when the brake heats up. This is so because when the brake drum cools 
down, resulting in decreased diameter, the brake drum touches the brake 
lining at the lowermost end of the brake shoe, i.e., at the end of the 
brake shoe which is nearest to the stationary brake shoe support 11, 
thereby pushing the brake shoe back into the position which it occupied 
prior to the temperature rise. This restores the original amount of play. 
Fashioning the inclined slots 9, 10 as elongated openings or slits brings 
additional advantages. For one thing, the dimensions and spatial 
orientation of the pins and the elongated openings can be so matched to 
each other that no adjusting elements are needed in order to provide the 
initially desired amount of play. In conventional brakes, the elements 
which coact with the inclined surfaces are in the form of adjustable 
eccentric elements, or the brake cylinder or the stationary support will 
be provided with screws or other means for adjusting the axial length 
thereof. The use of such adjusting elements can, in accordance with the 
present invention, be dispensed with, making the diameter of the abutment 
pins 14, 15 smaller than the width of the respective elongated opening 9, 
10, by an amount that is equal to the amount of the desired play of each 
brake shoe. The pins are so positioned on the brake casing or cover that 
those sides of the elongated openings which are directed away from the 
brake linings will engage the pins 14, 15, the first time the brake is 
vigorously actuated. In addition, the bending resistance of the pins 14, 
15, is selected to be such that when the brake is vigorously actuated for 
the first time, the pins will, if necassary, become permanently deformed 
until the brake lining lies against the brake drum 1. It will be 
appreciated that, in this way, the initial steps that need be taken for 
selecting the basic amount of play are significantly simplified, because 
now all that is necessary is that the pins be welded, riveted or otherwise 
be secured in place. This advantageous arrangement can be improved still 
further by increasing the width of the elongated openings 9, 10, in the 
direction in which the brake shoes are displaced as the result of wear of 
the brake lining. In this way, those sides of the elongated openings 9, 
10, which are remote from the brake linings will engage the pins 14, 15, 
only when the brake lining is new, so as to make it possible to adjust the 
amount of play of the brake by vigorously actuating the brake. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.