Disc brake with automatic adjusting device

In a disc brake with automatic adjusting device, a nut (12) is screwed to an axially slidable, unrotatable spindle (11). Nut (12) is connected to a coupling element (13). An auxiliary piston (32) is slidably mounted in a bore (31) of the brake piston (4) and is connected to the nut (12) by an axially lockable snap-in connection (35, 37, 38).

BACKGROUND OF THE INVENTION 
The present invention relates to a disc brake with automatic adjusting 
device. 
European patent No. 0 403 635 discloses an adjusting device of this type. 
In the prior art device, a nut screwed to a spindle is integrally formed 
as a combination unit along with a coupling element having a conical 
friction surface and an auxiliary piston, slidable in a bore of the brake 
piston. A device of this type necessitates exact coaxial positioning of 
the spindle, the combination unit and the bore of the brake piston. This, 
in turn, entails most accurate manufacturing tolerances and increased 
manufacturing costs. In addition, it is absolutely necessary that the 
portion of the combination unit used as the auxiliary piston can be 
rotated together with the nut. On the other hand, hydraulic sealing of the 
auxiliary piston is necessary which must be easily rotatable in the known 
device. This provision is also disadvantageous with respect to 
manufacturing costs. 
An object of the present invention is to improve a prior art disc brake 
with automatic adjusting device so that greater manufacturing tolerances 
are allowed and manufacturing costs are reduced. 
SUMMARY OF THE INVENTION 
This object is achieved by providing the auxiliary piston and the coupling 
element on two component parts being separately manufactured, and by 
connecting the component parts. In the coupling zone, a compensation of 
coaxialities is possible by means of a play which is to be predetermined 
between the two component parts. A snap-in connection, as claimed in claim 
2, permits a sealing arrangement of the auxiliary piston which need not be 
easily rotatable because the nut may be arranged on the snap-in connection 
so as to be rotatable relative to the auxiliary piston. When required, 
different materials for the nut and the auxiliary piston may even be used. 
In a preferred aspect of the present invention, the coupling element is 
integrally designed with the nut. This ensures a simple manufacture of the 
thread of the nut and the conical friction surface of the coupling 
element. The auxiliary piston may have a particularly simple shape in this 
case. 
In a first advantageous embodiment of the snap-in connection, the auxiliary 
piston is accommodated in a bore of the nut and has on its outside a 
circumferential annular groove which is in alignment with an internal 
circumferential annular groove in the bore of the nut. As a coupling 
element, an open metal ring is inserted into the annular grooves which 
clicks in elastically. 
The metal ring may be bent from a piece of wire, and it may have a circular 
cross-section or a rectangular cross-sectional shape. 
In a preferred aspect of the present invention, the snap-in connection can 
be disengaged, for example, when the auxiliary piston is urged in the one 
axial direction by a high hydraulic pressure and, on the other hand, the 
nut is retained in an opposite axial direction. Such a situation may occur 
when the nut cannot be rotated due to malfunction, or in adjusting devices 
which permit rotation of the nut only after hydraulic actuation. When, in 
such situations, a particularly long actuating travel of the brake piston 
occurs, an axially fixed auxiliary piston would slip out of its bore in 
the brake piston and cause complete failure of the hydraulic actuating 
devices. When, however, the snap-in connection is disengageable, the 
auxiliary piston will always remain in its bore in the brake piston. A 
disturbance as described above will cause only disengagement of the 
snap-in connection, but hydraulic leakage will be prevented. 
Disengagement of the snap-in connection is simply possible by at least one 
annular groove having a ramp-like chamfered groove wall, along which the 
metal ring slips out also in a radial direction when subjected to axial 
load. 
To fill hydraulic fluid into the brake cylinder, the air contained therein 
must be bled. Such a bleeding action is normally effected by removing the 
air prior to the fluid replenishment (`vacuum bleeding`) . Problems are 
involved with niches in the interior of the brake cylinder which are 
largely separated from the interior. For example, the nut screwed to the 
spindle forms together with the auxiliary piston an enclosed hollow 
chamber from which the air may escape only through slots having a small 
flow cross-section. In a preferred aspect of the present invention, the 
nut has a radial venting duct through which the air may escape more 
rapidly from the hollow chamber. In another variation of the present 
invention, the auxiliary piston has a venting groove which extends from 
the enclosed hollow chamber to run radially along the annular piston end 
surface. Thereafter, the groove extends in a paraxial direction on the 
external peripheral surface of the auxiliary piston until an area which is 
not covered by the nut. 
Embodiments of the present invention will be explained in more detail 
hereinbelow, taking reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
The Figures show a part of a brake housing 1 of a disc brake with an 
automatic adjusting device accommodated in the interior of a cylinder bore 
2 of a brake cylinder 3. Brake cylinder 3 is part of a hydraulic actuating 
device which includes a brake piston 4 that is axially slidable in brake 
cylinder 3. Brake piston 4 is sealed relative to the cylinder bore 2 by an 
elastic ring seal 5. Piston 4 is protected against contaminants by an 
elastic pleated bellows 6. Interior 7 of brake cylinder 3 is filled with 
hydraulic fluid. 
The hydraulic fluid in the interior 7 is pressurized for the hydraulic 
actuation of the brake cylinder 3. The hydraulic pressure urges the brake 
piston 4 out of the brake cylinder 3 (to the left in the drawing) to apply 
brake shoes (not shown) to a brake disc (not shown). 
The brake housing 1 also includes a mechanic actuating device 8 which 
generally comprises a rotatably mounted actuating shaft 9, a pressure 
member 10, an axially slidable and unrotatable spindle 11, and a nut 12 
screwed to the spindle 11. Nut 12 is integrally designed with the coupling 
element 13 having a conical friction surface 14. Inside the hollow brake 
piston 4 is a complementary friction surface 15 against which the conical 
friction surface 14 can be pressed. Spindle 11 and nut 12 are 
interconnected by a thread 16 without self-locking engagement. 
To actuate the mechanic actuating device 8, the actuating shaft 9 which is 
mounted in a roller bearing 17 devoid of friction is rotated 
counterclockwise. The pressure member 10 arranged in an eccentric recess 
18 is moved to the left, and this movement is transmitted to the spindle 
11. Spindle 11 has a guide portion 20 which is slidable in a guide bore 21 
of the brake housing 1 and is sealed hydraulically by a ring seal 19. 
Further, spindle 11 has a retainer 22 which is used as an abutment for one 
end of a wire spring 23. The other end of wire spring 23 bears against a 
sheet-metal bowl 24 which, in turn, is fixed in the brake housing 1 by a 
metal ring 25. Wire spring 23, by way of retainer 22, exerts an axial 
force on the spindle 11 to the right in opposition to the actuating 
direction. The spindle 11, urged to the left in the actuating direction, 
entrains the nut 12 and urges the friction surface 14 of the coupling 
element 13 against the friction surface 15 of the brake piston 4 so that 
the nut 12 is retained and supported unrotatably on the brake piston 4. 
Therefore, the axial force in the actuating direction is transmitted in 
full extent to the brake piston 4 which is thereby moved to the left in an 
axial direction to apply the brake shoes (not shown) to the brake disc 
(not shown). 
To terminate the mechanic actuation, the actuating shaft 9 is turned back 
clockwise, whereupon the wire spring 23, by way of the retainer 22, moves 
the spindle 11 back to the right in an axial direction. 
A second wire spring 26 is arranged inside the brake piston 4. One end of 
spring 26 is pressed against a circlip 27 which is unslidably attached to 
brake piston 4. The other end of spring 26 is pressed against a movable 
disc-type ring 28. The axially movable, unrotatable disc-type ring 28 (by 
way of a ball bearing 29) is urged axially against a bearing surface 30 of 
the rotatable nut 12. This action moves the nut 12 to the left, urging it 
with friction surface 14 against friction surface 15 of the brake piston 
4. The result is that the friction clutch provided between friction 
surfaces 14 and 15 is always closed in the non-applied condition of the 
disc brake. 
The brake piston 4 moves to the left when hydraulically operated. As soon 
as this movement of the brake piston 4, due to progressing wear of the 
brake linings, exceeds a small amount which is determined by the axial 
clearance of the thread 16 without self-locking engagement, the nut 12 is 
prevented from further axial movement by the spindle 11 which is in its 
inactive position. Continued axial movement of the brake piston 4 causes 
the friction clutch between the friction surfaces 14 and 15 to open. This 
makes the nut 12 rotatable in relation to spindle 11. Caused by the axial 
force that is exerted by the wire spring 26 on the nut 12 by way of ball 
bearing 29, nut 12 will turn on the spindle 11 by way of the thread 16 
without self-locking engagement until the axial movement of the nut 12 to 
the left is sufficient to restore engagement of the friction surfaces 14, 
15 and thereby terminate rotation of the nut 12. Of course, the axial 
force to the left generated by the spring 26 must not be in excess of the 
axial force to the right generated by spring 23. 
An auxiliary piston 32 is slidable in a bore 31 of the brake piston 4 and 
hydraulically sealed by a ring seal 33. Auxiliary piston 32 is exposed to 
hydraulic pressure on its right axial side and to atmospheric pressure on 
its left axial side. This is permitted by a small channel 34 which extends 
from the bottom of bore 31 to connect the interior with the outside 
chamber. The auxiliary piston 32 is connected to the nut 12 by an axially 
lockable snap-in connection. The snap-in connection is provided by a 
circumferential annular groove 35 on the outside of auxiliary piston 32, 
an internal circumferential annular groove 37 in a bore 36 of the nut 12, 
and an open metal ring 38 which is snapped into the aligned annular 
grooves 35 and 37. The metal ring 38 is bent from a piece of wire and has 
a round cross-section. 
In the modified embodiment of the present invention shown in FIG. 2, the 
metal ring 39 has a rectangular design in cross-section. Ring 39 is 
snapped on into a correspondingly modified annular groove 40 of nut 12 and 
a modified annular groove 41 of the auxiliary piston 32. To facilitate 
snapping on of the respective metal ring 38, 39, the nut 12 has a chamfer 
42 on the mouth of its bore 36. 
The function of the auxiliary piston 32 is to perform an axial movement to 
the left in relation to the brake piston 4 in the presence of high 
pressure. Due to the snap-in connection, the auxiliary piston 32 tends to 
entrain the nut 12 axially to the left. This causes closure of the 
friction clutch by the friction surface 14 abutting the friction surface 
15. The nut 12 which now became unrotatable, upon continued axial movement 
of the brake piston 4 to the left, tends to entrain the spindle 11 in the 
same axial direction. This action is not successful as long as the axial 
force to the left, generated by the hydraulic pressure on the auxiliary 
piston 32, is smaller than the axial force to the right which is applied 
to the spindle 11 by the wire spring 23 and by hydraulic pressure. When, 
however, the hydraulic pressure exceeds a predefined value, the force of 
the auxiliary piston 32 will exceed the counteracting force of the wire 
spring 23 and the hydraulic counteracting force, and the spindle 11 moves 
to the left together with the nut 12 and the brake piston 4. The adjusting 
device is not readjusted by rotation of the nut 12 on the spindle 11 in 
this case. 
The purpose of preventing readjustment under high hydraulic pressure, as 
described, is that a high hydraulic pressure produces extremely great 
clamping forces. However, the brake housing 1 inheres a certain elasticity 
and expands slightly when very high clamping forces are applied. If the 
longer actuating travel of the brake piston 4, which is due to the brake 
housing 1 expanding, was compensated by readjustment of the adjusting 
device, the disc brake would lock upon termination of the hydraulic 
actuation because the brake housing 1 deforms elastically to resume its 
initial shape, without the brake piston 4 being permitted to perform a 
corresponding return movement. 
In an embodiment of the present invention shown in FIG. 3, the nut 12 has a 
venting duct 43 which provides a connection between a hollow chamber 45, 
produced by the nut 12, the spindle 11 and the auxiliary piston 32, and 
the interior 7 of the brake cylinder 3. 
In the variation of the present invention shown in FIG. 4, the auxiliary 
piston 32 has a venting groove 44 which extends from the hollow chamber 45 
radially alongside the piston end surface 46, where it deflects in a 
paraxial direction to extend on the peripheral surface 47 of the auxiliary 
piston 32 until an area which is not covered by the nut 12.