Drum brake device

Fluid-pressure and analogous brake systems – Speed-controlled – Having a valve system responsive to a wheel lock signal

Reexamination Certificate

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Details

C303S011000, C303SDIG003, C303SDIG004, C303S010000

Reexamination Certificate

active

06328391

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drum brake device which is based on feedback control, and is able to constantly produce a stable braking force irrespective of the state of a brake friction surface, and is further adaptable for use in both an anti-lock braking system and a traction control system.
2. Discussion of the Related Art
Drum brake devices are widely employed in vehicle braking systems. A major reason for the drum brake device's use is its ability to automatically amplify a braking force (referred to as a self-amplifying function), as is observed in the uni-servo or duo-servo brake devices. This self-amplifying ability lessens the required brake operation force (e.g., a brake pedal depression force). However, this self-amplifying operation of the drum brake device is unstable. A proportional constant, which defines a relationship between a pedal depressing force and a final braking force, is very sensitive to the friction coefficient of the interface between the brake drum and the brake shoe. In cases where the inner surface of the brake drum is rusted after the vehicle travels in the rain, the wheels will be abruptly locked by applying an extremely small depression to the brake pedal.
To remove this defect, it is essential that the drum brake device have the ability to constantly produce a stable braking force at a predetermined boosting ratio in response to a brake operation force, even if the frictional characteristic of the interface between the brake drum and the brake shoe varies. One solution to this problem was proposed in JP-B-2-46424. The technique of this publication will be described with reference to
FIGS. 2 and 3
.
As shown, a drum brake device
1
is made up of a pair of brake shoes
3
and
4
, a wheel cylinder
6
, a link member
8
, a master cylinder
12
, and a fluid passage
13
. In the drum brake device
1
, the wheel cylinder
6
is improved to produce a stable braking force. The brake shoes
3
and
4
are oppositely disposed within an inner space of a cylindrical brake drum
2
. The wheel cylinder
6
, disposed between the opposed ends of brake shoes
3
and
4
, is used for expanding the ends of the brake shoes
3
and
4
so that they come into engagement with the inner surface of the brake drum. The link member
8
mutually links the other ends of the brake shoes
3
and
4
, and receives an anchor reaction force of one of the brake shoes
3
and
4
and transmits it to the other brake shoe. The master cylinder
12
generates a hydraulic braking pressure that corresponds in magnitude to a brake operation force F
1
(a depression force applied to a brake pedal
10
). The hydraulic braking pressure generated by the master cylinder
12
is introduced into the wheel cylinder
6
through the fluid passage
13
.
Details of the wheel cylinder
6
are illustrated in FIG.
3
. As shown, a cylinder body
17
includes a first cylinder
17
a
and a second cylinder
17
b
. These cylinders (
17
a
and
17
b
) are integrally formed, with the former (
17
a
) being located below the latter (
17
b
). The first cylinder
17
a
contains a slidable drive piston
15
. The second cylinder
17
b
contains a slidable control piston
16
. A hydraulic braking pressure is transmitted from the fluid passage
13
to a pressure chamber
20
via a control chamber
19
. The control chamber
19
is formed in the second cylinder
17
b
while the pressure chamber
20
is formed in the first cylinder
17
a
. The tip of the drive piston
15
contacts the primary shoe
3
, and presses the primary shoe
3
against the brake drum
2
by a thrust force P
1
. Thrust force P
1
corresponds to the hydraulic braking pressure supplied to the pressure chamber
20
.
The tip of the control piston
16
, which is in contact with the end of the secondary shoe
4
, receives an anchor reaction force P
2
from the secondary shoe
4
while the base end of the control piston
16
receives a hydraulic braking pressure supplied to the control chamber
19
. When an urging force corresponding to the anchor reaction force P
2
exceeds an urging force P
3
caused by the hydraulic braking pressure, the control piston
16
is displaced toward the control chamber
19
. Further, a valve body
24
is provided within the control chamber
19
. The valve body is used for opening and closing a communicating passage
22
which communicates the control chamber
19
with the pressure chamber
20
. When the control piston
16
is displaced toward the control chamber
19
, the communicating passage
22
is closed with the valve body
24
.
When the urging force P
3
(caused by the hydraulic braking pressure from the master cylinder
12
) is imparted or input to the control piston
16
, and the anchor reaction force P
2
is varied to a force defined by a predetermined boosting ratio, the control piston
16
is displaced toward the control chamber
19
to stop the supply of the hydraulic braking pressure to the pressure chamber
20
. As a result, the thrust force P
1
of the drive piston
15
is kept constant, a further increase of the anchor reaction force is prevented, and the braking force is stabilized.
In the conventional drum brake device mentioned above, the mechanism for controlling the anchor reaction force is incorporated into the wheel cylinder
6
. However, this type of drum brake device has the following problem. The structure of the brake device increases the size of the wheel cylinder
6
. The increase of the cylinder size makes it difficult to assemble the wheel cylinder
6
into the brake device of a small-size vehicle that has a brake drum with a small inner space. Additionally, the increased cylinder size further increases the weight of the brake device with the wheel cylinder assembled thereinto. For this reason, the conventional drum brake device which employs the unique mechanism for achieving braking force stabilization has found limited use in small-size vehicles. It is almost impossible to apply a common conventional drum brake device to various types of vehicles. Specifically, vehicles differ in body weight and in anchor reaction forces of the drum brake devices assembled thereinto. Accordingly, an object of the present invention is to provide a drum brake device which produces a stable braking force at a predetermined boosting ratio in response to a brake operation force input thereto, even if the frictional characteristics of the interface of the brake drum and the brake shoe varies. Additionally, the aforementioned object should be realized without increasing the device size. Furthermore, the drum brake device should be suitably applicable to a small-size vehicle having a brake drum with a small inner space.
SUMMARY OF THE INVENTION
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect of the present invention there is provided a drum brake device, comprising a brake drum; a pair of brake shoes oppositely disposed within the brake drum, each of the pair of brake shoes having a first end and a second end; a wheel cylinder for expanding the brake shoes, the wheel cylinder being disposed between the first ends of the brake shoes, and the second ends of the brake shoes being connected; a master cylinder outputting a hydraulic braking pressure; a fluid passage for introducing the hydraulic braking pressure output from the master cylinder to the wheel cylinder, the hydraulic braking pressure being dependent on a brake operation force; an input detector, coupled to the fluid passage, that detects the hydraulic braking pressure output from the mas

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