A brake system for a vehicle, such as a bus, truck or the like, typically includes an actuator and an application unit which presses friction material into contact with a brake disc or a brake drum assembly. Conventional brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions and an emergency or parking brake actuator which causes actuation of the brakes when power is removed. The parking brake actuator may include a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake. The emergency or parking brake may be a diaphragm or piston type brake.
Application of either the service or spring brakes is accomplished via a brake application unit which is activated by the service or spring actuator to apply friction material to a hub or disc.
Typically, the spring brake actuator is paired with the service brake actuator. When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm compresses the compression spring. A spring brake actuator rod is held in a retracted position by a relatively small return spring, thus not affecting the operation of the brake. When the brake is to be applied during normal driving operation, compressed air is provided to the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and causes the brakes to be applied with an application force which is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the compression spring acting on the spring brake actuator rod which in turn acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion may serve both as a parking brake and as an emergency brake.
Such common piggy back or combination designs lead to additional axial length of the brake actuator and the brake system. Further, air brake systems are typically designed with extra long strokes. Long stroke brakes provide heavy vehicle brakes with a greater margin of effectiveness for thick layers of friction material and to better combat brake fade. Stroke length relates to the performance of the service chamber.
Several trends in vehicle design are leading to changes in air brake actuator design. Increasingly, vehicle manufacturers are requiring additional features and capabilities in brake systems, while at the same time demanding that the brake system be provided in a smaller package while generating the same clamp force. Ideally, these demands will not necessitate a redesign of the application unit to accommodate the application of an input force at a substantial angle to the axis of operation of the application unit.
At the same time, the dimensions of the brake arrangements in the vehicle chassis are often critical. Modern brake systems for heavy vehicles often require space consuming electronic components such as onboard diagnostic sensors, electric motors and antilock brake modulators. These components, often necessary by regulation or by customer demand, occupy much of the limited space within the wheel well. In addition, suspension systems are also growing in complexity and size and in the number of components, further reducing the available space.
As wheel sizes remain constant, it is thus desirable to reduce the dimensions of the brake system. A particular requirement of vehicle manufacturers is reduction of actuator axial length. In the prior art, this has been accomplished with smaller caliper designs and reduction of excessive clearance between the pad and the disk.
As the stroke length of the actuator cylinder is reduced, the overall axial length of the brake system may also be reduced. Brakes with reduced axial dimensions are more easily adapted to different vehicles.
One approach to meet the needs of vehicle manufacturers has been to mount the actuator radially, at an angle approaching perpendicular to the application unit's axis of operation. Such radially mounted actuators may reduce the length of the brake system. A problem with this solution, however, is that it requires a redesign of the application unit in order for the application unit to generate sufficient clamp force from a radially applied input force.
While standard actuators operate along a substantially parallel axis with respect to the application unit, a radially mounted actuator operates at a greater angle with respect to the operation axis of standard application units. Further, radially mounted actuators are low volume parts compared to prior art axially mounted actuators, and are thus higher in cost. In addition to requiring a redesign of the application unit, radially mounted actuators also require alternative calipers and mounting apparatus.
The invention provides a compact brake system having a reduced axial dimension while maintaining substantially parallel axes of operation for the application unit and the actuator by shortening the stroke length of the actuator. The shortened stroke length may be as short as approximately 40 millimeters for a service brake. Similarly, the stroke length of the parking or emergency brake may be shortened to approximately 30 millimeters or less for additional reduction of axial length of the brake system.
Such a compact brake system can fit modern system architecture, and is particularly adaptive to the more restrictive space on front axle installations. Such a compact spring brake also may be suitable for sliding and fixed caliper applications and may be adapted for integrated antilock brake modulators.
Shortening the stroke length may reduce the mechanical advantage in the application unit. The mechanical ratio may be reduced to approximately 10:1 from a standard ratio of 15:1 for a conventional 24/24 spring brake chamber, an approximately 33% reduction in stroke length.
To overcome the decrease in mechanical advantage, the diameter of the actuator may be increased. A larger diameter actuator may house a larger diaphragm. A larger diaphragm is capable of generating increased actuator forces. Modifications may also be made to the application unit to compensate for the shorter stroke length and decreased mechanical advantage. The modifications could include altering the active length of the application unit lever and/or changing the eccentric ratios to alter the force multiplier ratio of the application unit.
In the past it has been normal to have a certain over-capacity regarding the stroke length of the actuator. The over-capacity provides the actuator sufficient stroke length to guarantee that the brake is actuated even as the friction material is consumed, if, for example, the adjustment mechanism fails. The over-capacity of the stroke length has meant that the actuator must also have an over-capacity with regard to the actuator dimensions. Thus, the axial length of the brake system is larger than it would have been without the over-capacity.
A sensing system to monitor the stroke of the actuator and provide a warning if the stroke differs from a reference value could reduce or eliminate the need for over-capacity. Because a possible malfunction may be detected early, the overcapacity of the stroke actuator may be reduced substantially, thus allowing the axial length of the actuator to be safely decreased.
According to the invention, the concept of having a rather large over-capacity of the stoke length of a brake actuator may be replaced by the technique of constantly monitoring the exact position of the brake actuator. Thus, the need for over-capacity of the brake actuator is avoided.