David E. Keyser Applications Coordinator
Keith Hogan OEM Sales
MICO, Inc. 1911 Lee Boulevard North Mankato, MN U.S.A. 56003-2507 Fax: +1 507 625 3212 Tel: +1 507 625 6426
ABSTRACT This paper will identify some prerequisites for brake actuation circuit design, describe three basic types of brake actuation circuitry, and will point out some common problems to be avoided in brake actuation circuit design for off-highway vehicles and equipment. It will deal only with the actuation of service brakes (as opposed to parking brake). Service brake selection, pneumatic, and vacuum powered brake systems are beyond the scope of this paper. PREREQUISITES Although we are assuming fully functional and operational brakes, this paper outlines six key prerequisites that need to be known in order to design the brake actuation circuit. These are: (1) stopping parameters, (2) required brake torque, (3) capacity of the service brakes, (4) brake line pressure, (5) volumetric requirements, and (6) required operator input effort. Because there are many brake actuation circuits possible for a given application, this information must be understood in order for the designer to choose the optimal circuit. In selecting the components for a brake actuation circuit the designer needs to start at the foundation (service) brakes and "work backward" to the method of actuation. By working backward the designer avoids the pitfall described below under the heading "Service Brake Capacity". Stopping Parameters The stopping parameters include deceleration rate, stopping distance, percentage of grade for operation, and vehicle speed. The desired stopping parameters, are used to determine the required brake torque. They can be sourced from one of several publications, such as the Society of Automotive Engineers (SAE), the International Standards Organization (ISO), Federal, State, Provincial Government agencies or specified by the vehicle designer. Brake Torque Requirement The torque required to stop the vehicle within the desired parameters can be calculated with the assistance of DESIGNING THE BRAKE SYSTEM STEP BY STEP, a 1976 SAE publication, 760637, by Fred W. Cords and John B. Dale, and MECHANICS OF VEHICLES, a 1957 Pento Publishing Co. publication by Jaroslav J. Taborek.
Service Brake Capacity The service brake capacity is expressed as the torque at a given brake line pressure and is generally a linear function. This information is acquired from the brake and or axle manufacturer. To avoid the pitfalls of either underbraking or overbraking it is important to calculate the actual required torque for the application. Do not assume that the manufacturer's specified maximum brake line pressure is the same as the required brake line pressure. This can be demonstrated in the following examples in which the brake torque requirement is above then below the torque capability of the service brake. Consider a vehicle that requires 680 N·m of brake torque. The torque capability of the service brakes is 450 N·m at a maximum pressure of 70 bar. Supplying the maximum 70 bar does not stop the vehicle as required, the vehicle is underbraked. Conversely, consider the torque requirement is now 110 N·m and the brake is still rated at 450 N·m and 70 bar. Supplying the 70 bar will stop the vehicle immediately (too severely). The torque requirement is satisfied with 25% of the available pressure resulting in aggressive, over sensitive brakes. These scenarios will change in magnitude depending on the style of actuation that you select. Through proper brake actuation and circuit design one can match the circuit to the service brakes and avoid poor performance characteristics such as these. Brake Line Pressure The brake line pressure is determined by the brake torque requirement and the service brake capacity. By establishing how much torque is developed at a given pressure, brake torque can...