Hydraulic Actuation System

The basic components of the hydraulic actuation system are illustrated in Figure 6.4, and include the brake pedal, booster (if fitted), master cylinder, hydraulic lines (pipes, tubes, hoses, etc.) and slave cylinders at the brakes. Since the hydraulic pressures generated are high (a typical design maximum is 12 MPa or 120 bar), all the system components must be designed with high stiffness, otherwise the brake pedal travel and ‘fluid consumption’ would be increased by too much elasticity in the system.

It is a legislative requirement (UN Regulations 13 and 13H) that every road vehicle has at least two separate brake actuation circuits, as illustrated in Figure 6.5, so that in the event of the failure of one circuit, the capability remains for the driver to stop the vehicle safely, albeit at a reduced level of performance as specified by the relevant legislation.

The simplest configuration for a two-axle rigid vehicle is a ‘vertical split’, where the front brakes are on one circuit and the rear brakes are on the other. The limitation of this design is that if the front circuit should fail, the adhesion available at the rear wheels may not be sufficient to provide the deceleration required by the legislation in the system part-fail condition. The vertical split can be satisfactory for rear wheel drive (RWD) and four-wheel drive (4WD) vehicles, but is usually not satisfactory for front wheel drive (FWD) vehicles because their weight distribution is concentrated on the front axle, especially when unladen. For FWD cars and light commercial vehicles the ‘diagonal split’ is better. This ensures that braking is always available at one front and one rear wheel, each on opposite sides of the vehicle. Because front brakes generally contribute a higher proportion of the vehicle braking force than the rear brakes (i.e. X1 > X2), in the part-fail condition where only one circuit is providing the braking force, the vehicle is subject to a strong yaw moment, which can be minimised by appropriate steering geometry and suspension design, usually in the form of zero or negative offset steering. For vehicles where neither the vertical nor the diagonal split is satisfactory, other configurations can be used.

Referring to Figure 6.4, the hydraulic pressure generated by the master cylinder is transmitted via the hydraulic brake lines to the slave cylinders, which provide the actuation force at each brake. The principle is the same for hydraulic actuation of a drum brake and a disc brake, with the difference that the disc brake requires a larger diameter slave cylinder than the drum brake because of the lower brake factor. In a drum brake hydraulic slave cylinder, the hydraulic seal is fitted to the piston and sliding contact occurs between the seal and the cylinder bore as the brake shoes are actuated. In a disc brake, the hydraulic seal fits in a groove machined in the cylinder bore. Under normal operation the displacement of the disc brake slave piston is small enough so that it is accommodated by deflection of the seal instead of sliding.

The required maximum clamp force Ncmax is:

The pedal ratio (Rp) is usually designed to conform with the vehicle manufacturer’s in-house standards, which are based upon the boundaries and restrictions.

The maximum movement (travel) of the brake pedal (xpmax) is determined by the distance between the ‘off’ position and the floor or bulkhead, including any deformation or deflection of the brake pedal, pedal box and the floor or bulkhead. The maximum pushrod travel or full stroke depends upon the pedal travel and the pedal ratio (Rp) as shown in Equation (6.16) and must not exceed the maximum length of piston travel of the master cylinder, which is usually specified by its manufacturer:

If the maximum pedal travel (xpmax) is 100 mm and Rp = 4, the full stroke of the master cylinder would be 25 mm and the maximum length of piston travel of the master cylinder would in practice usually be specified as significantly more (e.g. 36 mm) to accommodate deformation and deflections of the system components under actuation forces.

Exercise: Using data given in Table 6.7 and driver pedal force, Fd = 65/250/500N, calculate: a) Primary and secondary master cylinder pressure (Rb = 1/4/6) b) Total front and rear axle braking force and torque c) Volume of brake fluid of the master cylinder d) Braking distribution ratio