The hydraulic brake system used in the automobile is a multiple piston system. A multiple piston system allows forces to be transmitted to two or more pistons in the manner indicated in figure 2-25. Note that the pressure set up by the force applied to the input piston (1) is transmitted
undiminished to both output pistons (2 and 3), and that the resultant force on each piston is proportional to its area. The multiplication of forces from the input piston to each output piston is the same as that explained earlier.
The hydraulic brake system from the master cylinders to the wheel cylinders on most automobiles operates in a way similar to the system illustrated in figure 2-26.
Figure 2-25: Principle of hydraulic brake
When the brake pedal is depressed, the pressure on the brake pedal moves the piston within the master cylinder, forcing the brake fluid from the master cylinder through the tubing and flexible hose to the wheel cylinders. The wheel cylinders contain two opposed output pistons, each of which is attached to a brake shoe fitted inside the brake drum.
Figure 2-26: Hydraulic brake system
Each output piston pushes the attached brake shoe against the wall of the brake drum, thus retarding the rotation of the wheel. When pressure on the pedal is released, the springs on the brake shoes return the wheel cylinder pistons to their released positions. This action forces the displaced brake fluid back through the flexible hose and tubing to the master cylinder.
The force applied to the brake pedal produces a proportional force on each of the output pistons, which in turn apply the brake shoes frictionally to the turning wheels to retard rotation.
As previously mentioned, the hydraulic brake system on most automobiles operates in a similar way, as shown in figure 2-26. It is beyond the scope of this manual to discuss the various brake systems.
Accumulators
An accumulator is a pressure storage reservoir in which hydraulic fluid is stored under pressure from an external source. The storage of fluid under pressure serves several purposes in hydraulic systems.
In some aircraft hydraulic systems it is necessary to maintain the system pressure within a specific pressure range for long periods of time. It is very difficult to maintain a closed system without some leakage, either external or internal. Even a small leak can cause a decrease in pressure. By using an accumulator, leakage can be compensated for and the system pressure can be maintained within an acceptable range for long periods of time.
Accumulators also damp out fluctuations in pressure due to the operation of services such as control surfaces and landing gear. They can supply extra pressure when all the hydraulic services are being operated at one time (flaps, control surfaces, landing gear etc.) and when the hydraulic pump is unable to cope. They can also be used in an emergency when all other hydraulic power pressure supplies (pumps etc) have failed. Thus a large modern aircraft can be controlled on accumulator power alone, for up to an hour.
Accumulators also compensate for thermal expansion and contraction of the liquid due to variations in temperature.
Figure 2-27: Hydraulic Accumulator
The accumulator consists of an air chamber, which is charged with air or nitrogen. This is called the pre-charge pressure and is usually about 1000 PSI. This pressure is measured when there is no hydraulic pressure. The air chamber is the under side of the piston shown in figure 2-27. With no hydraulic pressure, the air/nitrogen pressure will push the piston to the top of the accumulator. A pressure gauge may be attached to the accumulator to indicate the air/nitrogen pressure. When the hydraulic pumps are switched on, the hydraulic pressure (acting on top of the piston, in opposition to the air/nitrogen pressure) begins to rise. When the hydraulic pressure exceeds the air/nitrogen pre-charge pressure (1000 PSI), the piston will begin to move down and further compress the air/nitrogen pressure.
At all times that the hydraulic pressure is above the air/nitrogen pre-charge pressure of 1000 PSI, the air/nitrogen and the hydraulic pressures are equal. Thus when the hydraulic pressure has reached its working level of 3000 PSI, the air/nitrogen pressure is also 3000 PSI.
It is the additional pressure supplied to the air/nitrogen by the hydraulic pressure, which can be used to feed back the pressure to the hydraulic fluid if the hydraulic fluid pressure falls below that of the air/nitrogen.
However, when the air/nitrogen gauge indicates 1000 PSI, the hydraulic pressure is zero, since the air/nitrogen has expanded back to its original pre-charge pressure.
Problems
1. Calculate the pressure on a gas when a force of 3100 N is exerted on a piston of diameter 2 cm
2. Calculate the force exerted when a pressure of 1 bar acts on a piston of diameter 8 cm which has a piston rod of diameter 2 cm taking some of the piston area.
3. The piston face area in the hydraulic jack shown above is 0.3 sq.in. The rod cross sectional area is 0.1 sq.in. Calculate the force and direction the ram rod will move if a pressure of 12 PSI enters equally into both sides of the cylinder chamber.
4. A brake master cylinder has a piston diameter of 0.4 ins. It feeds pressure to 4 identical wheel cylinders, each having just one piston of diameter 2 ins. What is the force on one wheel brake when the driver applies a force of 80 Ibs to the master cylinder?
5. An hydraulic accumulator is charged with nitrogen to 600 PSI. The hydraulic pump is then switched on and it feeds 3000 PSI to the other side of the accumulator piston. What will be the new pressure on the nitrogen side of the accumulator?
A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury.