278 Chapter 10 Force and Force-Related Parameters
the brake shoes against the brake drums (rotator). The mechanical power is converted to
hydraulic power and back to mechanical power. We will discuss the idea of what we mean by
power later in this chapter and in Chapter 13. The typical hydraulic fluid pressure in a car’s
hydraulic brake lines is approximately 10 MPa (1500 psi).
Now let us formulate a general relationship among force, pressure, and area in a hydraulic
system such as the simple hydraulic system shown in Figure 10.23. Because the pressure is
nearly constant in the hydraulic system shown, the relationship between the forcesF 1 andF 2
could be established in the following manner:(10.17)
(10.18)
(10.19)
(10.20)
It is important to realize that when the forceF 1 is applied, the drive piston (piston 1) moves by
a distanceL 1 , while the driven piston (piston 2) moves by a shorter distance,L 2. However, the
volume of the fluid displaced is constant, as shown in Figure 10.23. Knowing that in an enclosed
system, the volume of the fluid displaced remains constant, we can then determine the distance
L 2 by which piston 2 moves, provided the displacement of piston 1,L 1 , is known.(10.21)
(10.22)
Moreover, a relationship between the speed of the drive piston and the driven piston can be for-
mulated by dividing both sides of Equation (10.22) by the time that it takes to move each pis-
ton. This leads to the following equation:(10.23)
whereV 1 represents the speed of the drive piston, andV 2 is the speed at which the driven piston
moves.V 2
A 1
A 2
V 1
L 2
A 1
A 2
L 1
fluid volume displacedA 1 L 1 A 2 L 2
F 2
A 2
A 1
F 1
P 1 P 2
F 1
A 1
F 2
A 2
P 2
F 2
A 2
P 1
F 1
A 1
F 1 A 1 A 2 F 2
L 1
L 2
■Figure 10.23
An example of a simple hydraulic
system.62080_10_ch10_p251-302.qxd 5/22/10 12:32 AM Page 278
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