Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

10.5 Pressure and Stress —Force Acting Over an Area 279

Next, we will explain the operation and components of manually activated and pump-driven

hydraulic systems. Examples of these types of hydraulic systems are depicted in Figures 10.24(a)

and (b). Figure 10.24(a) shows a schematic diagram of a hand-driven hydraulic jack. The system

consists of a reservoir, a hand pump, the load piston, a relief valve, and a high-pressure check

valve. To raise the load, the arm of the hand pump is pushed downward; this action pushes the

fluid into the load cylinder, which in turn creates a pressure that is transmitted to the load piston,

and consequently the load is raised. To lower the load, the release valve is opened. The amount

by which the release valve is opened will determine the speed at which the load will be lowered.

Of course, the viscosity of the hydraulic fluid and the magnitude of the load will also affect the

lowering speed. In the system shown, the fluid reservoir is necessary to supply the line with as

much fluid as needed to extend the driven piston to any desired level.

The hydraulic system shown in Figure 10.24(b) replaces the hand-activated pump by a

gear or a rotary pump that creates the necessary pressure in the line. As the control handle is

moved up, it opens the passage that allows the hydraulic fluid to be pushed into the load cylin-

der. When the control handle is pushed down, the hydraulic fluid is routed back to the reser-

voir, as shown. The relief valves shown in Figure 10.24 could be set at desired pressure levels to

control the fluid pressure in the lines by allowing the fluid to return to the reservoir as shown.

Example 10.12 Determine the load that can be lifted by the hydraulic system shown in Figure 10.25. Use

g9.81 m/s

2

. We use Equation (10.20) to solve this problem.

F 2 8829 N 1 m 2 kg 2 1 9.81 m/s

2

2 1 m 2 900 kg

F 2 

A 2

A 1

F 1 

p 1 0.15 m 2

2

p 1 0.05 m 2

2 1 981 N^2 8829 N

F 1 m 1 g 1 100 kg 2 1 9.81 m/s

2

2 981 N

Load

Load

Release

valve

(closed)

Hand-activated

pump

High pressure

check valve

(open)

Control

lever

Relief

valve

(closed)

Pump

Reservoir

(a) (b)

■Figure 10.24 Two examples of hydraulic systems: (a) A hand-activated system. (b) A gear or rotary pump system.

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