Virtually, all-hydraulic circuits are essentially the same regardless of the application. There are six basic components required for setting up a hydraulic system:
1. A reservoir to hold the liquid (usually hydraulic oil)
2. A pump to force the liquid through the system
3. An electric motor or other power source to drive the pump
4. Valves to control the liquid direction, pressure and flow rate
5. An actuator to convert the energy of the liquid into mechanical force or torque,
to do useful work. Actuators can either be cylinders which provide linear
motion or motors which provide rotary motion and
6. Piping to convey the liquid from one location to another.
Figure 9.1 illustrates the essential features of a basic hydraulic system with a linear hydraulic actuator.
The extent of sophistication and complexity of hydraulic systems vary depending on the specific application.
Each unit is a complete packaged power system containing its own electric motor, pump, shaft coupling, reservoir and miscellaneous piping, pressure gages, valves and other components required for operation.
Power steering is another automotive application developed by Bendix Corporation. This is used in conjunction with a conventional type steering gear. The hydraulic power cylinder is mounted at any convenient place where it can be connected to act directly on the steering tie rod or equivalent linkage member (Figure 9.4). Power for steering is applied in the most simplest and direct way as straight-line motion to the existing steering linkage of the vehicle.
The control valve of the two-unit type installation is mounted in one of the ball joints, usually at the Pitman arm. A small movement in the valve serves to open and close the hydraulic ports and thus operate the double acting power cylinder. Installation of the power cylinder and control valve can be made without changing the existing geometry of the steering linkage.
In effect, the existing steering system including the steering gear remains intact. Likewise the system is free to operate entirely by physical effort, when the engine is not running and in the absence of hydraulic pressure.
This system was developed by Bendix Corporation as a solution to the typically crowded engine compartments consisting of larger vacuum units. Figure 9.3 contains a schematic of this system.
The basic system consists of an open center spool valve and a hydraulic cylinder assembled in a single unit. The power steering pump supplies the operating pressure. Hydro-boost provides power assist to operate the dual master cylinder braking system.
Normally mounted on the engine compartment firewall, it is designed to provide specific brake feel characteristics throughout the wide range of pedal force and travel. A spring accumulator stores energy for reverse stops.
A hydraulic system has four major advantages, which makes it quite efficient in transmitting power.
1. Ease and accuracy of control: By the use of simple levers and push buttons, the operator of a hydraulic system can easily start, stop, speed up and slow down.
2. Multiplication of force: A fluid power system (without using cumbersome gears, pulleys and levers) can multiply forces simply and efficiently from a fraction of a pound, to several hundred tons of output.
3. Constant force and torque: Only fluid power systems are capable of providing a constant torque or force regardless of speed changes.
4. Simple, safe and economical: In general, hydraulic systems use fewer moving parts in comparison with mechanical and electrical systems. Thus they become simpler and easier to maintain.
In spite of possessing all these highly desirable features, hydraulic systems also have certain drawbacks, some of which are:
• Handling of hydraulic oils which can be quite messy. It is also very difficult to completely eliminate leakage in a hydraulic system.
• Hydraulic lines can burst causing serious human injuries.
• Most hydraulic fluids have a tendency to catch fire in the event of leakage, especially in hot regions.
It therefore becomes important for each application to be studied thoroughly, before selecting a hydraulic system for it. Let us now discuss some of the most important and common hydraulic system applications.