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.
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 gauges, valves and other components required for operation.
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.
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.
Two types of heat exchanger are used to cool hydraulic oil: (1) shell-and-tube and (2) finned tube. The shell-and-tube (Fig. 8.1) has a series of tubes inside a closed cylinder. The oil flows through the small tubes, and the fluid receiving the heat (typically water) flows around the small tubes. Routing of the oil can be done to produce a single pass (oil enters one end and exits the other end) or a double pass (oil enters one end, makes a u-turn at the other end, and travels back to exit at the same end it entered).
The finned tube exchanger (Fig. 8.2) is used for oil-to-air exchange. The air may be forced through the exchanger with a fan or may flow naturally. If an oil cooler is used on a mobile machine, it is the finned tube type.
Oil coolers are not built to withstand pressure; they are mounted in the return line in an off-line loop. The two options used are shown in Figs. 8.3a and 8.3b. In Fig. 8.3a, the system pump flows oil through the heat exchanger in the return line. This arrangement works well for many circuits. The exchanger is sized to give only a small pressure drop at rated flow. The circuit shown in Fig. 8.3b has a separate low-pressure pump to flow oil through the heat exchanger.
More complex circuits can have significant pressure pulses in the return line. These pulses hammer the heat exchanger and, over time, the joints fracture and begin to leak. If significant (greater than 10 psi) pulses are measured in the return line, the circuit shown in Fig. 8.3b should be used. Here, a separate pump is used to circulate oil from the reservoir through the heat exchanger and back to the reservoir. This circulating pump does not have to build pressure (only the 15 psi or so is required to flow fluid through the exchanger); therefore, it can be an inexpensive design. Any kind of pump is satisfactory if it is rated for the needed flow rate and has seals that are compatible with the fluid properties.
There are three types of failure in a fluid power system.
1. Degradation. The performance of the component degrades over time as surfaces wear, clearances increase, and leakage increases.
2. Intermittent. Valves stick and then break loose such that operation is intermittent.
3. Catastrophic. Catastrophic failure occurs when a major component breaks apart. Often, debris causes the failure of other components, and a total replacement of the circuit is required.
Proper selection, placement, and servicing of contamination control devices will eliminate an estimated 80% of all system failures. Maintaining system cleanliness is a key issue in the operation of all fluid power systems, and particularly high-pressure oil circuits.
There are four sources of contamination in hydraulic fluid.
1. Built-in contamination. This is contaminate that was left in the system when it was assembled. It can range from a piece of teflon tape to a piece of welding slag.
2. Contaminated new oil. Contaminate is introduced during the manufacture and subsequent handling of oil. If this is not removed, it enters the system.
3. Ingressed contamination. This contaminate can enter with air flowing into the reservoir through the breather cap, or it can ride in on a cylinder rod. No rod seal can totally prevent the entrance of particles. Also, whenever a line is disconnected or the system is opened in any way, there is the potential for contamination to ingress.
4. Internally generated contaminate. Particles removed from the interior surface of the components will circulate in the system until they are removed. Each impact of one of these particles with a surface causes more damage. This phenomenon is known as the wear regeneration cycle.
Internally generated contamination causes damage in the following ways:
Abrasive wear. The particles that break loose from the hardened surfaces of the components are very hard. These particles bridge across the clearance between two moving surfaces and abrade one or both surfaces.
Adhesive wear. As discussed in the previous section, high temperature reduces oil viscosity and thins the film between moving parts. These parts then adhere, or stick together, and damage results.
Fatigue wear. Particles that bridge a clearance can cause a stress concentration. This stress concentration eventually causes a crack to form (Fig. 8.4), and this crack spreads until part of the surface breaks away. This type of surface failure is called a spall.
Erosive wear. Particles in a stream of moving fluid erode away the surface of the metering edge (Fig. 8.5).
Cavitation wear. When the pump compresses fluid that contains air bubbles, the bubbles implode. The resulting shock wave impacts the surface and, over time, these impacts cause damage. Cavitation wear and aeration wear are sometimes discussed as separate wear phenomena, but they are quite similar.
Corrosive wear. Chemical attack of the surface causes a loss of material. A simple example is water condensation in the reservoir that causes the metal to rust. More complex reactions than the oxidation of iron occur, and all these reactions are grouped under corrosive wear.
A temperature switch is an instrument that automatically senses a change in temperature and opens or closes an electrical switching element when a predetermined temperature level is reached. Figure 6.47 is an illustration of a common type of temperature switch which has an accuracy of ±1 F maximum.
This temperature switch is provided with an adjustment screw at the top end in order to change the actuation point. In order to facilitate its mounting on the hydraulic system whose temperature is to be measured, the bottom end of the switch is provided with threads. As in the case of pressure switches, temperature switches can also be wired either normally open or normally closed.
Flowmeters are used to measure flow in a hydraulic circuit. As shown in Figure 6.49, flowmeters mainly comprise of a metering cone and a magnetic piston along with a spring, for holding the magnetic piston in the no-flow position.
Flow meters are normally not bi-directional in nature. They in fact act as check valves and block flow in the reverse direction. Initially the fluid entering the device flows around the metering cone, exerting pressure on the magnetic piston and spring. With increase in flow in the system, the magnetic piston begins compressing the spring and thereby indicates the flow rate on a graduated scale.
Leaky fittings are a cause for concern in hydraulic circuits especially with increase in the number of connections. This is where manifolds play a very important role. Their incorporation in a hydraulic circuit helps drastically reduce the number of external connections required. Figure 6.50 shows a simple manifold commonly employed in hydraulic systems.
In the case of modular valve stacking, the manifolds used are provided with common pressure and return ports, with each valve station being incorporated with individual A and B work ports. Manifolds are normally specified according to system pressure, total flow, number of work stations and valve size and pattern.