Hydraulic System There are only following three basic methods of transmitting power : electrical，mechanical ,and fluid power. Most applications actually use a combination of the three methods to obtain the most efficient overall system. To properly determine which principle method to use, it is important to understand the characteristic of each method. For example，hydraulic systems on the long distance economically deliver power more than mechanical systems. However, fluid systems are restricted to shorter distances than electrical systems. Hydraulic power transmission systems are concerned with the generation, modulation, and control of sculptress and flow, and in general such systems include: l. Pumps convert available power from the prime mover to hydraulic power at the actuator. 2. Valves control the direction of pump- flow, the level of produced power, and the amount of fluid-flow to the actuators. The power level is determined by controlling both the flow and pressure level. 3. Actuators convert hydraulic power to usable mechanical power output at the point required. 4. The medium is a liquid, provides rigid transmission and control as well as lubrication of components, sealing in valves, and cooling of the system. 5.Connectors link the various system components, provide power conductors for the fluid under pressure, and fluid flow return to tank（reservoir). 6. Fluid storage and conditioning equipment ensure sufficient quality and quantity as well as cooling of the fluid. Hydraulic systems are used in industrial applications such as stamping presses, steel mills and general manufacturing, agricultural machines, mining industry, aviation, space technology, deep-sea exploration, transportation, Narine technology, and offshore gas and petroleum exploration. In short, many people get somehow benefiting from the technology of hydraulics. The secret of the success and widespread use of the hydraulic system is its versatility and manageability. Fluid power is not hindered by the geometry of the machine as is the case in mechanical systems. Also, power can be transmitted in almost limitless quantities because fluid systems are not so limited by the physical limitations of materials as are the electrical systems. For example, the performance of an electromagnet is limited by the saturation limit of steel. On the other hand, the power limit of fluid systems is limited only by the strength capacity of the material. Industry is going to depend more and more on automation in order to increase productivity. This includes remotion and direct control of production operations, manufacturing processes, and materials handling. Fluid power is the muscle of automation because of the advantages in the following four major categories. 1. Ease and accuracy of control. By the use of simple levers and push buttons, the operator of a fluid power system can readily start , stop ,speed up or slow down, and provide any desired horsepower with tolerances as precise as one ten-thousandth of an inch. 2. Multiplication of force. A fluid power system (without using cumbersome gears, pulleys, and levers) can multiply force simply and efficiently create the output from a fraction of an ounce to several hundred tons. 3. Constant force or torque. Only fluid power systems are capable of providing constant force or torque regardless of speed changes. It produces the work output moving from a few inches per hour to several hundred inches per minute, or from a few revolutions per hour to thousands of revolutions per minute. 4. Simplicity, safety, economy. In general, fluid power systems use fewer moving parts than comparable mechanical or electrical systems. Thus, they are simpler to maintain and operate. This, in turn, maximizes safety, compactness, and reliability. For example, a new power steering control design has made all other kinds of power systems obsolete on many off-highway vehicles. The steering unit consists of a manually operated directional control valve and meter in a single body. Because the steering unit is fully fluid-linked, without the link of mechanical linkages, universal joints, bearings, reduction gears, etc. This provides a simple, compact system. In addition, very little input torque is required to produce the control needed for the toughest applications. This is important where limitations of control space require a small steering wheel and it becomes necessary to reduce operator fatigue. Additional benefits of fluid power systems include instantly reversible motion, automatic protection against overloads, and infinitely variable speed control. Fluid power systems also have the highest horsepower per weight ratio of any known power source. In spite of all these highly desirable features of fluid power, it is not a panacea for all power transmission problems. Hydraulic systems also have some drawbacks. Hydraulic oils are messy, and leakage is impossible to comp