【正文】 6, it is easy to achieve linear motion / 7, it is easy to achieve the automation of machines, when the joint control of the use of electrohydraulic, not only can achieve a higher degree of process automation, and remote control can be achieved. The shortings of the hydraulic system: 1, as a result of the resistance to fluid flow and leakage of the larger, so less efficient. If not handled properly, leakage is not only contaminated sites, but also may cause fire and explosion. 2, vulnerable performance as a result of the impact of temperature change, it would be inappropriate in the high or low temperature conditions. 3, the manufacture of precision hydraulic ponents require a higher, more expensive and hence the price. 4, due to the leakage of liquid medium and the pressibility and can not be strictly the transmission ratio. 5, hydraulic transmission is not easy to find out the reasons for failure。 hydraulic control part contains a variety of control valves, used to control the flow of oil, pressure and direction。 flow control valves including throttle, adjusting the valves, flow diversion valve sets, etc.。 special antenna technology giant with control devices, measu rement buoys, movements such as rotating stage。 speed turbine power plant installations, nuclear power plants, etc.。 iron and steel indu stry metallurgical machinery, lifting equipment, such as roller adjustment device。 rather into the cylinder, making the latter subject to variations in the pump delivery due to changing workloads. Bleedoff circuits should not be used in applications where there is a possibility of the load running away. Types of Flow Controls Flow control valves fall into two basic categories: pressure pensated and nonpressure pensated. The latter being used where load pressures remain relatively constant and feed rates are not too critical. They may be as simple as a fixed orifice or an adjustable needle for free valve, although more sophisticated units may even include a check valve for free flow in the reverse direction. Use of nonpressure pensated valves is somewhat limited, since flow through an orifice is essentially proportional to the square root of the pressure drop across it. This means that any appreciable change in the work load would affect the feed rate. Pressure pensated flow controls are further classified as restrictor and bypass types. Both utilize a pensator or hydrostat to maintain a constant pressure drop across an adjustable throttle. The ByPass Typebines overload protection with pressure pensated control of flow. It has a normally closed hydrostat which opens to divert fluid, in excess of the throttle setting, to the tank. Pressure required by the work load is sensed 25 in the chamber above the hydrostat and together with a light spring tends to hole it closed. Pressure in the chamber below the hydrostat increase duo to restriction of the throttle and cause is to rise diverting any excess flow to tank when the difference in pressure is sufficient to overe the spring. This difference, usually 20 psi, is maintained across the throttle providing a constant flow regardless of the work load. Some horsepower saving is acplished in that the pump need operate at only 20 psi above work load pressure. Overload protection is provided by an adjustable spring loaded poppet which limits the maximum pressure above the hydrostat, causing it to function as a pound relief valve whenever work load requirement exceed its setting. The bypass flow control can only be used in a meterin circuit. If used for metering out, exhaust oil which could not get through the throttle would be diverted to tank permitting the load to run away. The Restrictor Type Flow Controlalso maintains a constant 20 psi differential across its throttle by means of a hydrostat. In this valve, the hydrostat is normally epen and tends to close off blocking all flow in excess of the throttle setting. In these units the work load pressure acts with a light spring above the hydrostat to hold it open. Pressure at the throttle inlet and under the hydrostat tend to close it, permitting only that oil to enter the valve that 20 psi can force through the throttle. Because of their tendency close off when flow tales to exceed the throttle setting, restrictor type valves may be used in meterin, meterout and bleedoff circuits. Unlike the bypass type , two or more restrictor valves may be used with the same pump since the excess pump delivery returns to tank through the relief valves. When placed in cylinder lines an integral check valve is optional to provide free flow for a rapid return stroke. One would not be required for valves placed in the main supply line, the tank line of a directional valve or when they are used in bleedoff circuits. Temperature Compensated Flow Control Valve Flow through a pressure pensated flow control valve is subject to change 26 with variations in oil temperature. Later design Vickers valves incorporate a temperature. Although oil flows more freely when it is hot, constant flow can be maintained by decreasing the size of the throttle opening as the temperature rises. This is acplished through a pensating rod which lengthens with heat and contracts when cold. The throttle is a simple plunger that is moved in and out of the control port. The pensating is installed between the throttle and its adjuster. This design also is available with a reverse freeflow check valve. Remote Flow Control Valves Remote flow control valves permit adjustment of the throttle size by an electrical signal. The throttle spool is linked to armature of a torque motor and moves in response to signal to the torque motor. Operation is otherwise the same as a pressure pensated flow control