【正文】 (4)國(guó)內(nèi)市場(chǎng)國(guó)際化程度日益提高，國(guó)外公司紛紛進(jìn)入中國(guó)市場(chǎng)參與競(jìng)爭(zhēng)，加上國(guó)內(nèi)私營(yíng)、合作經(jīng)營(yíng)、個(gè)體、三資等企業(yè)的崛起，給國(guó)有企業(yè)造成愈來愈大的沖擊。 發(fā)展走勢(shì) 隨著社會(huì)主義市場(chǎng)經(jīng)濟(jì)的不斷深化，液壓、氣動(dòng)和密封產(chǎn)品的市場(chǎng)供求關(guān)系發(fā)生較大變化，長(zhǎng)期來以“短缺”為特征的賣方市場(chǎng)已基本成為以“結(jié)構(gòu)性過?！睘樘卣鞯馁I方市場(chǎng)所取代。從總體能力看，已處于供大于求的態(tài)勢(shì)，特別是一般低檔次液壓、氣動(dòng)和密封件，普遍供過于求；而主機(jī)急需的技術(shù)含量高的高參數(shù)、高附加值的高檔產(chǎn)品，又不能滿足市場(chǎng)需要，只能依賴于進(jìn)口。在我國(guó)加入WTO后，其沖擊有可能更大。因此，“十五”期間行業(yè)產(chǎn)值的增長(zhǎng)，決不能依賴于量的增長(zhǎng)，而應(yīng)針對(duì)行業(yè)自身的結(jié)構(gòu)性矛盾，加大力度，調(diào)整產(chǎn)業(yè)結(jié)構(gòu)和產(chǎn)品結(jié)構(gòu)，也就是應(yīng)依靠質(zhì)的提高，促進(jìn)產(chǎn)品技術(shù)升級(jí)，以適應(yīng)和拉動(dòng)市場(chǎng)需求，求得更大的發(fā)展。氣壓系統(tǒng)氣壓系統(tǒng)是用壓力氣體傳遞和控制動(dòng)力，正如名稱所表明的那樣，氣壓系統(tǒng)通常用空氣（不用其他氣體）作為流體介質(zhì)，因?yàn)榭諝馐前踩?、成本低而又隨處可得的流體，在系統(tǒng)部件中產(chǎn)生電弧有可能點(diǎn)燃泄漏物的場(chǎng)合下（使用空氣作為介質(zhì)）尤其安全。在氣壓系統(tǒng)中，壓縮機(jī)用來壓縮并供應(yīng)所需的空氣。壓縮機(jī)一般有活塞式、葉片式和螺旋式等類型。壓縮機(jī)基本上是根據(jù)理想氣體法則，通過減小氣體體積來增加氣體壓力的。氣壓系統(tǒng)通常考慮采用大的中央空氣壓縮機(jī)作為一個(gè)無限量的氣源，這類似于電力系統(tǒng)中只要將插頭插入插座便可獲得電能。用這種方法，壓力氣體可以從氣源輸送到整個(gè)工廠的各個(gè)角落，壓力氣體可通過空氣濾清器除去污物，這些污物可能會(huì)損壞氣體組件的精密配合部件如閥和氣缸等，隨后輸送到各個(gè)回路中，接著空氣流經(jīng)減壓閥以減小氣壓值適合某一回路使用。因?yàn)榭諝獠皇呛玫臐?rùn)滑劑（包括20%的氧氣），氣壓系統(tǒng)需要一個(gè)油霧器將細(xì)小的油霧注射到經(jīng)過減壓閥減壓的空氣中，這有助于減小氣動(dòng)組件精密配合運(yùn)動(dòng)件的磨損。由于來自大氣中的空氣含不同數(shù)量的水分，這些水分是有害的，它可以帶走潤(rùn)滑劑引起過分磨損和腐蝕，因此，在一些使用場(chǎng)合中，要用空氣干燥器來除去這些有害的水分。由于氣壓系統(tǒng)直接向大氣排氣，會(huì)產(chǎn)生過大噪聲，因此可在氣閥和執(zhí)行組件排氣口安裝消聲器來降低噪聲，以防止操作人員因接觸噪聲及高速空氣粒子有可能引發(fā)的傷害。用氣動(dòng)系統(tǒng)代替液壓系統(tǒng)有以下幾條理由：液體的慣性遠(yuǎn)比氣體大，因此，在液壓系統(tǒng)中，當(dāng)執(zhí)行組件加速減速和閥突然開啟關(guān)閉時(shí)，油液的質(zhì)量便是一個(gè)潛在的問題，根據(jù)牛頓運(yùn)動(dòng)定律（力等于質(zhì)量乘以加速度），產(chǎn)生加速運(yùn)動(dòng)油液所需的力要比加速同等體積空氣所需的力高出許多倍。液體比氣體具有更大的粘性，這會(huì)因?yàn)閮?nèi)摩擦而引起更大的壓力和功率損失；另外，由于液壓系統(tǒng)使用的液體要與大氣隔絕，故它們需要特殊的郵箱和無泄漏系統(tǒng)設(shè)計(jì)。氣壓系統(tǒng)使用可以直接排到周圍環(huán)境中的空氣，一般來說氣壓系統(tǒng)沒有液體系統(tǒng)昂貴。然而，由于空氣的可壓縮性，使得氣壓系統(tǒng)執(zhí)行組件不可能得到精確地速度控制和位置控制。氣壓系統(tǒng)由于壓縮機(jī)局限，其系統(tǒng)壓力相當(dāng)?shù)停ǖ陀?50psi），而液壓力可達(dá)10000psi之高，因此液壓系統(tǒng)可以是大功率系統(tǒng)，而氣動(dòng)系統(tǒng)僅用于小功率系統(tǒng)，典型例子有沖壓、鉆孔、提升、沖孔、夾緊、組裝、鉚接、材料處理和邏輯控制操作等。附錄BHydraulic SystemThere are only three basic methods of transmitting power：electrical,mechanical,and fluid applications actually use a bination of the three methods to obtain the most efficient overall system. To properly determine which principle method to use, it isimportant to know the salient features of each type. For example, fluid systems can transmit power more economically over greater distances than can mechanical types. However,fluid systems are restricted to shorter distances than are electrical systems.Hydraulic power transmission system are concerned with the generation, modulation,and control of ptrssutr and flow,and in general such systems include: which convert available power from the prime mover to hydraulic power at the actuator. which control the direction of pumpflow,the level of power produced,and the amount of fluidflow to the actuators. The power level is determined by controlling both the flow and pressure level. which convert hydraulic power to usable mechanical power output at the point required. medium,which is a liquid, provides rigid transmission and control as well as lubrication of pinents, sealing in valves, and cooling of the system. which link the various system ponents, provide power conductors for the fluid under pressure,and fluid flow return to tank(reservoir). storage and conditioning equipment which ensure sufficient quality and quantity as well as cooling of the fluid.Hydraulic systems are used in industrial applications such as stamping presses, steelmills,and general manufacturing,agricultural machines, mining industry, aviation, spacetechnology,deepsea exploration, transportation,marine technology,and offshore gas and petroleum exploration. In short, very few people get through a day of their lives without somehow benefitiong from the technology of hydraulics.The secret of hydraulic system’s success and widespread use 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 flluid systems are not so limited by the physical limitations of materials as are the electrical systems. For example ,the performance of an electromangnet is limited by the saturation limti 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 productivityl This includes remote and direct control of production poerations, manufacturing processes, and materials handling. Fluid power is the muscle of automation because of advantages in the following four major categories.1 Ease and accuracy of control. By the use of simple levers and push buttons, the poerator of a fluid power system can readily start , stop ,speed up or slow down, and position forces which provide any desired horsepower with tolerances as precise as one tenthousandth of an inch.2 Multiplication of force. A fluid power system (without using cumbersomegears, pulleys, and levers) can multiply forces simply and efficiently from a fraction of an ounce to several hundred tons of output.3 Constant force or torque. Only fluid power systems are capable of providing constant force or torque regardless of speed changes. This is acplished whether the work output moves a few inches per hour, several hundred inches per minute, a few revolutions per hour, or thousands of revolutions per minute.4 Sinplicity, safety, economy. In general, fluid power systems use fewer moving parts than parable mechanical or electrical systems. Thus, they are simpler to maintain and operate. This ,in turn, maximizes safety, pactness, and reliability. For example, a new power steering control designed has made all other kinds of power systems obsolete on many offhighway vehicles. The steering unit consists of a manually poerated directional control valve and meter in a single body. Because the steering unit is fully fluidlinked, mechanical linkages, universal joints, bearings, reduction gears ,etc. areeliminated. This provides a simple, pact 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 bees 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 powe