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Hydraulic System
There are only 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 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 pressure and flow, and in general such systems include:
1.Pumps which convert available power from the prime mover to hydraulic power at the actuator.
2.Valves which control the direction of pump-flow, the level of power produced, and the amount of fluid-flow to the actuators. The power level is determined by controlling both the flow and pressure level.
3.Actuators which convert hydraulic power to usable mechanical power output at the point required.
4.The medium, which is a liquid, provides rigid transmission and control as well as lubrication of components, sealing in valves, and cooling of the system.
5.Connectors which 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 which 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 , marine technology , and offshore gas petroleum exploration . In short, very few people get through a day of their lives without somehow benefiting 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 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 remote and direct control of production operations, manufacturing processes, and materials handling. Fluid power is the muscle of automation because of advantages in the following four major categories.
Ease and accuracy of control. By the use of simple levers and push buttons, the operator of a fluid power systems can readily start, stop, speed up or slow down, and position force which provide any desired horsepower with tolerances as precise as one ten-thousandth of an inch.
Multiplication of force. A fluid power system (without using cumbersome gears, pulleys, and levers) can multiply forces simply and efficiently from a fraction of an ounce to several hundred tons of output.
Constant force or torque. Only fluid power systems are capable of providing constant force or torque regardless of speed changes. This is accomplished 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.
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 designed 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 sterring unit is fully fluid-linked, mechanical linkages, universal joints, bearings, reduction gears, ect . are eliminated. This provides a simple,compact systems.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 sterring 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 completely. Also, most hydraulic oils can cause fires if an oil leak occurs in area of hot equipment.
There are only 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 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 pressure and flow, and in general such systems include:
Pumps which convert available power from the prime mover to hydraulic power at the actuator.
Valves which control the direction of pump-flow, the level of power produced, and the amount of fluid-flow to the actuators. The power level is determined by controlling both the flow and pressure level.
Actuators which convert hydraulic power to usable mechanical power output at the point required.
The medium, which is a liquid, provides rigid transmission and control as well as lubrication of components, sealing in valves, and cooling of the system.
Connectors which link the various system components, provide power conductors for the fluid under pressure, and fluid flow return to tank (reservoir).
Fluid 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, steel mills , and general manufacturing , agricultural machines , mining industry , aviation , space technology , deep-sea exploration ,transportation , marine technology , and offshore gas petroleum exploration . In short, very few people get through a day of their lives without somehow benefiting 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 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 remote and direct control of production operations, 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 operator of a fluid power systems can readily start, stop, speed up or slow down, and position force which 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 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 accomplished 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. 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 designed 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 sterring unit is fully fluid-linked, mechanical linkages, universal joints, bearings, reduction gears, ect . are eliminated. This provides a simple,compact systems.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 sterring 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 completely. Also, most hydraulic oils can cause fires if an oil leak occurs in area of hot equipment.
液壓系統(tǒng)
僅有以下三種基本方法傳遞動力：電氣，機械和流體。大多數(shù)應用系統(tǒng)實際上是將三種方法組合起來而得到最有效的最全面的系。為了合理的確定采取哪種方法，重要的是了解各種方法的顯著特征。例如液壓系統(tǒng)在長距離上比機械系統(tǒng)更能經(jīng)濟的傳遞動力。然而液壓系統(tǒng)與電氣系統(tǒng)相比，傳遞動力的距離較短。
液壓動力傳遞系統(tǒng)涉及電動機，調(diào)節(jié)裝置和壓力和流量控制，總的來說，該系統(tǒng)包括：
泵：將原動機的能量轉(zhuǎn)換成作用在執(zhí)行部件上所謂液壓能。
閥：控制泵產(chǎn)生流體的運動方向，產(chǎn)生的功率的大小，以及到達執(zhí)行部件液體的流量。功率大小取決與對流量和壓力大小的控制。
執(zhí)行部件：將液壓能轉(zhuǎn)換成可用的機械能。
介質(zhì)即油液：可進行無壓縮傳遞和控制，同時可以潤滑部件，使閥體密封和系統(tǒng)冷卻。
聯(lián)結(jié)件：聯(lián)結(jié)各個系統(tǒng)部件，為壓力流體提供功率傳輸通路，將液體返回油箱（貯油器）。
油液貯存和調(diào)節(jié)裝置：用來確保提供足夠質(zhì)量和數(shù)量并冷卻的液體。
液壓系統(tǒng)在工業(yè)中應用廣泛，例如沖壓，鋼類工件的磨削及一般加工業(yè)，農(nóng)業(yè)，礦業(yè)，航天技術(shù)，深?？碧剑\輸，海洋技術(shù)，近海天然氣和石油勘探等行業(yè)，簡而言之，在日常生活中很少有人不從液壓技術(shù)中得到某種益處。
液壓系統(tǒng)成功而又廣泛使用的秘密在于它的通用性和易作性。液壓動力傳遞不會像機械系統(tǒng)那樣受到機器幾何形體的制約，另外，液壓系統(tǒng)不會像電氣系統(tǒng)那樣受到材料物理性能的制約，它對傳遞功率幾乎沒有量的限制。例如，一個電磁體的性能受到鋼的磁飽和極限的限制，相反，液壓系統(tǒng)的功率僅僅受材料強度的限制。
企業(yè)為了提高生產(chǎn)率將越來越依靠自動化，這包括遠程和直接控制生產(chǎn)操作，加工過程和材料處理等。液壓動力之所以成為自動化的重要組成分，是因為它有如下主要的四種優(yōu)點：
1. 控制方便精確 通過操作一個簡單的操作桿和按鈕，液壓系統(tǒng)的操作者便能立即啟動，停止，加減速和能提供任意功率，位置精度為萬分之一英寸的位置控制力。
2. 增力 一個液壓系統(tǒng)（沒有使用笨重的齒輪，滑輪和杠桿）
能簡單有效地將不到一盎司的力放大產(chǎn)生幾百噸力的輸出。
3. 恒力和恒扭矩 只有液壓系統(tǒng)能提供不隨速度變化的恒力或恒扭矩，它可以驅(qū)動對象從每小時移動幾英寸到每分鐘幾百英寸，從每小時幾百轉(zhuǎn)到每分鐘幾千轉(zhuǎn)。
4. 簡單，安全，經(jīng)濟 總的來說，液壓系統(tǒng)比機械或電氣系統(tǒng)使用更少的運動部件，因此，它們運行與維護簡單。這使的系統(tǒng)結(jié)構(gòu)緊湊，安全可靠。例如一種用于車輛上的新型動力轉(zhuǎn)向控制裝置已淘汰其他類型的轉(zhuǎn)向動力裝置，該轉(zhuǎn)向部件中包含有人力操作方向控制閥和分配器。因為轉(zhuǎn)向部件是全液壓的，沒有萬向節(jié)，軸承，減速齒輪等機械連接，這使得系統(tǒng)簡單緊湊。
另外，只需輸入很小的扭矩就能產(chǎn)生滿足極惡劣工作條件所需的控制力，這對于因操作空間限制而需要方向盤的場合很重要，這也是減輕司機疲勞度所必需的。
液壓系統(tǒng)的其他優(yōu)點包括雙向運動，過載保護和無級變速控制，在已有的任何動力系統(tǒng)中液壓系統(tǒng)亦具有最大的單位質(zhì)量功率比。
盡管液壓系統(tǒng)具有如此高性能，但它不是可以解決所有動力傳遞問題的靈丹妙藥。液壓系統(tǒng)也有些缺點，液壓油有污染，并且泄露不可能完全避免，另外如果油液滲漏發(fā)生在灼熱設備附近，大多數(shù)液壓油能引起火災。
氣壓系統(tǒng)
氣壓系統(tǒng)是用壓力氣體傳遞和控制動力，正如名稱所表明的那樣，氣壓系統(tǒng)通常用空氣（不用其它的氣體）作為流體介質(zhì)，因為空氣是安全、成本低而又隨處可得的流體，在系統(tǒng)部件中產(chǎn)生電弧有可能點燃泄露物的的場合下（使用空氣作為介質(zhì)）尤其安全。
在氣壓系統(tǒng)中，壓縮機用來壓縮并供應所需的空氣。壓縮機一般有活塞式、葉片式和螺旋式等類型。壓縮機基本上是根據(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?？紤]采用大的中央空氣壓縮機作為一個無限量的氣源，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工廠的各個角落，壓力氣體可通過空氣氣濾器除去污物，這些污物可能會損壞氣動組件的精密配合部件如閥和氣缸等，隨后輸送到各個回路中，接著空氣流經(jīng)減壓閥以減小氣壓值適合某一回路使用。因為空氣
不是好的潤滑劑（包括20%的氧氣），氣壓系統(tǒng)需要一個油霧器將細小的油霧注射到經(jīng)過減壓閥減壓的空氣中，這有助于減少氣動組件精密配合運動件的磨損。
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，它可以帶走潤滑劑引起過分磨損和腐蝕，因此，在一些使用場合中，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，會產(chǎn)生過大噪音，因此可在氣閥和執(zhí)行組件排氣口安裝消聲器來降低噪音，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)的危害。
用氣動系統(tǒng)代替液壓系統(tǒng)有以下幾條理由：液體的慣性遠比氣體大，因此，液壓系統(tǒng)中，當執(zhí)行組件加速和減速和閥突然開啟關(guān)閉時，油液的質(zhì)量便是一個潛在的問題，根據(jù)牛頓運動定律（力等于質(zhì)量乘以加速度），產(chǎn)生加速運動油液所需的力要比加速同等體積空氣的力高出許多倍4。液體比氣體具有更大的粘性，這會因為內(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，故他們需要特殊的油箱和無泄露系統(tǒng)設計。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的速度控制和位置控制。氣壓系統(tǒng)由于壓縮機局限，其系統(tǒng)壓力相當?shù)停ǖ赜?50psi）,而液壓力可達1000psi之高，因此液壓系統(tǒng)可以是大功率系統(tǒng)，而氣動系統(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、鎦接、材料處理和邏輯控制操作等。
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