【正文】 nts In such a joint the bolt has first been tightened to produce an initial tensile preload, after which the external tensile load and the external shear load are applied. 課文分析 在這種螺紋聯(lián)接中，首先把螺栓上緊以產(chǎn)生一預(yù)緊載荷初拉力，而后施加外拉力載荷和剪切載荷。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints The effect of the preload is to place the parts in pression for better resistance to the external tensile load and to create friction between the parts to resist the shear load. 課文分析 預(yù)載荷的作用是使被連接件處于壓應(yīng)力狀態(tài)以更好地抵抗外部拉伸載荷和產(chǎn)生摩擦力以抵抗剪切載荷。 The importance of preloading of bolts cannot be overestimated. A high preload improves both the fatigue resistance of a bolted connection and the locking effect. 螺栓預(yù)加載荷的重要性不能被過高地估計(jì)。較高預(yù)載荷即提高螺栓聯(lián)接的抗疲勞能力又改善鎖緊作用。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints Having learned that a high preload is very desirable in important bolted connections, we must next consider means of assuring that the preload is actually developed when the parts are assembled. 課文分析 已經(jīng)知道：高預(yù)載荷在重要的螺栓聯(lián)接中是非常希望的。下一步我們要考慮的是，當(dāng)要裝配零件時(shí)，如何確保設(shè)計(jì)情況下該預(yù)載荷的產(chǎn)生。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints If the overall length L of a bolt with cross area A can actually be measured with a micrometer when it is assembled, the bolt elongation d due to the preload F can be puted using the formula d=FL/AE, where E is the modulus of elasticity of the bolt material. 課文分析 如果橫截面積為 A、總長度為 L的螺栓裝配時(shí)，實(shí)際上是可以用千分尺來測量的，由于預(yù)載力 F的作用而使螺栓伸長量為 d， d可以利用公式 d＝ FL／AE來計(jì)算，式中 E是螺栓材料的彈性模量。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints Then simply tighten the nut until the bolt elongation through the distance d. This assures that the desired preload has been attained. 課文分析 那么只要旋緊螺母直至使螺栓伸長達(dá)到 d。這就保證了已經(jīng)達(dá)到了所希望的預(yù)緊載荷。 However, the elongation of a screw cannot usually be measured because the threaded end may be in a blind hole. 然而，螺釘?shù)纳扉L量通常是不能測量的，因?yàn)槁葆敹瞬靠赡苁窃诿た變?nèi)。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints It is also impractical in many cases to measure bolt elongation. In such cases the wrench torque required to develop the specified preload must be estimated. 課文分析 在許多情況下，去測量螺栓的伸長也是不切實(shí)際的。在這樣情況下，能產(chǎn)生具體預(yù)載荷的扭矩扳手必須能測定出該預(yù)緊載荷。 Then torque wrenching, pneumaticimpact wrenching, or the turnofthenut method may be used. 扭矩扳手，氣動(dòng)沖擊扳手，或螺帽旋動(dòng)圈數(shù)扳手法等可用于測定預(yù)載荷。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints The torque wrench has a builtin dial, which indicates the proper torque. 課文分析 扭矩扳手有一個(gè)內(nèi)置式指示盤，以便顯示正確的扭矩。 With impact wrenching the air pressure is adjusted so that the wrench stalls when the proper torque is obtained, or in some wrenches, the air automatically shuts off at the desired torque. 用沖擊扳手時(shí)，要調(diào)節(jié)空氣壓力，當(dāng)獲得正確扭矩時(shí)，扳手即停止轉(zhuǎn)動(dòng)，或者有些氣動(dòng)扳手，當(dāng)達(dá)到希望的扭矩時(shí)，壓縮空氣自動(dòng)關(guān)掉。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints The turnofthenut method requires that we first define the meaning of snug tight. 課文分析 要使用螺母轉(zhuǎn)動(dòng)圈數(shù)法首先要求我們確定適度緊的意義。 The snugtight condition is the tightness attained by a few impacts of an impact wrench, or the full effort of a man using an ordinary wrench. 適度緊的狀態(tài)就是借助于沖擊扳手沖擊幾次所達(dá)到的預(yù)緊度，或一個(gè)男人用普通扳手盡全力上緊的程度。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints Having attained the snugtight condition, all additional turning develops useful tension in the bolt. 課文分析 已經(jīng)達(dá)到適度緊狀態(tài)時(shí)候，所有的附加旋動(dòng)會(huì)在螺栓中產(chǎn)生有益張緊力。 The turnofthenut method requires that one putes the fractional number of turns necessary to develop the required preload from the snugtight condition. 旋動(dòng)螺母圈數(shù)法要求；必須先旋動(dòng)零點(diǎn)幾轉(zhuǎn)以便產(chǎn)生在適度緊基礎(chǔ)上所要求的預(yù)緊載荷。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints For example, for heavy hexagon bolts the turnofthenut specification states that the nut should be turned a minimum of 180 deg, from the sungtight condition. 課文分析 例如，對(duì)于重型六角頭螺栓的旋轉(zhuǎn)螺母圈數(shù)法的技術(shù)要求稱：螺母應(yīng)從適度緊狀態(tài)起最少旋動(dòng)到 180度。 Note that this is also about the correct rotation for the wheel nuts of a passenger car. 注意這也是關(guān)于客車車輪螺母預(yù)緊的正確轉(zhuǎn)數(shù)。 機(jī)械工程專業(yè)英語 Lesson 19 Screws, fasteners and joints