发动机类毕业设计外文翻译


1 Summarize Outline uses a more compact design along with the engine and has in a big way, the engine produces the waste heat density also obviously increases along with it. Some essential regions, if around a row of tyre valve radiates the question to have first to consider, the cooling system even if appears the small breakdown also possibly to create the disaster in such region consequence. The engine cooling system radiation ability generally should satisfy when the engine full load radiation demand, because this time engine produces the quantity of heat is biggest. However, when partial loads, the current capacity which the cooling system can have the power loss, which the water pumping station provides the refrigerant current capacity surpasses needs. We hoped starts the starting time to be as far as possible short. Because engine time discharges pollutant more, the oil consumption is also big. The cooling system structure has a more tremendous influence to the engine cold starting time. 2 Characteristics of modern engine cooling system Modern engines series characteristic tradition cooling system function reliably protects the engine, but also should have the function which the improvement fuel economy and reduces discharges. Therefore, the modern cooling system must synthesize under the consideration the factor: Engine interior friction loss; Cooling system water pump power; Burning boundary condition, like combustion chamber temperature, complete density, complete temperature. The advanced cooling system uses systematized, the modular design method, the overall plan considered each influence factor, causes the cooling system both to guarantee the engine normal work, and enhances the engine efficiency and the reduction discharges. 2.1 The temperatures set point Temperatures hypotheses firing in

bursts motive operating temperature limit value are decided by a row of

tire valve the peripheral region maximum temperature. The most ideal situation is according to the metal temperature but is not the refrigerant temperature control cooling system, like this can protect the engine well. Because the cooling system hypothesis cooling temperature is by the full load time most is big is the foundation, therefore, engine and cooling system in partial loads time is at not too the perfect condition, when urban district travel and low speed travel, can have the high oil consumption and discharge. Supposes the fixed point through the change refrigerant temperature to be possible to improve the engine and the cooling system in partial loads time performance. According to a row of tyre valve the peripheral region temperature limit value, may elevate either reduce the refrigerant or the metal temperature supposes the fixed point. Elevates or reduced temperature all respectively has the characteristic, this is decided the goal which achieved to the hope. 2.2 Enhances the temperature Enhances the temperature to suppose the fixed-point enhancement operating temperature to suppose the fixed point is one kind of comparison the method which welcome. Enhances the temperature to have many merits, it directly affects the engine loss and the cooling system effect as well as the engine discharging formation. Will enhance the operating temperature to enhance the engine Mac reduce the engine to rub wears, reduces the engine fuel oil consumption. The research indicated that, the engine operating temperature to rubs the loss to have the very tremendous influence. Discharges the temperature the refrigerant to enhance to 150 ℃ , causes the cylinder temperature to elevate to 195 ℃ , the oil consumption drops 4%-6%. Maintains the refrigerant temperature in 90-115 ℃ scope, causes the engine machine oil the maximum temperature is 140 ℃, then oil consumption in partial loads time drops 10%. Enhances the operating temperature also obvious influence cooling system the potency. Enhances the refrigerant or the metal

temperature can improve the engine and disperse the steam heat transfer transmission the effect, reduces the refrigerant the speed of flow, reduces the water pump the rated power, thus reduces the engine the power dissipation. In addition, may select the different method, further reduces the refrigerant the speed of flow. 2.3 Reduce the temperatures set point Reduced temperatures suppose

the fixed point to reduce the cooling system the operating temperature to be possible to enhance the engine charge efficiency, reduces the inlet temperature. This to the combustion process, the fuel oil efficiency and discharges advantageously. The reduced temperature supposes the fixed point to be allowed to save the engine movement cost, enhances the part service life. The research indicated, if the cylinder head temperature reduces to 50 ℃, the ignition angle of advance may 3 ℃ A but not have the engine knock ahead of time, the charge efficiency enhances 2%, the engine operational factor improvement, is helpful to the optimized compression ratio and the parameter choice, obtains the better fuel oil efficiency and discharges the performance. 2.4 Precise cooling system Precise cooling systems precise cooling

system mainly manifests in the cooling jacket structural design and in the refrigerant speed of flow design. In precise cooling system, hot essential area, if around a row of tyre valve, the refrigerant has an greater speed of flow, the heat transfer efficiency is high, the refrigerant gradient of temperature changes slightly. Such effect comes from to reduce these place refrigerant channels the lateral section, enhances the speed of flow, reduces the current capacity. The precise cooling system design key lies in the determination cooling jacket the size, the choice match cooling water pump, guaranteed the system the radiation ability can satisfy when the low speed big load essential region operating temperature demand. The engine refrigerant speed of flow range

of variation is quite big, from time 1 m/s to maximum work rate time 5 m/s. Therefore should considered the cooling jacket and the cooling system whole that, mutually supplemented, display biggest potential. The research indicated that, uses the precise cooling system, in the engine entire work rotational speed scope, the refrigerant current capacity may drop 40%. Covers the cooling jacket to the air cylinder the precise design, may make the ordinary speed of flow to enhance from 1.4m/s to 4 m/s, greatly enhances the cylinder cover or cap thermal conductivity, cylinder cover or cap metal temperature drop to 60 ℃. 2.5 Divergences types cooling system Divergences types cooling system divergence type cooling system for other one kind of cooling system. In this kind of cooling system, the hine oil temperature, will cylinder cover or cap friendly cylinder body cools by respective return route,

the cylinder cover or cap friendly cylinder body has the different temperature. The divergence -like cooling system has the unique superiority, may cause engine each part to suppose the fixed-point work at the most superior temperature. The cooling system overall efficiency achieves in a big way. Each cooling return route will suppose under the fixed point or the speed of flow in the different cooling temperature works, will create the ideal engine temperature distribution. The ideal engine hot active status is the cylinder head temperature lower but the air cylinder body temperature relative is higher. The cylinder head temperature is lower may enhance the charge efficiency, increases. The temperature is low also greatly may promote completely to burn, reduces CO, HC and the NOx formation, also enhances the output. The higher air cylinder body temperature can reduce the friction to lose, directly improves the fuel oil efficiency, indirectly reduces in the cylinder the peak value pressure and the temperature. The divergence type cooling system may cause the cylinder cover and the cylinder body temperature

differs 100 ℃. The cylinder temperature may reach as high as 150 ℃, but the cylinder cover temperature may reduce 50 ℃, reduces the cylinder body to rub loses, reduces the oil consumption. The higher cylinder body temperature causes the oil consumption to reduce 4%-6%, when partial loads HC reduces 20%-35%. When the damper all opens, the cylinder cover and the cylinder body temperature supposes the definite value to be possible to move to 50 ℃ and 90 ℃, improves the fuel oil consumption, the power output from the whole and discharges. 2.6 Controllable cooling system Controllable engine cooling system tradition engine cooling system belongs to the passive form, the structure simple or the cost is low. The controllable cooling system may make up at present cooling system the insufficiency. Now the cooling system design standard solves time the full load radiation problem, thus partially shoulders time the oversized radiation ability will cause the engine power waste. This to the light vehicle said especially obvious, these vehicles majority time all under the partial loads go in the urban district, only uses the partial engine power, causes a cooling system higher loss. In order to solve the engine to get down the hot question in the peculiar circumstance, the present cooling system volume was bigger, causes the evaporation efficiency to reduce, has increased the cooling system power demand, lengthened the engine during warm machine-hour. The controllable engine cooling system generally includes the sensor, the execution and the electrically controlled module. The controllable cooling system can act according to the engine working condition adjustment cooling quantity, reduces the engine power loss. In the controllable cooling system, the execution for the cooling water pump and the thermostat, generally and the control valve is composed by the electrically operated water pump, may act according to requests to adjust the cooling quantity. Temperature sensor for a system part, but rapidly bequeaths the engine hot condition

the controller. Controllable installment, if the electrically operated water pump, may suppose the temperature the fixed point from 90 ℃ to enhance to 110 ℃, saves 2%-5% fuel oil, CO reduces 20%, HC reduces 10%. When steady state, the metal temperature ratio tradition cooling system is high 10 ℃, the controllable cooling system has the quicker response ability, may cool the temperature to maintain is supposing the fixed point ±2 ℃ the scope. From 110 ℃ drops to 100 ℃ only needs 2 s. The engine during warm machine-hour reduces 200s, the cooling system operating region draws close to the work limit region, can reduce the engine cooling temperature and the metal temperature undulation scope, reduces circulates the fatigue of metal which the hot load creates, lengthens the component life. 3 Conclusion In front of 3 conclusions introduced several kind of advanced cooling systems have the improvement cooling system performance the potential, can enhance the fuel oil efficiency and discharge the performance. The cooling system can control the nature is improves the cooling system the key, can the controlling expression to the engine structure protection essential parameter, like the metal temperature, the refrigerant temperature and the machine oil temperature and so on can control, guarantees the engine to work in the safety margin scope. The cooling system can make the rapid reaction to the different operating mode, the most earth saves the fuel, reduces discharges, but does not affect the engine overall performance. Looked from the design and the operational performance angle that, divergence type cooling and precise cooling unifies has the very good prospects for development, both can provide the ideal engine protection, and can enhance the fuel oil efficiency and discharge the nature. This kind of structure is advantageous to forming the engine ideal temperature distribution. Directly to a cylinder cover

or cap row of tyre valve around the supplies refrigerant, reduced the cylinder head temperature change, causes the cylinder cover temperature distribution to be evener, also can maintains the machine oil and the cylinder body temperature at the design operating region, has a lower friction to damage the pollution withdrawal. method as follows: 1st, the cooling system function, is part of quantity of heats which absorbs the engine part carries off, guaranteed the diesel engine various components maintain in the normal temperature range. function and maintenance maintenance 2nd, the cooling water should be does not contain dissolves the Xie salt the soft water, like clean river water, rain water and so on. Do not use hard water and so on the well water, water seepage or sea water, guards against produces, causes the engine to radiate not good, question occurrence and so on air cylinder heat. 3rd, with the funnel when joins the cooling water the water tank, must prevent the water splashes to on the engine and the radiator, prevented on the radiator fin and the organism accumulates the dust, smears, affects the cooling effect. 4th, if when the engine lacks the water causes the hyperpyrexia, cannot immediately add water, should cause the engine idling speed to revolve 10 □15 minutes, after the uniform temperature slightly reduces, slowly does not join the cooling water in the engine situation. 5th, the winter, the water tank planted agent adds the hot water. After the start should surpass 40 degree-hour the slow revolution to the water temperature to be able to work. After the work had ended, must put the completely cooling water. 6th, must regularly eliminate in the water tank, must frequently scour the sludge to the forced-air cooling engine radiator fin, dirty is filthy. The radiator fin cannot damage, after if damages must promptly cooling system

replace, in order to avoid influence radiation effect. 4 Lathes Lathes are machine tools designed primarily to do turning, facing and boring, Very little turning is done on other types of machine tools, and none can do it with equal facility. Because lathes also can do drilling and reaming, their versatility permits several operations to be done with a single setup of the work piece. Consequently, more lathes of various types are used in manufacturing than any other machine tool. The essential components of a lathe are the bed, headstock assembly, tailstock assembly, and the leads crew and feed rod. The bed is the backbone of a lathe. It usually is made of well normalized or aged gray or nodular cast iron and provides s heavy, rigid frame on which all the other basic components are mounted. Two sets of parallel, longitudinal ways, inner and outer, are contained on the bed, usually on the upper side. Some makers use an inverted V-shape for all four ways, whereas others utilize one inverted V and one flat way in one or both sets, They are precision-machined to assure accuracy of alignment. On most modern lathes the way are surface-hardened to resist wear and abrasion, but precaution should be taken in operating a lathe to assure that the ways are not damaged. Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed. The headstock is mounted in a foxed position on the inner ways, usually at the left end of the bed. It provides a powered means of rotating the word at various speeds . Essentially, it consists of a hollow spindle, mounted in accurate bearings, and a set of transmission gears-similar to a truck transmission—through which the spindle can be rotated at a number of speeds. Most lathes provide from 8 to 18 speeds, usually in a geometric ratio, and on modern lathes all the speeds can be obtained merely by moving from two to four levers. An increasing trend is to provide a continuously

variable speed range through electrical or mechanical drives. Because the accuracy of a lathe is greatly dependent on the spindle, it is of heavy construction and mounted in heavy bearings, usually preloaded tapered roller or ball types. The spindle has a hole extending through its length, through which long bar stock can be fed. The size of maximum size of bar stock that can be machined when the material must be fed through spindle. The tailsticd assembly consists, essentially, of three parts. A lower casting fits on the inner ways of the bed and can slide longitudinally thereon, with a means for clamping the entire assembly in any desired location, An upper casting fits on the lower one and can be moved transversely upon it, on some type of keyed ways, to permit aligning the assembly is the tailstock quill. This is a hollow steel cylinder, usually about 51 to 76mm(2to 3 inches) in diameter, that can be moved several inches longitudinally in and out of the upper casting by means of a hand wheel and screw. The size of a lathe is designated by two dimensions. The first is known as the swing. This is the maximum diameter of work that can be rotated on a lathe. It is approximately twice the distance between the line connecting the lathe centers and the nearest point on the ways, The second size dimension is the maximum distance between centers. The swing thus indicates the maximum work piece diameter that can be turned in the lathe, while the distance between centers indicates the maximum length of work piece that can be mounted between centers. Engine lathes are the type most frequently used in manufacturing. They are heavy-duty machine tools with all the components described previously and have power drive for all tool movements except on the compound rest. They commonly range in size from 305 to 610 mm(12 to 24 inches)swing and from 610 to 1219 mm(24 to 48 inches) center distances,

but swings up to 1270 mm(50 inches) and center distances up to 3658mm(12 feet) are not uncommon. Most have chip pans and a built-in coolant circulating system. Smaller engine lathes-with swings usually not over 330 mm (13 inches ) –also are available in bench type, designed for the bed to be mounted on a bench on a bench or cabinet. Although engine lathes are versatile and very useful, because of the time required for changing and setting tools and for making measurements on the work piece, thy are not suitable for quantity production. Often the actual chip-production tine is less than 30% of the total cycle time. In addition, a skilled machinist is required for all the operations, and such persons are costly and often in short supply. However, much of the operator’s time is consumed by simple, repetitious adjustments and in watching chips being made. Consequently, to reduce or eliminate the amount of skilled labor that is required, turret lathes, screw machines, and other types of semiautomatic and automatic lathes have been highly developed and are widely used in manufacturing. 5 Limits and Tolerances Machine parts are manufactured so they are interchangeable. In other words, each part of a machine or mechanism is made to a certain size and shape so will fit into any other machine or mechanism of the same type. To make the part interchangeable, each individual part must be made to a size that will fit the mating part in the correct way. It is not only impossible, but also impractical to make many parts to an exact size. This is because machines are not perfect, and the tools become worn. A slight variation from the exact size is always allowed. The amount of this variation depends on the kind of part being manufactured. For examples part might be made 6 in. long with a variation allowed of 0.003 (three-thousandths) in. above and below this size. Therefore, the part could be 5.997 to 6.003 in. and still be the correct size. These are known

as the limits. The difference between upper and lower limits is called the tolerance.

1 概述 随着发动机采用更加紧凑的设计和具有更大的比功率, 发动机产生的废热密 度也随之明显增大。一些关键区域,如排气门周围散热问题需优先考虑,冷却系 统即便出现小的故障也可能在这样的区域造成灾难性的后果。 发动机冷却系统的 散热能力一般应满足发动机</B>满负荷时的散热需求, 因为此时发动机产生的热 量最大。然而,在部分负荷时,冷却系统会发生功率损失,水泵所提供的冷却液 流量超过所需的流量。 我们希望发动机冷启动时间尽可能短。因为发动机怠速时 排放的污染物较多, 油耗也大。 冷却系统的结构对发动机的冷启动时间有较大 的影响。 2 现代发动机冷却系统的特点 传统冷却系统的作用是可靠地保护发动机, 而还应具有改善燃料经济性和降 低排放的作用。为此,现代冷却系统要综合考虑下面的因素:发动机内部的摩擦 损失;冷却系统水泵的功率;燃烧边界条件,如燃烧室温度、充量密度、充量温 度。先进的冷却系统采用系统化、模块化设计方法,统筹考虑每项影响因素,使 冷却系统既保证发动机正常工作,又提高发动机效率和减少排放。 2.1 温度设定点 发动机工作温度的极限值取决于排气门周围区域最高温度。 最理想的情况是 按金属温度而不是冷却液温度控制冷却系统,这样才能更好地保护发动机。 由 于冷却系统设定的冷却温度是以满负荷时最大散热率为基础, 因此,发动机和 冷却系统在部分负荷时处于不太理想状态,如市区行驶和低速行驶时,会产生高 油耗和排放。 通过改变冷却液温度设定点可改善发动机和冷却系统在部分负荷时

的性能。 根据排气门周围区域温度极限值,可升高或降低冷却液或金属温度设定 点。 升高或降低温度点都各有特点,这取决于希望达到的目的。 2.2 提高温度设定点 提高工作温度设定点是一种比较受欢迎的方法。 提高温度有许多优点,它 直接影响发动机损耗和冷却系统的效果以及发动机排放物的形成。 提高工作温度 将提高发动机机油温度, 降低发动机摩擦磨损,降低发动机燃油消耗。研究表 明,发动机工作温度对摩擦损失有很大影响。将冷却液排出温度提高到 150℃, 使气缸温度升高到 195℃, 油耗则下降 4%-6%。将冷却液温度保持在 90-115℃范 围内,使发动机机油的最高温度为 140℃,则油耗在部分负荷时下降 10%。提高 工作温度也明显影响冷却系统的效能。 提高冷却液或金属温度会改善发动机和散 热气热传递传递的效果,降低冷却液的流速, 减小水泵的额定功率,从而降低 发动机的功率消耗。此外,可采用不同的方式, 进一步减小冷却液的流速。 2.3 降低温度设定点 降低冷却系统的工作温度可提高发动机充气效率,降低进气温度。这对燃烧 过程、 燃油效率及排放有利。降低温度设定点可以节省发动机运行成本,提高 部件使用寿命。 研究表明, 若气缸盖温度降低到 50℃, 点火提前角可提前 3℃A 而 不发生爆震,充气效率提高 2%,发动机工作特性改善,有助于优化压缩比和参 数选择,取得更好的燃油效率和排放性能。 2.4 精确冷却系统 精确冷却系统主要体现在冷却水套的结构设计与冷却液流速的设计中。 在精 确冷却系统中, 热关键区, 如排气门周围, 冷却液有较大的流速, 热传递效率高, 冷却液的温度梯度变化小。这样的效果来自缩小这些地方冷却液通道的横截面, 提高流速,减少流量。精确冷却系统的设计关键在于确定冷却水套的尺寸,选择 匹配的冷却水泵, 保证系统的散热能力能够满足低速大负荷时关键区域工作温度 的需求。 发动机冷却液流速的变化范围相当大,从怠速时的 1 m/s 到最大功率时的 5 m/s。故应将冷却水套和冷却系统整体考虑,相互补充,发挥最大潜力。 研究表明,采用精确冷却系统,在发动机整个工作转速范围,冷却液流量可 下降 40%。对气缸盖上冷却水套的精确设计,可使普通冷却道的流速从 1.4m/s

提高到 4 m/s,大大提高气缸盖传热性,将气缸盖的金属温度降低到 60℃。 2.5 分流式冷却系统 分流式冷却系统为另外一种冷却系统。在这种冷却系统中,气缸盖和气缸体 由各自 的液流回路冷却,气缸盖和气缸体具有不同的温度。分流式的冷却系统 具备特有的优势, 可使发动机各部分在最优的温度设定点工作。冷却系统的整体 效率达到最大。 每个冷却回路将在不同冷却温度设定点或流速下工作,创造理想 的发动机温度分布。 理想的发动机热工作状态是气缸盖温度较低而气缸体温度相对较高。 气缸盖 温度较低可提高充气效率,增大进气量。温度低且进气量大可促进完全燃烧,降 低 CO,HC 和 NOx 的形成,也提高输出功率。较高气缸体温度会减小摩擦损失, 直接改善燃油效率, 间接地降低缸内峰值压力和温度。分流式冷却系统可使缸盖 和缸体温度相差 100℃。气缸温度可高达 150℃,而缸盖温度可降低 50℃,减 少缸体摩擦损失,降低油耗。较高的缸体温度使油耗降低 4%-6%, 在部分负荷 时 HC 降低 20%-35%。 节气门全开时,缸盖和缸体温度设定值可调到 50℃和 90℃,从整体上改善燃油消耗、功率输出和排放。 2.6 可控式发动机冷却系统 传统的发动机冷却系统属于被动式的,结构简单或成本低。可控式冷却系统 可弥补目前冷却系统的不足。 现在冷却系统的设计标准是解决满负荷时的散热问 题, 因而部分负荷时过大的散热能力将导致发动机功率浪费。这对轻型车辆来说 尤为明显, 这些车辆大多数时间都在市区内部分负荷下行驶,只利用部分发动机 功率,引起冷却系统较高损耗。为解决发动机在特殊情况下过热的问题,现在的 冷却系统体积较大,导致冷却效率降低,增大了冷却系统的功率需求,延长了发 动机暖机时间。可控式发动机冷却系统一般包括传感器、执行器和电控模块。可 控式冷却系统能够根据发动机工作状况调整冷却量,降低发动机功率损耗。在可 控式冷却系统中, 执行器为冷却水泵和节温器,一般由电动水泵和液流控制阀组 成,可根据要求调整冷却量。温度传感器为系统的一部分,可迅速把发动机的热 状态传给控制器。 可控式装置,如电动水泵,可将冷却系温度设定点从 90℃提高到 110℃,节 省 2%-5%的燃油,CO 减少 20%,HC 减少 10%。稳定状态时,金属温度比传统冷却

系统的高 10℃,可控式冷却系统具有较快的响应能力,可将冷却温度保持在设 定点的±2℃范围。 从 110℃下降到 100℃只需 2 s。 发动机暖机时间减少到 200s, 冷却系统工作范围更贴近工作极限区域, 能够缩小发动机冷却温度和金属温度的 波动范围,减少循环热负荷造成的金属疲劳,延长部件寿命。 3 结论 前面介绍的几种先进冷却系统具有改善冷却系统性能的潜力, 能够提高燃油 经济性和排放性能。 冷却系统的能控性是改善冷却系统的关键,能控性表示对发 动机结构保护的关键参数,如金属温度、冷却液温度和机油温度等能够控制,确 保发动机在安全限度范围内工作。冷却系统能够对不同工况作出快速反应,最大 地节省燃料、降低排放,而不影响发动机整体性能。 从设计和使用性能角度看, 分流式冷却与精密冷却相结合具有很好的发展前 景,既能提供理想的发动机保护,又能提高燃油经济性和排放性。这种结构有利 于形成发动机理想的温度分布。 直接向气缸盖排气门周围供给冷却液,减少了气 缸盖温度变化, 使缸盖温度分布更加均匀,也能将机油和缸体温度保持在设计的 工作范围,具有较低的摩擦损失和污染排放量。 冷却系统的功能及维护保养方法如下: 1、冷却系统的功能, 就是将发动机零件吸收的一部分热量带走,保证柴油 发动机各零件维持在正常的温度范围内。 2、冷却水应是不含溶解盐的软水,如清洁的河水、雨水等。不要用井水、 泉水或海水等硬水,以防产生水垢,引起发动机散热不良,气缸过热等问题 发生。 3、用漏斗将冷却水加入水箱时,应当防止水飞溅到发动机与散热器上,防 止散热片和机体上积尘、弄脏,影响冷却效果。 4、若因发动机缺水而引起温度过高时,不能马上加水,应使发动机慢速运 转 1 0—1 5 分钟,等温度稍降低后,在发动机不息火的情况下慢慢加入冷 却水。 5、冬季,水箱内应加热水。 启动后应慢速运转至水温超过 40 度时才能工 作。工作结束后,必须放尽冷却水。 6、要定期清除水箱内的水垢,对风冷发动机的散热片要经常擦洗污泥、脏

垢。散热片不可损坏,若损坏后要及时更换,以免影响散热效果。 4 车床 车床主要是为了进行车外圆、车端面和镗孔等项工作而设计的机床。车削很 少在其他种类的机床上进行, 而且任何一种其他机床都不能像车床那样方便地进 行车削加工。 由于车床还可以用来钻孔和铰孔,车床的多功能性可以使工件在一 次安装中完成几种加工。 因此, 在生产中使用的各种车床比任何其他种类的机床 都多。 车床的基本部件有:床身、主轴箱组件、尾座组件、溜板组件、丝杠和光杠。 床身是车床的基础件。 它能常是由经过充分正火或时效处理的灰铸铁或者球 墨铁制成。它是一个坚固的刚性框架,所有其他基本部件都安装在床身上。通常 在床身上有内外两组平行的导轨。 有些制造厂对全部四条导轨都采用导轨尖朝上 的三角形导轨(即山形导轨),而有的制造厂则在一组中或者两组中都采用一个 三角形导轨和一个矩形导轨。 导轨要经过精密加工以保证其直线度精度。为了抵 抗磨损和擦伤, 大多数现代机床的导轨是经过表面淬硬的,但是在操作时还应该 小心,以避免损伤导轨。导轨上的任何误差,常常意味着整个机床的精度遭到破 坏。 主轴箱安装在内侧导轨的固定位置上,一般在床身的左端。它提供动力,并 可使工件在各种速度下回转。 它基本上由一个安装在精密轴承中的空心主轴和一 系列变速齿轮(类似于卡车变速箱)所组成。通过变速齿轮,主轴可以在许多种转 速下旋转。大多数车床有 8~12 种转速,一般按等比级数排列。而且在现代机床 上只需扳动 2~4 个手柄,就能得到全部转速。一种正在不断增长的趋势是通过 电气的或者机械的装置进行无级变速。 由于机床的精度在很大程度上取决于主轴。因此,主轴的结构尺寸较大,通 常安装在预紧后的重型圆锥滚子轴承或球轴承中。主轴中有一个贯穿全长的通 孔,长棒料可以通过该孔送料。主轴孔的大小是车床的一个重要尺寸,因此当工 件必须通过主轴孔供料时,它确定了能够加工的棒料毛坯的最大尺寸。 尾座组件主要由三部分组成。底板与床身的内侧导轨配合,并可以在导轨上 作纵向移动。 底板上有一个可以使整个尾座组件夹紧在任意位置上的装置。尾座 体安装在底板上, 可以沿某种类型的键槽在底板上横向移动,使尾座能与主轴箱

中的主轴对正。尾座的第三个组成部分是尾座套筒。它是一个直径通常大约在 51~76mm(2~3 英寸)之间的钢制空心圆柱体。通过手轮和螺杆,尾座套筒可以 在尾座体中纵向移入和移出几个英寸。 车床的规格用两个尺寸表示。第一个称为车床的床面上最大加工直径。这是 在车床上能够旋转的工件的最大直径。 它大约是两顶尖连线与导轨上最近点之间 距离的两倍。 第二个规格尺寸是两顶尖之间的最大距离。车床床面上最大加工直 径表示在车床上能够车削的最大工件直径, 而两顶尖之间的最大距离则表示在两 个顶尖之间能够安装的工件的最大长度。 普通车床是生产中最经常使用的车床种类。 它们是具有前面所叙的所有那些 部件的重载机床,并且除了小刀架之外,全部刀具的运动都有机动进给。它们的 规格通常是:车床床面上最大加工直径为 305~610mm(12~24 英寸);但是,床 面上最大加工直径达到 1270mm(50 英寸)和两顶尖之间距离达到 3658mm 的车 床也并不少见。这些车床大部分都有切屑盘和一个安装在内部的冷却液循环系 统。小型的普通车床—车床床面最大加工直径一般不超过 330mm(13 英寸)-被设计成台式车床,其床身安装在工作台或柜子上。 虽然普通车床有很多用途, 是很有用的机床,但是更换和调整刀具以及测量 工件花费很多时间,所以它们不适合在大量生产中应用。通常,它们的实际加工 时间少于其总加工时间的 30%。此外,需要技术熟练的工人来操作普通车床,这 种工人的工资高而且很难雇到。 然而,操作工人的大部分时间却花费在简单的重 复调整和观察切屑过程上。因此,为了减少或者完全不雇用这类熟练工人,六角 车床、 螺纹加工车床和其他类型的半自动和自动车床已经很好地研制出来,并已 经在生产中得到广泛应用。 5 极限与公差 机械零件被制造因此它们是可互换的。换句话说,每一种机械零件或装置被 制成一定的大小和形状来适用于其它型号的机器。为了使零件具有互换性,每一 个零件都做成一个尺寸来用正确的方法与对应的零件相配。这不仅不可能,而且 是许多零件都做成一个尺寸是不切实际的。这是因为机器不是完美的,而工具会 磨损。 相对于正确尺寸的一点偏差通常是允许的。这个偏差的大小依赖于被制零 件的种类, 比如一个零件可能是 6 英寸, 上下偏差是 0003 英寸 (三千分之一) 。

因此这个偏差可以是 5997 英寸到 6003 英寸之间并仍能保持正确尺寸。 这就是 偏差。上偏差和下偏差之差即是公差。 公差是零件尺寸的最大变化量, 基本尺寸是允许变动量和公差范围而衍生的 尺寸限制。 有时偏差只允许一个方向的变动,它允许公差在孔或轴上变化而不会 严重影响配合。当公差在两个方向上都变化时,称为完全偏差(正和负)。完全 偏差是分开的, 并且在基本尺寸的每一边都会有。而极限尺寸只有最大尺寸和最 小尺寸。因此,公差是这两个尺寸之差。


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