【菜鸟成长记】20岁后，我在机械的每一天

黑森林的鹿

【20160319】Commonly Used Links and Joints
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" v" U( j+ d7 W. q5 H- _6 Y+ kEccentric Crank
8 I3 |3 ~1 v( c% {/ r' EOn many mechanisms, the required length of a crank is so short that it is not feasible to fitsuitably sized bearings at the two pin joints. A commonsolution is to design the link as an eccentric crankshaft, as shown in Figure 1.23a. This is the design used in most engines and compressors.
5 Z1 p4 N3 Q; L偏心曲柄，由于尺寸过小而不能保证铰合处销钉尺寸合适。

- C- r* W0 E+ bThe pin, on the moving end of the link, is enlarged such that it contains the entire link. The outside circumference of the circular lobe on the crankshaft becomes the moving pin joint, as shown in Figure 1.23b. The location of the fixed bearing, or bearings, is offset from the eccentric lobe. This eccentricity of the crankshaft, e , is the effective length of the crank. Figure 1.23c illustrates a kinematic model of the eccentric crank. The advantage of theeccentric crank is the largesurface area of the moving pin, which reduces wear.
6 e9 n' F2 a0 y) P' g$ }- N& A, U1 f因此将该销钉放大，以至于包裹整个曲柄。这个大销钉中心与曲柄转轴中心间的距离即为有效的曲柄长度。这样配置的好处在于移动销钉的巨大的接触面积，这减少了磨损。
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Pin-in-a-Slot Joint
A common connection between links is a pin-in-a-slot joint, as shown in Figure 1.24a. This is a higher-order joint because it permits the two links to rotate and sliderelative to each other. To simplify the kinematic analysis, primary joints can be used to model this higher-order joint. The pin-in-a-slot joint becomes a combination of a pin joint and a sliding joint, asin Figure 1.24b. Note that this involves adding an extra link to the mechanism. In both cases, the relative motion between the links is the same. However, using a kinematic model with primary joints facilitates the analysis.
注意滑槽是高副，包含一个滑动副和一个转动副。
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Screw Joint
* e& v* E7 [6 h1 g# J: RA screw joint, as shown in Figure 1.25a, is another common connection between links. Screw mechanisms are discussed in detail in Chapter 12. To start with, a screw joint permits two relative, but dependent, motions between the links being joined. A specific rotation of one link will cause an associated relative translation between the two links. For example, turning the screw one revolution may move the nut along the screw threads a distance of 0.1 in. Thus, only one independent motion is introduced.
螺纹连接只有一个自由度，因为轴向运动和转动是相关的。
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A screw joint is typically modeled with a sliding joint, as shown in Figure 1.25b. It must be understood that out-of plane rotation occurs.However, only the relative translation between the screw and nut is considered in planar kinematic analysis.
可以简化为平面铰接，但必须注意还有空间旋转，虽然在研究时不考虑。
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黑森林的鹿

haneast 发表于 2016-3-18 17:05
1 W( b* a6 J# s大学六级刚过500，现在看到学妹一贴下来的英语各种666，表示十分惭愧，现在也天天看扇贝，看到学妹的水平才 ...

并没有多少水平，离自由交流还有很大距离……就像必威APP精装版下载各位前辈说的，机械这一行外语非常重要，还需要多多努力！学长也加油！早日实现自己的梦想呢！

黑森林的鹿

【20160320】关于做的一个简单实验。。。
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+ J9 s$ W- f; U) ^这学期期待已久的机械原理实验课！要求根据实验室提供的小机构画出机构简图，然而其中的一个。。。
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: _1 b$ ^( ^. E  W虽然标注着“双冲头机构”，但是……这根本不是双冲头机构好吗！！！怎么算自由度都是0啊！
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( \+ X6 [" L% w( `7 i) t  W1 ~; U图示有9个构件，12个低副约束，没有高副约束，因此根据公式：
M=3（n-1）-2PJ-PH=3×8-2×12-0=0
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但是！这货明明转得可欢乐了呢！
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别的组都画完走了，我们才看出来，这机构造的根本就不对！左下角的杆件，其上端的点应与销钉的运动轨迹一致，也就是，应该是绕机架最左边的点，定轴转动！然而它又是被限制在滑槽的杆件上的点，所以只能上下移动！这根本就是矛盾机构啊！然而它居然很润滑地动来动去，害得我们好久没看出来问题在哪……
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; T& t1 P+ e! c当然我们自己本事没到家不能一眼看出来是一方面，但是另一方面，这可是给学生上课的教具啊！都是错的！这体现了什么，想想都不寒而栗……我一直说我喜欢德国，别的不说，就从这一个小小的教学用机构上，人家能做出个矛盾机构？还一直安然无恙地摆在实验室里？真的不能怪我崇洋媚外啊，差距在那呢。
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哎……心累……不怕努力，只怕自己努力半天人家也不正眼看你……连给个实验仪器都这么糊弄……
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however1241

女性的坚持，不知道最近怎么养了

の小南灬

你是直接copy课件的吗？
编辑的好整齐

黑森林的鹿

【20160321】Special Cases of the Mobility Equation
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Mobility is an extremely important property of amechanism. Among other facets, itgives insight into the number of actuators required to operate a mechanism. However, to obtain correct results, special care must be taken in using the Gruebler's equation. Some specialconditions are presented next.
5 t' {4 m2 P6 b. o自由度计算中有些特殊情况需要格外注意。
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Coincident Joints

Some mechanisms have three links that are all connected at a common pin joint, as shown in Figure 1.28. This situation brings some confusion to kinematic modeling. Physically, one pin may be used to connect all three links. However, by definition, a pin joint connects two links.
同一销钉连接三个杆件的情况，需要算作两个销钉。因为按照定义，一个销钉连接两个杆件。
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( K* C! L5 q9 P! @* Z6 }Exceptions to the Gruebler's Equation
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Another special mobility situation must be mentioned. Because the Gruebler's equation doesnot account for link geometry, in rare instances it can lead to misleading results. One such instance is shown in Figure 1.31.
自由度计算公式也有例外，比如由几何因素引起的错误结果，一个例子如图所示。
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! [* t! W& U" ^2 o7 P% j1 bNotice that this linkage contains five links and six pin joints. Using Gruebler's equation, thislinkage has zero degrees of freedom. Of course, this suggests that the mechanism is locked. However, if all pivoted links were the same size, and the distance between thejoints on the frame and coupler were identical, this mechanism would be capable of motion, with one degree of freedom. The center link is redundant, and because it is identical in length to the other two links attached to the frame, it does not alter the action of the linkage.
5个杆件6个销钉，按自由度计算公式其自由度为零。这意味着机构锁定，然而若所有转轴等长，并且与机架相连的销钉到连杆的距离相等，这个机构是可以运动的，有1个自由度。中间的曲柄是多余的，并不影响机构的运动。
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, e( m$ q+ A* ^8 _Idle Degrees of Freedom

) |$ u/ U0 r  dIn some mechanisms, links exhibit motion which does not influence the input and output relationship of the mechanism. These idle degrees of freedom present another situation where Gruebler's equation givesmisleading results. An example is a cam with a roller follower as shown in Figure 1.32. Gruebler's equationspecifies two degrees of freedom (4 links, 3 pins, 1 higher-order joint). With an actuated cam rotation, the pivoted link oscillates while the roller follower rotates about its center. Yet, only the motion of the pivoted link serves as the output of the mechanism. The roller rotation is an idle degree of freedom and not intended to affect the output motion of the mechanism. It is a design feature which reduces friction and wear on the surface of the cam. While Gruebler's equation specifies that a cam mechanism with a roller follower has a mobility of two, the designer is typically only interested in a single degree of freedom. Several other mechanisms contain similar idle degrees of freedom.
1 {- X1 B, d% J& ^% ?多余自由度，有的机构中含有这样的杆件，其运动对输出运动不产生影响， 则其为多余自由度。如图机构含4个杆件，3个销钉，1个高副约束，输入端凸轮转动，输出端杆件摆动，中间滚轮从动件绕其中心转动。然而只有杆件的摆动是机构的输出，滚轮的滚动是多余自由度，不影响机构输出。这样设计是为了减小凸轮表面的摩擦。自由度计算公式给出，该机构有2个自由度，然而事实上我们只对一个感兴趣。还有其他例子，不作赘述。
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黑森林的鹿

however1241 发表于 2016-3-21 08:22
女性的坚持，不知道最近怎么养了

/ M  U) u9 A5 O7 h% J- R* r0 |9 @感谢关注！还在坚持！不过机械课之外的其他课负担越来越重了……哎，还需要努力，门门都不能丢！
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黑森林的鹿

の小南灬 发表于 2016-3-21 16:31
你是直接copy课件的吗？
编辑的好整齐

$ u9 Q4 U. R* j4 o& j" i0 M小南~好久不见排版啊……本人小小的强迫症，喜欢排版什么的，不过不是课件哦，自己手打呢，我们课件是介样的……

however1241

黑森林的鹿 发表于 2016-3-21 19:18
感谢关注！还在坚持！不过机械课之外的其他课负担越来越重了……哎，还需要努力，门门都不能丢！

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