【菜鳥成長記】20歲后，我在機械的每一天

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黑森林的鹿

161^#

樓主| 發表于 2016-3-19 22:46:14 | 只看該作者

haneast 發表于 2016-3-18 17:05
: W8 z _4 Y& F4 N+ R大學六級剛過500，現在看到學妹一貼下來的英語各種666，表示十分慚愧，現在也天天看扇貝，看到學妹的水平才 ...

并沒有多少水平，離自由交流還有很大距離……就像論壇各位前輩說的，機械這一行外語非常重要，還需要多多努力！學長也加油！早日實現自己的夢想呢！

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黑森林的鹿

162^#

樓主| 發表于 2016-3-20 16:11:35 | 只看該作者

【20160320】關于做的一個簡單實驗。。。

這學期期待已久的機械原理實驗課！要求根據實驗室提供的小機構畫出機構簡圖，然而其中的一個。。。

雖然標注著“雙沖頭機構”，但是……這根本不是雙沖頭機構好嗎！！！怎么算自由度都是0啊！

圖示有9個構件，12個低副約束，沒有高副約束，因此根據公式：
M=3（n-1）-2PJ-PH=3×8-2×12-0=0

但是！這貨明明轉得可歡樂了呢！

別的組都畫完走了，我們才看出來，這機構造的根本就不對！左下角的桿件，其上端的點應與銷釘的運動軌跡一致，也就是，應該是繞機架最左邊的點，定軸轉動！然而它又是被限制在滑槽的桿件上的點，所以只能上下移動！這根本就是矛盾機構啊！然而它居然很潤滑地動來動去，害得我們好久沒看出來問題在哪……

當然我們自己本事沒到家不能一眼看出來是一方面，但是另一方面，這可是給學生上課的教具啊！都是錯的！這體現了什么，想想都不寒而栗……我一直說我喜歡德國，別的不說，就從這一個小小的教學用機構上，人家能做出個矛盾機構？還一直安然無恙地擺在實驗室里？真的不能怪我崇洋媚外啊，差距在那呢。

哎……心累……不怕努力，只怕自己努力半天人家也不正眼看你……連給個實驗儀器都這么糊弄……

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however1241

163^#

發表于 2016-3-21 08:22:26 | 只看該作者

女性的堅持，不知道最近怎么養了

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の小南灬

164^#

發表于 2016-3-21 16:31:01 | 只看該作者

你是直接copy課件的嗎？
編輯的好整齊

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黑森林的鹿

165^#

樓主| 發表于 2016-3-21 19:17:12 | 只看該作者

【20160321】Special Cases of the Mobility Equation

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.
自由度計算中有些特殊情況需要格外注意。

Coincident Joints9 U0 }" y$ } I. p- {0 [3 l

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.
同一銷釘連接三個桿件的情況，需要算作兩個銷釘。因為按照定義，一個銷釘連接兩個桿件。

Exceptions to the Gruebler's Equation" e. v2 o8 Y3 U* r

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.
自由度計算公式也有例外，比如由幾何因素引起的錯誤結果，一個例子如圖所示。

Notice 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個桿件6個銷釘，按自由度計算公式其自由度為零。這意味著機構鎖定，然而若所有轉軸等長，并且與機架相連的銷釘到連桿的距離相等，這個機構是可以運動的，有1個自由度。中間的曲柄是多余的，并不影響機構的運動。

Idle Degrees of Freedom
) N+ j8 d5 {; A6 ]0 v: q
In 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.
多余自由度，有的機構中含有這樣的桿件，其運動對輸出運動不產生影響，則其為多余自由度。如圖機構含4個桿件，3個銷釘，1個高副約束，輸入端凸輪轉動，輸出端桿件擺動，中間滾輪從動件繞其中心轉動。然而只有桿件的擺動是機構的輸出，滾輪的滾動是多余自由度，不影響機構輸出。這樣設計是為了減小凸輪表面的摩擦。自由度計算公式給出，該機構有2個自由度，然而事實上我們只對一個感興趣。還有其他例子，不作贅述。