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[分享] 直线机构(STRAIGHT -LINE MECHANISMS)

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发表于 2019-11-17 22:51:15 | 显示全部楼层 |阅读模式
本帖最后由 muoubear* 于 2019-11-17 22:53 编辑

STRAIGHT -LINE MECHANISMS 直线机构

A very common application of coupler curves is the generation of approximate straight lines. Straight-line linkages have been known and used since the time of James Watt in the 18th century. Many kinematicians such as Watt, Chebyschev, Peaucellier, Kempe, Evans, and Hoeken (as well as others) over a century ago, developed or discovered either approximate or exact straight-line linkages, and their names are associated with those devices to this day.
耦合曲线的一个非常普遍的应用是近似直线的生成。自18世纪James Watt的时代以来,直线连杆就被人们所熟知和使用,许多运动学家,如一个世纪以前的Watt,Chebyschev,Peaucellier,Kempe, Evans, 和Hoeken(还有其他人),发展或发现了近似或精确直线连杆,直到今天,他们的名字都与这些机构密切相关。
The first recorded application of a coupler curve to a motion problem is that of Watt's straight-line linkage, patented in 1784, and shown in Figure 3-29a. Watt devised his straight-line linkage to guide the long-stroke piston of his steam engine at a time when metal-cutting machinery that could create a long, straight guideway did not yet exist. * This triple-rocker linkage is still used in automobile suspension systems to guide the rear axle up and down in a straight line as well as in many other applications.
第一个有记载的耦合曲线应用是Watt直线连杆机构,1784申请的专利,如图3-29a所示。Watt设计了直线机构用来引导蒸汽机的长冲程活塞的一个周期,金属切断机械可以形成一个实际并不存在的长而直的导轨。这种三摇杆连杆机构在汽车悬架系统中仍在使用,用来引导后桥进行上下直线运动,该机构在其他方面也有应用。
Figure 3-29.png
Richard Roberts (1789-1864) (not to be confused with Samuel Roberts of the cognates) discovered the Roberts' straight-line linkage shown in Figure 3-29b. This is a triple-rocker. Chebyschev (1821-1894) also devised a straight-line linkage-a Grashof double-rocker-shown in Figure 3-29c.
Richard Roberts (1789-1864) (不要和Samuel Roberts混淆)发现的Robert直线连杆机构见图3-29b。这是一个三摇杆。Chebyschev (1821-1894) 也设计了一个直线连杆机构,Grashof双摇杆如图3-29c所示。
The Hoeken linkage [16] in Figure 3-29d is a Grashof crank-rocker, which is a significant practical advantage. In addition, the Hoeken linkage has the feature of very nearly constant velocity along the center portion of its straight-line motion. It is interesting to note that the Hoecken and Chebyschev linkages are cognates of one another. t The cognates shown in Figure 3-26 (p. 116) are the Chebyschev and Hoeken linkages.
Hoeken连杆机构如图3-29d所示,是一个Grashof曲柄摇杆机构,这个有显著的实用优势。此外,Hoeken连杆机构沿着其直线运动的中心点上具有接近匀速的特性。有趣的是,Hoecken和Chebyschev连杆机构是彼此同源。图3-26所示为Chebyschev和Hoeken连杆机构的同源性。
Figure 3-26.png
These straight-line linkages are provided as built-in examples in program FOURBAR. A quick look in the Hrones and Nelson atlas of coupler curves will reveal a large number of coupler curves with approximate straight-line segments. They are quite common.
这些直线机构作为内置示例在4杆程序中被提供。耦合曲线的Hrones和Nelson图集中揭示了大量的近似直线段的耦合曲线。这些曲线相当常见。
To generate an exact straight line with only pin joints requires more than four links. At least six links and seven pin joints are needed to generate an exact straight line with a pure revolute-jointed linkage, i.e., a Watt's or Stephenson's sixbar. A geared fivebar mechanism, with a gear ratio of -1 and a phase angle of 1t radians, will generate an exact straight line at the joint between links 3 and 4. But this linkage is merely a transformed Watt's sixbar obtained by replacing one binary link with a higher joint in the form of a gear pair. This geared fivebar's straight-line motion can be seen by reading the file STRAIGHT.5BRinto program FIVEBAR,calculating and animating the linkage.  Peaucellier * (1864) discovered an exact straight-line mechanism of eight bars and six pins, shown in Figure 3-2ge. Links 5, 6, 7, 8 form a rhombus of convenient size. Links 3 and 4 can be any convenient but equal lengths. When OZ04 exactly equals OzA, point C generates an arc of infinite radius, i.e., an exact straight line. By moving the pivot Oz left or right from the position shown, changing only the length of link 1, this mechanism will generate true circle arcs with radii much larger than the link lengths.
要仅用销轴运动副生成精确的直线,需要多于4连杆。用纯旋转副联结的连杆机构,至少需要6个连杆和7个销轴运动副,才能生成精确的直线,例如,Watt或Stephenson的6杆。齿轮构成的5杆机构,用齿轮比率为-1和相位角为1t弧度,将在连杆3和4之间的运动副处生成精确的直线。但这种连杆仅仅是一个转换的Watt 6杆机构,通过用高副齿轮副替换了二元连杆获得。可通过读取文件STRAIGHT.5BR到5杆程序,计算和动画连杆,可以看到齿轮构成的5杆直线运动。Peaucellier(1864) 发现了一个8杆和6销的精确直线机构,如图3-29e所示,连杆5,6,7,8形成了一个大小适中的菱形。连杆3和4可以是任意方便长度,但两连杆长度要相等。当O2O4等于O2A时,P点生成一个半径无限大的圆弧,即一条精确的直线。通过向左或向右移动支点O2的位置,仅更改连杆1的长度,此机构将生成半径远大于连杆长度的圆弧。
动图参考如下:
Figure 3-29.gif
Figure 3-26.gif


Figure 3-26.jpg

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