These linkages convert rotary to straight-line motion without the need for guides.
Image 1: An Evans’ linkage has an oscillating drive arm that should have a maximum operating angle of about 40 degrees. For a relatively short guideway, the reciprocating output stroke is large. Output motion is on a true straight line in true harmonic motion. If an exact straight-line motion is not required, a link can replace the slide. The longer this link, the closer the output motion approaches that of a true straight line. If the link-length equals the output stroke, deviation from straight-line motion is only 0.03 percent of the output stroke.
Image 2: A simplified Watt’s linkage generates an approximate straight-line motion. If the two arms are of equal length, the tracing point describes a symmetrical figure 8 with an almost straight line throughout the stroke length. The straightest and longest stroke occurs when the connecting-link length is about two-thirds of the stroke, and arm length is 1.5 times the stroke length. Offset should equal half the connecting-link length. If the arms are unequal, one branch of the figure-8 curve is straighter than the other. It is straightest when a/b equals (arm 2)/(arm 1).
Image 3: Four-bar linkage produces an approximately straight-line motion. This arrangement provides motion for the stylus on self-registering measuring instruments. A comparatively small drive displacement results in a long, almost-straight line.
Image 4: A D-drive is the result when linkage arms are arranged as shown here. The output link point describes a path that resembles the letter D, so there is a straight part of its cycle. This motion is ideal for quick engagement and disengagement before and after a straight driving stroke.
Image 5: The “Peaucellier cell” was the first solution to the classical problem of generating a straight line with a linkage. Within the physical limits of the motion, AC X AF remains constant. The curves described by C and F are, therefore, inverse; if C describes a circle that goes through A, then F will describe a circle of infinite radius—a straight line, perpendicular to AB. The only requirements are that: AB = BC; AD = AE; and CD, DF, FE, EC be equal. The linkage can be used to generate circular arcs of large radius by locating A outside the circular path of C.
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