Four-bar Straight-line Linkage of Roberts

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The four-bar linkage showcased here belongs to the category of Watt's straight-line mechanisms, designed for his steam engine. In this specific model, a fixed support houses the immovable link. The two rotary links possess identical lengths. Attached to the central link is an extension that approximates a straight line; at times, this appendage resembles a "W" shape and is referred to as a 'W' straight-line mechanism in certain literature. However, it should be noted that this straight-line depiction is not entirely accurate. To achieve an exact straight-line representation, one can resort to Peaucellier's eight-bar planar linkage (Model S-35) or Surrat's spatial six-bar linkage.