Four-bar Straight-line Linkage of Roberts

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This is another four-bar linkage in the genre of Watt's straight-line linkage for his steam engine. In this model, the grounded link is part of the fixed support. The two rotary links are of equal length. An extension off the connecting or center link generates an approximate straight-line. This extension is sometimes in the form of a "W" and in some books is called a 'W' straight-line mechanism. As with all four-bar linkages it, the straight-line is not exact. An exact straight-line can be drawn with the eight-bar planar linkage of Peaucellier (Model S- 35) or the spatial six bar linkage of Surrat.