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This post is about the various Straight Line Mechanisms which are used to obtain either approximate straight lines or exact straight lines. By definition, a straight line mechanism is used to obtain a constrained straight line motion of a point, either by using linkages or other mechanical elements. **Approximate Straight Line Mechanisms :** These straight line mechanisms do not give an exact straight line. The straight line motion obtained by these mechanisms are somewhat accurate over a considerable distance of travel.

**Watt’s Mechanism**: This mechanism was used by James Watt in early steam engines to guide and constrain the piston rod in a cylinder to move in a straight line. It consists of 4 links attached as follows. The 2 longest links (In White) are of equal lengths. The motion of a point on the middle link gives an approximate straight line.**Scott Russel Mechanism :**This mechanism is another straight line mechanism which consists of 4 major parts : The long link(Blue), The crank(Red) and the sliding pair of slider(Yellow) and the frame(Grey). This mechanism can generate an exact straight line if the link dimensions are correct. The locus of the straight line is the same as the travel of the slider for 90 degrees rotation of the crank.**Grasshopper Mechanism**: This mechanism is a modification of the Scott-Russell Mechanism. In this mechanism, the slider is replaced by a link traveling in an arc about another fixed point. It is named so because of its resemblance to a grasshopper’s hind legs!**Tchebicheff’s Mechanism**: This is a fairly well known mechanism for approximate straight line motion. It was invented by the 19th century mathematician Pafnuty Tchebicheff who studied theoretical problems in kinematic mechanisms. It consists of 4 links connected as shown.

**Exact Straight Line Mechanisms : **These mechanisms always give an exact, mathematically correct straight line motion.

**Peaucellier Mechanism :**This mechanism consists of 7 interconnected links and the frame which work together to constrain the motion at the point to an exact straight line. The links shown in the same colour are of equal lengths. The point connecting the two red links is the tracing point which gives an exact straight line motion.

**2. Hart’s A-frame : **This mechanism is another exact straight line generating mechanism. It was invented and published by Harry Hart in 1874.This mechanism uses 6 links which is less than the number of links in the Peaucellier Mechanism. Hope you enjoyed reading this! Cheers!