# Which of the following mechanisms produces an exact straight line motion?

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Which of the following mechanisms produces an exact straight line motion?
1. Watt mechanism
2. Grasshopper mechanism
3. Peaucellier's mechanism
4. Tchebichiff mechanism

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Correct Answer - Option 3 : Peaucellier's mechanism

Explanation:

• 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.
1. Watt’s Mechanism
2. Grasshopper Mechanism
3. Tchebicheff’s Mechanism
• Exact Straight Line Mechanisms: These mechanisms always give an exact, mathematically correct straight-line motion.
1. Peaucellier Mechanism (Only Turning Pairs)
2. Hart’s A-frame (Only Turning Pairs)
3. Scott Russel Mechanism (Consisting of One Sliding Pair)

• The pantograph is a four-bar mechanism used to enlarge or reduce drawings.
• Peaucellier Mechanism: It consists of 8 links, connected by turning pairs.
• Harts Mechanism: It consists of six links, connected by turning pairs. This mechanism has a great practical disadvantage that even when the path of tracing point is short, a large amount of space is taken up by the mechanism.
• Grasshopper Mechanism: This mechanism is a modification of modified Scott-Russel’s mechanism with the difference that the point P does not slide along a straight line, but moves in a circular arc with centre O.