Questions: Linkages and Mechanisms

Question 1

A four-bar linkage has one bar fixed to the ground and the driving bar rotates continuously. What type of motion does the output bar produce?

• A. It always rotates continuously too
• B. It depends on the relative lengths of the bars -- it might oscillate or rotate
• C. It always moves in a straight line
• D. It stays still

Answer: (B)

The type of output motion in a four-bar linkage depends on the relative lengths of all four bars. If the shortest bar is the driver and it can rotate fully, the output bar may also rotate (double crank) or oscillate (crank-rocker). The geometry determines the behavior.

Question 2

A pair of scissors is an example of a linkage mechanism.

• T. True
• F. False

Answer: True

Scissors are a simple linkage: two bars (the blades) connected by a pivot point (the screw). Your fingers apply force at one end, and the cutting edges move at the other end. The pivot acts as a fulcrum, making scissors simultaneously a linkage and a lever.

Question 3

How does a car's windshield wiper convert the motor's rotary motion into the wiper's back-and-forth sweeping motion?

Model answer: The motor rotates a small crank arm continuously. This crank is connected by a linkage to the wiper arm, which is constrained to pivot at its base. The linkage converts the motor's continuous rotation into the wiper's oscillating (back-and-forth) sweep. This is a classic crank-rocker mechanism.

The four-bar linkage in the wiper system has the motor crank as the input (rotating fully), the car body as the fixed link, a connecting rod, and the wiper arm as the output (oscillating). The geometry is chosen so the wiper sweeps through the desired angle and pauses briefly at each end of its stroke.