For procedures, see your mechanical laboratory manual.

Title: In-line Slider Crank Mechanism


Aim

To investigate the connection the Moment "M" applied to the crank "r" and the force "F" applied to the slider.

Theory

An in-line slider crank has its slider positioned so the line of travel of the hinged joint of the slider passes through the base joint of the crank. This creates asymmetric slider movement back and forth as the crank rotates.

The displacement of the end of the connecting rod is approximately proportional to the cosine of the angle of rotation of the crank when it is measured from the top dead center (TDC). So the reciprocating motion created by a steadily rotating crank and connecting rod is approximately simple harmonic motion.




Assuming Î¸ is positive counter-clockwise, the virtual work done by M for an increase Î´Î¸ is Mδθ. Assuming x is positive to the left, the virtual work done by F for an increase Î´x is Fδx the total virtual work is zero. Hence Î´u = Mδθ + Fδx = 0

When the crank is driven by the connecting rod, a problem arises when the crank is at the top dead center (0°) or the bottom dead center (180°). At these points in the crank's cycle, a force on the connecting rod causes no torque on the crank. Therefore, if the crank is stationary and happens to be at one of these two points, it cannot be started moving by the connecting rod. For this reason, in steam locomotives, whose wheels are driven by cranks, the connecting rods are attached to the wheels at points separated by some angle, so that regardless of the position of the wheels when the engine starts, at least one connecting rod will be able to exert torque to start the train.

References

  1. Slider Crank Mechanism, wikipedia.org.
  2. Types of linkage mechanisms, researchgate.net.
  3. Hartenberg, R.S. & J. Denavit (1964) Kinematic synthesis of linkages, New York: McGraw-Hill, online link from Cornell University.
  4. Myszka, David (2012). Machines and Mechanisms: Applied Kinematic Analysis. New Jersey: Pearson Education.