The lengths of the links comply with the conditions: A͞B=A͞D and B͞C=C͞D. Link 3 makes one revolution to two revolutions of link 1. At the extreme positions (dead points) axes B and D of the links coincide and there is no positive motion of the mechanism unless some device is provided to pass through these dead points or unless the driving link has sufficient flywheel mass.
$543$LW,4L$