The unbalanced forces along the line of stroke for the two cylinders constitute a couple about the centre line YY between the cylinders as shown in Fig.
This couple has swaying effect about a vertical axis and tends to sway the engine alternately in clockwise and anticlockwise directions. Hence the couple is known as swaying couple.
Let a = Distance between the centre lines of the two cylinders.
The swaying couple is maximum or minimum when (cosθ m ~ in θ) is maximum or minimum. For (cos θ m sinθ) to be maximum or minimum,
Thus, the swaying couple is maximum or minimum when me = 45° or 225°. m Maximum and minimum value of the swaying couple