Crane Stability & Rotational Overturning

To investigate crane stability and rotational overturning.

Definition and Theory
One way a crane can fail is due to rotational overturning, resulting in the crane tipping over. When a load is lifted a distance from the edge of the crane it causes a rotation about that point called a moment. For this reason, most cranes need to be counter balanced. To calculate the overturning moment, the load P is multiplied by the horizontal distance H the load extends beyond the edge of the crane. To counter balance this moment, a load is applied to the other side of the crane a horizontal distance that equals the overturning moment.

  • M = Moment
    (rotation about a point)
  • P = Load
  • d = Actual Distance from
    the rotation point to the
    counter balance weight
  • D = Total Distance (1d, 2d or 3d)
  • M = P x D


  • SM = Sum of the Moments = 0
    (about the rotation point
    the crane will not tip over)
  • SM = (P x 1d) - (.5P x 2d) = 0
  • (P x 1d) = (.5P x 2d)


Laboratory Worksheet