What is the critical angle for lifting loads?

• A. 90 degree angle
• B. 60 degree angle
• C. 45 degree angle
• D. 120 degree angle

Answer: (D)

When lifting with two slings, how the load is shared depends on the angle between the sling legs. The vertical components of the two sling tensions must add up to the weight being lifted. If the angle between the legs is φ, and the tension in each leg is T, then 2T cos(φ/2) equals the weight. The “critical” point is reached when each leg carries the full load, meaning T equals the weight. Plugging that in gives W = 2W cos(φ/2), so cos(φ/2) = 1/2, which yields φ/2 = 60°, and φ = 120°. At this angle, each leg pulls with a force equal to the weight; any larger angle causes the required leg tension to increase beyond the weight, raising the risk of overload. So the commonly cited critical angle between the legs is 120 degrees (which corresponds to 60 degrees from vertical for each leg).