Why does a basket hitch sling configuration double a sling's vertical rated capacity?
Question: Could you please explain the engineering reasoning on why a basket hitch sling configuration doubles a sling’s rating if both ends of the sling are vertically parallel to each other when going from the load to the load hook/spreader beam? See image 3 below:
Answer: There are actually two questions within your request. We will address them separately.
First question: Why does the lifting capacity of a vertical basket hitch configuration double a sling’s lifting capacity?
Answer: The tension in a sling (neglecting friction) is equal along every portion of the sling, even if that sling (or rope) wraps around a curve, such as in a basket hitch configuration. Suppose a sling is rated for 1 Ton: When used in a basket hitch configuration, both ends of the sling are able to support the lifted load. EACH END can support its rated capacity of 1 Ton because the sling never has more than 1 Ton of tension (remember, the tension in a sling is equal throughout). Adding up the net supported weight results in a capacity of 2 Tons. 2 Tons can be supported with the 1 Ton sling because the tension in the sling is still limited to 1 Ton.
Here’s another helpful way to look at it: Picture the sling as if it were the running rope on a crane. To increase the lifting capacity of the running rope a larger wire rope is not the solution, instead, the running rope is reeved through additional pulleys. When a single part line is reeved through a block to make it two parts, the wire rope can safely lift twice as much as the single part line. The net capacity of a basket hitch works in the same way.
Second question: Why does a basket hitch require both ends of the sling to be vertical to achieve its full lifting capacity (which is double the sling’s load rating)?
Answer: The sling must be vertical because any deviation from vertical causes additional tension in the sling (in keeping with basic trigonometry). The vertical component of the sling’s tension is the equal to the tension in the sling and the lifted load… but only when the sling is vertical. As it becomes angled, only a portion of the tension in the sling contributes to lifting the load. The remainder of the sling’s tension is used up in a side load that does nothing to support the vertical load. Since the lifted load isn’t getting any smaller with the changing angle, the tension in the sling must increase with the increasing angle, thereby reducing the load the sling is able to lift.
Rigging institute would like to thank Culley Parris, P.E. – Coffman Engineers, Inc., Spokane WA for his assistance in answering these questions.