CRANECRITIQUECORNER
SLING ANGLES This month’s topic:
How Do Sling Angles Effect Tension
by: Dave Barnhouse
A monthly crane and rigging informative column for all personnel directly or indirectly involved with crane safety. Each month we will attempt to explain a different technical issue pertaining to crane operations here on Guam, addressing the sometimes over-looked or misunderstood topics by management and operators alike. When lifting a load there are two important factors that must be considered regarding the rigging. These are the weight of the load and the capacity of the slings. The competent rigger knows that this sling capacity may change depending on the configuration of the rigging, more specifically the sling angle. When sling angle is considered, the angle referred to is the angle measured between horizontal and the sling leg. This angle is very important and can have a dramatic effect on the rated capacity of the sling. As illustrated in the table below, when this angle decreases, the load on each leg increases. In other words, the smaller the angle between the sling leg and the horizontal, the greater the load on the sling leg. The minimum angle allowed is 30 degrees. This principle applies whether one sling is used to pull at an angle, in a basket hitch or for multi-legged bridle slings. The rated capacity of a sling must never be exceeded. The rated capacity is based both on sling fabrication components (minimum breaking force of rope used, splicing efficiency, number of parts of rope in sling and number of sling legs) and sling application components (angle
of legs, type of hitch, D/d ratios, etc.) If you are using one wire rope sling in a vertical hitch, you can utilize the full rated lifting capacity of the sling, but you must not exceed that lifting capacity. If you are using two wire rope slings in a vertical hitch (called a 2-legged bridle hitch) in a straight lift, the load on each leg increases as the angle between the leg and the horizontal plane decreases. Whenever you lift a load with the legs of a sling at an angle, you can calculate the actual load per leg by using the following three-step formula. These calculations assume that the center of gravity is equal distance from all of the lifting points, and the sling angles are the same. If not, more complicated engineering calculations are needed. 1. Divide the weight of your total load by the number of legs you are using. This gives you the load per leg if the lift were being made with all legs lifting vertically. 2. Measure the angle between the legs of the sling and the horizontal plane. 3. Multiply the load per leg that you calculated in step 1 by the load factor for the leg angle you are using. Use the Load factor guidelines table below to determine the load factor. The result is the actual
load on each leg of the sling for this lift and angle. The actual load must never exceed the sling’s vertical rated capacity. Note that at 30° the Load Angle Factor is 2. This means the tension will double if the sling length is at 30°, meaning if a two part bridle was used to lift a 10,000 lb. load, each leg share would be 5,000 lb. times the L.A.F. of 2 resulting in 10,000 lb. tension on each leg. This is especially important when lifting heavier loads. In this example, 2 ea. 7,500 lb. capacity slings would thought to be sufficient since the combined capacity is 15,000 lb. In reality each leg would be overloaded by 2,500 lb. Horizontal Angle
L.A.F.
5° 10° 15° 20° 25° 30° 35° 40° 45° 50° 55° 60° 65° 70° 75° 80° 85° 90°
11.49 5.75 3.861 2.924 2.364 2.00 1.742 1.555 1.414 1.305 1.221 1.155 1.104 1.064 1.035 1.015 1.004 1.00
Low sling angles also contribute to compression forces to the anchor points on the load. While this may not be an issue with some loads, it may have disastrous results with loads such as a cargo container which are meant to lift vertically only. If a heavy container is lifted with a low angle bridle sling it will easily fold up like a piece of cardboard.
It’s best to maintain a 60° sling angle if possible as the sling tension will only increase 15.5%. It is easily to assure this angle by using the sling lengths equal to or more then the sling attachment point to point length on the load.
28 | JULY2012
CONSTRUCTION NEWS BULLETIN
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