Sound lifting practices.
There are four primary factors to take into consideration when lifting a load.
They are: (1) the physical parameters of the load; (2) the number of legs and the angle they make with the horizontal; (3) the rated capacity of the sling; and (4) the condition of the sling.
Physical parameters of the load
The size of the object to be lifted, and particularly the location of lifting points, will affect sling selection. The weight of the lift, while a critical component, is only part of the information. The location of the center of gravity is also necessary to determine sling loadings.
If the load has small diameter corners, protective blocking or softeners must be used so that sling capacity isnt reduced. Also, if lifting a painted object or an object with a finished surface, padding or softeners may be needed between the sling and the load to protect the load.
Number of legs and angle with the horizontal
As the angle formed by the sling leg and the horizontal decreases, the rated capacity of the sling also decreases. 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.
Rated Capacity
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 capactity 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.
Three-step formula for calculating load per sling leg
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.
- 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.
- Measure the angle between the legs of the sling and the horizontal plane.
- 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 on the next page 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 slings vertical rated capacity.
Examples
- Total load is 1,000 lbs. divided by two legs 500 lbs. load per leg if vertical lift.
- Horizontal sling angle is 60 degrees.
- Multiply 500 lbs. by 1.154 load factor (from table) = 577 lbs. actual load per leg.
- Total load is 1,000 lbs. divided by two legs 500 lbs. load per leg if vertical lift.
- Horizontal sling angle is 45 degrees.
- Multiply 500 lbs. by 1.414 load factor (from table) = 707 lbs. actual load per leg
- Total load is 1,000 lbs. divided by two legs 500 lbs. load per leg if vertical lift.
- Horizontal sling angle is 30 degrees.
- Multiply 500 lbs. by 2 load factor (from table) = 1000 lbs. actual load per leg
WARNING: Slings shall not be used with horizontal angles less than 30°
Condition of sling
Each sling must be inspected daily. If the sling does not pass inspection do not use.