Fundamentals and equations of crane and rigging equipmentTemitayo Oketola | November 26, 2021
Cranes and rigging equipment are essential material handling systems in various applications, including construction sites, ports and industrial facilities. They perform the simple function of safely lifting and transporting heavy materials from one place to another.
But as simple as cranes might seem to operate, a lot of work and effort goes into planning a lifting operation. There are several things an engineer must know and several factors that a rigger must consider when planning a lifting operation.
Types of rigging equipment
For any operation that uses a crane to lift materials, rigging equipment is used to attach the load to the crane. These pieces of equipment include shackles, hooks, slings, ropes, chains and spreader bars.
Shackles connect rigging hardware, slings, chain, lifting devices and other rigging components to the load. They have a U-shape with a clevis pin or bolt securing the opening, as shown in Figure 2 below.
Hooks are quite similar to shackles in function, but they differ in design and application requirements. Hooks have a more basic design than shackles and are easier to set up. In addition, hooks typically withstand more twisting force than shackles, making them ideal for material handling applications that feature high torque.
A lifting sling comprises a cable, rope or chain used in conjunction with a crane to secure the load and facilitate lifting operations.
A spreader bar is a below-the-hook lifting device that stabilizes and supports the load, minimizing damage to the load and other pieces of rigging hardware.
Planning a lifting operation: 3 factors to consider
1. Weight of the material
As a rule, the weight of the part being lifted must be lesser than the lifting capacity of the shackles, slings and other rigging and crane equipment. Riggers can easily obtain the weight of a product by:
- Checking the product to see if its weight is marked
- Using an industrial scale
- Checking the shipping documentation or bill of landing
But for engineers in the design phase of a product that will be lifted using cranes, the weight can be estimated by multiplying the calculated volume of the product by the density of the material used to make the product. This is expressed mathematically below:
Volume is measured in ft3
Density is measured in lb/ft3
Table 1 presents the densities of some common materials today.
So, consider a scenario where there is a need to lift a steel sphere with a proposed radius of 2 ft. The volume of the sphere is obtained to be 33.514 ft3, and the estimated load weight is 16422.187 lb. Hence, for the particular operation, engineers must use a crane with a lifting capacity that is considerably higher than the load weight.
2. Sling Angle and Sling Angle Factor
Sling angle is the angle measured between the horizontal plane of the load being lifted and the sling leg, as shown here.
Engineers should keep in mind that as the sling angle reduces, the tension within the two sling legs increases. So, if riggers were to casually lower the sling angle without considering the horizontal tension during a rigging operation, the results could be catastrophic. As a rule, avoid rigging loads where the sling angle is less than 45°.
Engineers can determine the required sling size using a coefficient called the sling angle factor. The sling angle factor can be calculated by simply diving the sling length by the sling height, as shown below:
The force on each sling can then be calculated using:
3. Center of Gravity
The center of gravity is a point in an object where all the weight of the object is concentrated or where it can be hoisted perfectly balanced. As a rule, riggers must ensure that the load's center of gravity is directly below the hook before the load is lifted, as this ensures the stability of the lifting operation.
Engineers can locate the center of gravity of loads by calculation or trial and error method (learn more).
When the details of the load, center of gravity and sling angle have been correctly established, an engineer or a rigger can then decide on the type and capability of the crane to be used.