Incorrect calculation of the carrying capacity when lifting concrete blocks or steel beams often leads to rupture of the branches of the slings at the stage of separation of the object from the ground. The critical factor here is not only the declared weight of the cargo itself, but also the angle of divergence of the branches, which crane operators often ignore when planning a seizure. If load-carrying capacity Slings are selected without taking into account the coefficient, depending on the angle of inclination, the real load on each branch can exceed the permissible one and a half to two times.
To ensure safety, it is necessary to immediately determine the weight of the object being raised and its dimensions in order to choose the correct sling scheme. An error in determining the center of gravity or ignoring dynamic loads during a jerk can be fatal for equipment and personnel. In this guide, we will discuss the selection algorithm. load-grabbing devicesThis will eliminate emergencies at the construction site or warehouse.
Determination of the mass and center of gravity of the cargo
The primary task before the start of rigging is to know the exact weight of the object being raised. In the technical documentation for equipment or in the accompanying invoices for construction materials is always indicated nominal massHowever, in practice, there are often situations where the real weight differs from the passport due to additional fasteners, ice or dirt. Ignoring these factors leads to overloading of the lifting mechanism and the slinging elements.
Particular attention should be paid to the definition of the center of gravity, especially when working with asymmetrical loads or long structures. Incorrect installation of the hook relative to the center of mass will cause roll, slippage sling-loop Or uncontrolled rotation of an object in the air. For complex loads, it is recommended to make a preliminary estimate or use special weighing sensors on the crane.
โ ๏ธ Attention: It is strictly forbidden to lift a load whose weight is unknown or exceeds the carrying capacity of the crane and the slings used. Visual weight assessment "by eye" is unacceptable.
When working with bulk materials in containers or liquids in containers, it is necessary to take into account the possibility of shifting the center of gravity during the lifting process. The liquid medium creates a dynamic load that can significantly change the nature of the cargo behavior. In such cases safety-box Slings should be maximum, and the sling scheme - exclude the possibility of overflowing contents.
Effect of branch divergence angle on load
One of the most critical parameters influencing the choice of slings is the angle between branches. Many people mistakenly believe that if the cargo weighs 4 tons, and four branches are used, then each one accounts for 1 ton. This is true only in the perfect vertical position. As the branches diverge, the load on them increases exponentially, and utilization The load capacity is plummeting.
At a branch divergence angle of up to 60 degrees, the load is distributed relatively evenly, and standard load-carrying tables can be used. However, when the angle is increased to 90 degrees, the load on each branch increases by about 40% compared to the vertical position. If the angle reaches 120 degrees, the load on each branch becomes equal to the weight of the entire load, which often leads to destruction. rigging.
Branch load formula
To calculate the load on one branch, the formula is used: S = (Q / n) * (1 / sin(ฮฑ)), where S is the load on the branch, Q is the weight of the load, n is the number of branches, ฮฑ is the angle of inclination of the branch to the vertical.
To minimize risks, it is recommended to use traverses that allow you to keep the branches of the slings in a vertical position even when lifting overall loads. This allows you to use the full load-carrying capacity slings without dangerous angular coefficients. If the use of a traverse is not possible, it is necessary to choose slings with a margin of tonnage corresponding to the maximum angle of divergence.
Types of slings and their load capacity
The modern market offers different types of slings, each of which has its own limitations and advantages depending on the weight of the cargo. Cable slings They are distinguished by flexibility and the ability to fit the load tightly, but are afraid of sharp edges and high temperatures. Their load capacity depends on the diameter of the rope and the weaving scheme.
Chain slings They have high strength and resistance to mechanical damage, which makes them ideal for working with rolled metal and products with sharp edges. They withstand higher temperatures, but have a lower safety margin compared to cable analogues. Textile slings (ribbon) light, do not damage the surface of the cargo, but require special care when working with abrasive materials.
When choosing the type of equipment, it is necessary to take into account not only the mass, but also the operating conditions: the presence of aggressive environments, temperature and frequency of lifting cycles. For the chemical industry, for example, the resistance of the material of slings to reagents is critical, while in metallurgy, heat resistance comes to the fore. The right selection branch-material It extends the life of the equipment and guarantees safety.
Safety factor and standard
The safety of lifting operations is regulated by strict standards that determine the minimum permissible safety margin. For general purpose slings, this ratio is usually at least 6:1 for rope and 4:1 for chain products. This means that the burst load must be several times higher than the workloadThe product specified in the passport.
Reduction of the stock factor can occur during operation due to wear, corrosion or mechanical damage. Regular defect The survey allows to identify critical changes in the structure of the material. If more than 10% of the broken wires are found on the rope at the length of the roll, such a sling is subject to immediate rejection, regardless of its original carrying capacity.
| Sling type | Min. stock-rate | Temperature limit | Resistance to UV |
|---|---|---|---|
| Cable (steel) | 6:1 | 400ยฐC | Tall. |
| Chain. | 4:1 | 500ยฐC | Tall. |
| Textile (polyester) | 7:1 | 80ยฐC | Low. |
| Textile (polyamide) | 7:1 | 100ยฐC | Medium |
It is important to understand that the stock ratio is not a recommendation, but a mandatory requirement. safety standards. Excessive workloads, even for a short time, can cause irreversible deformations in the structure of the metal or polymer fibers, which will lead to sudden destruction in the next lifting cycle. The marking on the sling tag should be clearly readable and the use of products without a tag is prohibited.
Algorithm of Sling Selection for Specific Tasks
The process of selecting equipment should be based on a consistent analysis of the parameters of the cargo and working conditions. First, the mass and dimensions are determined, then the sling scheme is selected and the load on the branch is calculated taking into account the angle. Only then is the type of sling selected with the appropriate nominal capacity. Violation of the sequence of steps often leads to errors.
Simplified tables can be used for standard cargoes, but individual calculation is required for unique objects. In this case, the presence of protruding parts that can damage the sling, or the need to use intermediate gaskets, is taken into account. Specialized seizures can be more effective than universal slings, if we are talking about the serial lifting of the same type of products.
โ๏ธ Checklist before climb
When selecting, it is also important to consider the length of the branches. Too short slings can create an excessive angle of divergence, and too long can lead to entanglement or uncontrolled swaying. The optimal length ensures a stable position of the load and a minimum angle of inclination of the branches to the vertical. Balancing Cargo in space is a key skill of a slinger.
Operation and storage of cargo-grabbing devices
Even a properly selected sling will not last long if the rules of its operation and storage are violated. After completion of the work, the tooling must be cleaned of dirt and oil, then removed in a specially designated place, protected from moisture and direct sunlight. Incorrect storage Textile slings, for example, can cause fading and loss of strength of the material.
Regular inspection allows you to identify early signs of wear: stretching of chain links, the appearance of "shagginess" on ropes or cuts on tapes. Any deviation from the norm requires the product to be decommissioned. Recording and periodic testing is a mandatory requirement for enterprises using the load-lifting machinery.
โ ๏ธ Attention: It is forbidden to build up slings by welding or twisting, as well as to use homemade devices without factory marking and testing. This is a direct safety violation.
Timely replacement of worn out equipment saves money in the long run, preventing downtime due to accidents and damage to expensive goods. Investments in quality slings from proven manufacturers pay off with reliability and long service life. Remember that Safety is always a priority for the cost of equipment.
Main conclusion: Selection of the sling by weight of the cargo is impossible without taking into account the angle of divergence of branches. Always choose slings with a margin of safety exceeding the design load by at least 1.5 times at angles of more than 60 degrees.
Frequently Asked Questions (FAQ)
How to calculate the load capacity of the sling at an angle of 90 degrees?
At a branch divergence angle of 90 degrees, the load on each branch increases by about 1.4 times compared to the vertical position. If you lift a load weighing 2 tons with two branches, then each branch will have a load of not 1 ton, but about 1.4 tons. Slings should be chosen to accommodate this increased load.
Can I use slings with damaged markings?
No, the operation of slings without a tag or with unreadable markings is strictly prohibited by safety regulations. Without a tag, it is impossible to confirm the load capacity, the date of the last test and compliance with the standards. Such products are subject to write-off or re-certification in a specialized organization.
How often should the slings be tested?
The frequency of the tests depends on the type of slings and operating conditions. A full technical survey is usually carried out every 12 months. However, a daily visual inspection before the start of the shift is a mandatory procedure for the slinger. If defects are found, the sling is immediately removed.
What to do if the weight of the cargo is unknown?
Cargo lifting with unknown weight is prohibited. It is necessary to use technical passports, invoices or weigh on car scales / crane scales before the main lift. In extreme cases, it is possible to make a test separation of the cargo to the minimum height to assess the weight according to the indications of crane devices, if they are calibrated.