Lifting lugs are designed mounting points that help carry an object or equipment. In the world of engineering, industrial-scale lifting lugs move large containers and other heavy loads. They are also essential in the mobility of crates, containers, and other devices on large scale operations like oil rigs. In this article you will learn about how lifting lugs are designed, different types, and how they are fabricated, installed, and tested.
Lifting Lug Design
Lifting lugs are designed with the load and range of load that it can carry in mind. As the typical industrial applications of a lifting lug pose immense safety risks in the incident of failure, heavy load analysis calculations must be considered when making one. The load calculations take into consideration all the different ways a lifting lug may fail. Common modes of failure are tension, shear, bearing, and hoop tension.
Tension failure occurs along the axis perpendicular to the force of lift and must be considered along all the possible angles of lift, for which a maximum load may assigned. This load will designated the largest load the lug and shackle may carry safely at that specific angle of engagement.
Shear failure occurs on the axis parallel to the force of lift. In the case of shear failure, the size of the hook and the area it takes up on the inside of the lug must be considered for the material strength and dimensional requirements to prevent failure when lifting. If the hook is too small, the lug may fail as the hook may easily shear the lug or vice versa.
Bearing failure deals with the failure of the hook or shackle itself. It requires that the sizing and material of the shackle or hook be in accordance with the lifting lug to prevent any failures. Having adequate fill area in the void, sometimes called shackle engagement, ensures the hook does not move and encounter adverse forces during the lift.
Hoop Tension Failure
Hoop tension failure considers the possibility of the hook slicing right through the centerline of the lug. This consideration deals more so with the dimensions of the two corresponding with each other as well as the material strength of both to be sufficient for the weight they will carry.
Another important consideration when designing a lifting lug is the location on the vessel the lug will be placed. Calculating the locations to prevent any sort of buckling or unnecessary stress is crucial to safely operating with heavy loads.
Types of Lifting Lugs
The types of lifting lugs depend heavily on the application the lug is being used. For instance, there are detached lifting lugs which are secured by a bolt and attached lugs which are welded on. Another consideration would be the type of load being carried by the lifting lug.
For smaller lifts, lifting lugs may be bolted on or detached for easily removing and adjusting mounting points to lift the specified load. Any total weight below two tons is considered a small load.
For small lifts, such as dumping hoppers, lifting lugs maybe already welded onto the container or attached. These are ideal for items that will often be lifted, or in this case dumped.
For larger industrial scale lifts, lifting lugs may be detached for easy access, adjustments, and maintenance. The main difference for industrial-grade lift lugs is the construction of the lug for different lifting angles and for much larger loads.
Fabrication and Installation
For the large number of design considerations made when designing a lift lug, the fabrication process must be equally mindful of those requirements. The typical lift lug is made of steel for which the load of application would determine the type of steel. As the modulus of elasticity and stress associated with heat and cold are important factors to consider when fabricating a lifting lug, choosing a steel in accordance is crucial. The material yield strength is also an important consideration for choosing the type of steel used for a lifting lug.
The two main installations of a lifting lug are: bolt-on (detached) or weld-on (attached). When welding on a lifting lug, proper welds must be made to ensure security when lifting the load. Typically pressure vessels, tanks and hoppers are fitted with attached lifting lugs. For bolt-on lifting lugs, the typical application is on larger shipping containers and other heavy industrial equipment. Proper installation is crucial to prevent lug failure.
Lifting Lug Testing
To ensure the proper load-carrying ability of a lifting lug, they must be tested. According to OSHA, the occupational safety and health administration, lifting lugs must have a minimum five to one safety factor meaning that each lifting lug must be able to hold five times the amount of their safety rated load.
Load testing is required before a lug is sent out, although the test typically 1.25 to 2 times the rated capability. . Manufacturers must also specify the angle of lift required during the lift and the load rating for each angle. Similarly, stress tests as different temperatures must be conducted so that manufacturers may specify conditions in which the lifting lug may fail. Testing ensures safety and the proper operation of the lugs.