The term deflection can be formally defined as any kind of deviation from an allocated or known course. Deflection can refer to something being deviated by coming off its course or even by placing it aside or any other possible movements. It may not seem like a very impactful word, but when we give it the context of a crane and its design, it becomes immensely significant. Overhead cranes have a lot to do with deflections, and hence it is considered a primary parameter for consideration.
Deflections and overhead cranes
When we talk about overhead cranes, the definition of deflection would slightly differ in its meaning. It would refer to the relative horizontal or vertical displacement of a part or member of the overhead crane. Now one might think, how can this be so impactful? Well, to understand that, we need to learn more about the terms horizontal and vertical deflections and how they can be calculated. Furthermore, it is also imperative to learn how specifications related to organizations differ from one crane to the other and also from one association to the other. Fortunately, this article deals with just that.
What is Vertical Deflection in Overhead Crane?
Okay, let us start with the basics. Deflection Criteria can be defined as the highest amount of vertical deflection ratio that can be allowed for a lifting device; in our context, the overhead crane. Needless to say, that horizontal deflection varies from vertical deflections in the considered scale. Yet both aspects must be considered in the case of enclosed track bridge cranes. Any part of the overhead crane standing vertically is directly impacted by vertical deflection. This includes parts such as the wall, column, mast, etc.
The majority of the systems are manufactured using an approximate deflection as the manufacturers have little command over the installation procedure. Hence they cannot really control the foundation rigidity, standard variation in thickness tolerances for the piping, steel plate, tubing, and sheet metal. That is why some of the variations below or above deflections are manufacturer defined and has to be accepted as normal. Having said that, the safety level of the lifting products and their ability to handle loads are maintained at their optimum levels when we follow the standard manual while installing overhead cranes and the manufacturer’s installation maintenance manual while maintaining it. Performance standards are also improved in this process.
The vertical deflection criterion remains the same for most systems, although the terminology might differ. For enclosed track workstation overhead cranes, the deflection limit can be calculated using the term L/900; L is the length of the crane. Hence if we consider an overhead crane having a length or span of 25000 mm, then its deflection limit would be equal to 27.8 mm.
What are Horizontal deflection criteria?
It is referred to as the maximum amount of deflection ratio that can be allowed for an overhead crane or the runway. True to its nomenclature, the horizontal deflection criteria affect the horizontal parts of the overhead crane. This has to be considered for track systems that are enclosed, such as workstation jib booms and bridge cranes. Now let us get into numbers.
The majority of crane manufacturers design a max value of horizontal deflection for overhead cranes of L/750. The L here stands for the span of the crane. The deflection limit is considered to be 750. For instance, if we consider a span of 25000 mm, then its horizontal deflection would be 33 mm.
Other requirements for overhead crane manufacturers
Manufacturers are obliged to adhere to many more standards when it comes to manufacturing overhead cranes. They have to take into account many theoretical considerations and make their systems undergo multiple checks such as vertical and horizontal deflection analysis, stress analysis of its different parts. The basic requirements for these systems are that they should conform to the theoretical understanding, and their structural response (static) should be able to preserve the original crane structure’s response. In such a scenario, the system will pass the tests.
The rigidity requirement with respect to horizontal deflection is one such test, which is imperative to prevent oblique movement of the crane.
Load tests
All overhead cranes that have been installed post-January 1999 or have been modified majorly after that, they have to be mandatorily are load-tested before it can be put to service. IS 3177 and IS 807 defined deflection tests instruct that structural deflections have to test with 100 per cent rated capacity loads. If it then does not exceed the allowable deflection measure as defined by the design standard applicable, it shall be passed for usage. Furthermore, the load has to travel over the entire length of the runways and bridge throughout these load tests.
Conclusion
Vertical and horizontal deflection criteria are a significant part of every overhead crane’s operating principle. Various tests are performed, and the cranes are calibrated henceforth before they can be certified for usage in the factories and warehouses. The criteria help us to ascertain specific parameters of these tests and provide us with standards of performance and safety the cranes have to adhere to.
