Purchasing a crane based solely on capacity will result in most installations being either over worked or under worked. The number in the specification sheet is the beginning in determining what is actually required.
Consider the heaviest single lift the crane will ever have to make. Design to the average lift, and the crane will be insufficient for the day that a significantly larger load will need to be handled. It is generally impossible to increase the capacity of a crane by swapping out one of the components. It will be necessary to consider what lifting accessories will be used. Instead of financial resources on a crane with just enough capacity, design cranes to handle larger loads.
Consider what the load will be doing beyond just sitting on the ground. The loads on the crane will be much larger if the weight is being lifted in a stopped position, or if the weight is being stopped after a sudden swing. A crane that is designed without taking the dynamic factors into consideration will be reaching its limits on all lifts.
Next, we look at headroom and hook height, which are metrics that are easy to miscalculate. You cannot assume that the height of the building is the same as the height available to lift. The crane, the hoist, and the headroom required above the load all take up that space along with the load itself. The best way to avoid designing a crane that barely fits in a building but doesn’t have the capability to lift the load to the desired height, is to measure the clearance between the roof and the building floor and work from that.
Runway length and span are just as important as capacity, particularly at an overhead system. If the runway does not extend to the full working area, much of the purpose of installing a crane is lost, since the load must still be manually and/or mechanically moved the last distance. Span entails more than reach as well. A wider span requires stronger, heavier beams to avoid excessive bending, which alters the structural requirements of the crane and the building.
The most frequently overlooked specification is the duty cycle. This determines the strength of the construction of the crane rather than its lifting capability. A crane that only performs a small number of lifts in a day would utilize lighter-duty components than a crane that performs lifts continuously throughout the workday, even if both cranes are built to the same lifting capability. This is normally referred to as a duty rating by hoist manufacturers. This describes the number of starts the motor and brake can handle along with the amount of time the motor and brake are expected to be engaged in a given hour. If light-duty equipment is purchased for a constant heavily used application, that equipment will have a very short useful life. On the other hand, heavy-duty equipment purchased for a constant lightly used application is overspending for the application.
The floor loading is most frequently overlooked with mobile gantries or floor mounted jib cranes. The legs or the base of the cranes carry the full weight of the crane along with the load in a very small footprint. Not every floor is capable of supporting that concentrated weight without additional supports or spreading plates.
The floor loading, the span, the headroom, the dynamic allowance, duty cycle, etc. must all be correctly specified along with the heaviest load to determine the actual working load limit of the crane. If any of those elements are incorrect, while the crane may be capable of lifting the specified load, lifting that load is not done in theory.