There are numerous reputable suppliers that provide practical help, but fail to explain the basics, especially to someone purchasing lifting heavy equipment for the very first time. This article will try to provide simple advice on the different types of construction equipment available on the market, to aid the newcomers in choosing the appropriate equipment for a certain application. This time the center stage belongs to slewing crane, or swing jib crane as it is often called. Although often referred to as crane, slew crane is only a structure that offers support to a lifting machine, like electric hoist or chain block.
A slewing crane comprises a horizontal runway track cantilevered out from a vertical “king post” placed in bearings. The bearings and king post allow the runway track to slew. The machine is generally fixed on a trolley which moves along the horizontal runway track, also known as jib arm. There are many variations of jib crane models and here are the main options to deal with. First is the supporting method of the jib arm. The simplest is for the bearings to be attached right to an existing structure like a wall or building column. A wall mounted crane will be rotate up to 180 degrees, while column mounted will be limited to 270 degrees of slew.
Secondly, if the king post tilts from the vertical, the jib arm will have tendency to slew into the downhill position. An option to the column or wall mounted slewing jib is free standing jib crane with its own column. Usually, these types require special foundations of depth to withstand the flip over moment. Only short radius cranes with lowest capacity can be fixed right to the floor, but only if the floor is suitable and the conditions are good. The column of a free standing crane can be cast right into the foundation which also makes this the cheapest option. As well it keeps the space need around the base to a minimum. Like a mounted jib crane, the curve of slew of a free standing jib can rotate up to 270 degrees. Another option using a tubular column provide a total 360 degrees of slew.
Back to the jib arm, there are two design options, under-braced and over-braced. This relates to how the arm is supported. An over-braced one has a tension member from the king post to a point about 2/3 of the way down the jib arm. The benefits are a lighter structure which allows the machine to get as near as possible also giving maximum effective travel. An under-braced jib has the cantilever support below the runway beam. This keeps the headroom needed to the minimum but decreases the maximum effective travel and necessitates a heavier rolled beam.
A final consideration is the duty or classification of the slew cranes. They are divided into few classes base on the maximum number of operating cycles and the combination of the state of loading. Generally, the classification is to ensure a normal working life of the slewing crane.
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