What Is A Chicago Boom Derrick?

Chicago Boom Derrick
A Chicago boom can be mounted on a building frame during or after construction, on a tower, or on any frame. Indeed, Figure 1.19 shows one installed on a power plant stack.

When a boom or strut is assembled in the form of a Chicago boom, it can range from as little as 10 ft (3 m) to as much as 125 ft (38 m) in length, and capacities can range from a low of, say, ¼ ton (225 kg) to a practical upper limit of perhaps 35 tons (32 t).

In the not too distant past, booms were made of wooden poles. Short lightweight booms are easily and inexpensively made of single steel pipes fitted with the necessary attachments, but most booms are trussed, or latticed, structures of angle irons or tubing or a combination of the two. Aluminum and synthetic composite booms are plausible, too, where site conditions favors such unconventional materials.

The topping lift usually employs ordinary hoisting blocks; one is fitted at the boom tip held off with steel straps while the opposite end is mounted at the pivot fitting on the support structure also with straps.

The upper load block may consist of sheaves built into the boom head, or it may be a common block suspended on straps.  The purpose of straps is to allow clearance between the rope suspension system and adjoining boom elements through the full range of luffing motion.

A Chicago boom is able to hoist materials to a height above the boom foot, with the horizontal reach limited by the length of the boom and the swing arc by the host structure. Swing guys often are fitted to the boom tip and are run laterally on each side to a point of anchorage.

Wind, friction at the pivots, and the resistance of the opposing guy must be overcome when pulling on the line to swing. With a manual arrangement, the guys are fiber ropes arranged with several parts of line. Hand pulling through several parts can take several minutes to swing the boom through 90°. Where production economics justifies the expense to attain greater speed, mechanical swing systems are used.

In the typical installation, a two-drum winch is used to power the hoisting and topping motions, but when the work involves only lifting and swinging, the topping motion will not be needed. A fixedrope guy line can then be installed, or to make adjustment easier and to provide flexibility.

Visual control can be established when the winch is located at the floor level of the boom foot, particularly when the loads are to be hoisted to this floor, but the winch can be located at any level. When the winch is too large or too heavy to be lifted in the job-site material hoist or in the elevator of an existing building, it may be necessary to use a small temporary winch to operate the derrick in order to hoist the working winch.

Alternatively the winch can be positioned on the ground. When the winch is on the ground, the operator has direct communication with the ground crew and can have the boom and load in view at all times.

When the winch is on the same floor as the boom foot and the loads are to be hoisted to that floor, the operator has direct communication with the swing and load-landing crew and has the boom but not the load in view at all times.

Landing the load is often the most difficult part of the operation, so that direct and immediate communication at landing level is a distinct advantage. With less rope weight to overcome, overhauling weight can be reduced when the winch is at the higher elevation.

A Chicago boom is not a high-production machine. Its capital cost is very low and installation costs are moderate, but its crew size is large in relation to productive capacity, and its reach is limited. Typical applications are “on again, off again” rigging work or situations where the load placement is above the reach or capacity of mobile cranes. With quick-setup telescopic cranes reaching ever higher, the demand for these lifting machines has diminished.

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