CHICAGO BOOM DERRICK BASICS AND TUTORIALS

CHICAGO BOOM DERRICK BASIC INFORMATION
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.

THE BASIC HOISTING MECHANISM BASICS AND TUTORIALS

THE BASICS OF HOISTING MECHANISM
What Is Hoisting? How Hoisting Works?

Figure 1.1 shows a lifting device with a load attached to the lower block and the block in turn supported by two ropes, or parts of line, suspended from the upper block. Each rope must therefore carry half the weight of the load; this gives the system a mechanical advantage of 2. Had the load been supported by five ropes, the mechanical advantage would have been 5. Mechanical advantage is governed by the number of ropes actually supporting the load.

As parts of line are added, the force needed to raise or lower the load decreases, and load movement speed decreases as well. The blocks contain pulleys, or sheaves, so that the rope is in one continuous piece from the end attached to the upper block to the winding drum.

This makes the force in all parts of the rope uniform in a static system. The value of the rope load is found by dividing the weight of the lifted load by the mechanical advantage; in Figure 1.1 the lifted load would include the lower block, sometimes called the hook block.

When the distance between the upper and lower blocks is great, it is necessary to include the weight of the parts of line as well. The load in the rope is also equivalent to the force that must be generated at the winding drum in order to hold the load.

The effects of friction come into play as soon as the system is set into motion. Friction losses occur at the sheave shaft bearings and in the wire rope itself, where rope losses result when the individual wires rub together during passage over the sheave.

These losses induce small differences in load between each rope segment (i.e., each section of rope from sheave to sheave). The loss coefficient can vary from a high of about 4½% of rope load for a sheave mounted on bronze bushings to a low of as little as 0.9% for a sheave on precision ball or roller bearings. An arbitrary value of 2% is a reasonable approximation for sheaves on common ball or roller bearings when the rope makes a turn of 180°.

The tension in the rope at the winding drum is different when the load is raised and when it is lowered. Friction losses are responsible for this difference. When load-weighing devices that operate by reading the tension in the line to the drum are used, the variation is readily observed.

When an unloaded hook must be lowered, lowering will be resisted by friction, by the weight of the rope between the upper block and the deflector sheave, and by the inertia of the winding-drum mass. Mechanical advantage works in reverse in this case, as a mechanical disadvantage so to speak, so that the weight at the hook must exceed the rope weight multiplied by the mechanical advantage plus an allowance to overcome friction and inertia.