NOTE: The tag line winder may be used to controlthe twisting of the leads.4. Because these leads are not rigid, it is moredifficult to position the crane to set up for pile-drivingoperations.Underhung LeadsUnderhung leads are composed of exactly the samesections used for swinging leads. Underhung leads arebolted together on the ground, as described for swingingleads, and connected to the boom tip through the use oflead adapters (fig. 12-52). The boom tip sheaves areused to accommodate the pile and the pile-drivinghammer. All underhung leads have a standard bolt holelayout for bolting the lead adapters to the leads;however, the dimensions of the boom tip end of theadapters vary according to the make and model of thecrane. After the adapters are connected to the boom, theboom is raised to bring the leads to a vertical position(fig. 12-53). Long lead sections may require the use ofsupport equipment to raise the leads to a verticalposition.NOTE: Check the adapter bolts for tightness at thebeginning of each pile-driving shift.Adapter plates are mounted to the boom butt orcrane cab and on the bottom lead section for connectionof a fore-and-aft bottom brace, commonly known as acatwalk. The catwalk can be extended or telescoped tovarious lengths. It can be set to hold the leads verticalfor driving bearing piles or to hold them at an angle fordriving batter piles. In use, an underhung lead is heldby the boom at a fixed radius (fig. 12-54).Figure 12-53.-Underhung leads being raised by the boom.Figure 12-54.-Underhung leads.ADVANTAGES.— Some advantages of usingunderhung leads over other types of leads areas follows:1. They are lighter and generally less expensivethan the extended four-way type of lead.2. They require only a two-hoist drum crane.3. They provide accuracy in positioning leads invertical and fore-and-aft batter positions.4. They provide precise control of the leads duringpositioning operation.5. They reduce rigging time in setting up andbreaking down.6. They use boom tip sheaves.DISADVANTAGES.— Some disadvantages ofusing underhung leads are as follows:1. They cannot be used for side-to-side batterdriving.2. The length of pile is limited by boom length,since this type of lead cannot be extended above theboom tip.3. When long leads require the use of long boomlengths, the working radius that results may be excessivefor the capacity of the crane.4. They do not allow the use of a boom shorter thanthe lead.Extended Four-Way LeadsExtended four-way leads use the same intermediatelead sections as swinging and underhung leads. In placeof a 15-foot tapered section, an extended lead uses a12-39
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