NOTE: The tag line winder may be used to control
the twisting of the leads.
4. Because these leads are not rigid, it is more
difficult to position the crane to set up for pile-driving
operations.
Underhung Leads
Underhung leads are composed of exactly the same
sections used for swinging leads. Underhung leads are
bolted together on the ground, as described for swinging
leads, and connected to the boom tip through the use of
lead adapters (fig. 12-52). The boom tip sheaves are
used to accommodate the pile and the pile-driving
hammer. All underhung leads have a standard bolt hole
layout for bolting the lead adapters to the leads;
however, the dimensions of the boom tip end of the
adapters vary according to the make and model of the
crane. After the adapters are connected to the boom, the
boom is raised to bring the leads to a vertical position
(fig. 12-53). Long lead sections may require the use of
support equipment to raise the leads to a vertical
position.
NOTE: Check the adapter bolts for tightness at the
beginning of each pile-driving shift.
Adapter plates are mounted to the boom butt or
crane cab and on the bottom lead section for connection
of a fore-and-aft bottom brace, commonly known as a
catwalk. The catwalk can be extended or telescoped to
various lengths. It can be set to hold the leads vertical
for driving bearing piles or to hold them at an angle for
driving batter piles. In use, an underhung lead is held
by 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 using
underhung leads over other types of leads areas follows:
1. They are lighter and generally less expensive
than the extended four-way type of lead.
2. They require only a two-hoist drum crane.
3. They provide accuracy in positioning leads in
vertical and fore-and-aft batter positions.
4. They provide precise control of the leads during
positioning operation.
5. They reduce rigging time in setting up and
breaking down.
6. They use boom tip sheaves.
DISADVANTAGES.— Some disadvantages of
using underhung leads are as follows:
1. They cannot be used for side-to-side batter
driving.
2. The length of pile is limited by boom length,
since this type of lead cannot be extended above the
boom tip.
3. When long leads require the use of long boom
lengths, the working radius that results may be excessive
for the capacity of the crane.
4. They do not allow the use of a boom shorter than
the lead.
Extended Four-Way Leads
Extended four-way leads use the same intermediate
lead sections as swinging and underhung leads. In place
of a 15-foot tapered section, an extended lead uses a
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