Page 33 - Moving the Earth_ The Workbook of Excavation

LAND CLEARING AND CONTROLS

LAND CLEARING AND CONTROLS 1.33

If not mounted on a truck or other carrier, the winch should have a V-shaped towbar, or a sub-
frame by which it can be anchored. Planks should be provided to build up a base in line with the
pull, since if the line of pull is high, the winch will be lifted off the ground and will not be steady
enough to allow turning the handle.

MULTIPLE LINES

Snatch Blocks. If pulling stumps takes the full power of the tractor or winch, it may be advis-
able to use snatch blocks to obtain greater power at slower speed. These devices, also known as
blocks and as pulleys, are pulleys set in frames that are provided with one or two round hooks or
rings, usually on swivel connections. For most field work, single pulley wheels, with a latch
attachment permitting insertion of a cable at the side, are best, as cables usually carry attachments
too large for threading; a tedious job even when possible.
These blocks can be obtained in sizes to match any cable or strain. In large sizes they are very
heavy, and several workers, or a loader, or light winch or other lines may be used to move them.
Figure 1.25 shows several riggings using pulleys. If the lines are approximately parallel, and
the pulley bushings lubricated, each additional line will add about 90 percent to the single line
pull. This puts no extra strain on the winch cable, but the chokers holding the blocks must take
the combined pull of all the lines fastened to them.
The advantage obtained from the use of a block is decreased when the lines are not parallel,
becoming zero when the angle between the lines is 114 degrees. Still wider angles result in loss
of power.
The number of lines that can be put on one stump may be determined by the number of blocks
on hand, the space available for fastening them, the strength of the available anchorage, or the
amount of cable. Light machines may use six or eight blocks on a heavy stump.
Rigging is simplified by the use of series and sling blocks, as in Fig. 1.26A. The tractor line
passes around a block pulley to an anchor, doubling the pull at the block frame. This is attached

FIGURE 1.25 Stump-pulling layouts.