Page 104 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.54 THE WORK

Dozer Blade. The capacity of a bulldozer blade
cannot be computed exactly. The blade forms only
one of the six sides of a shapeless payload. Since the
load must slide or roll along the ground, friction is a
very important limiting factor on load size. Materials
with low internal friction or light weight, and down-
hill pushing favor big loads. Pushing through a slot
or between windrows, or with the blade almost
touching that of another dozer, increases load by
reducing side spillage.
A good average load may be said to be 20 per-
cent more than would be measured by the full width
and height of the blade, with a forward slope of 1-
to-1. The actual load would be higher in the center
and skimped in at the corners, averaging out as
above. See Fig. 2.46.
The blade on a 25-ton bulldozer might be 12 feet
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wide and 4 ⁄ 2 feet high. The profile of the theoretical
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load would be a right triangle 4 ⁄ 2 by 4 ⁄ 2 , with an
area of 10.25 square feet. Multiplying this by the
12-foot blade width, we have 123 cubic feet.
FIGURE 2.46 Bulldozer blade load.
Adding 20 percent for center bulges, this would be
148 cubic feet or 5.5 loose cubic yards. Different
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manufacturers rate blades of this size at 5 to 7 ⁄ 2 loose yards.
Under very good conditions the load might average 6 inches higher than the blade top, and
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extend forward on a 1 ⁄ 2 -to-1 slope. Then the profile would be 5 feet 7 ⁄ 2 feet or 19 square feet.
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Multiplying by 12-foot width, we get 230 cubic feet or about 8 ⁄ 2 yards loose.
Capacity of a blade is greatly affected by grade. Downhill the load tends to slide along with min-
imum pressure; uphill its friction rises and it spills more off the sides. Possible load, both from stand-
point of power to push it and ability to keep it in front of the blade, increases about 4 percent for each
percent of down slope. That is, a dozer might push double its normal load down a 25 percent grade.
Uphill the load falls off at first almost as rapidly, but the decline slows with steeper grades. A
half load might be pushed up a 20 percent grade, and a quarter load up 100 percent.
When a job is in progress, dozer blade capacity can be checked by counting the number of full
passes required to dig a known yardage of a bank, or by counting loads pushed into a pile and mea-
suring the pile.
The first result will be in bank yards, the second in loose yards.
Container Efficiency. A machine may go through many production cycles with less than its
rated load. The ratio of the container’s capacity to its actual load in loose yards is called the effi-
ciency factor of the bucket, bowl, or body.
An excavator may not be able to load its bucket or bowl fully because of hard digging, inade-
quate power, improper design, dull teeth, traction, heavy material, operator’s haste or careless-
ness, or combinations of these and other unfavorable conditions.
A truck may be run partly empty to enable it to climb steep grades, to reduce strain on rough
ground, to cross soft ground, to increase haul speed, or because of poor mechanical or tire conditions.
When material is coarse in proportion to bucket size, chunks may be partly supported by the sides
and each other, leaving excessive voids and reducing the actual load below that indicated by the
capacity of the container and the bulk of the load. The container efficiency factor (CEF) formula is
material in container
CEF
rated capacity of container
The amount of material in the container may be determined by careful measurement and/or
weighing of a number of individual loads, or by measuring either the bank or the fill to determine
the amount of material moved in a certain number of cycles.