Page 375 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.37

FIGURE 8.27 Turns may save time.

Intermediate Hauls. Some hauls are too short for normal scraper use and too long for dozers.
A crawler dozer’s output falls rapidly as length of haul increases. It becomes uneconomical
somewhere between 100 and 200 feet on level ground, although it may be used for much longer
distances. Because of complications of turning and distances required to load and to spread,
scraper use generally starts at 200 to 400 feet. These figures are for large machines. Small bull-
dozers lose effectiveness at lesser distances, and small scrapers are operated on much shorter runs.
Because of their higher speed, rubber-tire dozers may be used for substantially longer pushes
than crawlers. They are less efficient in a short run as, except in the easiest digging, the loading
part of their cycle is longer.
Scrapers can often be used more efficiently on very short hauls than is generally believed.
They can waste considerable time deadheading to turns, and carry undersize loads because of
short digging runs, and still move much more dirt than a dozer.
When the fill is too short for proper spreading, part of the load can be carried around the turn
and dropped on the way back to the cut.
Two-wheel drag scrapers, such as are used for land leveling, can be used shuttle fashion and
in easy digging will move about double the load of a dozer blade. Large models are very wide so
as to be inconvenient to transport and unable to work with other machines in ordinary roadways.
Haul Speed. Scrapers have advertised top speeds from 20 to 30 and even 40 miles per hour.
However, the average construction job does not permit much use of these high speeds, and even
when they are used, the average speed of straightaway hauling is very much lower.
Top speed is limited by the space needed to attain it. A loaded scraper or other hauler in its
highest gear has little surplus power for acceleration, after taking care of the rolling resistance of