Page 369 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.31

Snatch tractors show good results with experienced crews, but most contractors are better off
if they stick to pushing.

Scraper-Pushers. Scrapers may be equipped with pusher plates or blades so that they can push
each other to help in loading. This arrangement is usually not as efficient as using a separate push-
er. A scraper cannot deliver nearly the push of a tractor of the same price. There is a chain load-
ing effect that requires a long cut or better-than-average supervision.
There are a few jobs on which scraper-pushers may work out well. They may save or postpone
a big investment in a crawler tractor, or fill in time while a pusher is out of service.

Graders. A heavy grader equipped with a dozer blade may make a good pusher, and may be
used for this purpose in an emergency. Under most conditions, however, it is much more valuable
as a first-rate finishing and maintenance machine than as a second-rate pusher.

Loading on Grades. The effect of gravity is to pull a load going downhill to the extent of about
20 pounds per ton of weight per percent of grade. An uphill load will be held back to the same extent.
This will cause a pusher weighing 25 tons to gain or lose 1,000 pounds in net drawbar pull on
a 2 percent grade, while a 50-ton loader scraper will be affected doubly.
The difference between a 2 percent upgrade and 2 percent downgrade will be thousands of pounds
of pull, maybe 20 percent of the pusher’s power. Using the rule of thumb that 1 pound of push
produces 1 pound of dirt per minute in the scraper, direction on this grade would mean a difference
of 3 tons or 2 yards of earth in the load, or a proportionate amount of time in obtaining the load.
It is therefore profitable to keep the loading down grade even on a gentle slope, whenever job
conditions permit.

Pushing. The pusher is driven up behind the scraper in low or sometimes in second gear, and
contact with its bumper is made as smoothly as possible. The scraper unit, in low gear, applies as
much power as it can without spinning the wheels or drawing away from the pusher.
The pusher moves as fast as it can without making the scraper jackknife or either unit twist
sideward. Twisting or off-center pushing is likely to cause a dozer blade edge to rip into a tire.
Most pushing is done in low gear because it is important to keep going right through the pass.
Second gear may be used to advantage when the pusher is overpowered for the size of the scraper
or the load that it is to carry. There is an extra hazard of damage from making contact too roughly.
Cushion dozer blades may be used to reduce the shock.
Patterns. The simplest pusher loading pattern is called backtrack or shuttle loading. See
Fig. 8.23(A). The scraper drops its bowl at the beginning of the cut, and the pusher makes con-
tact. The scraper pulls and the pusher pushes until the desired load is obtained. The scraper bowl
is raised, informing the pusher operator that he or she is not needed, if the operator has not already
learned this from watching the load. The scraper tractor then shifts into higher gear and departs.
The pusher returns to the beginning of the cut. It may make this move in reverse, or by turn-
ing and using forward speeds. The length of the cut and the relative speeds the machine can make
in the highest usable reverse and forward speeds determine which should be used.
If the next scraper gets in loading position before return of the pusher, it will start to load itself,
as it can pick up part of a load readily, and will thereby decrease the distance the pusher must
come back in order to get behind it.
Another pattern, shown in (B), is suitable only for long cuts. After loading the first scraper, the
pusher waits for the next empty one to come up alongside it, pushes that until it is full; pushes the next
from its stopping point; and so on until the end of the cut is reached. The pusher is then turned and run
back to the beginning of the cut to start another series.
A third system, which is useful where there are more scrapers than the pusher can readily han-
dle, or where the dirt can be moved in two directions, is outlined in (C). Each scraper is loaded
moving in the opposite direction from the previous one, so that the pusher need only turn around
to be in position for the next push, instead of having to move back to a starting place.