Source: scholar.lib.vt.edu/
The scraper box was one of the simplest longwall-type systems. Originally
scraper boxes were used simply as haulage units on hand-worked longwalls in
moderately thin seams. The scraper box was made up of a box that is open at the top,
front, and bottom. A scraper blade was hinged into the rear of the box. When drawn
forward the blade took on a closed position preventing the contents from leaving the box.
When the box was drawn backwards the blade would rotate into an open position
allowing the box to pass over any objects in its path. To give the system more reach,
multiple scrapers were usually employed on the same wall. Each scraper carried its
contents to the end of its section of wall, dumping it where the next scraper could pick it
up and pass it along to the head gate of the wall.
One of the premier scraper box systems was the Haarman scraper box, which
utilized a heavy skid board to press the scraper box against the face. The caused the box
to take shallow cuts off the face each time it passed over it. In later systems the skid
board was removed and a heavy-duty chain that ran the length of the wall took its place.
In these methods the ends of the walls were kept slightly ahead of the center, in a bow
shape, to facilitate the movement of the chain. Since the tension in the chain kept the box
against the face cutting coal, this system came to be known as the "chain tension scraper
box" (Fig 4.5). By removing the skid board, the job of constantly moving the board was
also eliminated, hence greatly improving efficiency. The only job that remained for
personnel was the installation of roof supports. (Clark, 1982)
This system, despite all of its advances, still required the use of personnel on the
working face, to install roof supports. This gave the system a minimum seam height of
16 inches. The pulling forces needed to move the boxes on the face required a large
winch to generate them. Skid boards prevented easy access to the scraper boxes.
In Germany this system produced five tonnes per man shift. The tension used in
the chain varied from 4,000 to 8,000 pounds. This system had many problems that kept it
from being a widely used system. The shape of the wall coupled with the use of the
chain did not provide a sufficient normal force to generate an adequate rate of advance.
Also the bow shaped wall had extra stress accumulated on the lagging section of the wall.
This set up also limited the lengths of the wall and greatly hindered the economic
potential of this system. (Clark, 1982)
In any system where caving is desired behind the working section, it is important
to have a straight break line along which the roof can fail. Curved faces do not provide
this straight break line and make it more difficult for the roof to fail. If the roof fails to
break in a timely fashion, excessive stresses can accumulate on the supports. Keeping the
working section straight, or on-line, maintains responsive, predictable caving and
facilitates smooth operation of the system.
Figure 4.5 Chain Tension Scraper (Clark, Cauldon, and Curth, 1982) reproduced with permission