Lesson 1 the reading module

Read the text: A Bulldozer

A bulldozer is a crawler (caterpillar tracked tractor), equipped with a substantial metal plate (known as a blade), used to push large quantities of soil, sand, rubble, etc, during construction work. The term "bulldozer" is often used to mean any heavy engineering vehicle, but precisely, the term refers only to a tractor (usually tracked) fitted with a dozer blade. That is the meaning used herein.

The first bulldozers were adapted from farm Holt tractors that were used to plough fields. Their versatility in soft ground for logging and road building, lead directly to them becoming the armoured tank in the first war.

In 1923, a young farmer named James Cummings and a draftsman named J. Earl McLeod made the first designs for a bulldozer. A replica is on display at the city park in Morrowville, Kansas where the two built the first bulldozer.

By the 1920s, tracked vehicles became common, particularly the Caterpillar 60. To dig canals, raise earth dams, and do other earthmoving jobs, these tractors were equipped with a large thick metal plate in front. This thick metal plate (it got its curved shape later) is called a "blade". The blade peels layers of soil and pushes it forward as the tractor advances. Several specialized blades have been developed: for high volume loads such as coal, rakes to remove only larger boulders, or blades with razor sharp edges to cut tree stumps. In some early models the driver sat on top in the open without a cabin. These attachments, home built or by small equipment manufacturers of attachments for wheeled and crawler tractors and trucks, appeared by 1929, widespread acceptance of the bull-grader does not seem to appear before the mid-1930s, and the addition of powered down force made them the preferred excavation machine for large and small contractors alike by the 1940s, by which time the term "bulldozer" referred to the entire machine and not just the attachment.

Bulldozers grew more sophisticated as well. Important improvements include more powerful engines, more reliable drive trains, better tracks, raised cabins, and hydraulic (instead of early models' cable operated) arms that enable more precise manipulation of the blade and automated controls. As an option, bulldozers can be equipped with a rear ripper claw to loosen rocky soils or to break up pavement (roads). A more recent innovation is the outfitting of bulldozers with GPS technology, such as manufactured by Trimble Inc.or Mikrofyn for precise grade control and (potentially) "stakeless" construction.

The ripper is the long claw-like device on the back of the bulldozer. Rippers can come singly (single shank) or in groups or two or more (multi shank rippers). Usually, a single shank is preferred for heavy ripping. The ripper shank is fitted with a replaceable tungsten steel alloy tip.

Ripping rock lets the ground surface rock be broken into small rubble easy to handle and transport, which can then be removed so grading can take place. Agricultural ripping lets rocky or very hard earth be broken up so otherwise unploughable land can be farmed. For example, much of the very best land in the California wine country consists of old lava flows: with heavy bulldozers such as the Caterpillar D9 and the Caterpillar D11 the lava is shattered allowing practical agriculture for the wine industry. Also, hard earth can be ripped and decompacted to allow planting of orchards where otherwise trees could not grow on the land.

Sometimes a bulldozer is used to push another piece of earthmoving equipment known as a "scraper".