Text 3. Excavators

Match the following words and translations.

The universal excavator is an earth-moving machine that can be equipped with different attachments known under the following names: 1. shovel; 2. dragline; 3. backacter or backhoe; 4. grab or clamshell; 5. crane.

Conversion from one type to another is a comparatively easy operation.

Some decades ago only rope-controlled excavators were in use. Now there exists a tendency to replace the latter by more progressive hydraulically operated excavators. Especially widely used are now hydraulic backhoes.

In hydraulically operated backhoes hydraulic rams are used in place of ropes and winches to operate the boom and the dipper arm. The hydraulic system makes it possible to pivot the bucket on the dipper arm. This movement provided by an additional ram on the dipper arm gives the operator a means for selecting the proper angle of cutting for the bucket both at the start of the digging stroke and during the stroke to obtain optimum digging performance.

With the wheeled excavator, which is considerably lighter than the crawler mounted one, means to provide additional stability are incorporated. Before starting digging operation outrigger arms are operated hydraulically. Independent movement of the outrigger arms provides for leveling the machine on sloping ground, so that trenches with vertical sides can be cut perpendicular to the direction of the slope. In some models the revolving superstructure of the machine can be moved transversely in special guides, so that a trench can be dug parallel and close to a wall or some other obstruction, this movement being carried out hydraulically too.

A recent model incorporates a telescopic dipper arm: when extended it can dig deeper and has a greater discharge height and when retracted it produces its greatest

digging force at the bucket.

At present excavators are available in which all the digging motions as well as slewing are operated hydraulically. The crawler-mounted excavator has two other advantages over its rope-operated counterpart: with separate hydraulic motors for each track it is possible to put one track in reverse while the other is driven forward, so that the machine can be turned within its own length, and the boom can be of the adjustable radius type, so that the reach, digging depth and discharge height can be quickly varied to suit the job.