EXCAVATORS

Excavators are large backhoes used for heavier construction tasting. The types used in the NCF are either track mounted (fig. 9-53), truck carrier

Figure 9-54.-Truck carrier-mounted multipurpose excavator.

Figure 9-55.-Self-propelled wheel-mounted excavator.

mounted (fig. 9-54), or self-propelled wheel mounted fig. 9-55).

These excavators are hydraulic powered and consist of three structures: the revolving unit, the travel base, and the attachment.

REVOLVING UNIT

The revolving unit rests and revolves on a turntable and is normally a rectangular steel deck that carries the engine, the pumps, the attachments, the controls, and the operator's cab. The center of rotation is usually forward of the center of the revolving unit that places a major part of the revolving unit weight at the rear. This serves to counterbalance the weight and pull of the backhoe when performing excavation operations.

The swing axis is centered in the travel unit, so the rear edge of the revolving unit overhangs. This overhang must be accounted for when the revolving unit is rotated from side to side to avoid hitting personnel, equipment, and buildings.

The operator's cab is either mounted to the right or left of the boom and is the location of controls, gauges, and warning lights for all phases of operation. Some units may have fixed or removable front and side windows and a roof window that is helpful in watching out for and avoiding wires and tree branches. These windows should be cleaned during the prestart operation and anytime an amount of dust and dirt that has accumulated on the window obstructs your vision.

TRAVEL UNIT

The excavator travel unit may be track (crawler) mounted, truck mounted, or self-propelled wheel mounted. Of the three, the most common mounting is the track.

Rack Mounted

Track frames are single or double beams welded to the outer ends of the dead axles in the car body. The car body is a massive frame that includes the turntable and the dead axles or cross members that transmit its weight to the track flames. The track may be the link-shoe construction that is made up of a number of identical shoes cut and drilled at their ends, so they can be fastened together by pins (fig. 9-56). Wedge-shaped projections are cast into the upper surfaces of the shoes to provide a grip for the drive sprockets and to keep the tracks centered on the idler and rollers.

Figure _9-56.-Link-shoe construction.

Figure 9-57.-Track links and bolt-on shoes.

The other type of track is the roller chain with bolt-on shoes. Each linked pair is fastened together with a bushing at one end. A pin goes through the bushing and holds the overlapping ends of the next pair of links (fig. 9-57). The track is assembled on a hydraulic press that is able to force the slightly oversized pins and bushings into the links that very seldom work apart in service. The pins turn easily in the bushing, providing the necessary hinge action.

The propel, traction, or travel drive may come from a pair of live axles set across the center of the car body or a pair of reversible hydraulic motors fastened to the track frame.