mounted (fig. 9-54), or self-propelled wheel mountedfig. 9-55).These excavators are hydraulic powered and consistof three structures: the revolving unit, the travel base,and the attachment.REVOLVING UNITThe revolving unit rests and revolves on a turntableand is normally a rectangular steel deck that carries theengine, the pumps, the attachments, the controls, and theoperator’s cab. The center of rotation is usually forwardof the center of the revolving unit that places a majorpart of the revolving unit weight at the rear. This servesto counterbalance the weight and pull of the backhoewhen performing excavation operations.The swing axis is centered in the travel unit, so therear edge of the revolving unit overhangs. This overhangmust be accounted for when the revolving unit is rotatedfrom side to side to avoid hitting personnel, equipment,and buildings.The operator’s cab is either mounted to the right orleft of the boom and is the location of controls, gauges,and warning lights for all phases of operation. Someunits may have fixed or removable front and sidewindows and a roof window that is helpful in watchingout for and avoiding wires and tree branches. Thesewindows should be cleaned during the prestart operationand anytime an amount of dust and dirt that hasaccumulated on the window obstructs your vision.TRAVEL UNITThe excavator travel unit may be track (crawler)mounted, truck mounted, or self-propelled wheelmounted. Of the three, the most common mounting isthe track.Rack MountedTrack frames are single or double beams welded tothe outer ends of the dead axles in the car body. The carbody is a massive frame that includes the turntable andthe dead axles or cross members that transmit its weightto the track flames. The track may be the link-shoeconstruction that is made up of a number of identicalshoes cut and drilled at their ends, so they can befastened together by pins (fig. 9-56). Wedge-shapedprojections are cast into the upper surfaces of the shoesto provide a grip for the drive sprockets and to keep thetracks 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 withbolt-on shoes. Each linked pair is fastened together witha bushing at one end. A pin goes through the bushingand holds the overlapping ends of the next pair of links(fig. 9-57). The track is assembled on a hydraulic pressthat is able to force the slightly oversized pins andbushings into the links that very seldom work apart inservice. The pins turn easily in the bushing, providingthe necessary hinge action.The propel, traction, or travel drive may come froma pair of live axles set across the center of the car bodyor a pair of reversible hydraulic motors fastened to thetrack frame.Truck MountedThe revolving unit is carried on a turntable fastenedto a truck chassis. Some units may have an enginemounted in the revolving unit to provide power for theupper unit controls and a engine mounted in the truck tobe used for traveling. Some truck-mounted units mayonly have one engine used to power both the revolvingunit and truck.The truck-mounted excavator can ordinary swing ina full 360-degree rotation, but with most attachments, itcan work through only 270 degrees because ofinterference presented by the cab and the truck front.9-23
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