The upper structure is the rotating part of a hydraulic excavator, cable excavator, or mobile crane mounted on the undercarriage (chassis). It contains all essential components: operator cab, drive engine, hydraulic pumps, counterweight, and the attachment of work equipment (boom, arm, bucket).
The rotating connection between upper and lower structures is a large-diameter slewing ring bearing with internal teeth, driven by one or more swing motors. For hydraulic excavators in the 20-ton class, the slewing ring typically has a diameter of 1,200–1,500 mm. The slewing speed ranges from 10–14 rpm.
The counterweight at the rear of the upper structure balances the working load on the boom. In short-tail and zero-tail excavators, the counterweight is optimized so that the rear overhang does not extend beyond the track width — essential for work in confined areas such as inner-city construction sites or building demolition.