How an excavator turns without moving its tracks
The upper body of an excavator—the cab, engine, boom, and arm—sits on a large circular bearing above the tracked undercarriage. The bearing, often called a slewing bearing, carries the upper body’s weight while allowing it to pivot around a vertical axis. It is built to handle not just weight, but also the shifting forces created as the boom reaches out or lifts a load.
A hydraulic motor turns a small gear against a toothed ring attached to the rotating structure. That gear action swings the upper body around the bearing while the tracks can remain planted. The operator can therefore dig beside the machine, then turn to place the soil elsewhere without repositioning the entire vehicle.
Keeping power and control connected
The upper body still needs to receive power and send hydraulic force to the equipment below it. A central rotary joint—also called a swivel—lets hydraulic fluid pass between the stationary undercarriage and the rotating upper body without twisting the hoses into a knot. The bearing handles the mechanical load; the swivel keeps the connections working as the machine turns.
Together, these parts make repeated swinging practical. Instead of backing up after every scoop, an excavator can stay in one position and cycle between digging and dumping. That saves movement in tight work areas and helps keep the tracks on firmer, more predictable ground.
