LEXICONMachine components
Pivot pin
Also called: Boom pin, bucket pin, hinge pin
A hardened ground steel shaft that forms the hinge between two structural members of a machine — boom to arm, arm to bucket, cylinder to mount — carrying the entire load through a single axis.
What it does
Every articulation on an excavator is a pin joint, and each pin carries the full working load in double shear while rotating slowly under that load. The steel is usually case-hardened or induction-hardened with a tough core, then ground to size, and runs in a bronze or composite bush that is greased on a schedule. Pin diameters on mid-size machines run from 60 to 120 mm; larger machines use pins that two people cannot lift.
Why it becomes an object
A pin is a pure cylinder, ground and polished, with grease drillings, a retaining flat and often a keeper plate boss at one end. The contrast between a mirror-finished shaft and the crude fabrication it joined is stark, and it is the whole reason a pin reads well as an object. They are retired on ovality or scoring, at which point the wear is visible as a flat on one side — evidence of load direction, since a pin rotates through only a limited arc and always carries load in the same quadrant.
