From Boundary to Hydrodynamic Lubrication
In a plain bushing, the leader pin shaft contacts the bore surface directly (boundary lubrication). Even with grease, metal-to-metal contact occurs at microscopic peak points, causing wear. Oil grooves change this by creating channels that continuously supply oil to the contact zone.
As the pin slides, the groove geometry acts like a pump — pulling oil from the groove into the narrowing gap between pin and bore. When the oil film pressure exceeds the contact pressure, the surfaces separate entirely (hydrodynamic lubrication). The friction coefficient drops from 0.15 to 0.005 — a 30x reduction.
Groove Pattern Comparison
Helical: Best for high-cycle molds. The spiral path provides continuous oil supply during both insertion and withdrawal. Axial: Simplest design, gravity-fed. Adequate for standard molds. Cross-hatch: Best for intermittent operation — diamond pockets trap oil during idle periods.