A snap ring (or retaining ring) is a hardened spring-steel ring that seats into a machined groove on a shaft or inside a bore, providing a shoulder that axially locates a bearing, gear, or bushing. It replaces a shaft shoulder, threaded collar, or press fit — with a single low-cost part installed in seconds.

Two families dominate:

• External rings (shaft rings) — sit in a groove on a shaft and act against a part outboard of the groove. They compress radially inward for installation.
• Internal rings (bore rings) — sit in a groove inside a bore and act against a part inboard of the groove. They expand radially outward for installation.

Within each family, the common styles are:

• Tapered-section rings with lug holes (the classic "circlip," per DIN 471/472) — installed with snap-ring pliers. Tapered cross-section keeps the groove pressure uniform.
• Constant-section spiral rings — wound like a watch spring, no gap, no lugs, higher thrust capacity in the same groove.
• E-clips and C-clips — push on radially from the side; ideal when the shaft end isn't accessible.
• Bowed rings — deliberately dished to preload the retained part axially, eliminating end play.

Real-world example: Every automotive wheel bearing hub, every automatic transmission planetary carrier, and virtually every gearbox retains its internal bearings and thrust washers with snap rings. Pull apart a cordless drill's planetary gear head — you'll find an internal snap ring holding the ring gear into the housing.

Rule of thumb — thrust capacity: A properly seated tapered ring in a groove sized per the manufacturer's spec can handle an axial load roughly equal to the shear strength of the ring cross-section acting on the groove wall. As a rough estimate for a standard carbon-steel external ring on a hardened shaft:

Allowable thrust ≈ π × D × t × 40,000 psi, where D is shaft diameter and t is ring thickness. A 1" shaft with a 0.042" ring handles roughly 5,300 lbf — but only if the groove corner is sharp and the mating part has a chamfer no larger than the ring's own corner radius. A worn or radiused groove corner will let the ring roll out under a fraction of that load — the #1 failure mode in the field.

Watch out for: ring rotation at high speed (centrifugal force can dislodge unretained gaps — use spiral or self-locking rings above ~3,000 RPM), reused rings (they take a set and lose retention force), and grooves cut with a worn tool that leaves a bottom radius instead of a square shoulder.