You've seen these on cheap toys, casters, and appliance shafts: a flat stamped-steel disc with internal teeth, pressed onto a plain shaft with no groove. No snap ring pliers, no machining, no torque spec — just push it on with a socket or a mallet, and it stays forever. These are push-on retaining rings, sometimes called starlock washers, Palnuts, or push-nuts, and they're one of the cheapest permanent fasteners in industry.

How they grip: The internal teeth are stamped at an angle, sloping inward toward the direction of installation. When you press the ring onto the shaft, the teeth flex outward slightly and bite into the shaft surface. Try to pull the ring off, and the teeth dig harder — the geometry is a one-way ratchet. The holding force comes from elastic spring-back of the teeth combined with metal-to-metal friction and localized plastic deformation of the shaft.

Design rules of thumb:

• Shaft must be softer than the ring. Push-on rings are hardened spring steel (Rockwell C 47–52). The shaft should be mild steel, brass, aluminum, or plastic. Try to press one onto hardened dowel pin and the teeth skate.
• Shaft diameter tolerance is loose — typically the nominal shaft size ±0.005". The teeth accommodate the slop.
• Holding force ≈ 2–4× the axial installation force. A 1/4" push-on that takes 40 lb to install will resist ~120 lb of pull-off before the teeth slip.
• Single-use only. Removal permanently deforms the teeth. To remove: pry, cut, or grind — plan on destroying the ring.

Real-world example: Look at the axle of a shopping cart caster. The wheel spins on a plain steel axle with no groove, no cotter pin, no threads. A push-on cap on each end holds the wheel captive. Assembly time: two seconds per side with a pneumatic press. Cost: pennies. It will outlast the cart.

When to specify one: Light axial loads (under ~100 lb for small sizes), non-serviceable assemblies, high-volume consumer goods, and any application where machining a retaining ring groove would double the shaft cost. Common on furniture casters, small motor shafts, appliance linkages, toys, and lawn equipment.

When NOT to use them: Anything requiring field service, high cyclic axial loads, precise axial positioning (there's typically 0.010–0.030" of play), or hardened shafts. Vibration and thermal cycling can walk them loose over years — fine for a $12 caster, bad for a safety-critical joint.