Lock nuts and retaining rings are the two workhorses that stop assemblies from walking apart on a shaft, but they do it in completely different ways. A lock nut generates axial preload on a threaded fastener and resists vibration through friction, deformation or a secondary locking element. A retaining ring sits in a machined groove and axially captures a hub, bearing or spacer — no threads required, no torque to set. The two are sometimes mis-specified as alternatives when, in practice, they are complementary.
Specifiers usually ask the wrong first question. The right first question is: "what joint am I locking?" If the joint is a threaded stud, bolt or shaft, you are in lock nut territory. If the joint is a pushed-on component that needs a physical shoulder on the shaft, you are in retaining-ring territory. Getting that fork right up front saves a re-machined groove or a stripped thread.
Mechanism, Load Path and Where Each Fastener Belongs
A lock nut works on the threaded-fastener principle: tightening stretches the bolt and clamps the joint; the lock feature prevents the nut from rotating backward under vibration or thermal cycling. Common sub-types — nylon-insert (nyloc), all-metal deformed-thread, prevailing-torque stamped, two-jam (jam nut + regular nut), and castellated with a cotter pin — all attack the same problem by raising the breakaway torque above the prevailing vibration torque. Stamped retaining rings, by contrast, work on a snap-into-groove principle: the ring elastically deforms during installation, snaps into a groove, and presents a 360° shoulder that the retained part butts up against [S1]. The ring itself takes a pure radial load from groove contact and a small axial load from the retained part; the shaft takes the rest.
Because a retaining ring lives in a groove, the shaft or bore must be grooved to a defined width, depth and edge-break geometry — usually per DIN 471 (external/shaft) and DIN 472 (internal/bore) for metric circlips, or ANSI B27.7 for inch-series snap rings. A lock nut only needs a clean thread and a wrench flat. If the assembly does not already have a groove, the cost of adding one (lathe work, grinding, EDM) usually pushes the decision back toward a threaded fastener with a lock nut.
Comparison Table: Lock Nut vs Retaining Ring Across Four Decision Criteria
The table below is the decision short-list. Use it before opening a catalogue. [S1]
1) Joint type served. Lock nut: threaded fastener, bolt, stud, threaded shaft. Retaining ring: pushed-on hub, bearing, gear, bushing on a plain shaft or in a bore. You cannot replace one with the other without re-machining the joint.
2) Reuse and service. Lock nut: nylon-insert types are typically rated for 5–10 tightening cycles before the insert loses prevailing torque; all-metal deformed-thread and two-jam arrangements are reusable to bolt fatigue limits. Retaining ring: stamped carbon-steel rings are usually specified as single-use — over-spreading permanently opens the gap; spiral-wound rings are more tolerant of multiple installations. Stamped rings from Arcon and similar suppliers are routinely called out as one-time installation hardware [S1].
3) Radial envelope and stack height. Lock nut: adds the full nut height plus washer face; hex height is typically 0.5× the nominal thread size for standard hex. Retaining ring: adds a fraction of a millimetre radially once seated — a real advantage in compact bearing cartridges and small motor rotors. If stack height is the binding constraint, the retaining ring wins on a plain shaft; the lock nut wins where a thread already exists.
4) Standardisation and sourcing. Lock nut: ISO 7040 (nyloc type), DIN 985, DIN 980, ANSI B18.16.6 cover the majority of stocked patterns; inch and metric both widely available. Retaining ring: DIN 471/472, ANSI B27.7, and the smalley/sikes-pattern series dominate. Stamped retaining rings for OEM service such as Exmark/Toro mower spindles ship in 12-packs as direct-fit service parts [S2]. The lock-nut supply base is broader; the retaining-ring supply base is more concentrated.
Material, Finish and Temperature Envelope
Lock nuts are stocked in carbon steel (grade 8 / 10 / 12 ratings), 18-8 / A2 stainless, and 316 / A4 stainless for corrosive service. Nylon-insert variants lose prevailing torque above roughly 120 °C, so elevated-temperature joints (exhaust manifolds, turbo housings, oven conveyors) require all-metal deformed-thread, two-jam or castellated lock nuts. Stamped retaining rings from long-running US manufacturers such as Arcon are produced in spring steel, stainless 302/316 and beryllium copper for electrical/EMI applications, with finish options including phosphate, zinc and black oxide [S1]. Specialty rings for lawn-and-garden OEM replacement — the Exmark/Toro 32151-62 / E808523 / 1-808523 pattern — are commonly supplied in carbon spring steel with a protective finish in 12-piece service packs [S2].
For cryogenic and highly corrosive service, both fastener families shift to 316 / A4 stainless or nickel alloys; nylon inserts are ruled out below roughly −40 °C as the polymer becomes brittle. Where electrical continuity must be controlled, beryllium-copper retaining rings and all-metal lock nuts are preferred over nylon-insert parts, which are insulators.
Installation, Tooling and Reuse Discipline
Lock nuts need a torque wrench and a controlled torque value; under-torque and the joint creeps, over-torque and the bolt yields. Nyloc and prevailing-torque metal types are sensitive to installation speed — too fast and the nylon heats up and re-flows, reducing prevailing torque. A two-jam arrangement (a thin jam nut torqued against a regular nut) is the cleanest reusable lock for high-vibration joints, but it doubles the wrench operations. [S2]
Retaining rings need a dedicated plier set — internal snap-ring pliers for bore rings, external for shaft rings, plus tip-matching to the ring's lug style. Over-spreading a stamped ring is the most common field failure: the gap opens past yield and the ring no longer snaps cleanly into the groove. Spiral-wound retaining rings tolerate larger installation deformation and are the better choice where the ring will be removed for service. For deep-groove access, beveled-tip pliers are mandatory; straight tips will skate off the lug holes and damage the ring or the groove edge.
Both fastener families are unforgiving of mixed-and-matched hardware: a DIN 985 nyloc on an UNC bolt will cross-thread before it locks, and a DIN 471 ring in an ANSI B27.7 groove will be loose or pinched. Match the standard family across the joint.
Limits, Failure Modes and When NOT to Use Either
A lock nut cannot retain a part on a plain (unthreaded) shaft without a separate shoulder or step — the threads end and so does the clamping. A retaining ring cannot replace a lock nut on a threaded fastener; there is no thread for it to act on. For high-thrust bearings, a single retaining ring is also a poor choice on a stepped shaft under reversing axial load — the groove becomes a stress concentration and a fatigue crack initiator. In that case, specify a slewing-ring bearing or a paired thrust washer arrangement instead. [S3]
Failure modes worth flagging in a 2026 spec audit: (a) nyloc lock nuts re-used past their cycle rating — the insert no longer grips and the joint walks; (b) over-spread stamped retaining rings installed by field crews without calibrated pliers — the ring never seats at the design depth and the retained part shifts; (c) mixed-standard assemblies (DIN ring on ANSI groove, ISO lock nut on non-ISO thread) that pass a hand-feel check but fail under vibration. Audits that flag any of these three are catching real field failures, not paperwork failures.
Sourcing Signals and a 2026 Trackable Next Node
Stocked lock nuts from ISO/DIN/ANSI families ship in 24–48 h from major industrial distributors, with stainless variants running 1–2 weeks on specials. Stamped retaining rings for OEM-equivalent service (lawn-and-garden, automotive, electric-motor) continue to ship in pre-packaged multi-packs with OEM cross-references such as 32151-62 / E808523 / 1-808523 for Exmark and Toro spindle service [S2]; long-running US ring manufacturers such as Arcon continue to list the full stamped/wire/spiral/Eaton-type range and accept custom-quote RFQs on the same channel they have used for over fifty years [S1]. For specifiers cross-checking a retaining ring selection against current groove and material practice, the 2026 retaining-ring buying guide lays out the spec gates and material levers in detail. For fasteners on rotating shafts where a lock nut sets bearing preload, the bearing selection flow in the roller bearing vs angular contact bearing spec cut ties bearing choice to the lock-nut tightening sequence.