Disc spring washers — conical washer variants often listed alongside spring washer hardware in M6–M20 catalogue rows — received a 1-out-of-5 reusability score and a "High" lifetime-cost rating in Nord-Lock Group's bolt-securing comparison published 2026-05-16 [S1].
The same comparison scored the disc-spring geometry "Not Applicable" for locking ability, meaning it delivers preload retention through elastic deflection rather than the mechanical bite that a wedge-pair locking assembly produces [S1].
Disc Spring Geometry: Function and Scoring
A disc spring washer, also called a Belleville or conical spring washer, is a frustum-shaped washer that flattens under axial load and springs back when the bolt relaxes. Nord-Lock's evaluation table rates this category 4-out-of-5 for worker safety, "Medium" for initial cost, and 1-out-of-5 for reusability — a combination the vendor attributes to the loss of free height after a single torque cycle [S1].
Because the locking function is listed as "Not Applicable," the disc spring sits in a different functional bucket than a true sealing washer or wedge-style locking assembly; it stores energy but does not grip the mating surface.
Locking Assembly: Function and Engineering Basis
A locking assembly — typically a pair of cams or wedges plus a hardened washer face — produces a positive mechanical interlock between the bolt and the joint. Unlike a disc spring's elastic recovery, the wedge pair resists rotation by cam angle: a steeper cam angle yields a higher anti-loosening margin but a higher installation torque, and the joint stops loosening the instant the cam releases. [S1]
For vibration-loaded service (compressors, vibrating screens, rail bogies, marine driveline), specifying a locking assembly instead of an elastically deflected spring washer is the conventional refit when field-loosening events are recorded. Disc spring variants continue to be specified where the design intent is preload maintenance under thermal cycling or bolt elongation, not vibration loosening.
Selection Criteria: Vibration, Temperature, Reuse, Cost
Four criteria separate the two options in practice. (1) Anti-loosening: locking assemblies generate a positive mechanical lock; disc springs do not. (2) Reusability: 1-out-of-5 for the disc spring versus typically 5+ reuse cycles for a wedge-pair locking assembly when cam faces are inspected [S1]. (3) Thermal preload loss: disc springs compensate for bolt elongation and gasket creep; locking assemblies hold a fixed clamp force. (4) Total cost: disc spring initial cost is "Medium" but lifetime cost lands at "High" because of single-use replacement; locking assemblies front-load cost but amortize over many cycles [S1].
Comparison: Disc Spring Washer vs Locking Assembly
Direct comparison on the four decision criteria: (a) Anti-loosening under vibration — disc spring: relies on preload alone, no mechanical lock; locking assembly: positive cam-tooth or wedge pair lock, designed for transverse vibration. (b) Reuse cycles — disc spring: 1-out-of-5 per Nord-Lock; locking assembly: typically rated for multiple cycles when cam faces are intact. (c) Initial purchase cost — disc spring: "Medium"; locking assembly: higher unit price, amortized. (d) Lifetime cost — disc spring: "High" per Nord-Lock [S1]; locking assembly: lower per cycle. For joint designers running a Pareto cut on these four variables, the disc spring wins only on preload modulation and worker-safety handling.
Failure Modes and Field Constraints
Disc springs fail in three observable modes: permanent set after the first high-torque cycle, fatigue cracking at the inner edge under dynamic axial load, and over-deflection when stack height is miscalculated. A disc spring that has taken a permanent set no longer restores preload and behaves like a flat washer, dropping vibration margin to zero. [S2]
Locking assemblies fail by cam-tooth wear, surface brinelling on the mating washer face, and incorrect torque that pre-rotates the cam during installation. The remedy path differs: a disc spring that has flattened is scrapped; a worn locking assembly is rejected at inspection rather than at failure.
Standards and Sourcing Notes
Disc spring washers are commonly ordered against DIN 6796 (conical spring washers for bolted connections) and the related DIN 2093 disc-spring range; locking assemblies are typically called out by the manufacturer's part number because the cam geometry is proprietary. For sour-service hydrocarbon service, both options require NACE MR0175 material compliance regardless of geometry. [S3]
For buyers evaluating adjacent mechanical hardware, a shaft collar buying guide for 2026 and the shaft collar price and cost guide for 2026 cover the same bore, material, and mounting-style decision levers in the rotating-machinery domain.
Trackable signals for the next spec cycle: (1) revisions to DIN 6796 and DIN 2093 free-height tolerance bands, and (2) any new vendor data on disc-spring reusability ratings that would lift the current 1-out-of-5 score [S1].