Disc spring fatigue life is estimated by plotting the maximum tensile stress at the critical locations on the disc (point II and point III) against the preload tensile stress at the same location, then reading the intersection on the Modified Goodman style diagrams in DIN EN 16983 [S2][S9].
The standard covers three thickness bands for the published charts, under 1.25 mm, 1.25 to 6 mm, and above 6 mm up to 14 mm, and applies to spring steels with an outside-to-thickness ratio of 16–40 and an outside-to-inside diameter ratio of 1.8–2.5 [S2][S3].
Why points II and III, not point I, drive crack initiation
Points II (top inside diameter corner) and III (bottom outside diameter corner) are the two tensile stress maxima on a loaded disc, and fatigue cracks initiate at one of these two locations rather than at the compressive point I [S2][S5][S7].
Estimation requires the stress differential between preload and final load at the two locations; the location with the higher differential is the one used to read the chart, because that is the location expected to fail first [S2][S5]. For a DSC 50 x 25.4 x 2 spring cycled 15% to 75% of free height, differential at II is 795 N/mm² and at III is 876 N/mm², so point III governs and the diagram reads just below the 100,000 cycle line [S5].
Stress values at points II and III can be taken from the manufacturer's catalog (Figure 2 of SPIROL's white paper shows a worked table of σ_II and σ_III in N/mm² at deflections of 0.15 h₀, 0.5 h₀, 0.75 h₀ and h₀), or calculated from the formulas in DIN EN 16984, with E taken as 206,000 N/mm² and Poisson's ratio µ as 0.3 [S2].
Three group curves and how to read them
The diagrams are split by material thickness because fatigue strength of a disc scales with section size, so a 0.8 mm spring and a 12 mm spring cannot share a single curve: Group 1 covers t under 1.25 mm, Group 2 covers 1.25 to 6 mm, and Group 3 covers 6 to 14 mm [S2][S3].
The procedure is identical across groups: read the upper stress (maximum) on the Y-axis, the lower stress (preload) on the X-axis, plot the point, and read the cycle count on the logarithmic life axis. Example for a 15 x 5.2 x 0.4 spring (l₀ = 0.95) at 50% to 75% deflection: σ_III upper = 1,002 N/mm², σ_III lower ≈ 735 N/mm² by extrapolation, and the intersection sits above the 2,000,000 cycle contour [S3].
Charts are only validated for carbon spring steels per DIN EN 16983; corrosion and heat resisting alloys (stainless grades, Inconel-type alloys) require bespoke test data, and the major UK and European disc spring vendors explicitly recommend asking the supplier for those curves [S3].
Rules of thumb that keep the spring inside the chart envelope
Three operating rules are repeated across SPIROL, Belleville Springs, Springmasters, and FAN DISC: keep preload at a minimum of 15% of available deflection (h₀), cap the maximum deflection at 75% of h₀, and remember that increasing preload or reducing upper stress both push the operating point toward longer life [S3][S4][S6].
The same DSC 50 x 25.4 x 2 spring that fails just below 100,000 cycles at 15% to 75% deflection is rated above 2,000,000 cycles when the deflection range is tightened, demonstrating how steeply life rises once the stress window narrows [S2].
Stacking changes the heat balance: parallel stacks share the load but trap frictional and hysteresis heat, so parallel-stacked assemblies run hotter and fatigue faster than single discs at the same per-disc stress, while series stacks multiply deflection without raising per-disc stress [S2][S5].
Calculated worked example, step by step
For a Series B Group 2 DSC 50 x 25.4 x 2 disc spring, DIN EN 16983 catalog values at 0.15 h₀ deflection are σ_II = 128 N/mm² and σ_III = 264 N/mm², and at 0.75 h₀ deflection they are σ_II = 923 N/mm² and σ_III = 1,140 N/mm² [S2][S5].
Differential at II = 923 − 128 = 795 N/mm²; differential at III = 1,140 − 264 = 876 N/mm²; point III governs; on the Group 2 chart, plot (264, 1,140), the intersection lands just above the 10⁵ cycle line, so estimated life is slightly under 100,000 cycles [S2][S5].
The same spring with the same upper stress but with preload raised to 0.25 h₀ shifts the X-coordinate rightward, which on a Modified Goodman plot moves the operating point toward the longer-life region of the diagram [S3][S9].
Where the standard does not apply, and what to do instead
DIN EN 16983 / DIN 2093 charts cover carbon and silicon-chrome spring steels only; stainless grades such as X12CrNi 17 7 (1.4310) and X7CrNiAl 17 7 (1.4568), precipitation-hardened variants, and high-temperature alloys are outside the published curves and need factory fatigue test data [S3][S4].
Conical spring washers to DIN 6796 are a different product: they are designed for static thrust on bolted joints, and the SPIROL white paper explicitly warns they must not be used in fatigue-cycled applications [S2].
Geometric limits matter: outside-to-thickness ratio must be 16 to 40 and outside-to-inside diameter ratio must be 1.8 to 2.5, otherwise the standard's formulas and chart families are not valid for that part [S2][S5]. For reference, a disc spring used outside these ratios, or stacked in a way that traps heat, should be validated on a rig before being put into service.
Limitations, failure modes, and the lab confirmation step
Diagram-based estimates assume isothermal operation, room-temperature spring steel, and no surface defects; real disc springs fail from inclusions, decarburization, shot-peening residual stress relaxation at temperature, and fretting at the guide interface, none of which the chart captures [S2][S9].
Stacking in parallel shortens life because of self-generated heat; stacking in series can lengthen it because per-disc stress drops, but series stacks can buckle sideways if the guide clearance is too loose, a failure mode that is mechanical rather than fatigue-driven [S5][S6].
Vendor guidance from SPIROL is explicit: the published numbers are guidelines, not warranties, and laboratory testing under the actual duty cycle (frequency, stroke, environment) is required to confirm the estimate before committing the design to a high-cycle application [S2].
The underlying component specifications are covered under disc coupling, and construction machinery and equipment.
This topic is covered further in Particle Counter Count for ISO 7 Cleanrooms: Spec Map and Selection.