Under DIN EN 16983 (formerly DIN 2093), it is permissible to exceed the standard free-height tolerance on lo when the alternative would compromise the spring load at the test deflection, with the standard written so that force at lo - 0.75 ho is the controlling acceptance criterion [S1][S9].
The shift from DIN 2093 to DIN EN 16983 did not change the tolerance philosophy: outside and inside diameter limits follow h12/H12, the test deflection stays at 75% of free cone height ho, and the force tolerance band narrows from +/-25%/-7.5% in Group 1 down to symmetric +/-5% above 6 mm thickness [S8][S3].
Group 1, 2, 3: Why the Tolerance Bands Tighten With Thickness
DIN EN 16983 splits standard conical disc springs into three thickness groups, and the load tolerance tightens as the material thickens: Group 1 (under 1.25 mm) is +/-25% upper and -7.5% lower at the test height, Group 2 is +/-15%/-7.5% from 1.25 to 3.0 mm and +/-10%/-5% from 3.0 to 6.0 mm, and Group 3 (over 6.0 to 14.0 mm) is a symmetric +/-5% [S3][S1].
The intent is process-capability driven: a thin spring with t under 1.25 mm is harder to land on a tight force target, so the standard gives the manufacturer more upward room and only penalises springs that fall short of rated force. Once thickness passes 3 mm, the rolling, coining and lot-to-lot heat-treat spread drops, so the asymmetric band collapses to a +/-5% window that procurement and incoming inspection can actually enforce with a calibrated load cell [S3][S6].
Free Height lo and the lo - 0.75 ho Test Point
Force acceptance is taken at the installed height lo - 0.75 ho, not at the free height lo itself, because the conical disc only develops predictable force after the cone is partially flattened [S1][S3]. The free height tolerance in the standard is therefore a manufacturing reference, not the acceptance test: published values for Group 1 are +0.10/-0.05 mm, Group 2 ranges from +0.15/-0.08 mm at 1.25-2.0 mm thickness up to +0.30/-0.15 mm at 3.0-6.0 mm, and Group 3 sits at +/-0.30 mm [S3].
Because the test point is 75% of ho, the standard explicitly allows overall height to be slightly exceeded when needed to hold the rated force: a spring whose lo measures 0.05 mm high but lands within force tolerance is accepted, while a spring whose lo is in the band but falls outside the force window is rejected [S6][S9]. This is the practical reason the spring washer category specification carries a force clause that overrides the lo band on the print.
Thickness, Diameter and Concentricity: The Other Acceptance Gates
Outside and inside diameter follow h12/H12 per EN 16983, with published single-sided limits from 0.12 mm on a 3-6 mm OD up to 0.68 mm on 500-600 mm OD, applied symmetrically as a minus on the OD and a plus on the ID [S3][S8]. Concentricity is graded by OD range: 0.15 to 0.32 mm in the 3-50 mm OD band and 0.60 to 1.36 mm from 50 to 600 mm OD, with the standard reference to IT11 below 50 mm and IT12 above [S3].
Material thickness is the most tightly held geometric dimension because it directly sets the section modulus used in the load equation: Group 1 is +0.02/-0.06 mm (0.2-0.6 mm t) up to +0.03/-0.09 mm (0.6 to under 1.25 mm), Group 2 is +0.04/-0.12 mm (1.25-3.8 mm) and +0.05/-0.15 mm (3.8-6.0 mm), and Group 3 returns to a symmetric +/-0.10 mm for 6-14 mm thickness [S3][S4]. Buyers who try to verify a replacement spring on free height alone miss this gate: a spring that measures inside the lo band but outside the thickness band will fail the force test at lo - 0.75 ho [S2][S5].
Series A, B, C and the ho/t Ratio That Sets the Curve
Within each thickness group, DIN EN 16983 defines three series by the ratio of outside diameter to thickness (De/t) and free cone height to thickness (ho/t): Series A is De/t about 18 with ho/t about 0.4 (heavy load, short stroke), Series B is De/t about 28 with ho/t about 0.75, and Series C is De/t about 40 with ho/t about 1.3 (light load, long stroke) [S1].
This matters for tolerance review because the same physical spring in Series A and Series C can both pass the +/-5% force band at lo - 0.75 ho, yet the Series C spring will sit at a much higher installed deflection for the same load. Procurement documents that list only De x Di x t without naming the series end up with a dimensionally correct but functionally wrong part on receipt, and the incoming load check at 75% ho is the only place this is caught [S1][S2].
Material Codes, Hardness and the Indirect Effect on Force Window
Standard DIN 16983 materials used by volume manufacturers are C67S (1.1231, code B) and 51CrV4 (1.8159, code W), with austenitic stainless X10CrNi18-8 / 1.4310 (code D, equivalent to AISI 301 full hard) for corrosion service [S1][S5]. Hardness windows are HV 425-510 (HRC 43-50) on Group 1 carbon and alloy steel, and HRC 42-52 (HV 412-544) on Group 2, with zinc phosphate and oil the standard finish (code R) and a plain unoiled finish (code K) on stainless [S1].
Hardness is not a separate acceptance number on the print but it sets the modulus of elasticity and the yield stress that the force equation uses, so two springs with identical geometry and identical lo can sit on opposite sides of the +/-5% force band if one is at HV 425 and the other at HV 510. That is why a DIN 2093 incoming-inspection procedure that skips the load test and only checks dimensions is unsafe, and why the disc coupling assemblies and construction machinery hardware that use these washers all reference the force clause, not the geometry clause, as the controlling requirement [S1][S5].
Stacked Arrangements and How Force Tolerance Compounds
DIN 16983 covers single disc springs; for parallel stacks the force adds while the deflection stays that of one spring, and for series stacks the deflection adds while the force stays that of one spring, with mixed stacks the practical default in preloaded bolt and lamps and light fittings preload-retention designs [S1][S2].
The tolerance math does not get looser in a stack: if a Group 2 single spring is held to +/-15%/-7.5% at lo - 0.75 ho, a four-spring parallel stack is still held to the same per-spring band, which means the stack force can statistically drift further from nominal even though every individual spring is in spec. The standard's answer is the lo - 0.75 ho test point applied per spring before stack assembly, combined with a matched-stack option (springs from the same heat-treat lot) that several DIN 2093 suppliers offer for high-cycle or safety-critical service [S1][S9]. For a deeper look at how adjacent standards such as ASTM E2033/E2033M-24 drive NDT-related tolerancing, the computed radiography practice scope and selection criteria note provides a useful cross-reference on how tolerance bands are written in NDE documentation.
Inspection Workflow: What to Measure and What to Skip
A practical incoming-inspection sequence for a DIN 16983 (DIN 2093) lot starts with OD/ID on a calibrated optical or plug-and-ring gauge, moves to thickness with a micrometer on at least three points around the cone, then to free height lo between flat anvils, and finishes with the load test at lo - 0.75 ho on a calibrated compression tester [S3][S5][S9].
The free height lo value is recorded for traceability but is not the acceptance number, while the force at the test height is the pass/fail line; a lot can be accepted with lo 0.05-0.10 mm above the upper limit if every spring hits the force window, and a lot can be rejected with perfect lo if a single spring sits outside the +25%/-7.5% Group 1 or +/-5% Group 3 force band [S6][S9]. Two trackable signals to watch in the next sourcing cycle: the spread between the per-spring force reading and the published Group 2 +/-15%/-7.5% window, and the lot-level correlation between lo overshoot and force deviation, both of which feed back into whether the supplier is heat-treating within the published hardness range.