Inputs
Nominal size (M5 to M39, coarse thread) and strength class (04, 05, 5, 6, 8, 10, 12).
Bolts · proof load
ISO 898-2 Nut Proof Load
ISO 898-2 defines the tensile load a nut must withstand in the mandrel tensile test without significant permanent deformation, thread stripping, cracking or fracture. This proof load FP is the governing acceptance value -- it ensures the nut is at least as strong as a mated bolt of the corresponding strength class to ISO 898-1.
Mated bolt: ISO 4014 dimensions. Preload and tightening torque: Tightening Torque.
ISO898-2
02
Proof load
Enter nominal size and strength class.
Inputs and method
Determine nut proof load to ISO 898-2
Determine the tensile proof load of a coarse-thread nut from nominal size and strength class.
Calculation
A pure table lookup from Table 5 of the standard: proof load FP is fixed per combination of nominal size and strength class.
Example
M10 strength class 8: proof load FP = 50,500 N, thread pitch P = 1.5 mm.
Sources and limits: ISO 898-2:2022 (DIN EN ISO 898-2:2023-02), Table 5.
Determine nut proof load to ISO 898-2 in detail
Determine the tensile proof load of a coarse-thread nut from nominal size and strength class.
What does the proof load FP actually test?
In the ISO 898-2 mandrel tensile test, the nut is threaded onto a hardened test mandrel and loaded axially to the proof load FP, held for 15 seconds, then unloaded. The nut passes if it can then be run off by hand (or with at most half a turn of tool assistance) without significant permanent deformation, thread stripping, cracking or fracture. This is a pure tensile test of the thread engagement, not a statement about deformation under service load.
Formula and variables
A pure table lookup from Table 5 of the standard: proof load FP is fixed per combination of nominal size and strength class.
Choose the inputs correctly
Nominal size (M5 to M39, coarse thread) and strength class (04, 05, 5, 6, 8, 10, 12).
How to use the calculator
Select the nut's nominal size and strength class. The calculator returns the proof load FP from Table 5 and the corresponding thread pitch.
Worked example
M10 strength class 8: proof load FP = 50,500 N, thread pitch P = 1.5 mm.
How does the proof load relate to bolt selection?
Nut strength classes (04, 05, 5, 6, 8, 10, 12) are deliberately set so a nut of the matching class is at least as strong as a mated bolt of the same strength class to ISO 898-1 (e.g. nut class 8 with an 8.8 bolt). The standard itself notes in Annex C that a proof stress SP = FP/As back-calculated from these values doesn't consistently match historical figures -- so this calculator returns only the normative proof load FP, not a derived stress.
Proof load in newtons, thread pitch in millimetres.
Typical applications
Selecting the matching nut strength class for a given bolt strength class, acceptance testing of nut batches, verifying compatibility of a bolted joint.
Assumptions, limits and common mistakes
ISO 898-2:2022 (DIN EN ISO 898-2:2023-02), Table 5.
Common mistake: Don't use strength class 9 -- it was fully withdrawn in the 2022 edition. For low nuts (table footnote a), also note that the actual stripping force can fall below the proof load shown here for a nut of full load capacity.
Frequently asked questions
Which nut class matches an 8.8 bolt?
Strength class 8 -- this nut class's proof load is matched to the tensile strength of an 8.8 bolt of the same nominal size.
Is the proof load the same as the maximum service load?
No. The proof load FP is a one-time acceptance test of the nut itself, not a permissible service load for the bolted joint -- that comes from the bolt's preload calculation, not from this nut test.