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Calculated strength verification for springs and spring elements
FKM 2nd edition 2023
66 pages

The main innovations and additions in the 2nd edition are

• Addition of an appendix with fatigue strength diagrams (Goodman/Haigh diagrams) for standard helical compression springs with directly readable permissible stresses (result of the research project IGF 19693 BR based on calculations with the FKM guideline springs 1st edition)
• Addition of a table in the safety concept, which already contains calculated safety values for j_s (partial safety factor for reliability) for standard safety requirements, this reduces possible miscalculationsAddition of a calculation example for leaf springs
• Addition of a note on a further verification point on torsion springs
• Minor corrections and optimization of formulations

The calculation algorithm for springs is based on the calculation algorithm for non-welded components FKM Guideline: Calculated strength verification for machine components, 7th edition, VDMA Verlag, 2020 and was developed as part of the research project IGF 18495 BG (2017) "Calculated strength verification for springs and spring elements".
The FKM guideline for springs and spring elements
- is applicable in mechanical engineering and related areas of industry,
- describes the static strength verification, but essentially the fatigue strength verification as fatigue, time or fatigue strength verification,
- applies primarily to springs and spring elements made of oil tempered, patented drawn and corrosion-resistant wires with wire diameters of 1.0 to 12.0 mm or Springs and spring elements made of tempered and corrosion-resistant strip material with strip thicknesses of 0.1 to 2.0 mm,
- applies to springs and spring elements subject to torsional and bending stresses, preferably for helical compression springs, torsion springs and spiral springs (not for disc springs and stamped-bent parts) and
- applies uniformly to all cold-formed springs and spring elements.
A uniformly structured calculation procedure applies to all applications, which is based on the procedure in the FKM Guideline: Calculated strength verification for machine components, 7th edition, VDMA Verlag, 2020. The calculation algorithm consists of instructions, formulas, tables and explanations. The content with its comprehensive calculation options corresponds to the latest state of the art. The formula symbols used are adapted to recent developments. The calculation procedure is supplemented by examples for better comprehensibility.

The guideline was written
on behalf of the Forschungskuratorium Maschinenbau e.V. (FKM)
in the expert group on component strength
by
Prof. Dr.-Ing. Ulf Kletzin
Ilmenau University of Technology, Department of Machine Elements
Dr.-Ing.Ing. René Reich
Ilmenau University of Technology, Department of Machine Elements
Prof. Dr.-Ing. Matthias Oechsner
Darmstadt University of Technology, Institute of Materials Science
Dipl.-Ing.Ing. André Spies
Technical University of Darmstadt, Institute of Materials Science
further
Prof. Dr.-Ing. Brita Pyttel
Dipl.-Phys. Georg Hannig
Prof. Dr.-Ing. Roland Rennert
Prof. Dr.-Ing. Eckehard Kullig
M. Sc. Martin Petrich

Note: The first edition remains valid

Contents

0 General
0.0 Area of application
0.1 Fundamentals
0.2 Required verifications
0.3 Stresses and component types
0.4 Calculation procedure
1 Static strength verification
2 Fatigue strength verification with local stresses
2.0 General
2.1 Stress parameters
2.1.0 General
2.1.1 Stress components
2.1.2 Stress spectrum and S-N curve
2.1.2.1 Stress spectrum parameters
2.1.2.2 S-N curve parameters
2.1.3 Adaptation of the stress spectrum to the component S-N curve
2.1.4 Equivalent stress amplitude
2.2 Material parameters
2.2.0 General
2.2.1 Component standard values
2.2.2 Tensile compressive fatigue strength factor and shear fatigue strength factor
2.2.3 Temperature factor
2.3 Design parameters
2.3.0 General
2.3.1 Design factors
2.3.2 Estimated value of the notch coefficient
2.3.3 Calculation of the support numbers
2.3.4 Roughness factor
2.3.5 Surface layer factor
2.4 Component strength
2.4.0 General
2.4.1 Component fatigue strength
2.4.2 Component fatigue strength depending on medium stress
2.4.2.0 General
2.4.2.1 Amplitude of component fatigue strength
2.4.2.2 Medium stress sensitivity
2.4.2.3 Residual stress factor
2.4.2.4 Fatigue strength diagram
2.4.2.5 Medium-voltage factor
2.4.3 Component fatigue strength
2.4.3.0 General
2.4.3.1 Yieldable stress components
2.4.3.2 Component S-N curve
2.4.3.3 Fatigue strength factor
2.5 Safety factors
2.5.0 General
2.5.1 Confidence level of the database
2.5.2 Derivation of stress quantiles with higher reliability
2.5.3 Overall safety factor
2.6 Verification
2.6.0 General
2.6.1 Individual stress types
2.6.2 Composite stress types
3 Appendix
3.1 Determination of the highly stressed component surface
3.1.1 General
3.1.2 A 90 % method
3.1.2.1 Helical compression spring (DF)
3.1.2.2 Torsion spring (SF)
3.1.2.3 Spiral spring (SPF)
3.1.2.4 Leaf spring (BF)
3.1.3 Use of stress integrals
3.1.3.1 Torsion spring (SF)
3.1.3.2 Spiral spring (SPF)
3.1.3.3 Helical compression spring (DF)
3.2 Medium stress displacement
3.2.1 General
3.2.2 Normal stresses
3.2.3 Shear stresses
3.2.4 Typical residual stresses
3.3 Material tables
3.4 Calculation examples
3.4.1 Helical compression springs
3.4.1.1 General
3.4.1.2 Input variables.
3.4.1.3 Material parameters
3.4.1.4 Design parameters
3.4.1.5 Component strength
3.4.2 Coil spring
3.4.2.1 General
3.4.2.2 Input variables.
3.4.2.3 Static strength verification
3.4.2.4 Material properties
3.4.2.5 Design properties
3.4.2.6 Component strength
3.4.2.7 Safety concept
3.4.3 Torsion spring
3.4.3.1 General
3.4.3.2 Input variables.
3.4.3.3 Static strength verification
3.4.3.4 Material properties
3.4.3.5 Design properties
3.4.3.6 Component strength
3.4.3.7 Safety concept
3.4.3.8 Verification
3.4.4 Leaf spring
3.4.4.1 General
3.4.4.2 Input variables
3.4.4.3 Static strength verification
3.4.4.4 Material characteristics
3.4.4.5 Design characteristics
3.4.4.6 Component strength
3.4.4.7 Safety concept
3.4.4.8 Verification
3.5 Vibration resistance diagrams for standard helical compression springs.
3.5.1 Foreword
3.5.2 Oil tempered material
3.5.2.1 FDSiCr
3.5.2.2 VDSiCr
3.5.2.3 VDSiCrV
3.5.3 Patented drawn material
3.5.3.1 DH
3.5.4 Corrosion-resistant material
3.5.4.1 1.4310
3.5.4.2 1.4568
4.Formula symbols
4.1 Abbreviations
4.2 Indexes
4.3 Lower-case letters
4.4 Upper-case letters
4.5 Greek letters