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NASA TECHNICAL STANDARD

NASA-STD-5020A

w/CHANGE 1:

ADMINISTRATIVE/

EDITORIAL CHANGE

2019-02-11

Office of the NASA Chief Engineer

Approved: 2018-09-04

Superseding NASA-STD-5020

(Baseline)

REQUIREMENTS FOR THREADED FASTENING

SYSTEMS IN SPACEFLIGHT HARDWARE

NASA-STD-5020A w/CHANGE 1

DOCUMENT HISTORY LOG

Status Document

Revision Change Number

Approval Date Description

Baseline 2012-03-12 Initial Release Revision A 2018-09-04 Significant changes were made to this

NASA Technical Standard. It is recommended that it be reviewed in its entirety before implementation.

Key changes were: The format of the baseline version has been modified to provide better flow of the requirements language. This includes the order of the requirements from the baseline version.

Some requirements have been merged due to redundancy or deleted.

1 2019-02-11 Editorial Changes—Corrected decision box No. 2 in Appendix A.5, Figure 8, Determining Whether a Joint Separates before Rupture When Loaded Solely in Tension, to state Pp-max ≤ 0.75*Ptu-allow

(vs. “0.85”), which aligns with the previous revision and the justification in the text. Corrected the symbol for phi to ϕ (vs. (φ) in equation 47. Unbolded two equations in Appendix A.12.3.

FOREWORD

This NASA Technical Standard is published by the National Aeronautics and Space Administration (NASA) to provide uniform engineering and technical requirements for processes, procedures, practices, and methods that have been endorsed as standard for NASA programs and projects, including requirements for selection, application, and design criteria of an item.

This NASA Technical Standard is approved for use by NASA Headquarters and NASA Centers and Facilities, and applicable technical requirements may be cited in contract, program, and other Agency documents. It may also apply to the Jet Propulsion Laboratory (a Federally Funded Research and Development Center (FFRDC)), other contractors, recipients of grants and cooperative agreements, and parties to other agreements only to the extent specified or referenced in applicable contracts, grants, or agreements.

This NASA Technical Standard establishes criteria for ensuring the integrity of threaded fastening systems in launch and space vehicles and their associated equipment.

Requests for information should be submitted via “Feedback” at https://standards.nasa.gov.

Requests for changes to this NASA Technical Standard should be submitted via Marshall Space Flight Center (MSFC) Form 4657, Change Request for a NASA Engineering Standard.

______Original signed by________ ___________09/04/2018____________ Ralph R. Roe, Jr. Approval Date NASA Chief Engineer https://standards.nasa.gov/

TABLE OF CONTENTS

SECTION PAGE

DOCUMENT HISTORY LOG

FOREWORD

TABLE OF CONTENTS

LIST OF APPENDICES

LIST OF FIGURES

LIST OF TABLES

1. SCOPE

1.1 Purpose

1.2 Applicability

1.3 Tailoring

2. APPLICABLE DOCUMENTS

2.1 General

2.2 Government Documents

2.3 Non-Government Documents

2.4 Order of Precedence

3. ACRONYMS, ABBREVIATIONS, SYMBOLS, AND

DEFINITIONS

3.1 Acronyms, Abbreviations, and Symbols

3.2 Definitions

3.2.1 Definition of Variables

3.2.2 Definition of Terms

4. REQUIREMENTS

4.1 Fastening System Control Plan

4.2 Design Factors

4.3 Fastening System Preload

4.4 Strength Requirements

4.5 Fatigue Life

4.6 Locking Features

4.7 Part Selection Criteria

4.8 Quality Assurance

LIST OF APPENDICES

APPENDIX PAGE

A. EXPLANATION AND JUSTIFICATION OF FASTENER

ANALYSIS CRITERIA

A.1 Purpose of Appendix A A.2 Accounting for Preload Variation when Installing Fasteners with Torque

Control (Supplement to Section 4.3.1)

A.3 Short-Term Relaxation of Preload (Supplement to Section 4.3.1) A.4 Use of a Load-Introduction and Stiffness Factor (Supplement to Sections

4.4.1 and 4.4.2)

A.5 Bolt Analysis: Separation before Rupture (Supplement to Section 4.4.5). 60 A.6 Ultimate Margin of Safety for Tensile Loading with Linear Theory

(Supplement to Section 4.4.1)

A.7 Omission of Preload in Shear and Interaction Analyses (Supplement to Sections 4.4.1 and 4.4.4)

A.8 Theoretical Treatment of Interaction Equations (Supplement to Section 4.4.4)

A.9 Determining if Fastener Yielding is Detrimental for Separation or Joint Slip (Supplement to Section 4.4.2)

A.10 Margin of Safety for Joint Slip (Supplement to Sections 4.4.1 and 4.4.6) .. 73 A.11 Margin of Safety for Bolted Joint Separation (Supplement to Section

4.4.3)

A.12 Load Redistribution within a Fastened Joint

B. BEST PRACTICES FOR LOCKING FEATURES

B.1 Purpose of Appendix B B.2 Background and Introduction B.3 Best Practices for Mechanical Locking Features B.4 Best Practices for Prevailing Torque Locking Features B.4.1 General B.4.2 Prevailing Torque Locking Devices B.4.3 Inspect B.4.4 Installation and Verification B.5 Best Practices for Adhesive Locking Features B.5.1 Adhesives B.5.2 Process Validation B.5.3 Inspect B.5.4 Clean B.5.5 Prime (for anaerobic adhesives only) B.5.6 Installation

LIST OF APPENDICES (Continued)

APPENDIX PAGE

B.5.7 Verification B.5.8 General B.6 Best Practices for Free Spinning

C. JUSTIFICATION FOR LOW LIKELIHOOD OF FATIGUE

FAILURE

C.1 Purpose of Appendix C C.2 Justification

D. REFERENCES

D.1 Purpose of Appendix D D.2 Reference Documents

E. REQUIREMENTS COMPLIANCE MATRIX

E.1 Purpose of Appendix E

LIST OF FIGURES

FIGURE

PAGE

1 Logic Flow for Minimum Separation Factor of Safety 2 Dimensional Considerations in Selecting Fastening Hardware 3 Dimensions Used to Calculate the Geometric Load-Introduction

Factor

4 Preloaded Joint Represented by Springs 5 Preloaded Joint Represented by Two Springs in Parallel 6 Geometric Load-Introduction Factor 7 Predicting the Product nφ from Finite Element Analysis 8 Determining Whether a Joint Separates before Rupture When

Loaded Solely in Tension

9 How to Quantify Ductility of a Fastening System from a Tension Test

10 Preloaded Joint in which the Bolt Load Increases by a Relatively High Percentage of Applied Tensile Load

11 Separation before Rupture for a Brittle Threaded Fastening System .. 63 12 Separation before Rupture for a Ductile Threaded Fastening System . 63

LIST OF FIGURES (Continued)

FIGURE PAGE

13 Bolt Tensile Load versus Applied Tensile Load 14 Separation before Rupture with Linear Theory 15 Rupture before Separation with Linear Theory 16 Stress Distribution at Collapse due to Combined Bending and

Tension

17 Comparison of Interaction between Circular and Rectangular Cross Sections at Collapse

18 Effect of Fastener Yielding on the Separation Load 19 Loss of Preload Resulting from Fastener Yielding 20 Fastener Yielding under an Applied Load that Exceeds the

Separation Load

21a Lug with Clearance Holes Example 21b Rigid Body Response for Lug with Clearance Holes 22a Simplified Two-Bolt Plasticity Model 22b Assumed Plasticity Model 23 Difference in Response between Elastic and Elastic-Plastic Model 24 Two-Joint Separation Model 25 Bolt Forces during Loading of Two-Joint Separation Model 26 System Stiffness Due to Separation and Yielding 27 Shear Load Acting on Bolt from Slip Analysis 28 Contact Status at the Faying Surface during Ship 29 Locking Feature Selection Flowchart

LIST OF TABLES

TABLE PAGE

1 Analytical Adjustments of Maximum and Minimum Preloads 2 Nominal Preload Determination 3 Preload Variation Determination 4 Locking Feature Torque Verification 5 Two-Sided 90/95 Tolerance Limit Factors for a Normal Distribution

(Odeh and Owen, 1980)

6 Sample Data from 30 Torque-Tension Tests (Actual Test Data)

LIST OF TABLES (Continued)

TABLE PAGE

Hypothetical Enveloped Applied-Loading Spectrum for Fastener Fatigue Results of Fastener Fatigue Analysis for the Hypothetical Joint and Loading Spectra

REQUIREMENTS FOR THREADED FASTENING

SYSTEMS IN SPACEFLIGHT HARDWARE

1. SCOPE

1.1 Purpose

The purpose of this NASA Technical Standard is to specify requirements for design and analysis of threaded fastening systems in NASA spaceflight hardware.

1.2 Applicability

This NASA Technical Standard is applicable to all NASA programs and projects. This NASA Technical Standard may not be applicable to Ground Support Equipment (GSE); NASA Technical Standard NASA-STD-5005, Standard for the Design and Fabrication of Ground Support Equipment, is applicable to GSE. Additional requirements for fracture control, non-metallic structures, non-standard fasteners, and fasteners used in extravehicular activity (EVA) may apply.

This NASA Technical Standard is approved for use by NASA Headquarters and NASA Centers and Facilities, and applicable technical requirements may be cited in contract, program, and other Agency documents. It may also apply to the Jet Propulsion Laboratory (a Federally Funded Research and Development Center (FFRDC)), other contractors, recipients of grants and cooperative agreements, and parties to other agreements only to the extent specified or referenced in applicable contracts, grants, or agreements.

Verifiable requirement statements are designated by the acronym “TFSR” (Threaded Fastening Systems Requirement), numbered, and indicated by the word “shall”; this NASA Technical Standard contains 32 requirements. Explanatory or guidance text is indicated in italics beginning in section 4. To facilitate requirements selection by NASA programs and projects, a Requirements Compliance Matrix is provided in Appendix E.

1.3 Tailoring

Document tailoring of the requirements in this NASA Technical Standard for application to a specific program or project as part of program or project requirements and obtain formal approval by the delegated Technical Authority in accordance with NPR 7120.5, NASA Space Flight Program and Project Management Requirements.

2. APPLICABLE DOCUMENTS

2.1 General

The documents listed in this section contain provisions that constitute requirements of this NASA Technical Standard as cited in the text.

2.1.1 The latest issuances of cited documents apply unless specific versions are designated.

2.1.2 Non-use of a specifically designated version is approved by the delegated Technical Authority.

Applicable documents may be accessed at https://standards.nasa.gov or obtained directly from the Standards Developing Body or other document distributors. When not available from these sources, information for obtaining the document is provided.

2.2 Government Documents

National Aeronautics and Space Administration (NASA)

NPR 7120.5 NASA Space Flight Program and Project Management

Requirements

NASA-STD-6008 NASA Fastener Procurement, Receiving Inspection, and Storage Practices for Spaceflight Hardware

NASA-STD-6016 Standard Materials and Processes Requirements for Spacecraft

2.3 Non-Government Documents

American Society of Mechanical Engineers (ASME)

ASME B107.300-

Torque Instruments

National Aerospace Standard (Metric) (NASM)

NASM 1312-15 Fastener Test Methods, Method 15, Torque-Tension

References are provided in Appendix D.

https://standards.nasa.gov/

2.4 Order of Precedence

2.4.1 The requirements and standard practices established in this NASA Technical Standard do not supersede or waive existing requirements and standard practices found in other Agency documentation, or in applicable laws and regulations unless a specific exemption has been obtained by the Office of the NASA Chief Engineer.

2.4.2 Conflicts between this NASA Technical Standard and other requirements documents are resolved by the delegated Technical Authority.

3. ACRONYMS, ABBREVIATIONS, SYMBOLS, AND

DEFINITIONS

3.1 Acronyms, Abbreviations, and Symbols

°C degrees Celsius °F degrees Fahrenheit / divided by > greater than ≤ less than or equal to % percent π pi + plus ± plus or minus √ square root AC Advisory Circular AND Air Force-Navy aeronautical design ASME The American Society of Mechanical Engineers CRES corrosion-resistant steel dB decibel Eq. equation EVA extravehicular activity FAA Federal Aviation Administration FEA finite element analysis FFRDC Federally Funded Research and Development Center GSE ground support equipment HDBK Handbook Hz Hertz in inch(es) ksi kilopound per square inch lb Pound(s) LIF load-introduction factor MEK Methyl ethyl ketone MIL Military MSFC Marshall Space Flight Center

NAS National Aerospace Standard NASA National Aeronautics and Space Administration NASM National Aerospace Standard (Metric) NPR NASA Procedural Requirements NSTS National Space Transportation System PWA Printed wiring assembly SAE Society of Automotive Engineers SI Système International STD Standard T nominal effective torque TFSR Threaded Fastening Systems Requirement

3.2 Definitions

3.2.1 Definition of Variables

Γ uncertainty or scatter in initial preload Γa-max actual preload variation for maximum preload, as determined from a sample of torque-tension test data Γa-min actual preload variation for minimum preload, as determined from a sample of torque-tension test data φ stiffness factor σpi unbiased sample standard deviation Am minimum minor-diameter area cmax a factor that accounts for the maximum value of the fastener’s controlled installation parameter (e.g., effective torque), as allowed by the specified tolerance cmin a factor that accounts for the minimum value of the fastener’s controlled installation parameter (e.g., effective torque), as allowed by the specified tolerance

D nominal fastener diameter fbu design ultimate bending stress based on linear-elastic theory Fbu allowable ultimate flexural stress (bending) FF fitting factor Fpre preloaded bolts FSsep factor of safety for separation Fsu allowable ultimate shear stress FSu ultimate factor of safety FSy yield factor of safety Ftu allowable ultimate tensile stress Fty allowable yield tensile stress j test number kb stiffness of the bolt kc stiffness of the clamped parts local to the fastener Knom nominal (mean) nut factor m number of tests MSsep margin of safety for separation MSu ultimate margin of safety MSy yield margin of safety n load-introduction factor nf number of fasteners in a joint p thread pitch P′sep load that causes separation P′tu the applied tensile load that causes the fastener load to exceed the fastening system’s allowable ultimate tensile load if rupture occurs before separation

P′ty the applied tensile load that causes the fastener load to exceed the fastening system’s allowable yield tensile load, if yielding occurs before separation

Pp preload Ppc loss of preload from material creep Pp-max maximum preload Pp-min minimum preload Ppi-j initial preload Ppi-max maximum initial preload Ppi-maxa actual maximum initial preload from a sample of test data Ppi-min minimum initial preload Ppi-mina actual minimum initial preload from a sample of test data Ppi-nom nominal (sample mean) initial preload for a given effective torque Ppr short-term relaxation of preload PsL limit shear load Psu ultimate design shear load Psu-allow allowable ultimate shear load Pt applied tensile load Ptb tensile load in a preloaded bolt PtL limit tensile load Ptu ultimate design tensile load Ptu-allow allowable ultimate tensile load Pty-allow allowable yield tensile load PΔt-max maximum increase in preload due to temperature PΔt-min maximum decrease in preload due to temperature t half-thickness of the clamped members (minus any washers) Tbr-min minimum breakaway torque specified for the locking feature TL-max maximum locking torque (running torque) specified for the locking feature Tmax maximum effective torque Tmin minimum effective torque Ts-max maximum specified torque Ts-min minimum specified torque

3.2.2 Definition of Terms

Allowable Load: The maximum permissible load in a structural part or assembly. As applicable, it can be the specified strength of the hardware (e.g., in a fastener specification); a statistically based, lower-bound, load-carrying capacity derived from test data; or the load derived from the allowable stress of the materials.

Applied Load: Force or moment transferred across a joint; it does not include preload or changes in preload as a result of temperature change. (Note: “Applied load” is also referred to in literature as “external load,” “externally applied load,” or “service load.”)

Bolt Grip: For fasteners with a flat bearing surface head, the dimension from the bearing surface under the head to the end of the full diameter body, measured parallel to the axis of the fastener. For fasteners with a conical bearing surface head, the dimension from the top of the head to the end of the full diameter body, measured parallel to the axis of the fastener. Note that these definitions are similar to what some fastener standards use as the definitions for “Grip.”

Catastrophic Hazard: (1) A hazard that could result in a mishap causing fatal injury to personnel, and/or loss of one or more major elements of the flight vehicle or ground facility. (2) A condition that may cause death or permanently disabling injury, major system or facility destruction on the ground, or loss of crew, major systems, or vehicle during the mission

Critical Hazard: A condition that may cause severe injury or occupational illness, or major property damage to facilities, systems, or flight hardware.

Design Separation Load: The limit tensile load multiplied by a separation factor of safety.

Detrimental Yielding: Yielding that adversely affects fit, form, function, or integrity of the structure.

Effective Torque: A torque in excess of any running torque.

Factor of Safety: A multiplying factor to be applied to limit loads for purposes of analytical assessment (design factor) or test verification (test factor) of design adequacy in strength or stability.

Failure: Rupture, collapse, excessive deformation, or any other phenomenon resulting in the inability of a structure to sustain specified loads, pressures, and environments or to function as designed.

Fastener: For purposes of this NASA Technical Standard, a bolt or a screw that joins two or more parts and transfers load between them.

Fatigue: The cumulative irreversible damage incurred in materials caused by cyclic application of stresses and environments, resulting in degradation of load-carrying capability.

Fitting Factor: A supplemental factor of safety used in analysis of bolted joints to account for uncertainties in load paths and stresses.

Full Diameter Body: For purposes of this NASA Technical Standard, the ASME

B18.12-2001, Glossary of Terms for Mechanical Fasteners definition applies: “The unthreaded portion of a shank whose diameter is generally within the dimensional limits of the major diameter of the thread.” (Note: Some fastener specifications refer to the full diameter body as the “full cylindrical portion of the shank” or simply as a “shank.”)

Hardware Developer: Organization directly responsible for the design, manufacture, analysis, test, and safety compliance documentation of the hardware.

Limit Load: The maximum expected applied load, including load transferred across joints as a result of thermally induced loading of the structure; it does not include preload or changes in preload occurring as a result of temperature change.

Locking Feature: A device, chemical substance, or other physical characteristic added by design to one or more elements of a threaded fastening system to resist vibration-induced loosening or to provide retention against complete disengagement of the fastening elements.

Preload-induced friction forces between mating joint members, mating threads, or under the head or nut elements of a fastening system inherently resist loosening; however, within this NASA Technical Standard, the term “locking feature” is used to refer only to the added design items described above.

• Prevailing torque feature: Fastening system design that relies on friction to resist fastener rotation independent of preload (i.e., deformed thread or locking patch).

• Adhesive locking feature: Fastening system design that uses chemical compounds added at the time of assembly (i.e., anaerobic adhesive, epoxies, urethanes, etc.).

• Mechanical locking feature: Fastening system design employing non-friction elements usually involving a “hard stop” (i.e., cotter pins or safety wire).

Margin of Safety: A measure of a structure’s predicted reserve strength in excess of the design criteria. For a preloaded fastener, it represents the percentage that the design load (limit load multiplied by the applicable factor of safety) can increase before the design criteria are no longer satisfied.

Preload: The tensile force in a bolt and the equivalent compressive force in the joint members when there is no applied load.

Prevailing Torque or Running Torque: The torque required to overcome kinetic friction of the mating threads plus the torque required to overcome the locking feature when 100 percent of the locking feature is engaged and the fastener is unseated. This torque can be measured in either a loosening or a tightening direction while the mating threads are in relative motion.

Separation: The state of no compressive load between mating parts local to the fastener.

For a joint designed to maintain a seal, it is further defined as any condition that enables a liquid or gas to penetrate the seal at an unacceptable rate. Also referred to as “gapping.”

Separation-Critical Joint: A joint that fails to function as required if separated.

Separation Load: The minimum applied tensile load that causes separation.

Service Life: All significant loading cycles or events during the period beginning with manufacture of a component and ending with completion of its specified use. Testing, transportation, lift-off, ascent, on-orbit operations, descent, landing, and post-landing events are to be considered.

Service Life Factor (Life Factor): A multiplying factor to be applied to the maximum expected number of load cycles in the service life to determine the design adequacy in fatigue or fracture.

Shank: For purposes of this NASA Technical Standard, the ASME B18.12-2001 definition applies: “That portion of a headed fastener that lies between the head and the extreme point end.”

Threaded Fastening System: (Also referred to as “fastening system.”) An assembled combination of a fastener, an internally threaded part such as a nut or an insert, and also the region of all parts clamped between them, including washers, compressed by the fastener preload.

Ultimate Design Load: The product of the ultimate factor of safety, the fitting factor, and the limit load. Also referred to as “design ultimate load.”

Yield Design Load: The product of the yield factor of safety, the fitting factor, and the limit load. Also referred to as “design yield load.”

4. REQUIREMENTS

This section specifies general requirements for threaded fastening systems.

Within each requirement section, there are three sections of text. The actual requirement is shown in normal, non-italicized text along with a requirement label. Rationale for the requirement is shown as italicized text in brackets and is not to be considered a requirement.

Additional guidance for each requirement is provided below the rationale statement as italicized text. The guidance is likewise not considered to be a requirement and is not an endorsement of a particular process or equation. The intent of the guidance is to provide additional clarifying language, a demonstration of a principle using a simple condition, or possible techniques or processes to show compliance with the requirement.

4.1 Fastening System Control Plan

[TFSR 1] At the Preliminary Requirements Review, equivalent program milestone review, or project milestone review, each hardware developer shall submit a Fastening System Control Plan to the delegated NASA Technical Authority that:

a. Shows how the requirements in this NASA Technical Standard are to be satisfied;

and

b. Includes any organization-specific requirements and criteria for design, analysis, fastener installation, and verification; and

c. Captures or refers to organization-specific processes for ensuring quality and integrity.

[Rationale: A Fastening System Control Plan establishes a defined and approved agreement between the hardware developer and the delegated NASA Technical Authority. By agreeing to the plan early in the design, there is reduced likelihood of disagreements over verification strategies.]

Each hardware developer has the responsibility for submitting a Fastening System Control Plan.

The Fastening System Control Plan can be included as a portion of a broader structural verification plan.

4.2 Design Factors

4.2.1 Factor of Safety

[TFSR 2] Threaded fastening system hardware shall be designed using the structural factors of safety specified by the program or project for general structural design.

An example is NASA-STD-5001, Structural Design and Test Factors of Safety for Spaceflight Hardware.

[Rationale: Factors of safety are levied to provide margin between predicted service loads and the material strength.]

Factors of safety are applicable to limit load. A yield factor of safety, FSy, is applied in the assessment of yield strength, and an ultimate factor of safety, FSu, is applied in the assessment of ultimate strength. Factors of safety do not apply to fastener preload or changes in preload as a result of thermo-elastic deformation local to the fastening system hardware (for example, coefficient of thermal expansion mismatch between the bolt and joint).

4.2.2 Fitting Factor

[TFSR 3] Threaded fastening system hardware shall be designed using a fitting factor (FF).

[Rationale: A fitting factor is included in the strength and separation analysis of each part of the threaded fastening system to account for uncertainties in load paths and stresses.]

The factor of safety is multiplied by the fitting factor and typically applied to the tension and shear components, but not the preload, for fastened joints with multiple bolt locations. An example is shown in Eq. 15.

For purposes of analysis, joint members integral to a larger structural member are considered part of the threaded fastening system up to the point where their section properties become typical of the structural member away from the threaded fastening system.

Ultimate strength analysis of threaded fastening systems should include a fitting factor of at least

1.15 as a multiplier of the required ultimate factor of safety. The selection of a fitting factor for ultimate strength analysis should be strongly influenced by whether there is enough ductility in the critical failure mode to ensure load sharing between fasteners before any one fastening system fails. For example, consider a shear joint with metal joint members: Shear failure of a fastener can have little associated plastic deformation, whereas bearing failure of the joint member typically has considerable plastic deformation. Uneven load sharing can be the result of oversized bolt holes; but even if the bolts are in interference-fit holes, loads may not distribute evenly. If the margin of safety on bearing is lower than the margin of safety for bolt shear (referred to as a bearing-critical joint), shear loads will distribute more evenly between bolts before the highest-loaded bolt fails. A shear-critical joint typically warrants a larger fitting factor.

Yield strength analysis of threaded fastening systems whose performance is particularly sensitive to local yielding should include a fitting factor of at least 1.15 as a multiplier of the required yield factor of safety.

In threaded fastening systems where there is clearance between the fasteners and the bearing surfaces, there is a likelihood that some fasteners may not share the load equally; and some fasteners may be unloaded. The selection of the fitting factor or the prediction of load sharing should reflect the expected clearances between the fasteners and the bearing surfaces and the stiffness of the fastened joint members.

The value of the fitting factor may be reduced when there is less uncertainty in the load paths and stresses or there is little sensitivity to those uncertainties in the analysis of threaded fastening systems. A fitting factor of 1.0 may be adequate in the strength analysis of fastened joints when at least one of the following applies:

• The threaded fastening system’s strength is verified in an ultimate load test in which actual load paths and stresses are simulated in the joint and surrounding structure;

or

• The threaded fastening system’s load paths and stresses have been determined with detailed finite element analysis using modeling practices that have been correlated with tests of similar threaded fastening systems in which actual load paths and stresses were simulated and measured; or

• The threaded fastening system contains redundant load paths (e.g., multiple fasteners) and sufficient ductility to allow the load to redistribute before failure.

Separation analysis of joints that are separation-critical should include a fitting factor of at least

1.15 as a multiplier of the required separation factor of safety.

A fitting factor of 1.0 may be adequate in the strength analysis of fastened joints when at least one of the following applies:

• The threaded fastening system is not separation-critical; or

• The threaded fastening system is separation-critical but the joint’s functionality is verified in a test to limit load or greater on hardware representative of the flight design in which actual load paths and stresses are simulated in the joint and surrounding structure; or

• The threaded fastening system is separation-critical but the joint’s load paths and stresses have been determined with detailed finite element analysis using modeling practices that have been correlated with tests of similar threaded fastening systems in which actual load paths and stresses were simulated and measured.

4.2.3 Separation Factor of Safety

[TFSR 4] Threaded fastening system hardware shall be designed using the separation factor of safety (FSsep) specified by Figure 1, Logic Flow for Minimum Separation Factor of Safety.

Figure 1—Logic Flow for Minimum Separation Factor of Safety

[Rationale: A factor of safety greater than 1.0 applied to separation analysis provides additional margin between predicted service loads and the nominal or minimum expected preload in a fastened joint.]

A separation-critical joint is a joint that fails to function as required if separated. Examples of separation-critical joints include, but are not limited to, joints that must maintain contact to enable proper function of a system (e.g., thermal, electrical, fluid) or joints whose dynamic stiffness is reduced due to separation.

Primary structure with joints classified as “non-separation critical” and FSsep = 1.0 could separate when tested above limit load (e.g., qualification tests using 1.25 x limit load) producing alignment/performance issues, a shift in dynamic response, or both. In these scenarios, if the consequence of joint separation during testing can lead to a failure to satisfy test success criteria, then consider selecting a greater value for FSsep to produce a positive margin of safety on separation under test loads and environments, as well as flight loads and environments.

4.3 Fastening System Preload

Common methods of achieving a desired preload during installation are controlling the installation torque, controlling the turn angle of a nut or bolt head after the fastening system is seated, measuring the fastener’s change in length, and using strain gauges on the fastener body.

These methods are listed in order of increasing labor, cost, and accuracy. The most economical method, torque control, is the least accurate for controlling preload.

Refer to Appendix A for explanation and justification of fastener analysis criteria.

4.3.1 Maximum and Minimum Preload

[TFSR 5] Maximum and minimum preload calculations shall account for the preload variation associated with the method of developing initial preload, potential relaxation, creep, and the effects of maximum and minimum expected temperatures.

[Rationale: Many variables can affect the preload that is developed in a threaded fastening system during and after assembly. Proper characterization and use of the full range of possible preloads are necessary to ensure structural integrity is verified.]

Maximum and minimum preloads are calculated as

𝑃𝑃𝑝𝑝−𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑚𝑚𝑚𝑚 + 𝑃𝑃∆𝑡𝑡−𝑚𝑚𝑚𝑚𝑚𝑚 (Eq. 1)

𝑃𝑃𝑝𝑝−𝑚𝑚𝑝𝑝𝑚𝑚 = 𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑝𝑝𝑚𝑚 − 𝑃𝑃𝑝𝑝𝑝𝑝 − 𝑃𝑃𝑝𝑝𝑝𝑝 − 𝑃𝑃∆𝑡𝑡−𝑚𝑚𝑝𝑝𝑚𝑚 (Eq. 2) where Ppi-max and Ppi-min are the maximum and minimum initial preloads, respectively; the other variables are defined in Table 1, Analytical Adjustments of Maximum and Minimum Preloads.

Table 1—Analytical Adjustments of Maximum and Minimum Preloads

Symbol

Description Adjustment used when calculating maximum preload

Adjustment used when calculating minimum preload

Ppr Short-term relaxation of preload (often attributed to embedment of imperfectly matched surfaces); see Appendix A.3

Zero 5% of minimum initial preload for joints with all-metallic clamped parts, test-derived value for joints with any non-metallic parts or coatings)(1)

Ppc Loss of preload from material creep

Zero Calculated maximum expected preload loss from creep, if applicable

PΔt-max, PΔt-min

Change of preload with temperature (differential coefficients of thermal expansion and temperature dependence of elastic moduli)(2)

Calculated maximum increase in preload, PΔt-max, for maximum or minimum expected temperatures

Calculated maximum decrease (as a positive number) in preload, PΔt-min, for maximum or minimum expected temperatures

(1)For fastened joints with multiple faying surfaces, the assumption of 5 percent relaxation may be non-conservative. In these cases, it is recommended to perform testing to determine the relaxation or analysis that considers the creep-relaxation behavior of the fastening system design.

(2)An effective technique to minimize the change of preload with temperature is described in the SAE AIR 1754A, Washer, Thermal Compensating, Metric Series.

Maximum initial preload for strength and fatigue analyses is calculated as

𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑐𝑐𝑚𝑚𝑚𝑚𝑚𝑚(1 + 𝛤𝛤)𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑛𝑛𝑚𝑚 (Eq. 3) where Ppi-nom is the nominal (mean) preload applicable to installation, Γ is the preload variation, and cmax is a factor that accounts for the maximum value of the controlled installation parameter, as allowed by the specified tolerance. For example, if torque control is used and the effective torque is specified as 40 ± 2 N-m, then cmax = (40 + 2)/40 = 1.05.

For use in separation analysis of separation-critical joints and for fatigue analysis, minimum initial preload is calculated as

𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑝𝑝𝑚𝑚 = 𝑐𝑐𝑚𝑚𝑝𝑝𝑚𝑚(1 − 𝛤𝛤)𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑛𝑛𝑚𝑚 (Eq. 4) where cmin is a factor that accounts for the minimum value of the controlled installation parameter, as allowed by the specified tolerance. For example, if torque control is used and the effective torque is specified as 40 ± 2 N-m, then cmin = (40 – 2)/40 = 0.95.

For use in joint-slip analysis and separation analysis of joints that are not separation-critical, minimum initial preload is calculated as

𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑝𝑝𝑚𝑚 = 𝑐𝑐𝑚𝑚𝑝𝑝𝑚𝑚 �1 −

𝛤𝛤

�𝑛𝑛𝑓𝑓

�𝑃𝑃𝑝𝑝𝑝𝑝−𝑚𝑚𝑛𝑛𝑚𝑚

(Eq. 5) where nf is the number of fasteners in the joint. See Appendix A.2 for rationale.

In addition to the adjustments noted in Table 1, loss of preload may occur if the joint experiences cyclic slip. To minimize any such loss, the joint should be designed in any one of the following ways:

• Not to slip; or

• With minimal clearance fits achieved by specifying precision fasteners having a closely controlled or “close tolerance” full diameter body in conjunction with precision holes per National Aerospace Standard (NAS) 618, Fastener – Recommended Shank, Hole, and Head-to-shank Fillet Radius, Limits For; or

• With liquid thread fillers that cure properly; or

• With shear pins (dowel pins) or other dedicated shear-transfer devices.

4.3.2 Nominal Preload

[TFSR 6] Calculation of the nominal (mean) initial preload, Ppi-nom, shall be substantiated by tests of a minimum of six sets of the fastening system hardware per Table 2, Nominal Preload Determination, to determine the relationship between initial preload and the parameter controlled during installation (torque, turn-of-nut, turn-angle, or bolt stretch).

[Rationale: When using torque control, the torque-preload relationship has been shown to be sensitive to where lubrication is applied. Significant differences have been observed, for example, when lubricating only under the head compared to under the head and nut. When using turn-of-nut, turn-angle, or bolt stretch designs, the torque-preload relationship has been observed to be configuration dependent; therefore, tests should use hardware and processes that are identical to the flight design.]

Table 2—Nominal Preload Determination Torque

Control(1,2) Turn-of-Nut or

Turn-Angle Bolt

Stretch

The fastening system hardware has the same diameter and thread form, the same type and number of washers, same materials, and same nut/nut plate/insert as the flight assembly. The clamped part and washer that are adjacent to a non-rotating bolt head or non-rotating nut are not critical and are allowed to vary from the flight assembly.

R R R

Cleaning, lubricants, and lubrication process are the same as flight assembly.

R R NR

The fastening system hardware is the same specification (part number) as the flight assembly hardware.

NR R R

Installation process is the same as flight assembly.

NR R R

R=Required, NR=Desirable but not required

(1) At least three tests (install, torque, and removal) should be performed on each of the six sets of fastening system hardware for a total of eighteen tests, unless reuse is prohibited in the flight assembly.

(2) See NASM 1312-15, Fastener Test Methods, Method 15, Torque-Tension, for guidance for torque-tension testing.

4.3.3 Preload Variation

[TFSR 7] The preload variation, Γ, used to calculate the minimum and maximum initial preload shall be based on the criteria of Table 3, Preload Variation Determination.

[Rationale: Testing has demonstrated that when the same torque is applied, there is variability in the measured preload. To ensure safe operation of fastening system hardware, this variability should be accounted for in the design. A 90 percent probability and 95 percent confidence limit was selected to provide balance, analogous to a B-basis quantity, between conservatisms and limiting the required test matrix to a reasonable number of specimens.]

Table 3 – Preload Variation Determination Method Separation Critical Non-Separation Critical

Torque Control

Envelope of statistical basis(1) and variation to satisfy Table 2(2) while using

a. Lot-specific testing (testing the procurement lot of fastening system hardware that will be used for spaceflight) (3) or

b. Testing equal numbers of sets of fastening system hardware of the same specification as the flight-assembly hardware, from each of at least three procurement lots(3)

a. Statistical basis(1), or

b. Greater of

(1) 25 percent (if lubricated),

(2) 35 percent (if non-lubricated or as-received),

(3) Variation from tests to satisfy Table 2(2)

Turn-of- Nut or Turn Angle

Envelope of statistical basis(1) and variation to satisfy Table 2(2)

a. Statistical basis(1), or

b. Greater of

(1) 25 percent

(2) Variation from tests to satisfy Table 2(2)

Bolt Stretch

Envelope of statistical basis(1) and variation to satisfy Table 2(2)

a. Statistical basis(1), or

b. Greater of

(1) 10 percent

(2) Variation from tests to satisfy Table 2(2)

(1)90 percent probability and 95 percent confidence (two-sided distribution), with tests meeting the configuration requirements of Table 2.

(2)The variation from the tests to satisfy Table 2 is given by the minimum and maximum values of preload relative to the nominal preload such that Γa-max=Ppi-maxa/Ppi-nom-1 and Γa-min=1-Ppi-mina/Ppi-nom

(3)Each fastener to be installed, torqued, and removed at least three times unless reuse is prohibited for the flight assembly. When reuse is prohibited, only the data from the first cycle is used.

For non-separation critical joints, Table 3 provides several methods for determining the preload variation, Γ. The preferred approach is to use a statistical basis with 90 percent probability and

95 percent confidence for a two-sided distribution. However, to avoid excessive testing, it is permissible to use the alternate (not the statistical) method shown in Table 3.

When using the alternate (not the statistical) method for non-separation critical joints, if the test-measured variation exceeds the Γ values stated above, then the test-measured variation should be used or methods for reducing the preload variation should be pursued.

When using the alternate method for torque control, a fastener is considered to be lubricated if a solid-film, grease, or liquid lubricant is applied to the threads and to the turning bearing surfaces of the nut or the bolt head (or to the washer under the nut or the bolt head).

Appendix A.2 shows an example of how to calculate minimum and maximum preloads when preload is generated with torque control.

4.4 Strength Requirements

4.4.1 Ultimate Design Loads

[TFSR 8] Threaded fastening system hardware shall withstand ultimate design loads (limit load times the ultimate factor of safety and fitting factor) without failure when subjected to:

a. The accompanying service environments (for example, temperature) and

b. A coefficient of friction between clamped parts equal to zero (applicable only to verification by analysis, not applicable to verification by test) (see TFSR 13).

Withstanding ultimate design loads can be demonstrated by test or by analysis where a non-negative margin of safety is calculated.

Use of friction to react applied shear forces between the clamped parts is not permissible for ultimate load assessment.

[Rationale: Withstanding the ultimate design loads validates the integrity of the structure against failure in the expected service environment. Under ultimate load conditions, it is assumed that static friction is overcome and shear forces are reacted through bearing contact.]

Failure, as used in TFSR 8, is the inability to meet any performance requirements applicable at ultimate load. Examples of failure include rupture of the joint and leaking of a joint that can credibly lead to a catastrophic hazard.

Assessment for ultimate design loads addresses potential rupture in all elements of the threaded fastening system, including the fastener, the internally threaded part such as a nut or an insert, and the clamped parts. Assessment of a procured item such as a nut or a threaded insert should be based on the strength specified for that item rather than on thread-stripping analysis. Such items can expand under load, reducing the thread engagement areas.

Each internally threaded part used for design should have either a specified and controlled allowable tensile load, or an allowable tensile load derived from dedicated testing. Many threaded inserts have two allowable tensile loads that should be considered: the minimum allowed tensile load capability of the insert internal threads, and the minimum allowed tensile load for pullout of the insert from the parent material. One or both may be provided in the insert specification (or the procurement specification). The lower value should be used for strength analysis.

When performing ultimate analysis, the analysis should account for load redistribution due to geometric design features, localized yielding, separation, or slipping. The loads acting on individual members of the fastened joint are dependent on features of the joint design (e.g., fastener to bearing surface clearance), materials (e.g., a bearing-critical versus shear-critical joint design), and friction (resistance to slipping is highest under a preloaded member and drops with distance from the member). Examples of load redistribution are shown in Appendix A.12.

An insert’s allowable pull-out load depends on the material in which the insert is installed (parent material). The most common failure mode associated with pull-out is shear of internal threads in the parent material. The specifications (or procurement specifications) for most threaded inserts define how the allowable pull-out load is calculated such as by multiplying a specified minimum shear engagement area by the allowable ultimate shear stress of the parent material. Such an allowable pull-out load applies when the insert is installed in a solid, homogenous material. For inserts installed in nonhomogeneous or nonmetallic materials or in sandwich panels, allowable pull-out loads should be derived from test. Nuts should be limited to the load rating of the nut.

The margin of safety indicates how much the applied load can increase before the criteria are no longer satisfied. Simplistic equations for calculating the margin of safety for ultimate under axial load are

𝑀𝑀𝑀𝑀𝑢𝑢 =

𝑃𝑃𝑡𝑡𝑢𝑢−𝑚𝑚𝑎𝑎𝑎𝑎𝑛𝑛𝑎𝑎

𝐹𝐹𝐹𝐹 ∙ 𝐹𝐹𝑀𝑀𝑢𝑢 ∙ 𝑃𝑃𝑡𝑡𝑡𝑡

− 1 (Eq. 6) when separation occurs before rupture, and

𝑃𝑃𝑡𝑡𝑢𝑢′

𝐹𝐹𝐹𝐹 ∙ 𝐹𝐹𝑀𝑀𝑢𝑢 ∙ 𝑃𝑃𝑡𝑡𝑡𝑡

− 1

(Eq. 7) when rupture occurs before separation, where P′tu is the applied tensile load that causes the fastener load to exceed the fastening system’s allowable ultimate tensile load if rupture occurs before separation, and Ptu-allow is the allowable ultimate load for the fastening system.

Linear theory, empirical, or semi-empirical methods can be used to assess ultimate tensile strength of the fastening system and if rupture occurs before separation. Appendix A.6 provides further detail on this approach. With linear theory, the tensile load in a preloaded bolt, Ptb, increases proportionally with the applied tensile load per Eq. 8 until either any part in the fastening system ruptures or separation occurs.

𝑃𝑃𝑡𝑡𝑡𝑡 = 𝑃𝑃𝑝𝑝 + 𝑛𝑛𝑛𝑛𝑃𝑃𝑡𝑡 (Eq. 8) where Pp is the preload, Pt is the applied tensile load, n is the load-introduction factor (see Appendix A.4) that depends on the joint design and accounts for where the load is applied to the joint, and φ is the stiffness factor. The stiffness factor is 𝑛𝑛 = 𝑘𝑘𝑡𝑡 𝑘𝑘𝑡𝑡 + 𝑘𝑘𝑝𝑝

(Eq. 9) where kb is the stiffness of the bolt and kc is the stiffness of the clamped parts local to the fastener.

Based on Eq. 8 and the assumption of maximum preload, Pp-max, the applied tensile load that causes the bolt load to exceed the allowable ultimate tensile load for the fastening system, P′tu, is

𝑃𝑃𝑡𝑡𝑢𝑢′ =

𝑛𝑛𝑛𝑛

�𝑃𝑃𝑡𝑡𝑢𝑢−𝑚𝑚𝑎𝑎𝑎𝑎𝑛𝑛𝑎𝑎 − 𝑃𝑃𝑝𝑝−𝑚𝑚𝑚𝑚𝑚𝑚�

(Eq. 10) and the linearly projected load that causes separation when at maximum preload is

𝑃𝑃𝑠𝑠𝑠𝑠𝑝𝑝′ =

𝑃𝑃𝑝𝑝−𝑚𝑚𝑚𝑚𝑚𝑚

1 − 𝑛𝑛𝑛𝑛

(Eq. 11)

If P′sep is less than P′tu, linear theory predicts that separation would occur before rupture, and the ultimate margin of safety for tensile loading is calculated per Eq. 6. Conversely, Eq. 7 should be used when P′sep is greater than P′tu, as linear theory predicts rupture would occur before separation.

The allowable ultimate shear load for a fastener depends on whether or not the threads are in the shear plane. If threads are not in the shear plane, the allowable ultimate shear load per shear plane is commonly assumed to be half the value given in the fastener specification for double-shear joints, when applicable, or is calculated by

𝑃𝑃𝑠𝑠𝑢𝑢−𝑚𝑚𝑎𝑎𝑎𝑎𝑛𝑛𝑎𝑎 =

𝜋𝜋𝐷𝐷2𝐹𝐹𝑠𝑠𝑢𝑢

(Eq. 12) where Fsu is the allowable ultimate shear strength for the fastener material. If threads are in the shear plane, the allowable ultimate shear load for a fastener is calculated by

𝑃𝑃𝑠𝑠𝑢𝑢−𝑚𝑚𝑎𝑎𝑎𝑎𝑛𝑛𝑎𝑎 = 𝐹𝐹𝑠𝑠𝑢𝑢𝐴𝐴𝑚𝑚 (Eq. 13) where Am is the minimum minor-diameter area for the fastener threads. The ultimate margin of safety for shear loading of a fastener is

𝑃𝑃𝑠𝑠𝑢𝑢−𝑚𝑚𝑎𝑎𝑎𝑎𝑛𝑛𝑎𝑎

𝐹𝐹𝐹𝐹 ∙ 𝐹𝐹𝑀𝑀𝑢𝑢 ∙ 𝑃𝑃𝑠𝑠𝑡𝑡

− 1 (Eq. 14) where PsL is the limit shear load acting on the shear plane.

4.4.2 Yield Design Loads

[TFSR 9] Threaded fastening system hardware shall withstand yield design loads (limit load times the yield factor of safety and fitting factor) without detrimental yielding or detrimental deformation when subjected to the accompanying service environments (for example, temperature).

[Rationale: Withstanding yield design loads validates the integrity of the structure against yielding or deformations that could adversely affect the functionality of critical systems in the expected service environment.]

Withstanding yield design loads can be demonstrated by test or by analysis where a non-negative margin of safety is calculated.

Yield strength analysis is not required if there is no detrimental yielding or detrimental deformations at the yield design load. The assessment for yield design loads will address all elements of the threaded fastening system, including the fastener, the internally threaded part such as a nut or an insert, and the clamped parts.

Examples of detrimental deformations include

• Fastener yielding causing the joint to separate under an applied tensile load that is less than the design separation load.

• Fastener yielding causing the joint to suffer detrimental slip under an applied shear load that is less than the applicable design shear load.

• Deformations that degrade mission performance.

• Some other design-specific reason exists for why fastener yielding is detrimental (e.g., any fastener yielding that adversely affects the form, fit, or function of the design).

In a typical joint with preload generated by torque control, fastener yielding under combination of preload and applied load is not detrimental for separation (see first bullet above), given the separation analysis criteria specified in section 4.4.3. Rationale and exceptions for the above statement, as well as a discussion of the effects of fastener yielding on joint slip (see second bullet above), are provided in Appendix A.9.

When performing yield analysis, the analysis should account for load redistribution due to geometric design features, localized yielding, separation, or slipping. The loads acting on individual members of the fastened joint are dependent on features of the joint design (e.g., fastener to bearing surface clearance), materials (e.g., a bearing-critical versus shear-critical joint design), and friction (resistance to slipping is highest under a preloaded member and drops with distance from the member). Examples of load redistribution are shown in Appendix A.12.

In a linear analysis, the margin of safety indicates how much the applied load can increase before the minimum design factor of safety criterion is no longer satisfied.