NASA-STD-5020.pdf

NASA TECHNICAL STANDARD

National Aeronautics and Space Administration

Washington, DC 20546-0001

NASA-STD-5020

Approved: 03-12-2012

REQUIREMENTS FOR THREADED FASTENING

SYSTEMS IN SPACEFLIGHT HARDWARE

MEASUREMENT SYSTEM IDENTIFICATION:

METRIC/SI (ENGLISH)

APPROVED FOR PUBLIC RELEASE – DISTRIBUTION IS UNLIMITED

NASA-STD-5020

DOCUMENT HISTORY LOG

Status Document

Revision

Approval Date Description

Baseline 03-12-2012 Initial Release

FOREWORD

This 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 Standard is approved for use by NASA Headquarters and NASA Centers, including

Component Facilities and Technical and Service Support Centers.

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

Requests for information, corrections, or additions to this Standard should be submitted via

“Feedback” in the NASA Standards and Technical Assistance Resource Tool at http://standards.nasa.gov.

Original Signed By: 03-12-2012

Michael G. Ryschkewitsch Approval Date

NASA Chief Engineer

TABLE OF CONTENTS

SECTION PAGE

DOCUMENT HISTORY LOG

FOREWORD

TABLE OF CONTENTS

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

2.4.1 Document Conflicts

3. ACRONYMS AND DEFINITIONS

3.1 Acronyms and Abbreviations

3.2 Definitions

3.2.1 Definitions of Variables

3.2.2 Definition of Terms

4. GENERAL REQUIREMENTS

4.1 Strength

4.1.1 Ultimate Design Loads

4.1.2 Yield Design Loads

4.2 Fracture Control and Fatigue Life

4.3 Joint Separation

4.4 Locking Features

4.5 Fastening System Control Plan

5. DESIGN REQUIREMENTS AND CONSIDERATIONS

5.1 Materials

5.2 Specification of Lubricants, Coatings, and Sealants

5.3 Thread Form Compatibility

5.4 Use of Washers and Chamfered Bolt Holes

5.5 Locking Features

5.6 Thread Engagement, Dimensions, and Tolerances

TABLE OF CONTENTS (Continued)

SECTION PAGE

5.6.1 Fastener Length Selection for Thread Engagement

5.6.2 Bolt Grip Selection to Prevent Interference

5.7 Fastener Installation Specification and Control

5.8 Installation Torque Specification and Control

6. CRITERIA FOR ANALYSIS OF THREADED FASTENING

SYSTEMS

6.1 Nominal, Maximum, and Minimum Preload

6.2 Strength Under Ultimate Design Loads

6.2.1 Ultimate-Strength Analysis for Tensile Loading

6.2.2 Ultimate-Strength Analysis for Shear Loading

6.2.3 Ultimate-Strength Analysis for Interaction of Tension, Shear, and

Bending

6.3 Strength Under Yield Design Loads

6.4 Friction as a Load Path for Shear Loading: Joint-Slip Analysis

6.5 Joint Separation Analysis

7. QUALITY ASSURANCE

7.1 As-Built Documentation

7.2 Training

7.3 Installation Tools and Instruments

7.4 Calibration of Installation Tools and Instruments

7.5 Threaded Fastening System Hardware Inspection

7.6 Locking Feature Verification

7.7 Fastening System Hardware Procurement, Inspection, and Storage

APPENDICES

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 6.1)

A.2.1 Rationale for Eqs. (6.5b) and (A.2-3b)

A.3 Short-Term Relaxation of Preload (Supplement to Section 6.1)

A.4 Use of a Load-Introduction Factor (Supplement to Section 6.2.1.2)

A.5 Bolt Analysis: Separation Before Rupture (Supplement to Section 6.2.1) .. 48

A.6 Ultimate Margin of Safety for Tensile Loading with Linear Theory

(Supplement to Section 6.2.1.2)

A.7 Omission of Preload in Shear and Interaction Analyses (Supplement to

Section 6.2.2)

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

6.2.3)

A.9 Determining if Fastener Yielding is Detrimental for Separation or Joint

Slip (Supplement to Section 6.3)

A.10 Margin of Safety for Joint Slip (Supplement to Section 6.4)

A.11 Margin of Safety for Bolted Joint Separation (Supplement to Section

6.5)

A.12 Use of a Fitting Factor

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

B.5.7 Verification

B.5.8 General

B.6 Best Practices for Free Spinning

APPENDICES (Continued)

C. JUSTIFICATION FOR THE LOW-RISK FATIGUE

CLASSIFICATION

C.1 Purpose of Appendix C

C.2 Conclusions

D. REFERENCES

D.1 Purpose of Appendix D

D.2 Reference Documents

FIGURE PAGE LIST OF FIGURES

1 Logic Flow for Minimum Separation Factor of Safety

2 Dimensional Considerations in Selecting Fastening Hardware

3 Ultimate Strength Analysis of Fasteners Under Shear Loading

4 Dimensions Used to Calculate Load-Introduction Factor

5 Determining Whether a Joint Separates Before Rupture When Loaded

Solely in Tension

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

7 Preloaded Joint in Which the Bolt Load Increases by a Relatively High

Percentage of Applied Tensile Load

8 Separation Before Rupture for a Brittle Threaded Fastening System

9 Separation Before Rupture for a Ductile Threaded Fastening System

10 Bolt Tensile Load versus Applied Tensile Load

11 Separation Before Rupture with Linear Theory

12 Rupture Before Separation with Linear Theory

13 Stress Distribution at Collapse Due to Combined Bending and Tension

14 Comparison of Interaction Between Circular and Rectangular Cross

Sections at Collapse

15 Effect of Fastener Yielding on the Separation Load

16 Loss of Preload Resulting from Fastener Yielding

17 Fastener Yielding Under an Applied Load that Exceeds the Separation

Load

18 Locking Feature Selection Flowchart

LIST OF TABLES

TABLE PAGE

1 Analytical Adjustments of Maximum and Minimum Preloads

2 Two-Sided 90/95 Tolerance Limit Factors for a Normal Distribution

(Odeh and Owen, 1980)

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

4 Hypothetical Applied-Loading Spectrum Used to Justify the Low-Risk

Classification for Faster Fatigue…………………………………………... 82

5 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 Standard is to specify requirements for design and analysis of threaded fastening systems in NASA spaceflight hardware.

1.2 Applicability

This Standard is applicable to all NASA programs and projects. This Standard may not be applicable to ground support equipment. Additional requirements for fracture control, non metallic structures, non-standard fasteners, and fasteners used in extravehicular activity (EVA) may apply.

This Standard is approved for use by NASA Headquarters and NASA Centers, including

Component Facilities and Technical and Service Support Centers, and may be cited in contract, program, and other Agency documents as a technical requirement. This Standard may also apply to the Jet Propulsion Laboratory or to other contractors, grant recipients, or parties to agreements only to the extent specified or referenced in their contracts, grants, or agreements.

Requirements are numbered and indicated by the word “shall.” Explanatory or guidance text is indicated in italics beginning in section 4. This document shall not supersede applicable laws and regulations unless a specific exemption has been obtained by the Office of the NASA Chief

Engineer.

1.3 Tailoring

Tailoring of this Standard for application to a specific program or project shall be formally documented as part of program or project requirements and approved by the Technical

Authority.

2. APPLICABLE DOCUMENTS

2.1 General

The documents listed in this section contain provisions that constitute requirements of this

Standard as cited in the text.

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

2.1.2 Non-use of specific versions as designated shall be approved by the responsible

Technical Authority.

The applicable documents are accessible via the NASA Standards and Technical Assistance

Resource Tool at https://standards.nasa.gov or may be obtained directly from the Standards

Developing Organizations or other document distributors.

2.2 Government Documents

NASA

NASA-STD-5001 Structural Design and Test Factors of Safety for Spaceflight

Hardware

NASA-STD-5019 Fracture Control Requirements for Spaceflight Hardware

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 Document

American Society of Mechanical Engineers (ASME)

ASME B107.300

Torque Instruments

2.4 Order of Precedence

This Standard establishes requirements for design and analysis of threaded fastening systems in

NASA spaceflight hardware but does not supersede nor waive established Agency requirements found in other documentation.

2.4.1 Document Conflicts

Conflicts between this Standard and other requirements documents shall be resolved by the responsible Technical Authority.

3. ACRONYMS AND DEFINITIONS

3.1 Acronyms and Abbreviations

% percent

°F degrees Fahrenheit

AC Advisory Circular

AND Air Force-Navy Design

ASME The American Society of Mechanical Engineers

ASTM American Society for Testing and Materials

CRES corrosion resistant steel

EVA extravehicular activity

FAA Federal Aviation Administration

FS factor of safety

HDBK Handbook

JSC Johnson Space Center

MEK methyl ethyl ketone

MS margin of safety

MSFC Marshall Space Flight Center

MIL Military

NAS National Aerospace Standard

NASA National Aeronautics and Space Administration

NASM National Aerospace Standard (Metric)

NSTS National Space Transportation System

SAE Society of Automotive Engineers

STD Standard

VDI Verein Deutscher Ingenieure (German acronym)

3.2 Definitions

3.2.1 Definitions 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

Γ90/95 uncertainty or scatter in initial preload (calculated from torque-tension test data) selected using 90-percent probability and 95 percent confidence intervals δe elastic deformation of a fastening system at rupture under tensile loading δp plastic deformation of a fastening system at rupture under tensile loading φ stiffness factor µ coefficient of friction µ mean preload for a fastener mean total preload for a joint µnbp σ standard deviation of preload for a fastener σnbp standard deviation of total preload for a joint σpi unbiased sample standard deviation for initial preload τ shear stress

A cross-sectional area

Am minimum minor-diameter area

As cross-sectional area of the full diameter body

At tensile stress area a scaling factor applied to the external loads that causes a zero-margin condition 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 e minimum edge distance measured from center of hole to closest edge of any clamped part other than washers

Eb elastic modulus of the bolt material

Ec lowest elastic modulus of clamped parts excluding washers fbu design ultimate bending stress based on linear-elastic theory

Fbu allowable ultimate flexural stress (bending)

FF fitting factor

FS factor of safety

FSsep factor of safety for separation

Fsu allowable ultimate shear stress

FSu ultimate factor of safety

FSy yield factor of safety

Fsy yield stress in shear (not a typically encountered material property)

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 nut factor joint grip length less total thickness of washers (see figure 7)

Llp distance between the planes at which an applied tensile load is introduced to the joint m number of torque-tension tests (sample size)

M moment

L

Mp fully plastic moment

MS margin of safety

MSsep margin of safety for separation

MSslip margin of safety for joint slip

MSu ultimate margin of safety

MSy yield margin of safety n load-introduction factor

N axial force nf number of fasteners in a joint

Np fully plastic axial force p thread pitch

P′ allowable applied load

P′ sep load that causes separation

P′ su-allow allowable ultimate shear load

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

Pf friction load

PL limit load

Pp preload

Ppc loss of preload from material creep

Pp-max maximum preload

Pp-min minimum preload

Ppi initial preload

Ppi-j initial preload obtained in the j-th torque-tension test

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

PsL-joint the total limit shear load acting on the joint

Psu ultimate design shear load

Psu-allow allowable ultimate shear load

Pt applied tensile load

Ptb tensile load in a preloaded bolt

Pt-joint total applied tensile load acting on a joint

PtL limit tensile load

PtL-joint the total limit tensile load acting on the joint

Ptu ultimate design tensile load

Ptu-allow allowable ultimate tensile load

Pty yield design tensile load

Pty-allow allowable yield tensile load

Pu ultimate design load

Py yield design load

PΔt-max maximum increase in preload due to temperature

PΔt-min maximum decrease in preload due to temperature

Q shear

Qp fully plastic shear r radius s two-sided 90/95 tolerance limit factors for a normal distribution

(number of standard deviations from the mean used to calculate a value at 90 percent probability and 95 percent confidence)

Tbr-min minimum breakaway torque

TL-max maximum locking torque

Tmax maximum effective torque

Tmin minimum effective torque

Ts-max maximum torque specified on the engineering documentation

Ts-min minimum torque specified on the engineering documentation δe elastic deformation of the fastening system at rupture δp plastic deformation of the fastening system at rupture

3.2.2 Definitions of Terms

Allowable Applied Load: The maximum permissible applied load, which is derived either from the hardware specification (e.g., for fastening system hardware) or from the allowable stress of the materials.

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.”)

Body: For purposes of this Standard, the ASME B18.12-2001, Glossary of Terms for

Mechanical Fasteners, definition applies: “The unthreaded portion of the shank” of a threaded fastener.

Body Diameter: For purposes of this Standard, the ASME B18.12-2001 definition applies: “The diameter of the unthreaded portion of the shank of a threaded fastener.” Note that though it appears in some threaded fastener standards, per ASME B18.12-2001, the term “shank diameter” has no meaning for a threaded fastener, as the shank includes the fillet, neck, collar, body, shoulder, thread, point, etc.

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.”

Breakaway Torque: For fastening systems incorporating prevailing torque locking features, this is the torque required to initiate relative motion between mating threads when the self-locking mechanism is fully engaged and the fastener is unseated. For the purpose of this

Standard, this torque can be measured in either the tightening or loosening direction.

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 Standard, a fastener is 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 Standard, the ASME B18.12-2001 definition applies: “The unthreaded portion of a shank whose diameter is generally within the dimensional limits of the major diameter of the thread.” Note that 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.

Joint Grip: A term normally associated with structural bolting, it is the thickness of material or parts that the fastener is designed to secure when assembled. Note that this definition is similar to the ASME B18.12-2001 definition for “Grip.”

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 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 which 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).

Locking Torque: See “Running Torque.”

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. See Eq. (6-1).

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

Prevailing Torque: See Running Torque.

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: (Also referred to as “gapping.”) 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.

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 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. GENERAL REQUIREMENTS

This section specifies general requirements for threaded fastening systems.

4.1 Strength

a. Factors of safety shall be as specified in the applicable standard for the structural assembly (e.g., NASA-STD-5001, Structural Design and Test Factors of Safety for Spaceflight

Hardware).

b. A supplemental factor, referred to as a fitting factor (≥ 1.0), shall be applied. See

Appendix A.12 for guidance on selecting fitting factor values.

Factors of safety are applicable to limit load. Factors of safety do not apply to fastener preload or changes in preload as a result of thermoelastic deformation.

4.1.1 Ultimate Design Loads

All threaded fastening systems shall withstand ultimate design loads in conjunction with the applicable maximum expected range of environmental conditions without rupture. Section 6.2 provides criteria for ultimate-strength analysis. The ultimate design load, Pu, is calculated as

P = FF ⋅ FS ⋅ P u u L (4-1) where FF is the fitting factor, FSu is the ultimate factor of safety, and PL is the limit load.

Additional subscripts 't' and 's' are used to designate tensile and shear loads, respectively (e.g., "Ptu" for "ultimate design tensile load" and "PsL" for "limit shear load"). See Appendix A.12 for guidance on use of fitting factors.

4.1.2 Yield Design Loads

All threaded fastening systems shall withstand yield design loads in conjunction with the applicable maximum expected range of environmental conditions without detrimental yielding.

Section 6.3 provides criteria for yield-strength analysis. The yield design load, Py, is calculated as

P = FF ⋅ FS ⋅ P y y L (4-2) where FSy is the yield factor of safety. Additional subscripts 't' and 's' are used to designate tensile and shear loads, respectively (e.g., "Pty" for "yield design tensile load"). See Appendix

A.12 for guidance on use of fitting factors.

4.2 Fracture Control and Fatigue Life

All threaded fastening systems shall withstand the service life and service environment without fracture or fatigue failure.

Fracture control and fatigue life requirements for threaded fastening systems are the same as those in the general structural requirements levied by NASA on the hardware developer (e.g., NASA-STD-5019, Fracture Control Requirements for Spaceflight Hardware, and NASA-STD

5001). Note that fracture control requirements may not be levied by NASA for some projects, particularly projects for non-human-rated spaceflight hardware. Typically, verification of service life may be by any of the following methods:

a. Crack growth analysis per NASA-STD-5019.

b. Fatigue analysis using the fatigue analysis factor and service life factor specified in the applicable structural requirements (e.g., NASA-STD-5001).

c. Fatigue testing using the service life factor specified in the applicable structural requirements (e.g., NASA-STD-5001).

Alternatively, with the prior approval of the responsible Technical Authority, the low risk approach described below may be used for fasteners in lieu of fatigue analysis or testing. This approach does not relieve any fracture control requirements. A fastener has low risk of fatigue failure if all of the following are true:

• The fastener is made of A-286 alloy.

• The fastener has UNJ or MJ threads rolled after heat treatment and the limit tensile load multiplied by any factors applicable to fatigue analysis does not exceed 50 percent of the fastener’s allowable ultimate tensile load, or

• The fastener has UNR threads rolled after heat treatment and the limit tensile load multiplied by any factors applicable to fatigue analysis does not exceed 40 percent of the fastener’s allowable ultimate tensile load.

• The lowest elastic modulus for the material of any clamped part excluding washers is greater than one-third of the elastic modulus for the bolt material. (See discussion in section

A.5.)

• The minimum edge distance (measured from center of hole to closest edge of any clamped part other than washers) is at least 1.5 times the bolt diameter. (See discussion in section A.5.)

• The limit tensile load does not exceed the minimum preload calculated using Eqs. 6-3 and 6-5a.

• When threads are in the shear plane, either a dedicated shear transfer device, such as a shear pin (dowel pin), is present or a positive margin of safety for joint slip is shown per section 6.4.

See Appendix C for justification of the low-risk classification described above.

4.3 Joint Separation

a. Mechanical joints using threaded fastening system hardware shall withstand the design separation load in conjunction with applicable maximum or minimum temperatures without separation, using the minimum separation factor of safety, FSsep, as defined in figure 1, Logic Flow for Minimum Separation Factor of Safety. Section 6.5 provides criteria for separation analysis.

b. The joint separation factor of safety shall be multiplied by a fitting factor.

Figure 1—Logic Flow for Minimum Separation Factor of Safety

4.4 Locking Features

a. Regardless of the magnitude of preload, each threaded fastening system in spaceflight hardware shall incorporate a minimum of one locking feature that does not depend upon preload to function.

Devices such as jam nuts may not be suitable locking features to satisfy this requirement if they require preload to function effectively, or if a change in preload could compromise the locking performance. A redundant locking feature may be advisable for some joints as described in

Federal Aviation Administration (FAA) Advisory Circular (AC) 20-71, “Dual Locking Devices on Fasteners,” December 8, 1970.

b. Locking features shall be verifiable per section 7.6.

c. A mechanical locking feature shall be used on any bolt subject to rotation in operation.

A bolt is considered to be subject to rotation if it serves as an axis of rotation producing relative motion between the bolt and one or more components attached to the bolt. Friction-type locking features include prevailing torque and adhesive locking features which rely on friction to resist loosening or provide retention. On a bolt subject to rotation in operation, the relative motion between parts may produce forces capable of overcoming the locking friction, so friction-type locking features are not suitable except when they are used in conjunction with a mechanical locking feature such as safety wire. For reference, consult FAA AC 20-71, “Dual Locking

Devices on Fasteners,” December 8, 1970, and FAA AC 23.607-1, “Self-Locking Nuts on Bolts

Subject to Rotation,” August 24, 1984.

4.5 Fastening System Control Plan

Each hardware developer shall submit a Fastening System Control Plan to the responsible

Technical Authority at the Preliminary Requirements Review or equivalent program or project milestone review that:

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

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

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

5. DESIGN REQUIREMENTS AND CONSIDERATIONS

This section specifies design requirements for threaded fastening systems.

5.1 Materials

Materials used in threaded fastening systems shall comply with NASA-STD-6016, Standard

Materials and Processes Requirements for Spacecraft.

Material issues of particular importance for threaded fastening systems include galling, corrosion, stress-corrosion cracking, hydrogen embrittlement, creep, outgassing, and effects of atomic oxygen. These issues and others are addressed in NASA-STD-6016.

5.2 Specification of Lubricants, Coatings, and Sealants

All lubricants, coatings, and sealants used in threaded fastening systems, as well as their application processes, shall be specified and controlled on the engineering documentation per

NASA-STD-6016.

Lubrication of fasteners prior to assembly is often desirable and sometimes necessary to prevent galling or to reduce preload variation when using torque control. For a given applied torque, use of lubricants, coatings, or sealants can increase the preload developed in a mechanical joint over that of an unlubricated joint. The surfaces on which lubricants, coatings, or sealants are applied, or the surfaces to which they migrate, influence preload as well. Lubricants also reduce wear, inhibit fretting, and improve ease of installation and removal of fasteners.

Lubricants used in sensitive applications, such as fluid wetted components, optical systems, and some external spacecraft surfaces, can result in system contamination and performance degradation. Overuse of lubricants should be avoided, even in non-sensitive applications.

5.3 Thread Form Compatibility

Mating threaded parts shall have compatible thread forms.

The following are examples of incompatible thread combinations that can result in dimensional interference and may lead to seizure, inadequate strength, inadequate preload for a specified torque value, or inability to disassemble.

• Fine threads mated to coarse threads.

• Inch-based threads mated to metric threads.

• Mated threads with the same pitch and different nominal diameters.

• External UNJ threads mated to UN internal threads.

• External MJ threads mated to M internal threads.

5.4 Use of Washers and Chamfered Bolt Holes

a. The engineering documentation shall specify types, locations, and allowable quantities of washers.

b. Clearance shall be provided for the head-to-shank fillet radius as needed to avoid interference with the clamped parts through the use of chamfered holes or countersunk washers installed under bolt heads with the countersink facing the bolt head (see figure 2, Dimensional

Considerations in Selecting Fastening Hardware). Flush-head screws should be installed without washers under the head.

5.5 Locking Features

a. Locking features and their installation processes, including verification methods, shall be specified in the engineering documentation.

b. When using locking adhesives, whether as thread-locking compounds or staking materials, installation processes shall be developed and validated prior to implementation to ensure adhesives cure, adhere, and function as expected.

It should be recognized that some locking features are not a means of maintaining preload but rather are safety devices principally intended to resist rotational loosening and prevent loss of fasteners. Examples of the selection of locking features are provided in Appendix B.

5.6 Thread Engagement, Dimensions, and Tolerances

Dimensions, tolerances, and fastening system hardware shall be specified in the engineering documentation to control the features and issues described in the following subsections.

All parts of the threaded fastening system should be dimensionally compatible.

5.6.1 Fastener Length Selection for Thread Engagement

When the threaded fastening system incorporates a prevailing torque locking feature, the fastener length shall be sufficient for fully formed threads to engage the locking feature.

To ensure that all internal threads are fully engaged, the length of each fastener used with a nut, nut plate, or insert should be selected to extend a distance of at least twice the thread pitch, p, past the outboard end of the nut, nut plate, or insert. Figure 2 points out common methods of addressing dimensional issues in hardware selection for a fastener used with a nut.

To avoid thread stripping as the critical failure mode, thread engagement in an internally threaded part other than a nut, nut plate, or insert should be selected to ensure the minimum number of engaged complete threads such that the fastener would fail in tension before threads would strip. It is desirable to have the fastener fail in tension before the threads strip to facilitate ease of repair and to increase the likelihood that any failed hardware may be detected. In addition, fastener tensile failures typically exhibit more plastic deformation prior to rupture, which allows loads to redistribute between fasteners before failure occurs in any one threaded fastening system.

seated, measuring the fastener’s change in length, and using strain gages on the fastener body.

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

5.8 Installation Torque Specification and Control

The requirements in this section apply when preload is generated by torque control.

a. The engineering documentation shall specify the installation torque range or specify an applicable standard that defines the installation torque range.

b. The engineering documentation shall clearly identify when the installation torque is the torque above running torque.

When engineering documentation does not specify “above running torque,” the specified torque is understood to be the final torque indicated by the torque instrument. Running torque does not generate preload. Specifying torque above a measured running torque provides tighter control of preload. Refer to section 6.1.

The engineering documentation should specify whether torque is to be applied to the fastener head or the nut because the relationship between torque and preload is different depending upon which part is torqued. Whenever possible, torque should be applied to the nut. When torque is applied to the fastener head, the fastening system should be designed to ensure no binding occurs between any parts of the rotating shank and its clearance hole.

When it is important for joints with multiple fasteners to have an evenly distributed clamp load, an appropriate torque sequence along with incremental torquing should be specified on the engineering drawing or hardware-specific installation procedure. Example procedures for multi-fastener preloading can be found in MIL-HDBK-60, Threaded Fasteners-Tightening to

Proper Tension, section 5.7: Flange Joint Bolt Tensioning.

6. CRITERIA FOR ANALYSIS OF THREADED FASTENING

SYSTEMS

This section applies to analysis of a threaded fastening system based on the assumption that the applied load acting on that fastening system is known or is adequately predicted. The analysis performed to determine how applied loads distribute between fasteners is outside the scope of this document. Appendix A provides technical rationale for selected analysis criteria in this section. The analysis criteria herein apply only to fasteners made of ductile metallic materials.

Analysis of threaded fasteners made of brittle or nonmetallic materials should be based on test data.

As applicable, analysis results for ultimate strength (section 6.2), yield strength (section 6.3), joint slip (section 6.4), and joint separation (section 6.5) should be reported with a margin of safety, MS, per the general Eq. (6-1). The margin of safety indicates how much the applied load can increase before the criteria are no longer satisfied.

P'

MS = − 1

FF ⋅ FS ⋅ PL (6-1)

where P' is the allowable applied load and FS is the factor of safety.

All margins of safety shall be greater than or equal to zero (MS ≥ 0).

6.1 Nominal, Maximum, and Minimum Preloads

a. Analysis of threaded fastening systems shall address maximum and minimum preloads, accounting for the preload variation associated with the method of developing initial preload, potential relaxation, creep, and the effects of maximum and minimum expected temperatures, as shown in table 1, Analytical Adjustments of Maximum and Minimum Preloads.

Maximum and minimum preloads are calculated as

(6-2) P = P + P t max p − max pi − max Δ −

(6-3) P = P − P − P − P p − min pi − min pr pc Δ t − min where Ppi-max and Ppi-min are the maximum and minimum initial preloads, respectively; the other variables are defined in table 1.

Table 1—Analytical Adjustments of Maximum and Minimum Preloads

Description Symbol

Adjustment used when calculating maximum preload

Adjustment used when calculating minimum preload

Short-term relaxation of Ppr Zero 5% of minimum initial preload (embedment of preload for joints with imperfectly matched all-metallic clamped surfaces); see Appendix A.3 parts, test-derived value for joints with any non metallic parts or coatings

Loss of preload from Ppc Zero Calculated maximum material creep expected preload loss from creep, if applicable

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

PΔt-max, PΔt-min

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

* 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

(6-4) Ppi −max =c max (1+Γ)Ppi −nom 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

P = c (1− Γ)P (6-5a) pi −min min pi −nom 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

⎛ ⎞Γ P = c ⎜1− ⎟ P (6-5b) pi −min min pi −nom ⎜ n ⎟ f⎝ ⎠ 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:

(1) Not to slip (see section 6.4).

(2) 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.

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

b. 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 to determine the relationship between initial preload and the parameter controlled during installation (torque, turn-of-nut, turn-angle, or bolt stretch). Other requirements pertaining to these tests depend on the parameter controlled during installation:

(1) Torque control (see NASM 1312-15, Fastener Test Methods, Method 15, Torque-

Tension for guidance for torque-tension testing).

A. The fastening system hardware shall be of materials that are the same as used in the flight assembly.

B. The fastening system hardware shall be installed using the same lubricants and lubrication process as used on the flight assembly.

C. At least three tests (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) Turn-of-nut, turn-angle, or bolt-stretch control

A. The fastening system hardware shall be of the same specification (part number) as the flight assembly fasteners.

B. The installation process shall be the same as for the flight assembly.

c. For separation-critical joints, the preload variation, Γ, used to calculate the minimum initial preload shall be derived from test data at a statistical basis of 90 percent probability and 95 percent confidence (two-sided distribution). Test requirements depend on the parameter controlled during installation:

(1) Torque control. The tests performed to determine Γ shall meet the requirements of paragraph 6.1.b.1 and should conform to either of the following approaches:

A. Lot-specific testing (testing the actual procurement lot of fastening system hardware that will be used for spaceflight), with each fastener installed at least three times unless reuse is prohibited for the flight assembly.

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, with each fastener installed at least three times unless reuse is prohibited for the flight assembly.

(2) Turn-of-nut, turn-angle, or bolt-stretch control. The tests performed to determine

Γ shall meet the requirements of paragraph 6.1.b.2.

d. The preload variation, Γ, used to calculate maximum initial preload and to calculate minimum initial preload for joints that are not separation-critical shall be as follows, depending on the parameter controlled during installation:

(1) Torque control. Either of the following approaches may be used:

A. In absence of applicable data showing otherwise, Γ = 25 percent for fasteners lubricated at assembly, and Γ = 35 percent for non-lubricated and as-received.

If the actual extreme variation found in the tests performed per paragraph

6.1.b exceeds the Γ values stated above, then the actual extreme variation should be used or methods for reducing the preload variation should be pursued. 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).

B. Γ to be obtained from test data at a statistical basis of 90 percent probability and 95 percent confidence (two-sided distribution), with the tests conforming to the requirements of paragraph 6.1.b.1.

(2) Turn-of-nut or turn-angle control. Either of the following approaches may be used:

A. Γ = 25 percent or the actual variation from the sample mean found in the tests performed per paragraph 6.1.b., whichever is greater.

B. Γ to be obtained from test data at a statistical basis of 90 percent probability and 95 percent confidence (two-sided distribution), with the tests conforming to the requirements of paragraph 6.1.b.1.

(3) Bolt stretch control methods such as ultrasonic measurement, bolt elongation measurement, and instrumented bolts. Either of the following approaches may be used:

A. Γ = 10 percent or the actual variation from the sample mean found in the tests performed per paragraph 6.1.b.2, whichever is greater.

B. Γ to be obtained from test data at a statistical basis of 90 percent probability and 95 percent confidence (two-sided distribution), with the tests conforming to the requirements of paragraph 6.1.b.2.

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

6.2 Strength Under Ultimate Design Loads

Analysis for ultimate design loads shall address 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.

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 loads, one directly provided in the insert specification (or the procurement specification) and one calculated for pull-out based on data typically provided in the insert specification. The lower value should be used for strength analysis.

For inserts installed in nonhomogeneous or nonmetallic materials or in sandwich panels, allowable pull-out loads should be derived from test. 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.

6.2.1 Ultimate Strength Analysis for Tensile Loading

Ultimate strength analysis of a fastening system under applied tensile loading shall be performed as follows:

a. Do not include preload when separation occurs before rupture—see section 6.2.1.1.

b. Include preload when rupture occurs before separation—see section 6.2.1.2.

Whether separation would occur before rupture can be determined based on test or analysis.

Refer to Appendix A.5 for guidance.

6.2.1.1 Ultimate Tensile Strength Analysis for Separated Joints

If separation occurs before rupture, the ultimate margin of safety, MSu, for tensile loading is

Ptu −allow MS = − 1 u FF ⋅ FS u ⋅ PtL (6-6) where Ptu-allow is the allowable ultimate tensile load for the fastener, the internally threaded part, or the threaded interface, whichever is lower, and PtL is the limit tensile load.

6.2.1.2 Inclusion of Preload in Ultimate Tensile-Strength Analysis with Linear Theory

If it is not clear whether rupture occurs before separation, linear theory should be used to assess ultimate tensile strength of the fastening system. Appendix A.6 provides further detail on this approach. With such theory, the tensile load in a preloaded bolt, Ptb, increases proportionally with the applied tensile load per Eq. (6-7) until either any part in the fastening system ruptures or separation occurs.

P = P + nφ Ptb p t (6-7) where Pp is the preload, Pt is the applied tensile load, n is the load-introduction factor which 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 k φ = b k + kb c (6-8) where kb is the stiffness of the bolt and kc is the stiffness of the clamped parts local to the fastener. See Appendix A.4 for discussion of the load-introduction factor.

Based on Eq. (6-7) 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

' 1 −Ptu = (Ptu −allow Pp−max nφ (6-9) and the linearly projected load that causes separation when at maximum preload is

P ' p−max

P = sep 1− nφ

(6-10)

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. (6-6).

If P′ sep is greater than P′ tu, linear theory predicts that rupture would occur before separation, and the ultimate margin of safety for tensile loading is

P'

MS = tu − 1 u FF ⋅ FS u ⋅ PtL (6-11)

−allow MS u = Psu − 1 (6-14)

FF ⋅ FS

u

⋅ P sL where PsL is the limit shear load acting on the shear plane.

6.2.3 Ultimate Strength Analysis for Interaction of Tension, Shear, and Bending

For fasteners under simultaneous applied…