NASA-STD-5017A_Revalidated_w-Change_1.pdf

APPROVED FOR PUBLIC RELEASE – DISTRIBUTION IS UNLIMITED

METRIC/SI (ENGLISH)

NASA TECHNICAL STANDARD

NASA-STD-5017A

w/CHANGE 1:

REVALIDATED

w/ADMINISTRATIVE/

EDITORIAL CHANGES

2016-05-31

National Aeronautics and Space Administration

Approved: 2015-07-31 Superseding NASA-STD-5017

DESIGN AND DEVELOPMENT

REQUIREMENTS FOR MECHANISMS

NASA-STD-5017A W/CHANGE 1

DOCUMENT HISTORY LOG

Status Document Revision

Change Number

Approval Date Description

Baseline 2006-06-13 Initial Release Revision A 2015-07-31 General revision.

Language for nearly every requirement was altered for clarity.

Some requirements were deleted or combined with other requirements, and new requirements have been added based on lessons learned with the document and engineering state of the art.

Rationale was added for all requirements.

Appendix A, Best Practices for Mechanisms, was added for guidance.

1 2016-05-31 Revalidated w/Administrative/Editorial Changes—This NASA Technical Standard was reviewed and no technical changes resulted. Administrative changes to number requirements, add a Requirements Compliance Matrix as an appendix, and conform to the current template were made, along with editorial corrections.

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 may be cited in contract, program, and other Agency documents as a technical requirement. It may also apply to the Jet Propulsion Laboratory and other contractors only to the extent specified or referenced in applicable contracts.

This NASA Technical Standard establishes uniform design, development, and verification requirements for mechanisms and mechanism components whose correct operation is required for safety or mission success.

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 MSFC Form 4657, Change Request for a NASA Engineering Standard.

Original Signed By: 2015-07-31

Ralph R. Roe, Jr. Approval Date NASA Chief Engineer https://standards.nasa.gov/

SECTION

TABLE OF CONTENTS

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 AND DEFINITIONS

3.1 Acronyms and Abbreviations

3.2 Definitions

3.2.1 Definitions of Variables

3.2.2 Definitions of Terms

4. REQUIREMENTS

4.1 Tolerancing

4.2 Clearances

4.3 Torque and Force Margins

4.3.1 Servomechanism Margins

4.4 Stroke Margin

4.5 Electrical Bonding and Grounding

4.6 Lubrication

4.7 Structural Requirements

4.8 Bearings

4.9 Motors

4.9.1 Electrically Commutated Brushless Motors

4.9.2 Stepper Motors

4.9.3 Brush Motors

4.10 Springs

4.11 Gears

4.12 Dampers

SECTION

TABLE OF CONTENTS (Continued)

4.13 Separable Interfaces

4.14 Pulleys

4.15 Switches

4.16 Fasteners

4.17 Quick Release Pins

4.18 Released Degrees of Freedom

4.19 Threaded Interfaces

4.20 Heritage Mechanisms

4.21 Performance Testing

4.22 Qualification Testing

4.22.1 Design Life Testing

4.23 Acceptance Testing

4.24 Mechanism Installation

LIST OF APPENDICES

APPENDIX PAGE

A Best Practices for Mechanisms A.1 Purpose A.2 Best Practices A.2.1 Torque and Force Margins A.2.2 Lubrication A.2.3 Bearings A.2.3.1 Ball Bearings A.2.3.2 Roller and Needle Bearings A.2.3.3 Spherical Plain Bearings A.2.3.4 Plain Bearings A.2.3.5 Linear Bearings A.2.4 Motors A.2.4.1 DC Motor Types and Selection A.2.4.2 Stepper Motor Performance Analysis A.2.4.3 Torque Profile Test for Electronically Commutated Motors and Drive

Electronics

A.2.5 Springs A.2.6 Gears A.2.6.1 Harmonic Drives A.2.7 Fastening

TABLE OF CONTENTS (Continued)

LIST OF APPENDICES (Continued)

APPENDIX PAGE

A.2.8 Quick Release Pins A.2.8.1 Quick Release Pin History A.2.8.2 Quick Release Pin Failures in the Space Program A.2.8.3 Current Quick Release Pin Best Practices A.2.9 Inspection A.2.10 Qualification Testing A.2.10.1 Design Life Testing A.2.11 Acceptance Testing A.2.12 Protoflight Testing B References B.1 Purpose B.2 References B.2.1 Government Documents B.2.2 Non-Government Documents C Requirements Compliance Matrix

LIST OF FIGURES

FIGURE PAGE

1 Illustration of Length and Width for Various Examples of Mechanisms Using a Set of Guides

2 Illustration of Moment Arm, Effective Length, and Width for a Single Linear Bearing Application

3 Pulley Guard Illustration 4 Illustration of the Effect of Saturation on Actuator Output 5 Diagram of a Typical Quick Release Pin

TABLE

LIST OF TABLES

1 Minimum Torque/Force Margin Factors 2 Allowable Contact Stress for Bearing Materials Under Non-Operational Limit

Loads

3 Factors of Safety for Springs 4 Recommended Environmental Qualification Test Matrix for Mechanisms 5 Recommended Testing Sequence 6 Recommended Environmental Acceptance Test Matrix for

Mechanisms

DESIGN AND DEVELOPMENT

REQUIREMENTS FOR MECHANISMS

1. SCOPE

1.1 Purpose

The purpose of this NASA Technical Standard is to establish common National Aeronautics and Space Administration (NASA) design, development, and test requirements for mechanisms whose operation is required for safety or mission success.

1.2 Applicability

This NASA Technical Standard is applicable to space flight mechanisms, including valves and ordnance-operated mechanical devices that are designed, built, or acquired by or for NASA, though it may also serve as a useful guidance document for other systems such as ground support equipment (GSE). This NASA Technical Standard does not address human factors requirements.

Adherence to this NASA Technical Standard does not in and of itself exempt a mechanism from any fault tolerance or hazard control requirements. The requirements and best practices in this NASA Technical Standard may serve as a useful basis for evaluating rationale for variances to fault tolerance requirements that may be proposed for mechanisms.

This NASA Technical Standard is approved for use by NASA Headquarters and NASA Centers and Facilities and may be cited in contract, program, and other Agency documents as a technical requirement. It may also apply to the Jet Propulsion Laboratory and other contractors only to the extent specified or referenced in applicable contracts.

Verifiable requirement statements are numbered and indicated by the word “shall”; this NASA Technical Standard contains 96 requirements. Explanatory or guidance text is indicated in italics beginning in section 4. To facilitate requirements selection and verification by NASA programs and projects, a Requirements Compliance Matrix is provided in Appendix C.

1.3 Tailoring

[MR 1] Tailoring of this NASA Technical Standard for application to a specific program or project shall be formally documented as part of program or project requirements and approved by the responsible 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 [MR 2] The latest issuances of cited documents shall apply unless specific versions are designated.

2.1.2 [MR 3] Non-use of specifically designated versions shall be approved by the responsible Technical Authority.

The applicable documents are accessible at https://standards.nasa.gov, may be obtained directly from the Standards Developing Body or other document distributors, or 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

2.3 Non-Government Documents

None.

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.

2.4.2 [MR 4] Conflicts between this NASA Technical Standard and other requirements documents shall be resolved by the responsible Technical Authority.

3. ACRONYMS AND DEFINITIONS

3.1 Acronyms and Abbreviations

°C degrees centigrade AA arithmetic average ABEC Annular Bearing Engineering Committee ABMA American Bearing Manufacturers Association https://standards.nasa.gov/

AGMA American Gear Manufacturers Association BLDC brushless direct current CEVM consumable electrode vacuum melted CMG control moment gyroscope DC direct current DFL dry film lubricant EC electronically commutated EHD elastohydrodynamic EMC electromagnetic compatibility EMI electromagnetic interference EVA extravehicular activity FS factor of safety GSE ground support equipment HRC Rockwell C hardness HST Hubble Space Telescope Hz Hertz ISS International Space Station ksi thousand pounds per square inch MAC multiply alkylated cyclopentane MPa megapascals MR mechanisms requirement NASA National Aeronautics and Space Administration NLGI National Lubricating Grease Institute PCVD physical chemical vapor deposited PFPE perfluoropolyalkylether PIP push in and pull PTFE polytetrafluoroethylene VAR vacuum arc remelted VIM vacuum induction melted

3.2 Definitions

3.2.1 Definitions of Variables

σ standard deviation τ motor torque ferror frequency of harmonic drive output gear error Ι current drawn by a motor Kcrit life test criticality factor Kcycle the factor applied to number of cycles within a defined cycle range when calculating mechanism life test cycles Kf factor applied to each individual fixed resistive torque in a torque margin calculation Klub lubrication factor Km motor constant

Kt torque constant Kv factor applied to each individual variable resistive torque in a torque margin calculation N harmonic of interest Ncycle the number of cycles within a defined cycle range when calculating mechanism life test cycles P resistive power loss Rt resistance across motor terminals Tavail the minimum available torque or force generated by a mechanism at worst case environmental conditions at any time in its life Tf individual fixed resistive torques that are well-known and not strongly influenced by friction, temperature, life, or other highly variable phenomena.

Tv individual resistive torques that may vary over environmental conditions and life

3.2.2 Definitions of Terms

Bearing Preload: The equal and opposite axial load on each of two bearings or bearing sets mounted on a common axis.

Cold Welding: A phenomenon in which similar adjacent metal surfaces molecularly bond to one another given sufficient cleanliness, time, and contact pressure.

Contact Ellipse: The area of contact between the ball and raceway that occurs as a result of elastic deformation of both parts under load.

Coulomb Friction Torque: The parasitic torque in a bearing due only to the sliding friction generated by the relative motion between the balls and the raceways. Note:

Generally, this torque has to be assessed at low speeds because the effect of drag on the bearing from the liquid lubricant starts to affect the torque at higher speeds.

Deployable: A component that is moved from a stowed position on the spacecraft to an extended position while remaining connected to the spacecraft.

Design Factor of Safety: A multiplying factor to be applied to limit loads or stresses for the purposes of analytical assessment.

Detent Torque: The amount of magnetic torque that a motor produces to resist motion when it is not energized.

Dynamic Clearance: The minimum distance between two entities when the entities are in motion and subjected to service environments.

Dynamic Torque: The torque necessary to achieve a required acceleration of a mechanism.

Hard Preload: A bearing preload approach in which the bearings are clamped together without spring or diaphragm elements.

Holding Torque: The torque necessary to prevent motion of a mechanism under external load.

Kickoff Spring: A spring intended to overcome forces present during initial separation of a contacting interface.

L0.05 Life: The life at which 0.05 percent of the bearings in an application can be expected to have failed due to rolling contact fatigue or, alternatively, the life at which 99.95 percent of the bearings will still be operating. Note: The L0.05 life of the bearing is theoretical and may not represent actual service life of the bearing.

Lubricant: A material with low shear resistance that reduces friction and wear. Note:

Lubricants can include gases, reaction films, liquids, and solids.

Mechanical Stop: A feature intended to prevent a mechanism component from extending beyond a prescribed travel limit by physically impeding motion of the component, also known as a hard stop.

Mechanism: An assembly in which one mechanical part moves relative to another mechanical part.

Microstepping: A method of achieving smoother motion, smaller step angles, or more precise positioning of a stepper motor by using a controller to rotate the stator magnetic field through an arbitrary stepping angle that is less than the cardinal step size.

Motor Constant: A figure of merit used to evaluate a motor’s ability to transform electrical power to mechanical power and compare the relative efficiencies and output power capabilities of different motors, defined as

𝑲𝑲𝒎𝒎 =

𝝉𝝉

√𝑷𝑷

𝐨𝐨𝐨𝐨 𝑲𝑲𝒎𝒎 =

𝑲𝑲𝒕𝒕

�𝑹𝑹𝒕𝒕

where Km is the motor constant, τ is the motor torque, P is the resistive power loss, Kt is the torque constant, and Rt is the resistance across the motor terminals.

Pull-in Torque: The maximum constant torque for a given speed, inertial load, and controller under which a stepper motor will accelerate from rest to operating speed, stop, or reverse direction in synchronism with input pulses (i.e., without loss of steps). Note: Pull-in torque is determined using 100 percent pulse duty cycle unless defined otherwise.

Pull-out Torque: The torque at which a stepper motor begins to lose synchronization as its torque load is increased while operating at its desired speed. Note: Typically, a curve plotting torque versus step rate (or rotor speed) is produced. This curve represents the maximum torque that the stepper motor can supply to a load at any given speed. Any torque or speed required that exceeds this curve will cause the motor to lose synchronization. Pull-out torque is affected by drive voltage and phase switching techniques.

Quick-Release Pin: A pin with a fast-acting retention and release mechanism built into the pin. Note: Also known as a PIP (push in and pull) pin. Quick release pins come in a variety of forms but generally utilize a spring-loaded central shaft to actuate one or more retention balls that are retained in the housing via swages.

Separation Nut: A segmented nut in which the segments are held together for retention of a bolt and then allowed to release through a mechanical action that is triggered on command. The release may be triggered via pyrotechnics or, when low source shock is necessary, non-explosive means such as shape memory alloys.

Servomechanism: An automatic device that uses error-sensing negative feedback to correct the performance of a mechanism.

Spring Preload: See “Soft Preload.”

Starting Torque: The torque necessary to initiate motion in a mechanism.

Static Clearance: The minimum distance between two entities when the entities are at rest.

Step Stability Analysis: An analytical method of quantifying stepper motor system performance by evaluating dynamic response to step commands.

Structural Fastener: A fastener that is used for structural purposes only, is installed on the ground only, and whose configuration is not altered during flight.

Torque Constant: The ratio of the motor torque to the current drawn by the motor, defined as

𝑲𝑲𝒕𝒕 =

𝝉𝝉

𝑰𝑰

where Kt is the torque constant, τ is the motor torque, and I is the current drawn by the motor.

Torque Ripple: A periodic variation in torque as an element rotates.

Tribological Coating: A coating applied to a surface for the purpose of reducing friction or increasing wear resistance.

Yield Load: The product of the design limit load and the yield factor of safety.

4. REQUIREMENTS

4.1 Tolerancing

a. [MR 5] Dimensional tolerances on all moving parts and intentional interference-fit parts shall be established and documented via a dimensional analysis to ensure that proper functional performance is maintained under all natural and induced environmental conditions and configurations.

Tolerancing and dimensional analysis is important not only for ensuring external clearances, but also for ensuring proper mechanism function in the first place. Tolerancing is too often considered as an afterthought of the design during the drawing creation phase and established without a thorough understanding of the tolerance drivers. Establishing the tolerances via a documented dimensional analysis helps drive the understanding of the effects of tolerances and other factors, and allows for easy review and revision later.

b. [MR 6] The dimensional analysis shall account for the following:

(1) Manufacturing, assembly, and alignment tolerances.

(2) Temperature.

(3) Temperature gradients.

(4) Vibration.

(5) Deflections due to external loads.

(6) Deflections due to operational loads.

(7) Adjustability and rigging of the mechanism parts.

The factors to be considered in the dimensional analysis are similar to those considered for clearances. Additional factors may be appropriate for consideration based on individual applications. Verification that these factors have been considered is expected to include line items in the analysis documentation for each factor in the dimensional analysis, or if a particular factor does not apply in a given situation, rationale for its inapplicability.

4.2 Clearances

a. [MR 7] Static and dynamic clearance requirements between mechanism components and any other structure, component, thermal covering, and field of view shall be established and maintained.

b. [MR 8] Internal mechanism clearance requirements shall be established and maintained.

Maintaining clearances within and around mechanisms is necessary both to maintain proper mechanism function and to prevent the mechanism from causing problems with other systems.

The necessary clearances required have to be established to enable design and verification, and the design has to maintain those clearances.

c. [MR 9] The established clearance requirements shall account for the following:

(1) Manufacturing, assembly, and alignment tolerances.

(2) Temperature.

(3) Temperature gradients.

(4) Vibration.

(5) Deflections due to external loads, including gravity effects.

(6) Deflections due to operational loads.

(7) Deflections due to pressurization or depressurization effects, including thermal blanket billowing.

(8) Motion of cable harnesses, tubing, and sensor wiring.

(9) Environments arising from transportation.

(10) Adjustability and rigging of the mechanism parts.

Because many of the factors affecting the overall (dynamic) clearance are not present when inspections are performed, these effects have to be accounted and included in the static clearance specified on the drawings. Tolerancing, thermal expansion effects, and deflections are the most important factors to consider in establishing the clearances. Thermal blanket behavior is notorious for causing unexpected interferences with mechanisms. Additional factors may be appropriate for consideration based on individual applications. Verification that these factors have been considered is expected to include line items in the analysis documentation for each factor in the clearance analysis, or if a particular factor does not apply in a given situation, rationale for its inapplicability. Motion under transportation loads is often not considered; but clearances are important in that situation, too.

d. [MR 10] Clearance measurements shall be performed on the highest level of assembly possible.

To verify that the proper clearances exist, inspections of all the established clearances have to be made on the as-built hardware after installation or assembly of the components of interest. The measurement of each clearance should be made when the mechanism is in the configuration that generates the worst case for that clearance. If clearances cannot be directly measured, positional measurements that allow clearance to be calculated may be substituted.

4.3 Torque and Force Margins

Torque margin is defined as follows:

𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕 𝒎𝒎𝒎𝒎𝒕𝒕𝒎𝒎𝒎𝒎𝒎𝒎 = 𝑻𝑻𝒎𝒎𝒂𝒂𝒎𝒎𝒎𝒎𝒂𝒂

∑𝑲𝑲𝒇𝒇𝑻𝑻𝒇𝒇+∑𝑲𝑲𝒂𝒂𝑻𝑻𝒂𝒂

− 𝟏𝟏 (4-1)

Tavail is the minimum available torque generated by the mechanism at worst-case environmental conditions at any time in its life.

Tf are the individual fixed resistive torques that are well-known and not strongly influenced by friction, temperature, life, or other highly variable phenomena.

Tv are the individual resistive torques that may vary over environmental conditions and life.

Kf and Kv are factors applied to each individual resistive torque prior to summation per table 1, Minimum Torque/Force Margin Factors.

Table 1 – Minimum Torque/Force Margin Factors Origin of Factor Kf Kv

Value Obtained via Theory or Analysis 1.5 3.0 Value Obtained via Test of Flight-Like Hardware 1.25 2.0 Value for One-Spring-Out Case1 1.0 1.0 1Spring-driven mechanisms that utilize multiple springs nominally working together to provide torque may utilize a minimum Kf and Kv of 1.0 for the cases in which one of those springs fails. Prior to failure, the nominal (non-failure) factors still apply.

For linear devices, “Force” replaces “Torque” in the above equation and descriptions.

Tavail represents torques from actuators such as motors, springs, pyrotechnics, solenoids, heat-actuated devices, and other devices. Examples of fixed torques, represented by Tf, include accelerated inertias, motor detent torques, and unbalanced pressure loads limited by relief mechanisms; all other resistive torques tend to be variable enough that a higher factor is more appropriate and thus fall under Tv. Examples include static or dynamic friction, alignment effects, wire harness torques, damper drag, and variations in lubricant effectiveness, including degradation or depletion of lubricant over life.

This single equation can be used to calculate holding torque margin, starting torque margin, dynamic torque margin, and pull-in torque margin (for stepper motors).

For holding torque margin, Tavail is the actuator torque, while Tf and Tv are the torques that tend to disturb the mechanism.

For starting torque margin, Tavail is the actuator torque, while Tf and Tv are the resistive torques.

For dynamic torque margin, Tf is the torque required to accelerate an inertia by a given amount, and Tv are the resistive torques.

For pull-in torque margin, Tavail is the pull-in torque at a given drive rate, Tf is the maximum detent torque, and Tv is the total friction torque seen by the motor.

a. [MR 11] All calculated force and torque margins shall account for worst-case credible combinations of factors at end of life.

Because spacecraft mechanisms are exposed to many factors in combination that can deplete margin, calculation of force and torque margins also have to account for these factors in combination. It is recommended that the following considerations be included in margin calculations as they reflect phenomena that are frequently found to cause problems in margin calculation:

• Environmental conditions.

• Frictional effects.

• Possible changes in static and dynamic friction due to storage time.

• Alignment effects.

• Wire harness loads.

• Damper drag.

• Thermally induced distortions.

• Load-induced distortions.

• Variations in lubricity.

• Fluid pressure on the elastomers in viscous dampers.

• Supply voltage, motor, and controller parameters.

• Acceleration due to vehicle motion or maneuvers that can retard motion.

• Loading due to vibroacoustic environment.

b. [MR 12] The starting torque or force margin shall be greater than zero at all points of travel.

c. [MR 13] Dynamic torque or force margin shall be greater than zero at all points of travel.

d. [MR 14] Holding torque or force margin shall be greater than zero at all points of travel.

A positive torque or force margin ensures that the mechanism retains reserve torque or force that can be applied in the event of an unforeseen effect that robs motive force from the

e. [MR 15] If motors are used in the system, Tavail shall be measured at multiple points over the range of motion with the minimum supplied motor voltage and at the output of the prime mover, not including gear heads or gear trains affixed to the motor or within the

The torque margins must be calculated at the motor output because the resistive torques present in the gear heads can drive the minimum margin to be at the motor output rather than at the gear head output. Basing torque margin on the gear head output can give a false impression of the true torque margin.

f. [MR 16] Stepper motor stability margin from a step stability analysis shall be greater than zero.

A further discussion of stepper motor performance and stability analysis can be found in Appendix A. When stepper motor detent torque is used to maintain the position of the motor, the holding torque margin may be calculated via equation 4-1.

g. [MR 17] When stepper motor detent torque is used to maintain the position of a motor in the presence of vibratory disturbances, detent stiffness and motor damping shall be considered when determining the holding force margin.

The spring-damper nature of the detent torque requires special consideration when used in a holding torque application.

h. [MR 18] All torque and force margins shall be verified during an acceptance test at the highest possible level of assembly.

Torque and force margins are intended to ensure that the mechanism retains reserve torque or force that can be applied in the event of an unforeseen effect that reduces motive force from the mechanism. Therefore, as with any other capability of the mechanism, the minimum torque or force margin must be verified as intact prior to placement into service.

In practice, it is often difficult to test-verify the margin directly because of the difficulty of ensuring that worst-case parameters are all in effect or the inability to measure certain values separate from others. This often drives some portion of the verification to depend on calculation. In these cases, the margin should be calculated by using the worst stack-ups of tested factors and adjusting for factors not present in the test. The result has to be greater than zero.

There are many forms of torque margin equations in use in various standards. They each can be reformulated to appear like the others; the only difference among them is in the magnitude and nature of the different factors applied to the terms. This form was chosen due to its relative simplicity, its ability to handle several margin calculations with a single equation, and its suitability for application of maturity-based factors.

The required conservatism is included in the equation in the form of the factors, so a margin of zero indicates that requirements are met. A positive margin indicates that torque or force above that which is required for conservatism exists. To evaluate margin over the minimum torque or force required to operate with no conservatism included, the equation is calculated with all Kf and Kv values set to unity.

The theoretical/analytical factors listed in table 1, Minimum Torque/Force Margin Factors, are not intended to be used as “untested factors,” i.e., factors to be used when attempting to meet torque margin requirements by analysis only, without any testing to verify margin. Such an approach is prohibited by the requirement to test-verify margins in section 4.3.h. These factors are intended as higher uncertainty factors to be used in sizing and calculations until test-obtained values can be obtained for the torque values to which the factors are applied. This allows the required torque to be reduced as confidence in the system characteristics grows through testing.

When assessing failure tolerant cases, it is important to recognize that mechanisms that utilize multiple springs nominally working together to provide force or torque would end up delivering a much greater margin under normal operating conditions to show a positive margin after a failure. This margin can be excessive. For those cases, reduced conservatism in the factors is appropriate. The “one-spring-out” case may describe a missing spring or a broken coil as appropriate for the application. Redundancy will often be achieved with elements such as redundant motor windings, velocity-summed motor arrangements, or torque-summed motors that run at half current nominally. These elements do not work together at full power to provide force or torque. These types of implementations still require the nominal (non-failure case) Kf and Kv values from the table. Note that if the failed actuator produces a resistive torque after failure, this torque has to be included in the margin calculation for the failure case.

4.3.1 Servomechanism Margins

[MR 19] For servomechanism applications, performance margins shall be documented.

Servomechanisms may require motor performance that far exceeds that required by this torque margin equation to meet performance requirements, so the typical margin requirement alone is not sufficient. Though other applicable margins such as starting force margins should still be calculated and documented, servomechanisms will need a control system performance analysis, e.g., phase and gain margin analysis, to fully assess performance and margin.

4.4 Stroke Margin

Stroke margin is defined as follows:

𝒔𝒔𝒕𝒕𝒕𝒕𝒕𝒕𝒔𝒔𝒕𝒕 𝒎𝒎𝒎𝒎𝒕𝒕𝒎𝒎𝒎𝒎𝒎𝒎 = 𝒕𝒕𝒕𝒕𝒕𝒕𝒐𝒐𝒕𝒕𝒕𝒕 𝒔𝒔𝒕𝒕𝒕𝒕𝒕𝒕𝒔𝒔𝒕𝒕 𝒕𝒕𝒇𝒇 𝒕𝒕𝒕𝒕𝒕𝒕 𝒎𝒎𝒂𝒂𝒕𝒕𝒕𝒕𝒎𝒎𝒕𝒕𝒕𝒕𝒕𝒕 𝒔𝒔𝒕𝒕𝒕𝒕𝒕𝒕𝒔𝒔𝒕𝒕 𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒎𝒎𝒕𝒕𝒕𝒕𝒓𝒓 𝒕𝒕𝒕𝒕 𝒎𝒎𝒂𝒂𝒕𝒕𝒎𝒎𝒕𝒕𝒂𝒂𝒕𝒕 𝒓𝒓𝒕𝒕𝒔𝒔𝒎𝒎𝒕𝒕𝒕𝒕𝒓𝒓 𝒇𝒇𝒕𝒕𝒎𝒎𝒂𝒂𝒕𝒕𝒎𝒎𝒕𝒕𝒎𝒎

− 𝟏𝟏 (4-2)

a. [MR 20] Stroke margin shall be documented for all linear mechanisms.

Stroke margin helps to guarantee enough travel exists in a mechanism to accomplish its function in the presence of uncertainty. A ten percent stroke margin is frequently employed.

b. [MR 21] All stroke margins shall account for worst-case credible combinations of the following:

(1) Environmental conditions.

(2) Thermally induced distortions.

(3) Load-induced distortions.

(4) Mounting alignments.

(5) Tolerances.

Like force margin, stroke margin ensures that adequate travel is available to account for unforeseen effects, which can increase the stroke required. A variety of factors, including those specified in the requirement, can affect the dimensions of the assembly and stroke needed.

Therefore, these various factors are important considerations when determining the necessary travel for linear actuators, which include pin-pullers. In applications where stroke of the driven member is physically limited by a mechanical stop, this requirement still applies to the actuator, i.e., the actuator itself is to have stroke remaining when the mechanical stop is reached by the driven member. Assurance that a linear actuator remains engaged prior to actuation is covered by holding force margin (4.3.d), tolerancing (4.1), and adherence to structural requirements.

4.5 Electrical Bonding and Grounding

a. [MR 22] Bearings shall not be used to carry electrical current.

b. [MR 23] Gears shall not be used to carry electrical current.

These requirements are meant to preclude bearings and gears from being used as part of an intentional electrical distribution circuit or to carry other currents such as ground return currents, lightning currents, or plasma-induced surface currents. These components are not designed to carry electrical current and their geometry and lubrication makes it difficult for them to do so. Other current paths should be provided. Electrical currents produced by unintentional charging of bearing-supported hardware should be considered when evaluating this requirement.

c. [MR 24] Mechanisms shall include electrical bonding and ground paths between moving and stationary parts sufficient to meet electromagnetic environmental effects requirements.

It can sometimes be difficult to achieve an adequate bond or ground path when the interfaces are in motion, especially when bearings and gears are involved, since they are prohibited from carrying current. Care should be taken to ensure that the bonding and grounding scheme is able to perform as intended. Verification of this requirement is expected to consist of an analysis or test that demonstrates that an adequate bond or ground path exists to meet electromagnetic environmental effects requirements without using prohibited current paths.

4.6 Lubrication

a. [MR 25] All surfaces in contact that affect the performance of the mechanism while in relative motion shall be lubricated.

Lubrication is one of the most important factors in successful mechanism design and operation.

All contacting surfaces that are expected to move with respect to one another should be lubricated in some way, regardless of material choices, load, or life requirements. Use of dissimilar metallic materials for the wear surfaces, though strongly encouraged, is not an equivalent to or substitute for lubrication and does not meet the intent of this requirement. Refer to Appendix A for a discussion of lubricant selection and factors that should be considered.

b. [MR 26] The selection of lubricants for mechanisms shall include the following considerations:

(1) Lubricant property changes in storage or in a space environment.

(2) Creep properties of wet lubricants.

(3) Viscosity versus temperature properties of wet lubricants.

(4) Elastohydrodynamic (EHD) film thickness if operating in the EHD lubrication regime.

(5) Outgassing or potential breakdown products from wet lubricants that could cause contamination, such as on optical or thermal control surfaces.

(6) Possibility of polymerization of wet lubricants, particularly due to high contact pressures or contaminants.

(7) Required purity of the lubricant.

(8) Lubricant depletion (lubrication loss analysis) for wet lubricants or lubricant wear-out for dry lubricants.

(9) Dry lubricant debris generation.

(10) Compatibility of the lubricant with other materials, particularly other lubricants if used, during ground testing as well as in service.

(11) Operating temperature limits of the mechanism and the lubricant.

(12) Corrosion protection of the mechanism.

(13) Protection against galling and friction welding of the mechanism.

(14) Contact stress.

(15) Run-in requirements, such as rate of speed, load, and time duration.

(16) Coefficient of friction of the tribological system.

(17) The effect of other environments on the tribological system, such as humidity and salt spray.

Numerous factors have to be considered to make a proper choice of lubricant. Not all of these considerations apply to every case. Verification that these factors have been considered is expected to include a line item discussing each factor or the rationale for inapplicability of each factor in the compliance assessment for this NASA Technical Standard.

c. [MR 27] An evaporative loss analysis shall be performed to show that 90 percent of the initial lubricant quantity remains at end of life, not including lubricant degradation.

Because life testing evaluates cycle life and not calendar life, evaporative effects on lubricant availability typically cannot be evaluated with a life test alone.

d. [MR 28] The lubricant application process for each application shall be specified in the engineering documentation.

The quantity of lubricant used and method of application can be almost as important as the presence of lubricant in the first place. Too much can impede mechanism performance or create contamination problems, and too little can result in reduced life or inadequate performance.

4.7 Structural Requirements

a. [MR 29] Mechanisms classified as failure tolerant shall meet all structural requirements after failure of the mechanism to operate using full design factors of safety.

To be considered failure-tolerant, a mechanism has to meet all performance requirements after any failures commensurate with its required level of failure tolerance. Structural requirements or the redistribution of loads caused by the failure are sometimes overlooked or reduced design factors of safety are sometimes erroneously applied. While it is often permissible to use a reduced statistical bound on the loads after a failure (e.g., using 2σ loads instead of 3σ loads), the design factor of safety should not be reduced.

b. [MR 30] Engineering analyses shall account for the structural mounting boundary conditions, including:

(1) Stiffness.

(2) Mounting alignment tolerances.

(3) Temperature-induced distortions.

(4) Load-induced distortions.

(5) Interface friction.

Often, the structural analysis of a mechanism does not consider interface properties or assumes a rigid interface. Ignoring these properties can lead to failure in service when the interfaces create different environments than were considered in the analysis. Verification of the accounting for these items is expected to include a discussion of each of these items and how they were modeled, or why they were not modeled, in the structural analysis documentation.

c. [MR 31] Mechanism components shall maintain positive margins of safety under actuation force/torque stall conditions.

Many things can cause a mechanism to reach its stall torque or force. Designing the mechanism with enough strength to withstand stall ensures that the mechanism is undamaged by this situation and allows steps to be taken to recover the mechanism functionality. The usual factors of safety apply; i.e., the factors of safety are not to be reduced for this situation.

d. [MR 32] Non-jamming mechanical stops shall be incorporated into all mechanisms where exceeding required range of motion will result in detrimental effects to the mechanism or larger system.

Stops ensure that mechanisms do not travel farther than intended and cause problems or end up in an unrecoverable state. Soft stops such as software logic, open-loop control, and limit switches can be unreliable, so mechanical stops are important for maximum reliability.

Examples of a mechanism for which over-travel is impossible include filter wheels and gimbals with slip rings.

e. [MR 33] Mechanism components shall maintain a positive margin of safety with the appropriate factors of safety applied when subjected to worst-case transient loads from mechanical stop impact.

The impact against the mechanical stop can create elevated loads on other parts of the mechanism in addition to the stops themselves, and these loads have to be accounted for in the structural analysis. The contact of mechanical stops is often rapid enough that static analysis approaches can be unconservative and dynamic analysis will be necessary. A bounding worst-case load would include impact at maximum speed combined with stall torque.

f. [MR 34] If manipulator systems, payload operations, extravehicular or intravehicular activities, or other situations presenting a risk of inadvertent contact are present, then exposed mechanism components, protective shrouds and covers, and mounting structure shall be designed to accommodate inadvertent impact loads from these sources.

Designing for this possibility will ensure adequate margins against deformation that could cause a binding or jamming condition or inadvertent operation of the mechanism. Analysis of these cases should use full factors of safety. The particular load to be accommodated will be determined by the individual program.

4.8 Bearings

a. [MR 35] Ball bearings used in high precision or low torque ripple applications shall utilize raceways that meet Annular Bearing Engineering Committee (ABEC) 7, 7P, or 7T tolerances (or better) in accordance with American Bearing Manufacturers Association (ABMA) standards.

b. [MR 36] Nonstandard ball bearings or thin section ball bearings where ABMA tolerances do not apply that are used in high precision or low torque ripple applications shall have the manufacturer’s precision level most nearly equivalent to ABEC 7.

c. [MR 37] Ball bearings used in high precision or low torque ripple applications shall utilize balls of ABMA grade 10 or better.

d. [MR 38] Ball bearings used in high precision or low torque ripple applications shall utilize a raceway surface finish of 2.0 microinches arithmetic average (AA) or better.

e. [MR 39] Ball bearings used in low torque ripple or long life applications shall utilize material that has been consumable electrode vacuum melted (CEVM), vacuum induction melted (VIM), and/or vacuum arc remelted (VAR).

Bearing tolerances and raceway surface finish can have a strong impact on the performance of bearings where precision or low torque ripple is needed. ABEC 7 tolerances on the raceways have been shown to be adequate for these applications in most situations. However, one has to be careful to also specify the ball grade and raceway surface finish, which are not covered by the ABEC rating. Balls of poor grade can negate the benefits of tight bearing tolerances. Vacuum melting improves the cleanliness of bearing steels by eliminating non-metallic inclusions. When located at the contact surfaces, non-metallic inclusions create pits which degrade surface finish and increase torque ripple. When located at or below the contact surfaces, non-metallic inclusions are stress raisers from which fatigue cracks and spalls originate, shortening fatigue life. While not required, it should be noted that smoother raceway finishes can help a bearing transition to the EHD regime at lower speeds and thus can also help to extend life. It should be stressed that the above conditions are necessary to achieve high precision, low torque ripple, and/or long life applications but may not be sufficient.

f. [MR 40] The mean Hertzian contact stress on the most highly loaded element in a rolling element bearing shall remain less than or equal to the appropriate values in table 2, Allowable Contact Stress for Bearing Materials Under Non-Operational Limit Loads, when subjected to the non-operational limit load.

g. [MR 41] For materials other than those listed in table 2, an allowable contact stress shall be determined.

Table 2—Allowable Contact Stress for Bearing Materials Under Non-Operational Limit Loads

Bearing Material, Typical Hardness

Range

Mean Hertzian Contact Stress— High Precision, Low Torque Ripple

Applications

Mean Hertzian Contact Stress— Other Applications

440C Steel, 58-62 HRC 2310 MPa (335 ksi) 2760 MPa (400 ksi) 52100 Steel, 60-63 HRC 2480 MPa (360 ksi) 2960 MPa (430 ksi) M50 Steel, 62-64 HRC 2480 MPa (360 ksi) 2960 MPa (430 ksi) VIM CRU20 Steel, 66

HRC minimum

3790 MPa (550 ksi) 4070 MPa (590 ksi)

NOTE: For hybrid bearings using silicon nitride balls with steel rings, the allowable contact stress will be that of the steel used.

Stressing the bearing raceway material beyond its elastic limit creates a permanent indentation known as a brinell, which can be detected on the torque trace of the bearing and is unsuitable for quiet running applications. The values in the table for 440C and 52100 in high precision, low torque ripple applications were established by experiment and correspond to a brinell depth of 0.00003 times the ball diameter for 440C and 0.00005 times the ball diameter for 52100 (Leveille & Murphy, 1973). The values for VIM CRU20 were established experimentally using ball-on-plate indentation tests with a brinell depth threshold of 0.00005 times the ball diameter based on the previous work on 440C and 52100 (Park et al., 1998). Values for M50 are conservative limits based on hardness comparison with 52100. It is recommended that the above references be consulted to understand the configurations and bearing hardnesses tested before utilizing these limits.

The values in the table for non-quiet running applications are based on the load that produces brinell depths of 0.0001 times the ball diameter, as found in ABMA standards and elsewhere.

Experience shows that this brinell depth can be tolerated in most bearing applications without affecting fatigue life.

It should be emphasized that these allowables are for non-operational loadings only, e.g., random vibration loads during launch, and are NOT appropriate allowables for operating stress levels, which will generally be much lower and depend on a number of factors. See Appendix A for guidelines.

A yield factor of safety is used to account for uncertainty in determining the loading conditions on bearings.

If the design limit loads are derived from mass acceleration curves, it is acceptable to analyze the bearing using only the static loads derived from those curves.

h. [MR 42] Bearing fatigue life calculations shall be based on the L0.05 life when subjected to maximum time varying loads consistent with the conditions under which the L0.05 life was determined.

Experience indicates that the L0.05 life is sufficient to avoid fatigue life problems in life testing and service for space applications

i. [MR 43] The upper and lower extremes of the ball bearing contact ellipses shall be contained by the raceways.

Understanding the contact geometry and stress in a bearing is important for obtaining required bearing performance and life. Truncation of the contact ellipse can cause significant increases in stress that are not accounted for in traditional bearing analyses and can have correspondingly large impacts on load carrying capacity and life. See Appendix A for more on contact ellipse truncation.

j. [MR 44] All ball bearings shall be preloaded with the following exceptions:

(1) Four-point (gothic arch) bearings.

(2) Deep groove ball bearings for which it can be shown that the absence of preload on the deep groove ball bearing is not detrimental to the performance of the

Bearing preload eliminates free play, reduces runout of the rotating member, increases axial and radial stiffness, prevents fretting damage, reduces impact loading during vibration, increases the load sharing among rotating elements, and prevents ball skidding. It is recognized that there are situations in which preload is not advisable, and there are many bearing preload strategies and considerations when analyzing the effects of preload levels and unloading of balls. See Appendix A for guidance.

k. [MR 45] If axial sliding of a bearing ring is required to maintain preload, sliding shall be facilitated by methods such as a tribological coating or a lubricated sleeve.

Some preload schemes depend on a spring and one bearing ring that is free to slide axially. In such a case, sliding at this interface is required to achieve the desired preload and has to be guaranteed by design.

l. [MR 46] Bearing preload shall be measured once all the assembly steps that establish or affect bearing preload have been completed.

Measurement of preload needs to be verified after operations affecting bearing preload have been completed to ensure that the assembly operations did not adversely affect the preload and that the bearings function as desired in the fully-assembled configuration. It is highly recommended that the preload be measured at intermediate levels of assembly as well. Direct measurement is preferred but if bearing preload cannot be directly measured, indirect methods of assessing preload, such as measurement of stiffness or Coulomb friction torque, may be used.

m. [MR 47] Mechanisms utilizing guides or linear bearings shall use a length-to-width ratio of 2:1 or greater, unless it can be shown by analysis that a length-to-width ratio of less than 2:1 will not cause the mechanism to bind or undergo stick-slip motion taking into account the following:

(1) Possible friction coefficients.

(2) Contact forces.

(3) Actuating forces.

(4) Dynamically induced forces.

(5) Misalignments.

(6) Eccentric loading.

For mechanisms utilizing a set of linear bearings or guides, the length is defined as the distance between guide points or bearing centers along the axis of motion and the width is the lateral spacing between the guides or centerlines of the bearings, as depicted in figure 1, Illustration of Length and Width for Various Examples of Mechanisms Using a Set of Guides. For mechanisms utilizing a single linear bearing shaft or guide rail, the length is the effective length of the linear bearing or guide and the width is the moment arm defined by the distance from the load application point to the centerline of the rail as depicted in figure 2, Illustration of Moment Arm, Effective Length, and Width for a Single Linear Bearing Application.

A length-to-width ratio greater than 2:1 requires no analysis. However, care should be taken when using large length-to-width ratios within a linear bearing because shaft deflections over the length of the bearing can also cause binding problems.

Mechanisms guided by linear devices are sensitive to the geometry of the supports. Length-to-width ratios greater than 2:1 rarely have problems unless friction coefficients are abnormally high. Ratios less than 2:1 can be used successfully but require careful analysis and characterization of parameters to ensure the system does not bind or undergo stick-slip motion. Ratios less than 1:1 are suitable only for high-precision, low friction applications such as linear bearing systems. The effective bearing length is often not the overall length of the bearing, but the length of the portion of the bearing carrying the load, often called a “load zone.” Few manufacturers publish information about their bearings’ load zones, so it is often necessary to contact the company or make a conservative estimate of the load zone of a specific bearing. The same uncertainty factors used in the force margin analysis (e.g., on friction) should be applied to any bearing ratio analysis performed.