3.2.1 Definition of Variables
| Symbol | Definition | Section |
|---|---|---|
| uncertainty or scatter in initial preload | ||
| actual preload variation for maximum preload, as determined from a sample of torque-tension test data | ||
| actual preload variation for minimum preload, as determined from a sample of torque-tension test data | ||
| stiffness factor | ||
| coefficient of friction | ||
| shear stress | ||
| axial (tension) stress | ||
| stress tensor | ||
| unbiased sample standard deviation | ||
| area | ||
| minimum minor-diameter area | ||
| 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 | ||
| 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 | ||
| nominal fastener diameter | ||
| design ultimate bending stress based on linear-elastic theory | ||
| allowable ultimate flexural stress (bending) | ||
| fitting factor | ||
| preloaded bolts | ||
| factor of safety for separation | ||
| allowable ultimate shear stress | ||
| ultimate factor of safety | ||
| allowable yield shear stress | ||
| yield factor of safety |
| Symbol | Definition | Section |
|---|---|---|
| allowable ultimate tensile stress | ||
| allowable yield tensile stress j: test number | ||
| stiffness of the bolt | ||
| stiffness of the clamped parts local to the fastener | ||
| nominal (mean) nut factor m: number of tests | ||
| applied moment | ||
| fully plastic moment | ||
| margin of safety for separation | ||
| margin of safety for slip | ||
| ultimate margin of safety | ||
| yield margin of safety | ||
| load-introduction factor | ||
| axial force | ||
| number of fasteners in a joint | ||
| fully plastic axial force | ||
| thread pitch | ||
| load that causes separation | ||
| the applied tensile load that causes the fastener load to exceed the fastening system's allowable ultimate tensile load if rupture occurs before separation | ||
| the applied tensile load that causes the fastener load to exceed the fastening system's allowable yield tensile load, if yielding occurs before separation | ||
| friction load | ||
| preload | ||
| loss of preload from material creep | ||
| maximum preload | ||
| minimum preload | ||
| initial preload | ||
| maximum initial preload | ||
| actual maximum initial preload from a sample of test data | ||
| minimum initial preload | ||
| actual minimum initial preload from a sample of test data | ||
| nominal (sample mean) initial preload for a given effective torque | ||
| short-term relaxation of preload | ||
| limit shear load | ||
| total limit shear load acting on the joint | ||
| ultimate design shear load | ||
| allowable ultimate shear load | ||
| applied tensile load | ||
| total applied tensile load acting on the joint | ||
| total limit tensile load acting on the joint | ||
| tensile load in a preloaded bolt |
| PtL | limit tensile load |
| Ptu | ultimate design tensile load |
| Ptu-allow | allowable ultimate tensile load |
| Pty-allow | allowable yield tensile load |
| PΔt-max | maximum increase in preload due to temperature |
| PΔt-min | maximum decrease in preload due to temperature |
| Q | applied shear force |
| Qp | fully plastic shear force |
| t | half-thickness of the clamped members (minus any washers) |
| Tbr-min | minimum breakaway torque specified for the locking feature |
| TL-max | maximum locking torque (running torque) specified for the locking feature |
| Tmax | maximum effective torque |
| Tmin | minimum effective torque |
| Ts-max | maximum specified torque |
| Ts-min | minimum specified torque |
3.2.2 Definition of Terms
Allowable Load: The maximum permissible load in a structural part or assembly. As applicable, it can be the specified strength of the hardware (e.g., in a fastener specification); a statistically based, lower-bound, load-carrying capacity derived from test data; or the load derived from the allowable stress of the materials.
Applied Load: Force or moment transferred across a joint; it does not include preload or changes in preload as a result of temperature change. (Note: “Applied load” is also referred to in literature as “external load,” “externally applied load,” or “service load.”)
Bolt Grip: For fasteners with a flat bearing surface head, the dimension from the bearing surface under the head to the end of the full diameter body, measured parallel to the axis of the fastener. For fasteners with a conical bearing surface head, the dimension from the top of the head to the end of the full diameter body, measured parallel to the axis of the fastener. Note that these definitions are similar to what some fastener standards use as the definitions for “Grip.”
Catastrophic Hazard: (1) A hazard that could result in a mishap causing fatal injury to personnel, and/or loss of one or more major elements of the flight vehicle or ground facility. (2) A condition that may cause death or permanently disabling injury, major system or facility destruction on the ground, or loss of crew, major systems, or vehicle during the mission
Critical Hazard: A condition that may cause severe injury or occupational illness, or major property damage to facilities, systems, or flight hardware.
Design Separation Load: The limit tensile load multiplied by a separation factor of safety.
Detrimental Yielding: Yielding that adversely affects fit, form, function, or integrity of the structure.
Effective Torque: A torque in excess of any running torque.
Factor of Safety: A multiplying factor to be applied to limit loads for purposes of analytical assessment (design factor) or test verification (test factor) of design adequacy in strength or stability.
Failure: Rupture, collapse, excessive deformation, or any other phenomenon resulting in the inability of a structure to sustain specified loads, pressures, and environments or to function as designed.
Fastener: For purposes of this NASA Technical Standard, a bolt or a screw that joins two or more parts and transfers load between them.
Fatigue: The cumulative irreversible damage incurred in materials caused by cyclic application of stresses and environments, resulting in degradation of load-carrying capability.
Fitting Factor: A supplemental factor of safety used in analysis of bolted joints to account for uncertainties in load paths and stresses.
Full Diameter Body: For purposes of this NASA Technical Standard, the ASME B18.12-2001, Glossary of Terms for Mechanical Fasteners definition applies: “The unthreaded portion of a shank whose diameter is generally within the dimensional limits of the major diameter of the thread.” (Note: Some fastener specifications refer to the full diameter body as the “full cylindrical portion of the shank” or simply as a “shank.”)
Hardware Developer: Organization directly responsible for the design, manufacture, analysis, test, and safety compliance documentation of the hardware.
Limit Load: The maximum expected applied load, including load transferred across joints as a result of thermally induced loading of the structure; it does not include preload or changes in preload occurring as a result of temperature change.
Locking Feature: A device, chemical substance, or other physical characteristic added by design to one or more elements of a threaded fastening system to resist vibration-induced loosening or to provide retention against complete disengagement of the fastening elements. Preload-induced friction forces between mating joint members, mating threads, or under the head or nut elements of a fastening system inherently resist loosening; however, within this NASA Technical Standard, the term “locking feature” is used to refer only to the added design items described above.
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Prevailing torque feature: Fastening system design that relies on friction to resist fastener rotation independent of preload (i.e., deformed thread or locking patch).
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Adhesive locking feature: Fastening system design that uses chemical compounds added at the time of assembly (i.e., anaerobic adhesive, epoxies, urethanes, etc.).
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Mechanical locking feature: Fastening system design employing non-friction elements usually involving a “hard stop” (i.e., cotter pins or safety wire).
Margin of Safety: A measure of a structure’s predicted reserve strength in excess of the design criteria. For a preloaded fastener, it represents the percentage that the design load (limit load multiplied by the applicable factor of safety) can increase before the design criteria are no longer satisfied.
Preload: The tensile force in a bolt and the equivalent compressive force in the joint members when there is no applied load.
Prevailing Torque or Running Torque: The torque required to overcome kinetic friction of the mating threads plus the torque required to overcome the locking feature when 100 percent of the locking feature is engaged and the fastener is unseated. This torque can be measured in either a loosening or a tightening direction while the mating threads are in relative motion.
Separation: The state of no compressive load between mating parts local to the fastener. For a joint designed to maintain a seal, it is further defined as any condition that enables a liquid or gas to penetrate the seal at an unacceptable rate. Also referred to as “gapping.”
Separation-Critical Joint: A joint that fails to function as required if separated.
Separation Load: The minimum applied tensile load that causes separation.
Service Life: All significant loading cycles or events during the period beginning with manufacture of a component and ending with completion of its specified use. Testing, transportation, lift-off, ascent, on-orbit operations, descent, landing, and post-landing events are to be considered.
Service Life Factor (Life Factor): A multiplying factor to be applied to the maximum expected number of load cycles in the service life to determine the design adequacy in fatigue or fracture.
Shank: For purposes of this NASA Technical Standard, the ASME B18.12-2001 definition applies: “That portion of a headed fastener that lies between the head and the extreme point end.”
Threaded Fastening System: (Also referred to as “fastening system.”) An assembled combination of a fastener, an internally threaded part such as a nut or an insert, and also the region of all parts clamped between them, including washers, compressed by the fastener preload.
Ultimate Design Load: The product of the ultimate factor of safety, the fitting factor, and the limit load. Also referred to as “design ultimate load.”
Yield Design Load: The product of the yield factor of safety, the fitting factor, and the limit load. Also referred to as “design yield load.”