Fastener Quality Standards

The models tested in the FO Division wind tunnels must be assembled using high-quality fasteners of SAE grade 5 or more.

Ames requires using certified fasteners; if used exclusively and proof of certification is supplied to the Ames Test Manager in the form of a Certified Material Test Reports (CMTR), no further checking will be required. If a CMTR cannot be provided, then all noncertified critical bolts will be removed from the model for examination and Rockwell hardness verification during test installation.

Fastener Assembly

Critical fasteners must be assembled using a calibrated torque wrench. The fastener manufacturer's torque specification will be used if the full-rated strength of the fastener is required to maintain Ames required safety factors.

Structural Joint Drawings

Drawings for all structural connections must list the following:

• Strength and quality of fasteners

• Torque values for tightening screws and nuts

• All welded, soldered, brazed, bonded, or other nonbolted, structural-connection techniques must be listed, showing locations on drawings and exact fabrication specifications, as well as analyzed in the stress report

Mil Spec Standards for Joints

Joining components (including tubing) by methods other than welding, soldering, or bonding is to be accomplished as appropriate in compliance with military specification standards.

All joints must be inspected using the appropriate nondestructive inspection technique decided upon by the customer and Ames personnel.

At NASA's discretion certain joint designs located at critical load-carrying sections might not be permitted. Soft-soldered joints are not acceptable. Silver-soldered joints might be acceptable, depending on application.

Welded Joints

All welded joints must be designed and fabricated in compliance with the code of the American Welding Society. All welds must be verified by appropriate inspection techniques such as, but not limited to, magnetic particle inspection, X-ray, or dye penetrant methods.

Critical welds (those whose failure would result in model or facility damage) must be analyzed in the stress report. Include inspection certification as an addendum to the stress report.

Shear Loads (bolted joints)

Shear loads must be transmitted by keys, pins, pilots, or shoulders.

Bolt Preload

For bolt preload in bolted structural joints:

• Use manufacturer's recommended value.

• Avoid oscillating stresses in threads.

Thread Engagements

Critical fastener thread engagement with nuts and/or tapped holes must be sufficient to develop strength equal to the fastener or to the application design load with the appropriate safety factors applied.

Countersinks, Counterbores and Spot Faces

Model countersinks will be inspected during the test installation period to ensure that they are cut concentric to the threaded hole, have the proper countersink angle, and the fastener heads seat properly in the countersinks.

Counterbores and spot faces will likewise be inspected to verify that the contact surface does not bend or pry on the fastener body when it is tightened.

Small Screws

Fasteners of size #4 or less that are removed during a model change must be replaced with a new fastener.

Screw Joints

To assure tight joints between parts joined by screws, screws and threaded connectors must be sufficiently torqued to provide loads greater than the expected maximum separating forces.

Bolted Joints

Bolted joints with the primary function of transmitting moments must be designed in such a manner that the bolt preload divided by the joint contact area is at least 1.25 times the applied moment divided by the section modulus of the contact area.

Any bolt torque values that are different from the published vendor data must be derived in the stress analysis.

Fastener Locking

All structural bolted or screwed connections must be provided with positive mechanical locks such as:

• Locking inserts

• Self-locking-type nuts

• Safety wiring (drilled heads must be provided)

• Fastener adhesive such as Loctite (within rated temperature)

All bolted and screwed connections must meet these requirements, even if the connection is to be repeatedly disassembled during testing (e.g., changes in flap deflections).