CFS CFS Locking & Anti-Vibration Fasteners 1 — Questions and Answers
Question 1: Which of the following best describes the locking mechanism of a prevailing torque nut (e.g., nylon insert lock nut)?
- It uses a chemical adhesive to bond threads together
- It creates friction through a deformed or non-metallic insert that grips the bolt threads independent of clamping load (Correct answer)
- It relies entirely on high clamp load to prevent loosening
- It uses a locking wire threaded through the nut
Correct answer: It creates friction through a deformed or non-metallic insert that grips the bolt threads independent of clamping load
Prevailing torque nuts create a mechanical interference fit — through a nylon insert or deformed metal threads — that resists rotation independent of the joint's clamp load.
Question 2: A Belleville (conical spring) washer used as a locking device primarily functions by:
- Biting into the mating surface with serrations
- Maintaining a residual spring load that compensates for minor clamp load loss due to embedding or relaxation (Correct answer)
- Chemically bonding to the bolt shank
- Deforming the bolt threads upon installation
Correct answer: Maintaining a residual spring load that compensates for minor clamp load loss due to embedding or relaxation
Belleville washers act as spring elements that store elastic energy, compensating for small losses in joint preload caused by embedding relaxation or thermal cycling.
Question 3: Per IFI and industry standards, a nylon insert lock nut (Nylock) should generally NOT be reused after removal because:
- The nylon insert permanently deforms and loses its interference fit after the first installation (Correct answer)
- The nut corrodes after first use
- The threads stretch beyond reuse limits
- Reuse is permitted but requires re-torquing
Correct answer: The nylon insert permanently deforms and loses its interference fit after the first installation
The nylon insert deforms to match the bolt thread profile during initial installation; upon removal and reinstallation, the insert no longer provides consistent interference, reducing its locking effectiveness.
Question 4: What is the primary difference between free-spinning lock washers and prevailing torque fasteners in a joint assembly?
- Free-spinning lock washers create locking force before the nut contacts the bearing surface; prevailing torque fasteners do not
- Prevailing torque fasteners create locking resistance throughout tightening and loosening, while free-spinning lock washers only function under clamp load (Correct answer)
- Free-spinning lock washers are reusable; prevailing torque fasteners are not
- Prevailing torque fasteners are used only in aerospace; lock washers are used in general industry
Correct answer: Prevailing torque fasteners create locking resistance throughout tightening and loosening, while free-spinning lock washers only function under clamp load
Prevailing torque fasteners resist rotation at all times due to mechanical interference, whereas split or toothed lock washers rely on clamp load to bite into surfaces and lose effectiveness if clamp load drops.
Question 5: Which fastener locking method is considered most reliable in high-temperature environments where nylon inserts would degrade?
- Nylon insert lock nut
- Chemical thread-locking adhesive (e.g., Loctite)
- All-metal prevailing torque nut (distorted thread or beam-type) (Correct answer)
- Split lock washer
Correct answer: All-metal prevailing torque nut (distorted thread or beam-type)
All-metal prevailing torque nuts (e.g., ESNA or distorted-thread designs) maintain their locking function at elevated temperatures where nylon degrades and most thread-locking adhesives lose effectiveness.
Question 6: The Junker vibration test (DIN 65151) is used to evaluate fastener locking systems by:
- Applying axial tensile load cycles to the bolt
- Subjecting the joint to transverse (lateral) vibration loads that are most effective at causing self-loosening (Correct answer)
- Immersing the fastener in a salt spray chamber
- Measuring torque-tension relationships under static load
Correct answer: Subjecting the joint to transverse (lateral) vibration loads that are most effective at causing self-loosening
The Junker test applies cyclic transverse (lateral) displacements to a bolted joint, which is the most severe loading condition for inducing self-loosening, providing a standardized comparative test for locking fasteners.
CFS CFS Locking & Anti-Vibration Fasteners 1
Which of the following best describes the locking mechanism of a prevailing torque nut (e.g., nylon insert lock nut)?