CFS Fastener Selection & Application 3 — Questions and Answers
Question 1: What is the primary advantage of using a fine thread (UNF) over a coarse thread (UNC) bolt in the same diameter?
- Higher resistance to vibration loosening (Correct answer)
- Faster installation speed
- Greater resistance to cross-threading
- Better performance in soft materials
Correct answer: Higher resistance to vibration loosening
Fine threads have a smaller helix angle, which provides better resistance to self-loosening under vibration due to more thread engagement per length.
Question 2: A self-drilling screw (also called a Tek screw) eliminates which installation step compared to a standard screw?
- Countersinking
- Pre-drilling a pilot hole (Correct answer)
- Applying thread-locking compound
- Torque specification
Correct answer: Pre-drilling a pilot hole
Self-drilling screws have a drill-point tip that simultaneously drills and taps the mating material, eliminating the need to pre-drill.
Question 3: Which metric property grade bolt has the highest tensile strength?
- Grade 4.6
- Grade 5.8
- Grade 8.8
- Grade 12.9 (Correct answer)
Correct answer: Grade 12.9
Metric property class 12.9 has a minimum tensile strength of 1220 MPa, the highest of the common metric bolt grades.
Question 4: In which application would a shoulder bolt (stripper bolt) be the most appropriate fastener choice?
- Clamping two rigid plates together permanently
- Creating a precise pivot point or guide for moving parts (Correct answer)
- Attaching a nameplate to a panel
- Connecting high-tension wire rope
Correct answer: Creating a precise pivot point or guide for moving parts
Shoulder bolts have an unthreaded cylindrical shank that provides a precise bearing surface for rotating or sliding components.
Question 5: When fastening dissimilar metals, which primary concern must be addressed to prevent premature failure?
- Thread galling
- Galvanic corrosion (Correct answer)
- Hydrogen embrittlement
- Stress relaxation
Correct answer: Galvanic corrosion
When dissimilar metals are in contact in the presence of an electrolyte, galvanic corrosion occurs as the less noble metal corrodes preferentially.
Question 6: A wood screw and a sheet metal screw of the same size differ primarily in which way?
- Head style
- Thread form and pitch (Correct answer)
- Material grade
- Drive type
Correct answer: Thread form and pitch
Wood screws have coarser threads with a tapered shank, while sheet metal screws have sharper, finer threads that cut into thin metal material.
Question 7: Which type of washer is specifically designed to prevent galvanic corrosion between a fastener and the substrate?
- Hardened flat washer
- Belleville washer
- Isolating (isolation) washer (Correct answer)
- Fender washer
Correct answer: Isolating (isolation) washer
Isolating washers are made of non-conductive materials (such as nylon or neoprene) that electrically separate the fastener from a dissimilar metal substrate.
CFS Fastener Selection & Application 3
What is the primary advantage of using a fine thread (UNF) over a coarse thread (UNC) bolt in the same diameter?