CFS Torque & Tension Specifications 3 — Questions and Answers
Question 1: A fastener specification calls for 'prevailing torque' measurement. What does this assess?
- The torque needed to seat the head against the joint
- The frictional resistance torque of the locking element before clamp load is developed (Correct answer)
- The minimum breakaway torque after installation
- The torque at which the fastener reaches proof load
Correct answer: The frictional resistance torque of the locking element before clamp load is developed
Prevailing torque is the resistance offered by the locking feature (e.g., nylon insert, distorted thread) before the fastener engages the joint surface.
Question 2: According to VDI 2230 guidelines, the recommended bolt utilization rate for controlled tightening is approximately:
- 50% of proof strength
- 70–90% of proof strength (Correct answer)
- 100% of tensile strength
- 110% of yield strength
Correct answer: 70–90% of proof strength
VDI 2230 recommends using 70–90% of the fastener's proof strength to maximize clamp load while maintaining a safety margin.
Question 3: What is the effect of increasing the grip length of a bolt on joint stiffness?
- Increases joint stiffness
- Decreases joint stiffness (Correct answer)
- Has no effect on joint stiffness
- Increases stiffness only in shear joints
Correct answer: Decreases joint stiffness
A longer grip length means a more compliant (less stiff) bolt spring, which improves resilience to dynamic loads but reduces joint stiffness.
Question 4: When converting ft-lb to N·m for a torque specification, which conversion factor is correct?
- 1 ft-lb = 0.738 N·m
- 1 ft-lb = 1.356 N·m (Correct answer)
- 1 ft-lb = 8.85 N·m
- 1 ft-lb = 0.113 N·m
Correct answer: 1 ft-lb = 1.356 N·m
1 ft-lb equals 1.356 N·m; dividing by 1.356 converts N·m back to ft-lb.
Question 5: A torque audit (check torque) is performed after installation. The fastener moves at 95% of the original torque. What does this indicate?
- The fastener is overtightened
- The fastener likely has adequate preload within acceptable scatter (Correct answer)
- The fastener has completely lost preload
- The torque wrench is out of calibration
Correct answer: The fastener likely has adequate preload within acceptable scatter
Movement at 90–105% of specification torque during an audit typically indicates acceptable installation; significant shortfall suggests relaxation or improper initial torque.
Question 6: Which tightening method provides the most accurate and consistent bolt preload?
- Hand-torque wrench
- Impact wrench with torque stick
- Hydraulic tensioner (bolt elongation method) (Correct answer)
- Torque-angle method
Correct answer: Hydraulic tensioner (bolt elongation method)
Hydraulic tensioners apply direct axial load and measure elongation, bypassing friction variables entirely, giving ±5% or better preload accuracy.
Question 7: What is 'embedment relaxation' in a bolted joint?
- Loss of preload due to gasket creep under sustained load
- Gradual plastic flattening of surface asperities after initial tightening (Correct answer)
- Corrosion-induced thread loosening
- Thermal expansion of the bolt relative to the joint
Correct answer: Gradual plastic flattening of surface asperities after initial tightening
Surface asperities on mating faces plastically deform after torquing, causing micro-settling and a loss of 5–10% of initial preload.
CFS Torque & Tension Specifications 3
A fastener specification calls for 'prevailing torque' measurement.
What does this assess?