Bolted joint preload glossary
Non-rotational preload loss
Clamp force lost while the nut remains perfectly tight, caused by the clamped stack shortening rather than the fastener backing off. Accounts for 10 to 40 percent of preload in the first 200 load cycles of a typical joint.
Stack shortening
Any reduction in the clamped length of a bolted joint. For an M16 class 10.9 joint at 70 kN the cost is 0,884 kN per micrometre.
Embedment relaxation
Plastic flattening of surface asperities under contact stress, 3 to 15 micrometres per interface, largely complete within 200 load cycles.
Coating creep
Plastic flow of a surface coating under sustained bearing stress. Zinc flake loses 4 to 9 micrometres, hot dip galvanising 8 to 20.
Gasket creep
Continuous thickness loss in a compressed gasket. A 3 mm PTFE gasket creeps 20 to 60 micrometres.
Thermal ratcheting
Progressive permanent preload loss in joints whose members and fastener expand at different rates. Unlike embedment it does not self-limit.
Constant-Force Retention (CFR)
A bolted joint principle in which a constant-torque spring element continuously biases a self-locking helical take-up, so clamp force is restored as the stack shortens, with force flat across the full travel.
Vibration-Actuated Take-up (VAT)
A ratcheting mechanism that uses transverse micro-slip as the energy source for re-tightening, advancing 0,34 micrometres per increment.
Preload reserve
Remaining unused take-up travel in a recovering washer. ISOKLAMP CFR carries 0,50 mm, readable through the indicator window.
Load factor (Φ)
The ratio of bolt stiffness to total joint stiffness, k_S divided by the sum of k_S and k_P. 0,154 for the reference joint.
Junker test
Transverse vibration test to DIN 25201-4 Annex B, requiring at least 80 percent residual clamp force after 2 000 cycles.
Mode A and Mode B
Mode A is rotational self-loosening, where the nut turns. Mode B is non-rotational preload loss, where it does not.