A torque setting does not guarantee clamp load. Friction takes most of the effort, so the same wrench reading can leave one joint secure and another ready to loosen. Here is what to specify and check.
N·m
01Torque and preload
Torque is an indirect measure of bolt preload. A common estimate is T = K × F × d: torque T, preload F, nominal diameter d and nut factor K. Because K mainly reflects friction, a torque value is only meaningful alongside the joint's surface and lubrication conditions.
Where the torque goes
Share
Friction under the nut face
about 50%
Friction in the threads
about 40%
Actually converted to clamp load
about 10%
Friction accounts for most applied torque, so clamp load changes when surface conditions change.
Surface condition
K range
Bare, dry
0.20 ～ 0.30
Zinc plated
0.18 ～ 0.24
Dacromet
0.10 ～ 0.16
Molybdenum disulfide lubricant
0.10 ～ 0.15
General machine oil
0.15 ～ 0.20
Lubrication can produce a very different preload at the same torque setting.
02Tightening methods
Method
Clamp load scatter
Cost
Typical use
Torque control
±25 ～ 35%
Low
General use
Torque plus angle
±10 ～ 15%
Medium
Cylinder heads, critical joints
Yield point control
±5 ～ 10%
High
High-value joints
Bolt elongation measurement
±5%
High
Large bolts, flanges
Hydraulic tensioning
±5%
Very high
Wind turbines, pressure vessels
Torque-plus-angle control reduces, but does not eliminate, variation from friction.
03Why bolts loosen
Transverse vibration. Junker's test showed that once the mating faces slide sideways relative to each other, a bolt can back off within a few hundred cycles. This is the dominant loosening mechanism.
Embedment. Surface asperities flatten out, the grip length shortens, and preload drops. The rougher the surfaces and the more layers in the stack, the worse it gets.
Stress relaxation and creep. Heat, or materials like plastics and aluminium, deform under sustained load.
Differential thermal expansion. Dissimilar materials expand at different rates, so temperature cycling swings the preload.
04Locking devices and their limits
Device
Principle
Under transverse vibration
Split lock washer
Elastic compensation
Essentially useless; dropped from most specifications
Star / serrated washer
Bites into the surface
Limited, and it damages the surface
Nylon insert nut
Adds prevailing torque
Moderate; not for heat, not reusable
Threadlocker
Fills the gaps, then cures
Good; removable or permanent depending on grade
Wedge lock washer
Wedge angle exceeds the thread helix angle
Very good; locks geometrically
Higher preload
Keeps the faces from slipping at all
The real fix
Correct preload is the first defense against loosening; locking devices serve a separate role.
05What the drawing should specify
Bolt size and property class (8.8, 10.9, 12.9, or A2-70)
Tightening torque and its tolerance
Surface condition and lubrication (dry / oiled / which lubricant)
Tightening sequence, multi-bolt flanges go up in crossing passes
Anti-loosening method, and whether the fastener is reusable