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In modern industry, a reliable bolted connection and a trustworthy tightening tool are critical.  Before production the tool must be calibrated and certified, and it must be re-checked during use to achieve optimum tightening quality and standards.  Threaded-joint torque directly affects vehicle quality and operational reliability.  Many factors influence the final torque: fastener material and diameter, thread surface roughness, friction coefficients between the bolt/nut and the contact faces, tool accuracy, tool speed, tightening sequence, and—crucially—the joint’s own characteristics.

Joint characteristics are classified as soft or hard based on their influence on torque build-up.

Soft joint

• The joint uses soft materials or incorporates elastic elements (e.g., rubber gaskets).

• After the mating surfaces touch, the fastener must be turned 324° or more to reach the target torque.

• Torque decay can occur after tightening.

• Example: head-lamp screws with rubber washers.

Hard joint

• Joint members are stiff, rigid, and have smooth, well-mated contact faces.

• After contact, ≤ 27° of additional rotation reaches the target torque.

• Torque may “rebound” (over-tightening) after the driver stops.

• Example: engine-block metal-to-metal joints.

Standards

• ISO 5394:1994 defined soft as > 720° total rotation.

• ISO 5393:2017 re-defined it: from 10 % to 100 % of target torque, the angle change must be ≥ 324°.

• VDI/VDE 2647 uses 50 % → 100 % of target torque:
  – Hard joint < 30°
  – Soft joint < 360°

The apparent differences arise only from the reference angle used in each standard.