In safety-critical fields such as aerospace and automotive manufacturing, the quality of a single screw is not simply a matter of “torque to target”. Multi-step tightening strategies are usually required to raise final locking quality, but this places high demands on the intelligent screwdriver: it must deliver high-precision control and allow every step’s parameters to be set independently.
Taking the strategy “tighten–loosen, repeat three times” as an example, we analyse the hidden purpose of every single step.

1st cycle: 50 % target torque + immediate loosening
Eliminate initial clearance and release assembly stress

• The screw is tightened to 50 % of the final target torque so that the threads engage lightly and the joint faces touch, then it is completely loosened to a stress-free state.

• Purpose: remove “invisible obstacles” at the start of assembly. Engagement surfaces may carry machining burrs or oxide films, and microscopic gaps can exist between clamped parts. These cause “false seating”: the screw appears tight but tiny gaps or local load peaks remain.

• 50 % torque brings all parts into light contact; loosening lets the screw and parts spring back naturally, releasing the instantaneous elastic stress created by first contact.

2nd cycle: 70 % target torque + loosening
Calibrate load path and equalise load distribution

• Tighten again to 70 % of target torque. Now every thread flank is engaged, joint clearance is fully removed, and the faces are firmly seated; then loosen once more until the screw is stress-free.

• After the first cycle obstacles are gone, but “local overload” may still exist—especially when the lead angles of screw and nut differ slightly, so that only one or a few threads carry most of the load.

• 70 % torque forces all flanks to seat progressively, changing load from local concentration to full-thread distribution.

• Loosening releases torsional residual stress created during engagement; if left, this stress would later counteract the applied torque.

3rd cycle: final torque to specification
Stabilise the joint and ensure long-term reliability

• The screw is slowly tightened to the specified final torque, keeping the torque rise uniform. When the value is reached the tool stops; no further loosening is done.

• After the first two cycles the threads are fully and evenly engaged, no gaps or residual stresses remain. Applying the final torque now produces a stable axial preload and the residual torque fully meets the process requirement.

• Most elastic and assembly stresses were released in the first two loosening steps, preventing torque decay caused by slow stress relaxation in service—e.g. vehicle vibration or the high-temperature vibration inside an aero-engine, both of which can cause secondary deformation and loosening if stresses were not released.

A screw tightened with this sequence has stable thread contact and uniform load; even under complex service conditions it retains its clamp load and meets the core demand of “long-term reliability”.

The sequence is extremely demanding for the intelligent screwdriver:

• Must support fully programmable multi-step routines

• Must allow accurate setting of torque, speed and pause time (pause after loosening so stress can dissipate) for every step

• Must provide torque feedback to monitor actual torque in real time and guarantee that every step is executed exactly as specified.