Torque Control and Tightening Processes for Automatic Screw Machines: From Tightening Curves to Parameter Setting

In automatic screw machine applications, whether a screw is tight enough is not judged by feel but determined by torque control accuracy and the tightening strategy. Excessive torque easily causes stripped threads, cracked studs or deformed workpieces, while insufficient torque leaves a risk of loosening. This article explains torque control principles, tightening curves and process parameter setting in a systematic way.

1. Basic principles of torque control

Automatic screw machines usually use servo screwdrivers or electric drivers with torque feedback, collecting torque and angle signals in real time to form closed-loop control. When the actual torque reaches the set value, or the rotation angle reaches the preset angle, the system stops tightening immediately and records the torque curve for that fastening.

Torque control accuracy of mainstream models can reach within ±3%, and together with data upload this makes the process parameters of every screw traceable.

2. Three common tightening strategies

Torque method: stop when the set torque is reached. The most widely used method, suitable for most threaded connections; simple to implement, but relatively sensitive to fluctuations in the friction coefficient.

Angle method: first seat the joint at low torque, then rotate through a specified angle. Elongation is controlled through the rotation angle, giving better torque consistency, and it is suitable for applications requiring higher preload.

Yield point method: monitor the slope change of the torque-angle curve in real time and stop near the material yield point. This makes maximum use of bolt strength and is mostly used in demanding industries such as automotive and aerospace.

3. How to read a tightening curve

A normal tightening curve is usually divided into three stages: run-down, where torque rises slowly; seating, where torque rises quickly; and tightening, where the target torque is reached. If an abnormal peak appears during run-down, it usually indicates a tilted screw, a hole position deviation or damaged threads; if the slope of the tightening stage is abnormally flat, it may indicate stripped threads or an over-deep tapped hole.

By setting upper and lower torque windows and an angle window, the system can automatically judge NG and raise an alarm, intercepting missed screws, floating screws and stripped threads on the line.

4. Key points for setting process parameters

Before setting torque, establish the screw specification (M1.0-M6.0), the material being fastened (plastic, sheet metal, aluminium profile), the thread type (self-tapping or machine thread) and the working requirements (anti-loosening, sealing). In general, start by test-fastening standard parts, approach the target torque gradually, and verify with pull-out or torque testing.

It is recommended to record torque curves by batch and to calibrate the screwdriver periodically with a torque meter to ensure long-term consistency.

Honred Technology: torque closed loop and data traceability

Honred Technology's servo fastening models support dual closed-loop torque-angle control, multi-stage tightening speed and torque slope settings, and can save and export the tightening curve of every screw. They support MES integration and barcode traceability, and Honred provides process parameter advice and sample validation for self-tapping threads, thin plates, plastics and similar working conditions.