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DEPRAG Clamp Force Control: Constant clamp force despite fluctuating tightening torques

| Screwdriving technology | News articles

Challenges in modern screwdriving technology

Screwdriving technology is the most widely used joining technology. Ever more complex assembly tasks, increased demand for processing reliability and higher quality standards require sophisticated procedures which are clever enough to cope with these complicated requirements. Aluminium and other lightweight metals and synthetics are increasingly being used in the automotive industry, for example, for lightweight design.  Housings for pumps, airbags or interior components – all these products are usually assembled using thread-forming or thread-cutting screws.

Strongly contrasting tightening torques are generally required in these types of screw assemblies. The use of a torque controlled tightening procedure can cause wide variations in the resulting clamp force even with excellent shut-off accuracy. The new adaptive screwdriving procedure DEPRAG Clamp Force Control (CFC) works to counter this by eliminating all frictional influences up until the point of head seating. This thereby enables a significantly improved constancy of the clamp force. 

 

Detection of seating point and bolting for differential torque

Dabei setzt sich die vollständige Verschraubung aus dem Schraubmuster Kopfauflageerkennung und einer Verschraubung auf Differenzmoment oder einer Verschraubung auf Drehwinkel zusammen. Hauptbestandteil ist die Kopfauflageerkennung: Auf Basis des Drehmomentverlaufs wird kontinuierlich eine mathematische Bewertungsfunktion gebildet. Die Kopfauflage wird erkannt sobald diese Funktion einen fest definierten Grenzwert überschreitet.
The entire screw assembly consists of the screwdriving template seating recognition and an assembly to differential torque or assembly to angle. The main part is the seating recognition: mathematical evaluation functions are continuously built up on the basis of the torque process. Seating is recognised once this function exceeds a specified limit value.

The calculation procedure combines two fundamental advantages:

  • The calculation rule is robust in relation to coincidental fluctuations or increases during the torque procedure which do not occur as a consequence of the actual head seating.
  • Furthermore, the algorithm is universal so the user does not have to set any relevant parameters for the calculation. There is therefore no need for extensive preliminary tests and parameterisation.

 

 

  • The torque upper limit serves as the shut-off criteria for the screwdriving step.
  • The OK window for seating recognition can optionally be monitored using the seating point torque upper/lower limit.
  • The end values can either be saved as the torque and angle values at the point of seating or the end of the screwdriving step.
  • The end values of the screwdriving template seating recognition provide the reference for the following program section.

 

  • Screw assembly to angle can follow instead of screw assembly to differential torque.

 

 

Application area for the DEPRAG Clamp Force Control screwdriving process

The procedure for an EC-Servo screwdriver in combination with sequencing controller AST40 is mainly used for direct screw assemblies in metal or plastics. The occurring fluctuating tightening torques are caused for example by alterations in the screw or hole geometries or in the structure of the component material, varying surface characteristics of the screw thread and flexible elements or seating conditions.

 

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