Preload force measuring system flexSonic

A bolted connection works by pressing together the parts to be joined. The process force results mainly from the preload force of the screw. The preload force is the quality-determining parameter for a bolted connection. The fracture of a screw or the loosening of a bolted connection are the extreme consequences of an incorrectly set screw preload force.

Strain or tension of a screw influence the propagation velocity of ultrasonic waves. It is generally recognized, that this effect allows the determination of strain or tension as well as of the preload force.

The system flexsonic uses this principle, known as "acusto-elastic effect", to control the screwdriver as a function of the preload force.


  • Evaluation of ultrasonic wave propagation using specific ultrasonic transducers
  • Integration of the ultrasonic transducer into the screwdriver nut with consideration of suitability for volume production
  • Non-contact signal transmission, automated measurement and data analysis
  • Control of screwdriver and screw driving process unsing the recorded run time measurement results
  • Continuous comparison with estimated characteristic diagram

For technical details and descriptions of the system and the process please refer to specific EST flexsonic brochure.