Screw-joint Analysis

Safety for accurate tightening parameter, sequences and tools
  • determining the optimum process parameter
  • process reliability right from the start
  • analyzing settling conditions

In the screw-joint analysis it is all about the question:

  • What is the ideal torque for the screw assembly?
  • Which rotation speed should be applied?
  • What type of screwdriving tool best fits the task?

“In order to find out the best way to screw together a component, I have got to destroy it. I have to consciously apply excessive torque until screws or components are overstressed. Only this way can I determine the overload torque."

The test is now repeated ten or twenty times – always with original components.

Finding the friction losses:
The fastening of the screw causes friction, which depends on both the screw's geometry and the component's material. The friction changes the relationship between the torque and achieved pre-load force; it is one of the major unknown dimensions in the specification of the fastening parameters. The engineer in the laboratory gets to the bottom of it by loosening the screw and fastening it again. With the comparison of the torque's graph progression in the first and second screwdriving process, he can determine potential friction losses.

How to define settling conditions:
We can evaluate settling conditions, for example when a silicone flat-gasket is tightened to a pump with four screws. The Result: the pump leaks.

Application specific requirements:
If, for example, two sheet-metal pieces are assembled and the holes are not aligned with each other, then a high force has to be applied initially, to assure a true-fit for the two sheets. Once the screw-holes are aligned at the end of the screwdriving process, a lower torque is sufficient to finish the assembly. Screw assembly with a constantly high torque, as in the initial phase of this application, would result in unintentionally reaching the destructive overload torque and the screws or the sheet metal may be deformed. In this case too, the screw joint analysis highlights the problem and defines a reliable set of parameters and sequences.

At the end of this test series, after careful analysis, the results are made available for the manufacturer to see which screwdriving parameters and type of screwdriving tool are most suitable for this particular screw assembly task.

Additional Information on our screw-joint analysis can be found in our Product Catalog Services D 3330.