What is your algorithm for tightening screws

Further tightening torque test of a screw that has already been tightened

Further tightening torque test of a screw that has already been tightened

Correct screwing of a connection is an elementary step for quality control in a production process.

At the end of the production line, a critical step must be carried out for complete quality control in a production line: the measurement of the pulling torque.

Selecting the correct tightening strategy ensures that the connection has been tightened to the specified values. However, it remains completely unknown what happens in the connection after the screw connection. How much of the applied preload is retained in the connection?

In order to evaluate the tightening torque of a connection (e.g. digital torque wrench), a tool is required that offers one or more control strategies for measuring the tightening torque. The data must be tracked in a database with management software that creates statistical analyzes and reports in order to avoid errors through statistical process control.


 

STARTING POINT - TORQUE / ANGLE

Determining the breakaway point is the preferred method of measuring the pull-on torque.

In this strategy, a digital torque wrench applies torque to the screw to keep turning it. The torque is measured at the point where the screw begins to move.

In some cases, for example if a drop of glue blocks the screw, the breakaway point does not match the tightening torque. In fact, more torque is required to loosen the adhesive (or to overcome the friction if there is rust in the joint). Then the torque drops to increase again with rotation.

To carry out this test, a digital torque wrench with a torque sensor and a gyroscope for angle measurement is required, which has an algorithm for the automatic detection of the breakaway and pull-out torque, as shown above.

Some torque wrench models offer strategies to detect the pull-ahead torque using a torque / time algorithm rather than a torque / angle algorithm. The measurement carried out with this method is influenced by the operator and is therefore not completely objective, less repeatable and less reliable.

Before methods and intelligent digital torque wrenches were developed, the breakaway point at peak value or angle measurement provided a reliable reading, but it was less objective than automatic detection.

The peak torque value is influenced by the operator, as the torque / angle requires precise knowledge of the connection mechanics and the definition of the correct test parameters:


 

RELEASE - Tighten

Measuring the tightening torque with the breakaway point method could cause the screw connection to be tightened too tightly under test (Even if the tightening torque is normally less than the tightening torque, so it is likely that the tightening torque test will not exceed the nominal value).

In order to avoid the risk of the screw connection being too tight or in special applications with very large screws, the "Loosen - Tighten" method is preferable: Loosen the connecting piece by a few degrees (e.g. 10 °, but depending on the torque / angle of the Connection) and determine the torque to turn it back to its original position.

In the past, the position of the screw was marked with a mark before the test was performed manually:

With today's digital torque wrenches with gyroscopes, the operator can perform this strategy much more easily.


 

CHECKING THE MINIMUM TORQUE

Some automobile manufacturers tighten the screws that have already been tightened again with an articulated wrench. If the torque wrench clicks without rotating, this means that the clamping force has not been lost in the connection after the screwing process.

This method only provides an OK or a Not OK result. The value of the pull-on torque (no tracking) is not measured. In addition, when using a buckle wrench, there is always the risk that the screw connection will be tightened too tightly.

With a digital torque wrench with a gyro sensor, a quick check of the screw connection (no measurement of the tightening torque) and the applied torque is possible. It can also be determined whether a certain value (usually the minimum tightening torque) has been reached within a certain angular rotation (usually very small). In this way, checks are carried out quickly and automatically, and the torque wrench can detect if the screw connection has been tightened too tightly.