The right screwdriver guarantees top quality
Screw joint analysis assists in the selection of the best screwdriving tool
In the world of technology, screw assembly is the procedure most frequently used to join components. Every manufacturer fears errors in assembly. If there is a need for product recall due to faulty screw connections, the economic impact is considerable. Along with the appropriate training of personnel and the use of high quality materials, the selection of the most suitable screwdriving tool for each application plays a major role in ensuring process reliability during assembly.
A screwdriving task may only be executed reliably if precision screwdriving tools are used and the screwdriver has been optimally adapted to the application. Therefore, when making a selection, there is a wide range of criteria to consider – from flexibility and process reliability, to torque accuracy, production data acquisition, documentation requirements, investment needs, operating costs etc. In order to determine the necessary tightening parameters and to identify approaches to solutions, a comprehensive screw joint analysis is recommended.
The important issues to address during screw joint analysis are the ideal torque, the correct speed and the most suitable screwdriving tool for the application. DEPRAG Application Engineer Tobias Dirrigl explains: “In order to determine the best way to assemble a component, we need to destroy that component. We deliberately use excessive torque so that the screw or component breaks, thereby enabling us to establish the overload torque”. This process is repeated, using original components, until a reliable result has been ascertained.
Friction is generated as the screw is tightened. The extent of the friction is dependent on both the geometry of the fastener, and the material of the component. The friction changes the relationship between the torque and the pre-load force attained. It is one of the great unknown factors when defining the most suitable tightening parameters. The technician figures this out in the lab by repeatedly loosening and tightening the assembled screw. By comparing the curve characteristics of the torque during the first and second screwdriving procedure, he is able to detect potential friction losses.
Seating conditions are also recorded, for example when a motor is mounted on a pump unit with a silicone gasket. Even if the tightening torque has been designed so that the required pre-load force is attained, it will gradually be lost due to the “seating” of the silicone. Consequently: the pump is leaky. In this type of “soft joint” application, the technician tightens the screw to the defined tightening torque and then retightens it after a certain amount of time. The “prevailing torque” provides information about the seating conditions and their effect on the pre-load force. In such cases, application technicians recommend that screw assembly is carried out in two phases.
More information: DEPRAG SCHULZ GMBH u. CO.
