Reliable assembly processes are the core competence of modern screwdriving systems. Industrial customers want high-quality processing of fasteners for their expensive goods. With the highest quality benchmarks and optimised cycle times, screwdriving systems ensure thorough, reliable and fast joining of individual parts. Advance test parameters help to avoid product recalls. The top priority of a screw connection is to join two or more components – in the correct way: In the end, they have to act as a single component. The required clamping or pre-tension force has to be generated accurately and reproductibly. Different screw connections require different pre-tension forces. These are determined in advance so that all parameters for the screwdriving systems are correct. The objective here is to apply the calculated pre-tension force as accurately as possible, because precision work results in a reduction of materials and costs.

Screwdriving to depth

For screwdriving to depth, on timber connections for example, the head of the screw is countersunk.

Diagram of a screwdriving curve for screwdriving to depth

Torque-controlled screwdriving

The objective of torque-controlled screwdriving is to tighten the screw to a pre-tension force below the yield point. The screw is tightened until a preset torque has been reached. Then the screwdriving unit is switched off. For most of our customers’ applications, we use torque-controlled screwdriving.

Diagram of a screwdriving curve for screwdriving to torque

Rudolf, 1992

Angle-controlled screwdriving

For angle-controlled screwdriving, the screw is tightened until it reaches the plastic range. The screwdriving process starts with tightening of the screw until the pre-defined threshold torque (M) has been reached. This is when the angle measurement starts. The screwdriving unit is switched off when the pre-defined rotation angle has been reached. All calculations concerning angle-controlled screwdriving have to account for any – including extreme – weather conditions, temperature fluctuations and signs of wear in advance

Diagram of a screwdriving curve for screwdriving to rotation angle

Rudolf, 1992

Yield-point-controlled screwdriving (gradient screwdriving method)

The objective of the gradient screwdriving method is to tighten the screw to just before the yield point is reached. The switch-off here occurs based on a falling gradient, not on a torque or angle. The controller continuously calculates the gradient from the torque increase per rotation angle. The screwdriving process stops when the gradient falls by a pre-defined percentage of the maximum. This screwdriving process is still in its early days. This process could only be developed with powerful screwdriving process controllers with reaction times in the millisecond range. The advantage of this method is the highly precise shut-off just before the yield point is reached. This eliminates friction in the thread as much as possible. The drawback is that the shut-off torque can have a large spread depending on the application. This often poses a problem for traditional quality assurance.

Diagram of a screwdriving curve for screwdriving using the gradient screwdriving method

Rudolf, 1992