When pipes and fittings are connected during the installation of drinking water supply systems and when linking heating systems with one another, the pressing process is the most common method used. In the middle of the 1980’s, the pressing method was introduced by a German manufacturer. This was considered a revolutionary technology at the time because the connections were made with an extremely special tool. In the beginning, this process was only possible when an enormous amount of force was applied using a press wrench. Later, large dimensioned electro-hydraulic press machines were designed because the power delivered by the manual press wrench was no longer sufficient. Furthermore, the press machines allowed the force to be more precisely delivered. In addition, the pressing technology for pipe connections is quicker, simpler, more comfortable and less expensive compared to traditional connection techniques such as soldering or flanging.
All common pipe materials such as multi-layer composite pipes, plastic pipes, c-steel, and stainless-steel pipes, as well as pipelines made of copper, can be connected with metal and plastic fittings. Since then the distribution rate of pressing technology, for example in Germany, has grown to almost 80 percent. Almost all applications in piping construction are covered, such as drinking water, heating, compressed air and combustible gases using current pressing systems and can even be used in industrial pipeline construction.
How does pressing technology work?
Although the pressing method only describes a mechanical mode of action, the process is complex and, depending on the piping system, extremely varied. So, what really happens during pressing? A press jaw is applied to the section that is to be pressed and with the use of a respectable amount of power the upper portion of the connection is pressed onto the lower portion. This, or something similar, is the standard explanation. But what really happens?
Pressing technology – a mechanical connection
A pressed connection consists of a fitting and a pipe. The pipe is inserted into the fitting or the fitting is inserted into the pipe. A sealing ring or sealing element made of EPDM is normally used and it is inserted between the parts to be connected. During pressing, the exterior material ring is pressed onto the interior material ring. Thus, the tensile strength is reached and a twisting or warping of the connection is thereby prevented. If you look closely at picture 1 you will notice that during pressing the amount of the exterior rings material is significantly longer compared to the interior material ring. But during pressing a connection between both material rings should take place. For this to happen, the exterior material ring needs to be compressed. Deformation occurs because most pipe materials are not able to be compressed. There, where the exterior material is pressed on the interior material layer, is where the mechanical connection results. But this also results in excess material in another spot, which can be seen in picture 2. At the same time, pressing tolerances are balanced.
To prevent damage to the sealing ring, the other pressing effect needs to happen on a different spot than where the mechanical press effect is planned. To insure this, only press jaws that have been made specifically for press connections can be used. Naturally, there are many varieties of force- and positive locking press connections. For example, for a connection made between multi-layer composite pipes with an aluminium core that is inserted into a fitting, the aluminium pipe will be pressed – since the aluminium is able to be compressed. Other connections are also possible where optically fewer deformations result. With less material in the gap, there is less material that needs to be deformed. It therefore makes sense to make sure that the gap is reduced to an absolute minimum using an alternate sealing method. This system constructively uses the interlocking teeth that are made using the softer material (usually the pipe material) between the upper and lower part of the press connection to create the seal. The amount of force applied to the section needs to be precisely defined so a long-term seal is guaranteed. If too much force is applied unwanted notches can result.
Avoiding the source of errors
A considerable source of errors is using a tool incorrectly or using the wrong tool, synonymous with press jaw. If the pressing is done on the wrong section many portions of the connection can be damaged. To some extent this type of faulty connection can be sealed and mechanically secure. But there are always consequences to be expected, especially those that affect the service life of the unit.
As you can see in picture 3, the targeted introduction of the pressing force is essential for a compliant connection. Even the incorrect application of the press jaw can lead to the transfer of force onto the wrong section which can create a connection that can no longer be guaranteed. This type of faulty connection can also occur when using a press jaw with the wrong shape that has not been approved by the fitting manufacturer or press jaw manufacturer. An optimal connection can only be guaranteed when a fitting construction that has been tailored to the pressing contour is used.
Conclusion: During a press connection many parts are mechanically bonded. During this process the parts are highly stressed. As a matter of principle, only compatible press jaws are to be used and the manufactures recommendations for the location of the press connection should be followed. In most cases, deviations for these recommendations will result in limitations to the service life of the unit.
Author: Matthias Bambl. Product Management – Pressing Technology for ROTHENBERGER Werkzeuge GmbH, Deutschland, www.rothenberger.com.