Friction linings in the form of brake and clutch linings on the basis of organic binding agents play a very important role in modern engineering. Motion energy is converted to heat through friction, and the friction lining must display constant and uniform friction properties. There must not be any variation in compressive strength, shear strength, or material hardness across the entire lining, so strict requirements are placed on the homogeneity of the mixtures that are pressed into friction linings. It comes as no surprise then that renowned manufacturers are lining up for the best available mixing technology – which also allows dry and wet mixtures to be processed in the same machine. In 2020, EIRICH again set up several projects for new mixers, including for manufacturers in Southern and Eastern Europe. In almost all of these projects the mixers were configured to be ATEX compliant, which is almost always the standard in this industry.
Friction linings consist of a large number of raw materials, with up to 50 different materials in a single recipe. The substances display a huge variation in terms of their bulk weights – ranging from 50-100 g/l for fibers to over 4,000 g/l for metallic components. This places very high-quality requirements on the mixing technology.
The manufacturers’ recipes have been carefully developed and fine-tuned for the relevant application and the specific requirements of their customers. Ingredients include inorganic fillers, lubricants, and metals in the form of powders, grains, or fibers. Plastic fibers are also often used – for example on the basis of aramid fibers – that do not melt when exposed to heat, but instead carbonize. In addition there is also the binding resin, which carbonizes under the effects of heat during the course of production and joins all the particles to each other. New graphite-based binding systems, which produce a mechanical bond under pressure, work with low resin content or with no resin at all.
In all recipes, it is necessary to optimally mix very lightweight substances with very dense materials and fibers. For the mixer, this is a real challenge, particularly as the fibers have to be opened. It is known from the literature that demixing occurs during mixing with a “normal” mixer, because a demixing process is superimposed here over the actual mixing process. This happens particularly in mixtures where the raw materials have different densities. Investigations into the “Mixing time determination for a horizontal plowshare mixer” with two test substances showed that the maximum mixing time is limited to just 120 seconds. If the mixing time is extended beyond this, demixing of the two components will occur. This is explained by the different movement behavior of the components in the mixer; as a result, there is a gradual accumulation of the heavier components in the edge areas.
In practice, this means that the mixing times in this type of mixer cannot be freely chosen and that the best possible mixing quality in each case needs to be established in a series of measurements – but this is not usually the optimum mixing quality. These findings apply to all mixers in which the transport of the materials being mixed is performed via mixing tools. Demixed or inhomogeneous pressed compound material will lead to a spread in quality and will cause potential problems during subsequent use.
In order to achieve the optimum mixing quality and carry out mixing processes without demixing, manufacturers of friction linings regularly opt for mixing technology from Eirich. Here, the material being mixed is not transported by the mixing tools, but by the rotating pan. These mixers are used in a large number of industries whenever other mixing systems reach their design-imposed limits. Many investigations have shown that the Eirich technology can achieve mixing qualities that generally cannot be reached by other systems. The operating principle with a rotating mixing pan and material deflector prevents the formation of any poorly-mixed dead zones in the mixer, as every partial volume of the material mix is fed to the rotor tool. It also reliably rules out the occurrence of demixing in the mixer.
Since the processes of material transport (performed by the rotating pan) and mixing (performed by the mixing tool, which is known as the rotor) are separated from each other, the mixing tool can run at any speed – with tool speeds of up to 30 m/s depending on the process step. This is why this mixer does not require any fast-running, wear-intensive choppers in order to break down and solubilize fibers. It also means that Froude numbers can be achieved many times higher than what is possible with other mixing systems. As a consequence, an Eirich mixer can process any required consistency. With friction linings, this is generally dry mixes or mixes with resin solution – which are no problem for the mixer. Both mixes can be produced using the same machine. Even highly viscous resin solutions are processed effortlessly, which was previously very difficult or completely impossible with conventional mixers. And, of course, the latest generation of graphite-based friction linings can be optimally processed with this technology.
Customers – including renowned friction lining manufacturers from all around the world – also appreciate that these mixers offer low wear by design and are comparatively easy to clean. For them, this is regularly a further reason to stay loyal to Eirich technology whenever it is time to upgrade their production capacity. In most cases, a mixer ranging from 75 to 1,500 liters in terms of usable volume is used, generally in an ATEX-compliant configuration. Thanks to the design of the system, upscaling is also relatively straightforward. As a result, it is very easy to transfer recipes from small-scale test mixers to large production mixers.
Depending on the requirements, the mixers are finished with explosion protection (inside: zone 21 for wet processing, zone 20 for dry processing; outside: zone 22) in accordance with the European ATEX regulations. If required, the mixers can also be configured for gas explosion protection zones 0, 1 or 2. Where necessary, the mixers can also be equipped with inductive heating, which of course is also supplied in an ATEX-compliant form.
Test centers are available at all Eirich locations around the world, where the system-based advantages of this special mixing technology can be demonstrated first-hand to customers using their own raw materials. In comparison to other mixers, this usually results in significantly shorter mixing times, which in turn means that smaller mixers can be used – and all this with improved reproducibility of the mixing results.
Contact: Oliver Zeitner, e-mail: firstname.lastname@example.org