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With fibre-reinforced rubber coating

Flat gasket material for static applications
With fibre-reinforced rubber coating

Unfavourable operating conditions with superimposed dynamic effects can reveal weaknesses in conventional fibre-based flat gaskets. The flat gasket material MatriCS ensures optimum, long-term sealing for the most varied applications. Even with low surface pressures, the material exhibits excellent adaptability to the sealing surfaces, together with high temperature stability and no need for re-torquing of the sealed joint.

Matthias Eppler, Dr. Rolf Gladen

Beaded, sheet steel gaskets of different metals and with elastomer coatings of 25 µm or 60 µm fluorocaoutchouc (FKM) or nitrile rubber (NBR) are already well known under the trade name Retall [1]. The flat gasket material MatriCS (Matrix Coated Steel) combines the advantages of pure soft materials, e.g. compressibility, with those of Retall gaskets.
Construction of MatriCS and MatriCS plus
At present, two types of MatriCS gasket are produced. The standard version has a core of 0.25 mm thick structural steel. For MatriCS plus, the core is made of 0.20 mm thick stainless spring steel. The overall thickness is 0.5 mm with both versions. In contrast with Retall, the rubber coating is reinforced with fibres that give the material a compressibility and recovery approaching the values for various soft materials based on rubber-bonded fibres. This greatly improves its adaptability and temperature stability. More-over, MatriCS has a non-stick surface with a low coefficient of friction. This facilitates disassembly and allows relative component movement without damaging the coating.
Operating principles of gaskets
For most applications, MatriCS gaskets are used, but with special embossed beads. These ensure high line pressures even with low bolt forces, leading to very good sealability. Furthermore, considerably larger un-evennesses can be compensated than with unbeaded gaskets. The geometry and stiff-ness of the sealing lines (beads) are adapted individually to the components and to the desired sealing conditions. In order to achieve an optimum solution in every case, the gaskets are designed precisely to the customer’s specifications. With fibre-based gaskets, good sealing can become problematic particularly between the bolts. Areas with insufficient surface pressure can occur due to component deflection. In these areas, the gasket’s pores are not compressed sufficiently, with sub-optimal adaptability to the sealing surface. This results in leakage channels (Fig.1, bottom). On the other hand, beaded MatriCS gaskets have springy, multiple pressure lines in all areas of the sealed joint, that conform very well to any roughness and unevenness of the components (Fig. 1, top). At the points of high pressure around the bolts, the bead is almost pressed flat, whilst between the bolts the magnitude of the deformation decreases down to the point of least pressure. However, because of the gasket’s design, a leaky joint is practically impossible with this sealing method. Even roughly machined surfaces can usually be sealed reliably.
Advantages
The metal core ensures blow-out safety. Also important is the high temperature stability of MatriCS. Due to its special mixture, the coating of high-quality fibres and elastomer allows operational temperatures between -40 °C and about +220 °C (up to +300 °C for short periods). This is considerably higher than for Retall gaskets with a comparable elastomer coating (approx. 180 °C) or for pure NBR gaskets (120 °C).
One known disadvantage of fibre-based material, especially at higher temperatures, is gasket setting. Owing to the thin coating and the reinforcing fibres, the degree of setting with MatriCS gaskets is negligible. For example, at a testing temperature of 300°C, thermal deformation of no more than 5%, practically independent of the applied surface pressure, was observed. This ensures good sealability, even during long-term operation. Moreover, there is no need to re-torque the gasket after installation.
Suitable for difficult applications
MatriCS gaskets demonstrate their capabilities particularly effectively in difficult applications, as explained by the following practical example. Figure 2 shows the cylinder head, the valve plate, and the head gasket of a refrigeration compressor. Several factors precluded the use of a fibre-based gasket for this application:
• The web between the suction and pres-sure sides at the centre of the cylinder head is long and only has two bolts, these being some distance from the web and the ideal sealing line (imaginary line between two bolts). In this case, the bolts are also displaced from the actual sealing line. The surface pressure at the web is therefore low, and in addition the valve plate is not rigid.
• The high, shock-like start-up pressures cause an abrupt reduction in the surface pressure, making it likely that a non-reinforced soft gasket will rupture.
• The reduced sealing forces caused by the internal pressures under operational condi-tions lead to a diminished surface pressure that is below the level required for a fibre-based gasket.
With soft gasket materials, this situation will result in leaks between the suction and pressure sides. Furthermore, the gasket web was ruptured several times during operation because of insufficient constraint and stability.
The unfavourable pressure distribution in the case of the refrigeration compressor can be demonstrated by means of pressure-sensitive Fuji „Prescale“ film. For this purpose, the film is mounted between the cylinder head and the valve plate instead of a gasket. After the film is removed, areas with high surface pressure are identified by a deep red colouring. Areas with low surface pressure are light red or may even remain white [2].
This problem can be solved with MatriCS gaskets that have a stiff bead along the web. The bead produces a high line pressure that ensures a reliable seal. Moreover, the gasket’s metal core prevents rupturing.
A less-stiff bead along the gasket’s outer contour has the effect that the bolt force required for the critical low-pressure area at the web is not wasted at the edges. The lower part of Figure 3 shows the pressure distribution without a gasket. The areas of insufficient pressure at the web are clearly visible. The upper part shows closed pressure lines along the web bead after installing a MatriCS gasket. There are no gaps in the surface pressure, i.e. a tight seal is ensured.
Another application of MatriCS and MatriCS plus in the area of refrigeration compressors is sealing base plates, where start-up pressure shocks cause the plate to deflect considerably, possibly leading to leaks via the bolt holes if fibre-based gaskets are used. Especially in problematic cases like this, the special bead geometry ensures optimum sealing results. Furthermore, MatriCS gaskets with individual bead geometries provide very good sealing with controller flange gaskets. Leaks frequently occur here between the central hole and the high-pressure bore due to insufficient sur-face pressure.
Beaded MatriCS gaskets can provide a solution in the field of hydraulics, where the main problems are caused by high pressures and high dynamic loads. At present, rubber O-rings or metal rings with moulded-on rubber lips are widely used in hydraulic applications. The advantages of MatriCS gaskets over rubber O-rings lie firstly in the simpler, automatable installation procedure. Secondly, it is easy to check subsequently whether a gasket has been fitted. This is not possible with O-rings, because they are fitted between mating surfaces. In addition, the sealed joint remains elastic over a longer period, especially in the case of MatriCS plus, as the metal bead exhibits less fatigue than elastomers, resulting in a longer service life.
In general, gaskets made of MatriCS are the ideal solution for the following problematic applications:
• low mounting surface pressure or insufficient pressure distribution,
• narrow webs, particularly in combination with high pressures,
• considerable component deflection under working conditions or insufficient residual pressure due to the relief caused by internal pressures,
• dynamic forces acting on the joint (pressure changes, vibrations, etc.),
• changes in the sealing gap width due to frequent and severe temperature variations from which a soft-material gasket is unable to recover. These conditions are typically encountered with pumps, valves, compressors, small combustion engines, electric motors, transmissions and gas supply equipment.
Victor Reinz
++49/731/7 04 67 65
Further information cpp-225
Literature
[1] Dußler, K.; Gladen, R. : Flexibilität eingebaut. Ferderelastisches Metall-Dichtsystem für schwierige Abdichtaufgaben. „Chemie Technik 25“, page 54 (1996)
[2] Gladen, R.; Dußler, K.: A simple method for determining the mounting surface pressure with flat gaskets. Translation from „Dichtungstechnik 1“, page 30 (1998)
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