TYPE COUPLINGS AND FRETTING CORROSION
People say, that disc type couplings, opposite to for example rubber block type couplings, have no mating parts and therefore no wear. This is, however, not entirely true. Let us take for instance the drawing on the right hand side. This shows that when misaligned, the upper discs are in compression while the lower ones are under traction. This implicates that the upper discs get a little bit shorter while the lower ones are extended. Therefore we have movement.
This movement combined with the pressure on the bolts causes a phenomenon called fretting corrosion. It looks like stain and is a consequence of the rubbing of the discs under high pressure at bolt level. This phenomenon is general and can be found with almost all disc type couplings (even the ones in stainless steel) that have functioned under normal (and also abnormal) working conditions.
It is obvious that such a (normal) phenomenon will limit the life of the coupling. Extra tensions and cracks are created in the material.
One can therefore not consider every disc type coupling to have infinite life.
The solution to this problem is obvious. The contact between the discs has to be avoided. This is possible by putting very thin plates in between the discs (see drawing).
These discs are placed under the backing rings. One can make the remark that by doing this, contact between the discs is avoided, but contact between the plates and the discs is created. This is true. However, underneath the backing rings (which put pressure on the discs keeping them together), movement is not possible and therefore neither is fretting corrosion. (Remark: in reality the plates are much thinner in comparison with the discs. On the drawing they're thicker in order to show their position and way of working.
This solution is used in all Escodisc couplings. The plates are called fillers and are in stainless steel. It is a simple but effective solution : fretting corrosion is avoided. Because Escodisc couplings have only a limited number of discs, the number of fillers is also limited and therefore, the influence on the total thickness of the disc pack is negligable.
DISC TYPE COUPLINGS AND REACTION FORCES
When people speak about the advantages of disc type couplings, they will always refer to them as being maintenance free. A disc type coupling is the only coupling that, when properly designed, can work for a longer period without human intervention. This is of course true. Nevertheless, it is almost always forgotten that a coupling is a link between two machines. Therefore, when due to certain circumstances, misalignment occurs, this coupling will compensate for these misalignments. But not without consequences however.
The coupling will, influenced by changes in the system, start to react to the misalignment. This will create forces on bearings, mechanical seals and even shafts. The coupling will therefore have a direct influence on the functioning and the lifetime of the connected elements. That this influence depends on the type of coupling used (disc, gear, rubber element) is shown by the below test results.
Remark : the tests where performed on comparable (for what torque capacity is concerned) disc, gear and rubber element couplings under equal misalignment conditions. Tests performed by the University of Brussels.
It is obvious that the disc type coupling has the lowest reaction forces which are as good as not depending on the torque applied. For both the gear as the elastic coupling, reaction forces are very important and strongly depending on the torque. The disc type coupling will therefore, in comparison to other types of couplings, have a positive influence on the lifetime of bearings, mechanical seals, etc...
Escodisc couplings are designed (as described on p. 5-6) to keep the reaction forces, in the event of misalignment, to a minimum level. Therefore, Escodisc couplings will improve lifetime of connected equipment.
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