1. Field of the Invention
Axial seal rings serve the purpose of sealing shaft openings of machines. A force acting axially presses a rotating seal ring against a stationary seal ring. The rotating seal ring has a certain axial and radial clearance and is sealed onto the shaft by means of a secondary sealing element, and which is in most cases an O-ring, a lip-type packing or conical ring. This sealing element is subjected to differing stresses due to temperature, pressure, vibration, corrosion, swelling and shrinking, depending on the operation conditions of the machine.
2. Description of the Prior Art
The secondary sealing element is made predominantly of a rubber-type elastic material whose high elasticity has the advantage of better compensating for any roughness of the sealing surfaces, and, in particular of elastically absorbing machine vibrations. This guarantees a plane-parallel run of the seal faces of the rotating and the stationary seal ring. The seal rings used in pumps and agitators in the chemical and pharmaceutical industry frequently have to work with secondary sealing element materials possessing the necessary chemical stability, e.g. polytetrafluoroethylene, hereafter referred to as PTFE, at the cost of insufficient elasticity. These secondary sealing elements are frequently designed in the form of conical rings. Seal rings of this kind show a higher leakage than those with rubber-type elastic secondary seals. Moreover, by using conical rings, the axial mobility of the rotating seal ring on the shaft is impeded due to the large frictional surface. It is common practice to produce PTFE-O-rings and lip-type packings based on PTFE or O-rings with an elastic core and a PTFE covering.
The service-life of these sealing elements is limited predominantly by the operating temperature and the shaft vibrations in addition to the chemical reactions. Axial seal rings cause the development of heat at the sliding surface, which puts a stress on the secondary seal ring element. The amount of heat generated depends on the sliding properties of the materials used and the lubricating quality of the medium to be sealed, provided that the surface quality of both the rotating and the stationary seal rings is equal. In the chemical industry rotating and stationary seal rings are frequently produced in ceramic material, synthetic coal or graphite. In recent years, the development towards highly wear-resistant materials, like cemented tungsten carbide, or silicon carbide has continued. On coupling these materials, a higher friction coefficient than on coupling ceramics and coal is permitted. Utilizing highly wear-resistant materials by coupling tungsten carbide with tungsten carbide or tungsten carbide with silicon carbide in almost all cases results in a higher thermal load of the secondary sealing element of the seal ring. This leads to a more frequent failure of the seal ring owing to destruction or deformation of the secondary sealing element.
On the other hand, the secondary sealing element of the axial seal ring having a high sensitivity to dirt is incorporated into the seal, whereas by use of PTFE dirt produces grinding traces on the sealing element leading to channels which give rise to leakage. The disadvantages of the rubber-type elastic materials are the limited thermal and chemical stabilities. The disadvantage of being susceptible to dirt particles is true for all materials presenting no wear-resistance. Sealing elements made of PTFE possess high chemical and thermal stability. In continuous operation PTFE can be used up to about 200.degree. C. Its elasticity is very low. On the other hand, the material has undesired flow properties which will be raised under the influence of increasing temperature and pressure. To counteract this influence the PTFE seal is largely enclosed and furthermore the PTFE material is mixed with glass, graphite, carbon, ceramic or metallic materials. But the secondary sealing element cannot be enclosed totally. The rotating axial seal ring must have some small clearance on the shaft. This gap between the axial seal ring and the shaft is sufficient for the expansion of PTFE due to shaft vibrations. Secondary sealing elements with PTFE covering formed as O-rings possessing an elastomer core often fail, because the chemicals penetrate into the sealing element due to diffusion and in this way the elastic support is destroyed. Therefore secondary sealing elements of PTFE are also produced with a conical cross section. In that case a special design of the rotating axial seal ring for holding the key is necessary. But tapered secondary sealing elements impede the axial clearance and totally prevent the radial clearance of the axial seal ring. Another secondary sealing element, according to West German Offenlegungsschrift No. 27 55502 and U.S. Pat. No. 4,157,187 consists of an O-ring made of a metallic or rubber-type elastic core covered with graphite. This sealing element suffers from the disadvantages of the graphite being unstable with regard to heavily oxidizing agents, the instability of the elastomeric core and the inelastic behaviour of the graphite coating, which can be easily broken due to pressure and vibration loads. The axial seal ring must be given a special shape like for the tapered secondary sealing element, in order to provide readjustment of the seal.
As disclosed in U.S. Pat. No. 3,117,794, another flexible sealing ring is formed as a V and is positioned between a locating ring and a sealing member. The sealing member is positioned in such a manner to make axial and radial movements caused by the rotating shaft. The stress and the inside pressure of the machine effects the V-sealing ring. The sealing ring gets into the free space between the locating ring and a sliding ring and then it becomes fixed by both rings. Rubber elastically and use of a low elastic material also lead to a rapid deterioration of the sealing ring.
In French Pat. No. 1,131,832, a secondary sealing element is described which has two conical shoulders biased by spring force, is positioned between a driving device and an axial seal ring, and is adjusted under strength on the shaft. To diminish the static friction on the shaft, the sealing element is scratched out of the contact area with the shaft, to reduce the static friction. This sealing element seals radial to the shaft and axial to the conical shoulders of the axial seal ring. Between the driving device and axial seal ring there remains a space. In practice, the axial seal ring moves axially and radially. These movements are not done by the conical sealing face. This results in a clearance between the sealing face of the axial seal ring and the conical shoulders of the secondary sealing element and leads to leakage.
In German Offenlegungsschrift No. 1,775,727, a self-sealing mechanical seal is proposed which avoids a secondary sealing element. Further, there is arranged a springy ring body formed as a V or U and housing mounted on its free ends sliding rings, which are sealed axially to the shaft shoulder and to the casing. This springy ring body is stressed by torsion from the rotating shaft. This leads frequently to stress corrosion on the springy ring body, especially when stainless steel is used.
In U.S. Pat. No. 3,907,309 and German Patentschrift 1,204,575, there are also proposed self-sealing mechanical seals. These seals are constructed by a double sliding ring in both axial directions and their sliding surface is connected elastically. The disadvantage is their failure by stress corrosion, similar to the seal design of German Offenlegungsschrift No. 1,775,727. Further, these types of seal constructions do not guarantee adjustment of the sliding faces when vibrations of the shaft occur, especially by pumps to seal liquids and gases.
O-ring seals made of perfluoro-elastomers demonstrated permanent deformation in practical use owing to a fluid medium temperature of 140.degree. C. and the simultaneous influence of chemicals giving rise to a premature seal ring failure. Occasionally, the perfluoro-elastomer ring is radially split due to the material structure. The main disadvantage of the material is its expensive production technology. The manufacturing expenses for radial PTFE sealing elements amounts to only 1/10 to 1/20 compared to the perfluoro-elastomer gaskets.
On using mechanical seals it has been proven helpful to coat the shaft itself or the shaft tube protector by the help of a wear-resistant and non-corrosive coating is necessary because gaps represent preferred regions of corrosion attack. Hence ceramics or other metal oxides like alumina or chromic oxide will be applied. But these oxide-type materials have a worse thermal conductivity than the basic materials of the shafts or the shaft tube protectors. This is the reason for a heat storage at the secondary sealing element.