Secondary sealing element with U-shaped sealing ring for mechanical seals

A secondary sealing element for axial seal rings, is resistant to stresses due to temperature, pressure, vibration, corrosion, swelling and shrinking. This sealing element is produced of non-rubber-type elastic and non brittle material and its shape, in cross-section, consists of a ring, with a pair of arms to give it an elastic reaction. The double-arm ring forms three radial-type sealing surfaces and is completely locked into a chamber. Special material with low static friction and higher thermal conductivity than rubber material is used. Furthermore the dissipation of heat, produced by the axial sliding rings, will be increased through the double contact area to the shaft by the arms in comparison to an O-ring, and through a wear-resistant and non-corrosive layer on the shaft having at least the thermal conductivity of the basic material.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
The object of the present invention is therefore to provide a secondary 
sealing element for axial seal rings having the necessary elasticity and 
chemical stability. Furthermore it has to show low susceptiblity to 
pressure, contamination and high temperature. The sealing element should 
enable slight axial and radial clearance of the axial seal ring in order 
to elastically absorb vibrations and compressive impact loads of the 
machine shaft. The subject of this invention is a secondary sealing device 
for an axial seal ring assembly having a chamber therein with a 
cylindrical surface wall surrounding the axial shaft, the secondary 
sealing device comprising: 
(a) a biased axial sliding and adjustment ring positioned in the chamber; 
and 
(b) a sealing ring positioned around the shaft in the chamber between the 
cylindrical surface wall and the sliding and adjustment ring, formed of a 
material with good sliding low-elasticity, and non-brittle 
characteristics, having a generally U-shaped cross-section with two arms 
arranged symmetrically or unsymmetrically to a plane perpendicular to the 
axis of rotation of the shaft, and pressed into sealing engagement against 
the cylindrical surface wall and the shaft by the bias of the sliding and 
adjustment ring thereagainst and the resulting changes in the diametric 
dimensions of the sealing ring. 
Especially efficient is the use of materials for the sealing element which 
have a higher thermal conductivity than the well-known rubber-type elastic 
materials. This decreases the thermal stress of the sealing element 
through an increased heat transfer to the shaft or the protective tube in 
the sealing element region, with a coating which exhibits at least the 
thermal conductivity of the basic material. 
Moreover, it is advantageous to shape the sealing element in such a way 
that a chamber is obtained to accommodate foreign matter. The sealing 
element has to permit a small axial and radial clearance of the axial seal 
ring in order to compensate elastically for vibrations and compressive 
impact loads of the machine shaft. These requirements are realized by a 
good sliding behaviour and elastic reaction of the sealing element. 
The sealing element corresponding to the present invention which is made of 
material with low elasticity and oversizing interior and outside diameter 
can be installed into the chamber created by the axial seal ring and the 
shaft without damaging the sealing element due to the elastic behaviour 
gained by its shape. Thereby an assembling safety as good as with seal 
rings made of elastomers is obtained. For instance a glass-fibre 
reinforced PTFE sealing element manufactured with oversize does not result 
in blocking the rotating axial seal ring, in addition the axial and radial 
clearance required is maintained, and the seal ring does not open under 
the influence of vibration and compressive impact loads. The sealing 
element acts like an elastomer ring. Though made of rigid material, the 
oversized sealing element does not exhibit any loss of material when the 
axial seal ring is slipped onto the shaft and the sealing element, because 
the latter adjusts itself elastically. On the other hand, a PFTE 
glass-reinforced non-elastic formed sealing element of a conical form, 
such as described in French Pat. No. 1,131,832 has to be produced with 
very close manufacturing tolerances in order to ensure sealing. That is 
extremely difficult in the case of PTFE or reinforced PTFE. 
It is advantageous to design the radial contact surfaces of the sealing 
element in spherical form which results in an adaptation to the surfaces 
to be sealed on the machine shaft and at the sliding rings. 
The sealing element, consisting of two arms connected at one end, seals 
onto the machine shaft surface like a double O-ring sealing. In this way a 
highly loaded sealing area is doubly secured. In addition, the two arms 
provide a space for absorbing dirt. 
In contrast with an O-ring, this sealing element at the same time increases 
the heat transfer owing to the two fold contacting surface on the shaft. 
On using suitable coatings a further improvement of heat removal can be 
obtained in the seal face region from the sealing element to the machine 
shaft or to the protective shaft tube. The coating should be done by use 
of materials having at least the thermal conductivity of the basic 
material. Alloy steel with 18% Ni and 10% Cr has a thermal conductivity of 
46 W/mK, alumina oxide (99% Al.sub.2 O.sub.3) 21 W/mK and molybdenum 123 
W/mK. 
The use of metal oxides or ceramic materials for coating results in 
decreasing the thermal conductivity of the contact surfaces between 
sealing element and machine shaft. 
However, the use of metallic materials for coating shafts or protective 
shaft tubes (e.g. molybdenum or chromium-nickel) results in establishing a 
wear-resistant and non-corrosive layer which decreases the heat 
accumulation in the area of the sealing element contact surfaces. It is 
practical to extend the coating area to at least ten times the width of 
the sealing element in order to guarantee a good heat dissipation on the 
surface. 
Rubber-type elastic materials usually have a bad sliding behaviour on the 
shaft, in fact a high static friction. In an extreme situation the O-ring 
will be curled. With a spherical locating surface of a less elastic 
material, the sealing element will slide on the shaft very well, for 
example when using PTFE, the axial and radial clearance required will be 
improved. 
The sealing element according to the present invention is produced to the 
best advantage that both arms which abut to a connecting ring have a 
rounded shape in the attachment area to the connecting ring. In this way a 
splitting of the double arm annulus due to a notch effect is avoided. The 
radial sealing element proposed will preferably be dimensioned in such a 
way that an utilization for seal rings with a rubber-type elastic radial 
O-ring is possible. The sealing element is locked into the axial seal ring 
chamber provided and is fixed by means of a reaction ring. Thereby the 
manufacture of a specially shaped axial seal ring for the sealing element 
according to the standarization obtained has a special economic 
importance. Apart from the production savings gained, the expenditure for 
storing is decreased both for producer and consumers. 
For the production of this radial sealing element certain plastic materials 
are particularly suitable depending on the chemical stability required 
with respect to the heat and pressure stress. The predominant material is 
PTFE filled with glass, graphite or carbon. Other materials, e.g. of 
ceramic or metallic characteristics, can also be added, provided they 
possess the chemical stability required. The filling diminishes the flow 
behaviour of the pure PTFE and simultaneously the heat conductivity of the 
material, (e.g. PTFE filled with artificial coal or graphite resp.) could 
be increased threefold to fourfold, i.e. up to about 0.9 W/mK. That means 
also a fourfold heat conductivity improvement compared with rubber. 
The chemical stability of carbon or graphite with regard to strong 
oxidizing agents is not ensured. In such cases the graphite or carbon 
content of the PTFE has to be reduced if possible even to zero. Then the 
sealing element is made of pure PTFE or glass-filled PTFE in order to 
decrease the flow behaviour. A further possibility is the addition of 
metallic materials, e.g. chromium-nickel in powdered form, resulting in 
better thermal conductivity. In any case the elastic behaviour of the 
double-arm ring is preserved as an essential advantage. 
PTFE with a glass content of 5 to 75%, PTFE with a graphite or carbon 
content of 15 to 30%, and a mixture of PTFE with 15 to 30% glass and 5 to 
10% graphite or carbon, are advantageous. In addition, a mixture made up 
of graphite with a PTFE content of 5 to 30% is suitable. Furthermore it is 
convenient to produce other mixtures in any proportions as well as to add 
metal powders to PTFE or to one of the above mentioned mixtures in amounts 
of 0.5 to 95% 
Both arms of the sealing element according to the present invention, are 
arranged symmetrical or unsymmetrical with respect to the centre line of 
the cross section. The divergence angles of the arms with regard to the 
centre line may amount to 0.5.degree. up to 89.5.degree.. It is preferred 
that the arrangement of the arms is symmetrical, with the thickness of the 
arms being uniform, i.e., the internal and external sides extend parallel. 
It is possible to design the arms in the shoulder region at the connecting 
ring thinner and the free arms thicker whenever necessary because of the 
low elasticity of the material. This results in a softer double-arm ring 
resilience. By designing the arms thicker in the shoulder region and 
tapeing them in the direction of the free ends, the double-arm ring will 
have less resilience. 
The sealing element according to the present invention, may also be used 
for a seal ring assembly whose axial seal ring is slipped onto a hollow 
shaft or it is applied for the stationary ring. In both cases it is 
efficient to arrange the two free arm ends outwards. The manufacture of 
the radial sealing element, which comprised two arms connected with a ring 
at each end, is carried out to the best advantage from one piece of a 
hollow cylinder, provided that non-metallic and non brittle material is 
used. 
The double-arm ring can also be produced from a pure metallic material. In 
this case, the cross section may take the shape of a widened-out hairpin, 
possessing drop-shaped thickenings at both ends. Moreover, it can be 
realized that the double-arm rings made of a metallic material have the 
same cross section shape as for sealing elements made of nearly pure PTFE 
or the mixtures mentioned. It is favorable to use thin sheet-metal members 
and connected at their ends to form a ring by means of an appropriate 
welding process. In this case the cross section of the radial sealing 
element has a hollow profile.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIG. 1 illustrates a secondary sealing ring element 1 for axial seal rings 
according to the invention made of a low-elastic, non-metallic and 
non-brittle material. The sealing element is rotationally symmetrical with 
regard to axis A and has a plane of symmetry B. The body consists of two 
arms (2) connected by a ring (3). Both arms form a freespace (4) and the 
surfaces of the free arms ends (5) as well as the external ring surface 
(6) are designed spherically. 
FIG. 2 shows a sealing element as in FIG. 1 installed into a mechanical 
seal, which is assembled in a machine casing (11). Assembly of the device 
takes place from the direction indicated by (12). In the casing, a 
stationary seal ring (13) with sealing element (14) is inserted. The 
rotary axial sealing (15) is guided by a casing (16) attached to the shaft 
(17) with rotational axis C. For accommodating the sealing element 
according to the present invention a recess (18) is provided in the axial 
seal ring (15). 
An adjustment ring (19) presses with the help of a spring (20) towards the 
sealing element (1). 
The sectional drawing, FIG. 3, shows a version of the sealing element (FIG. 
1) with a thickening of the two arms (2) towards their free ends. 
The sectional drawing, FIG. 4, shows a version of the sealing element (FIG. 
1) with a tapering of the two arms (2) towards their free ends. 
The sectional drawing of FIG. 5 shows a version of the sealing element 
according to FIG. 1 where the free arms (2) are directed outwards away 
from the axis A. 
The versions of FIGS. 3, 4, 6, and 7 can also be made with outwardly 
directed free arms (2) as illustrated in FIG. 5. 
The sectional drawing of FIG. 6 shows a sealing element version which can 
be applied when using metallic materials. 
The sectional drawing of FIG. 7 shows a version of the sealing element, in 
which the arms have different angles to the plane B. 
EXAMPLE 
A secondary sealing element according to the invention, an embodiment of 
which is illustrated in FIG. 1, is produced through machining from a 
hollow PTFE cylinder containing 25% graphite. The graphite content 
increases the thermal conductivity from 0.23 W/mK (pure PTFE) up to 0.93 
W/mK. The interior diameter is manufactured with minus-tolerance and the 
outer diameter with plus-tolerance. The arms (2) of FIG. 1 and the plane 
of symmetry B form an angle of 30.degree.. Both arms have a thickeness of 
1.7 mm. 
When the ring is slipped over the machine shaft both arms are spread out 
and the interior diameter is adjusted to the shaft diameter. The outer 
diameter decreases and the lever collar can be slipped over without any 
difficulty. Only by pressing the adjustment ring (19) shown in FIG. 2 do 
the external and internal boundary surfaces of the double-arm sealing ring 
develop their complete sealing configuration and sealing characteristics. 
This sealing ring can also be used as a secondary sealing element (14) in 
order to seal the stationary seal ring (13) shown in FIG. 2. In this, the 
use of the version of the sealing ring in FIG. 5 is recommended. Then the 
contact surfaces of the stationary seal ring (13) and the casing (11) (cf. 
FIG. 2) may have the same angle as the arms of the sealing element.