Seal for a joint or juncture – Seal between relatively movable parts – Circumferential contact seal for other than piston
Reexamination Certificate
2002-08-13
2004-04-20
Miller, William L. (Department: 3677)
Seal for a joint or juncture
Seal between relatively movable parts
Circumferential contact seal for other than piston
Reexamination Certificate
active
06722659
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a shaft seal, arranged between a shaft and a housing, having at least one sealing lip which bears with a sealing-lip supporting surface against the shaft surface moving relative to the seal. A predetermined surface pressure is applied to the sealing lip and shaft surface interface by a pre-stressing element.
2. Background and Summary of the Invention
In the prior art, a multitude of sealing arrangements for providing a seal between two components moving relative to each other has been disclosed. The sealing arrangement seals spaces containing different media or pressure conditions-from each other.
FIG. 1
shows diagrammatically, by way of example, such a sealing arrangement according to the prior art used in a field of use, which is frequently encountered. In the case illustrated, a gap between a shaft
1
and a housing part
2
is shown, through which shaft
1
, rotating relative to the housing
2
, is guided. These are to be sealed in such a manner that a medium contained in a space
3
cannot pass into a space
4
, and vice versa. Arranged in space
3
is a shaft bearing
5
, for example a grooved ball bearing, which supports shaft
1
in the opening of the housing
2
and is lubricated by a suitable lubricant. In contrast, space
4
is exposed to environmental influences and so spray water and dirt can pass into it, the intention being to prevent them from entering into space
3
.
To mutually seal spaces
3
and
4
, there is provided in the gap, which is defined by a shaft surface
7
and a surface
8
of the housing
2
, a shaft seal. Shaft seal
10
consists of an elastomeric sealing material and has essentially a U-shaped cross-sectional configuration. A first limb
11
of the shaft seal
10
bears in a sealing manner against surface
8
of housing
2
and is secured thereon—for example, by means of compression. A second limb
12
is in sealing contact with shaft surface
7
. First and second limbs
11
and
12
, respectively, are connected to each other by a base section
13
. Furthermore, an L-shaped stiffening element is provided for reinforcing first limb
11
and base section
13
. In addition, shaft seal
10
, according to
FIG. 1
, has a protective lip
15
which is arranged at that end of second limb
12
which is adjacent to base section
13
, and comes to bear against shaft surface
7
.
Second limb
12
bears with its outer end against shaft surface
7
at a predetermined contact pressure force, with the result that shaft surface
7
can rotate relative to shaft seal
10
and can also move in the axial direction. The contact pressure force is determined by the restoring force, which depends on the elastic properties of the seal material and the pre-stressing of second limb
12
with respect to shaft surface
7
, and/or by the tangential force of a helical tension spring
16
which is inserted into shaft seal
10
at the outer end of second limb
12
and presses the outer end of second limb
12
against shaft surface
7
with a predetermined force.
A variant of shaft seal
10
according to
FIG. 1
, in which a protective lip
15
is not provided, is illustrated in cross section on an enlarged scale in FIG.
2
. In particular, a sealing lip
20
, formed at the outer end of second limb
12
, can be seen in FIG.
2
.
A body element
21
of sealing lip
20
is defined by two side surfaces
22
and
23
tapering toward each other. Body element
21
has an essentially triangular cross section, against which spring
16
presses, as is indicated by an arrow in FIG.
2
. At the tip of body element
21
, sealing lip
20
bears with an annular, relatively narrow sealing-lip supporting surface
24
against shaft surface
7
.
Grinding seals of this type, such as shaft seal
10
described above, provide a reliable seal, particularly if the wear of sealing lip
20
is small on account of the surface quality of the shaft surface
7
and/or on account of the lubrication of the sealing edge or sealing-lip supporting surface
24
. In contrast, increased friction of the seal on opposite surface
7
has a disadvantageous effect on account of the temperature increase associated therewith and the effects of wear caused as a result. The wear leads to a reduced sealing action of shaft seal
10
, which, as will be explained in greater detail with reference to
FIGS. 3A
to
3
C,
4
and
5
, depends substantially on the surface pressure at sealing-lip supporting surface
24
.
FIGS. 3A
,
3
B and
3
C each illustrate profiles of sealing lips
27
,
28
and
29
, which are arranged on a sealing body
25
, bearing against an opposite surface
26
. They differ in “sharpness”. These profiles have been used for the measurements illustrated in
FIGS. 4 and 5
. The profiles of
FIGS. 3A
,
3
B and
3
C differ in each case merely by the different point radii R
1
, R
2
and R
3
of sealing lips
27
,
28
and
29
, respectively. Sealing lip
27
, according to
FIG. 3A
, has a point radius R
1
=0.1 mm; sealing lip
28
, according to
FIG. 3B
, has a point radius of R
2
=0.2 mm; and, sealing lip
29
, according to
FIG. 3C
, has a point radius of R
3
=0.3 mm. The remaining parameters of the profiles: the rectangular cross-sectional shape of sealing body
25
with a height HO and a length LO, the overall length L
1
, the 30° angle with respect to the axis of symmetry of the side surfaces
22
and
23
of the sealing lips, and the sealing materials are essentially identical for all of the profiles of
FIGS. 3A
to
3
C.
FIG. 4
shows the distributions of the surface pressures in the sealing gap X
R1
, X
R2
and X
R3
for the different point radii R
1
, R
2
and R
3
, respectively.
FIG. 5
illustrates the rise in the maximum value of the surface pressure in the sealing gap Y
R1
, Y
R2
and Y
R3
for the point radii R
1
, R
2
and R
3
, respectively, as a function of compression distance.
The curves according to
FIGS. 4 and 5
show a marked dependence of the surface pressure in the sealing gap on the point radius, the maximum value of the surface pressure decreasing with increasing point radius R
1
→
R
2
→
R
3
. In the case of relatively small point radii, a relatively large surface pressure is obtained. Accordingly, sealing action improves with the geometry otherwise unchanged. Laboratory tests also show that the sealing lips having the smallest point radii have the highest seal tightness. It should also be noted that the variants having the smallest point radii achieve the greatest surface pressures with, at the same time, the smallest reaction forces.
In summary, it follows from this that seal tightness of a sealing arrangement having a resilient sealing material depends substantially on the surface pressure in the sealing gap, which in turn depends on the “sharpness” of the sealing lip and the contact pressure force.
The wear of sealing lip
20
, which influences the surface pressure of sealing-lip supporting surface
24
, is therefore critical to the service life of the sealing arrangement or shaft seal
10
. The wear depends on the relative speed of sealing lip
20
with respect its respective shaft surface
7
, on the roughness of shaft surface
7
bearing against sealing lip
20
, and on the wear properties of the sealing material.
The roughness of shaft surface
7
is reduced over time by sealing lip
20
rubbing against it, since the sealing lip grinds in a running surface on shaft
1
. Even after a short running time, sealing lip
20
produces a finely polished region on shaft surface
7
. Subsequently, sealing lip
20
is subject to a greatly reduced wear or virtually no at all. In the case of structures with small axial relative displacements of shaft surface
7
, this region is very narrow. A structure of this type can be achieved if, for example, shaft seal
10
on shaft
1
is situated directly next to a fixed shaft bearing, such as, for example, a tapered roller bearing.
In the case of structures in which shaft seal
10
on shaft
1
is far away from a fixed bearing of
Brehob Diana D.
Miller William L.
LandOfFree
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