Piston or rod seal

A piston or rod seal comprising a scraper ring 2 which is made of a PTFE material and supported in a first groove 1 open toward the relatively moving machine part. This ring is pressed against the relatively moving machine part 5 by an O-ring 4 made of an elastomeric material and supported on the bottom 3 of the first groove 1. On the side facing the relatively moving machine part, the scraper ring 2 is bounded by at least two sealing edges 7 separated by a second groove 6 and, on the side facing the O-ring, it is bounded by a conical surface 8. The second groove 6 has a rounded bottom having a radius R which approximately corresponds to the radial depth T of the second groove 6. The conical surface overlaps the radial planes of all sealing edges 7 while avoiding any considerable changes in directions.

BACKGROUND OF THE INVENTION 
The present invention relates to a piston or rod seal comprising a scraper 
ring made of a PTFE material and supported in an open first groove, the 
latter being disposed radially with respect to a relatively moving machine 
part. An O-ring made of an elastomeric material and disposed on the bottom 
of this first groove presses the scraper ring against the machine part. On 
its side facing the piston or rod, the scraper ring has at least two 
sealing lips separated by a second groove. These sealing lips contact the 
piston rod with sealing edges, and on their side facing the O-ring, they 
have conical surfaces with an external diameter which axially increases in 
the direction toward the side facing away from the pressure. 
A piston rod of this type is known from the German Patent Publication No. 
DE-OS 36 03 669. The sealing action achieved therewith is less than 
satisfactory, especially over long-term use. 
SUMMARY OF THE INVENTION 
It is an object of the invention to further develop a piston or rod seal of 
the type described above such that the sealing action be substantially 
improved over a long-term service life. 
This object, as well as other objects which will become apparent from the 
discussion that follows, are achieved, with a piston or rod seal of the 
aforesaid type and in accordance with the present invention by providing a 
rounded bottom in the "second groove" of the scraper ring with a radius 
which approximately corresponds to the radial depth of the second groove. 
Further, while avoiding any substantial change in direction, the conical 
surface overlaps at least the radial planes of all sealing edges as a 
continuous surface. The profile of the scraper ring is thus not subject to 
"notch effects" in the intermediate zone between the sealing edges coming 
from a radial direction either from the inside or the outside. 
Deformations during use of the seal in accordance with the invention are 
thus largely prevented. The relative association of the sealing lips to 
the surface of the machine part to be sealed is then largely retained over 
an extended service life. This is very advantageous to achieving a good 
sealing action. Moreover, during use in accordance with the invention, the 
O-ring, which is made of an elastomeric material, is supported on the 
bottom of the first groove and essentially determines the elastic contact 
pressure of the sealing edges against the relatively moving machine part. 
The O-ring is thus not subject to any static deformation exceeding its 
bearing load capacity. Rather, the O-ring is capable of following the back 
and forth movements of the relatively moving machine part in an 
elastically yielding manner. It can relax in short time intervals which 
ensures a superior and uniform contact pressure of the sealing lips 
against the machine part to be sealed over a longer period of time. 
The O-ring has a circularly defined profile and a diameter which is less 
than the axial length of the first groove in which it is supported. In a 
rod seal, it rests, with the side facing radially toward the outside, on 
the bottom of the first groove. This bottom surrounds the rod to be sealed 
as a concentric cylinder surface. With its inside surface, the O-ring 
contacts the scraper ring on the conical surface thereof. The latter 
surface overlaps at least the radial planes of all sealing edges as a 
continuous surface while avoiding any considerable change in direction, 
and it has an axial length which corresponds at least to that of the 
cylindrical surface. This ensures a good relative mobility of the O-ring 
which in turn ensures that, during the power stroke, the sealing lips, 
succeeding one another in the direction toward the chamber to be sealed, 
are pressed against the rod to be sealed with the contact pressure 
gradually decreasing. The generation of a liquid pressure in the second 
groove, which exceeds the pressure level in the chamber to be sealed 
during the power stroke, is thus avoided. 
The second groove is preferably perpendicularly spaced-apart from the 
conical surface. This spacing should be at least three times, 
advantageously at least five times, as large as the radial depth of the 
groove. Undesired deformations of the scraper ring during the power stroke 
can thus be anticipated. 
The conical surface which concentrically surrounds the piston rod 
preferably has a conical angle between 10.degree. and 20.degree.. This 
ensures a sufficient relative mobility of the O-ring over longer periods 
of time in addition to a sufficient contact pressure of the piston rod to 
be sealed. 
In the area of its axial front surfaces, the scraper ring can be provided 
with collars which radially protrude toward the outside, thus preventing 
the arrangement, which surrounds the scraper ring and the O-ring, from 
coming apart. This simplifies the assembly of the entire unit 
considerably. The aforesaid applies analogously to a piston seal. Such an 
analogous application of the respective features will be understood by a 
person skilled in the art. 
The preferred embodiments of the present invention will now be described 
with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The piston rod seal of FIG. 1 comprises a scraper ring 2 made of 
PTFE-material, e.g. a filled PTFE, which is supported in a first groove 
radially open toward the inside. An O-ring 4 made of an elastomeric 
material and resting on the bottom 3 of the first groove 1 presses this 
scraper ring against the piston rod 5 to be sealed. On its side facing the 
piston rod 5, the scraper ring 2 is provided with two sealing edges 7 
bounding the sealing lips and separated from one another by a second 
groove 6. On its side facing the O-ring 4, the scraper ring 2 is bounded 
by a conical surface 8. The external diameter of the latter increases in 
axial direction toward the side 9 facing away from the pressure. The first 
groove 1, the O-ring 4, the scraper ring 2 and the piston rod 5 have a 
rotational-symmetrical configuration and are concentrically associated 
with one another. 
The bottom 10 bounding the second groove 6 of the scraper ring 2 in radial 
direction toward the outside has a radius R which corresponds to 
approximately the radial depth T of groove 6. The latter has very small 
dimensions and, regardless of the diameter of the piston rod 5 to be 
sealed, always ranges between 0.2 and 0.4 mm, advantageously 0.3 mm. On 
its outside, the scraper ring is bounded by a conical surface 8 which, as 
a continuous surface, overlaps at least the radial planes 11 and 12 of 
both sealing edges 7 while avoiding any considerable change in direction. 
In the represented embodiment, the conical angle of the conical surface 8 
is 18.degree.. 
The second groove 6 is perpendicularly spaced-apart from the conical 
surface 8 at a distance A which is approximately six times the size of the 
radial depth T of the second groove. This and the rounded bottom 10 
provided in the second groove of the scraper ring 2 account for good 
dimensional stability. This ensures a uniform association between the 
sealing lips and the piston rod 5 to be sealed over a longer period of 
time. Advantageously, the radial depth T of the second groove is 0.20 to 
0.35 times as large as the axial distance B between adjacent sealing 
edges. 
The embodiment shown in FIG. 2 is similar to that of FIG. 1. In this 
embodiment, however, the scraper ring has three sealing edges 7' disposed 
in tandem in the axial direction which improves the sealing action. 
The embodiment of FIG. 3 is distinguished from the embodiments of FIGS. 1 
and 2 essentially in that in the area of its axial bounding surfaces, the 
scraper ring 2" is provided with collars protruding toward the exterior in 
the radial direction. These have an external diameter which is larger than 
the internal diameter of the O-ring 4" in a relaxed state. When the entire 
unit is not yet assembled, the O-ring 4" and the scraper ring 2" thus form 
a self-contained unit which facilitates mounting. 
There has thus been shown and described a novel piston or rod seal which 
fulfills all the objects and advantages sought therefor. Many changes, 
modifications, variations and other uses and applications of the subject 
invention will, however, become apparent to those skilled in the art after 
considering this specification and the accompanying drawings which 
disclose the preferred embodiments thereof. All such changes, 
modifications, variations and other uses and applications which do not 
depart from the spirit and scope of the invention are deemed to be covered 
by the invention, which is to be limited only by the claims which follow.