Shaft and radially sealing ring with cold flexed seal disk and a method of its use for sealing the crankshaft of a two-cycle engine

A radially-sealing ring has a sealing element of a non-elastomeric plastic, preferably of PTFE, planar, annular disk having plane-parallel surfaces held at its outer circumference in a stiffening ring and cold flexed in the axial direction to form a cuff at its inner circumference which engages the shaft with bias. The disk has a thickness of from 0.3 to 0.8 mm and an inside diameter approximately 1.5 to 7 times its thickness less than the diameter of the shaft. The distance between the shaft and the line about stiffening ring where the disk is held is 2 to 7 times as great as the thickness of the disk. The radially-sealing ring is preferably for sealing the crankshaft of a two-cycle engine.

BACKGROUND OF THE INVENTION 
The invention relates to a shaft and sealing ring, the sealing ring having, 
more particularly, a cold flexed seal disk, and a method of its use for 
sealing the crankshaft of a two-cycle engine. 
A known seal ring is produced from an annular disk of a non-elastomeric 
plastic, preferably polytetrafluoroethylene (PTFE). The outer 
circumference of the disk is in a stiffening ring and the inner 
circumference of the disk is rolled in the axial direction so as to form a 
kind of cuff which engages the shaft under bias. 
German Federal patent publication OS No. 25 53 290 describes a sealing ring 
of the above-described kind. It has a seal element produced from a ribbed 
but otherwise planar, annular disk. The seal element is rolled at its 
inner circumference toward the sealed chamber to form a cuff-like end 
section with its ribs contacting the shaft for pumping any leakage thereat 
back to the sealed space. The attainment of good sealing action depends, 
for this reason, on the shaft's turning at the correct speed. 
If the circumferential speed of the shaft is too great, intensified pumping 
action of the ribs toward the sealed space results. This can cause 
lubrication failure around the dynamic sealing zone of the ribs, as well 
as the pumping of dust from the surroundings into the dynamic sealing zone 
and even into the sealed chamber. All of these events are undesirable and 
can result in the premature failure of the seal. 
If the circumferential speed of the shaft is too little, decreased pumping 
action of the ribs toward the sealed space results. Below a certain 
threshold velocity, therefore, the dynamic sealing zone can shift toward 
the outside, epsecially at the interstices between individual ribs, to 
allow leakage or contamination. Particularly when the shaft is at rest or 
vacuum builds up in the sealed space, these effects are plain to see. Both 
are undesirable. 
It is not possible, therefore, to use such a seal for sealing the 
crankshaft of a two-cyle engine in its crankcase, because vacuum is 
produced in its crankcase. The outside air and the contaminants it 
contains then easily pass through the dynamic sealing zone of the seal 
into the "sealed" crankcase. The presence of the ribs intensifies this 
problem. 
SUMMARY OF THE INVENTION 
An object of the invention is, therefore, improving a seal of the kind 
described above such that the described disadvantages are prevented. The 
resulting seal should provide good sealing action regardless of whether 
the sealed shaft is at rest or in rotation, even when the direction of the 
pressure differential across the seal changes, thus to be very well suited 
for sealing the crankshaft of a two-cycle engine. 
For this, the invention proposes a seal having a planar, annular, 
non-elastomeric plastic, sealing-element disk the inside of which is cold 
flexed about a shaft. The disk has a thickness of from 0.3 to 0.8 mm and 
an inside diameter that is from about 1.5 to about 7 times the thickness 
of the disk less than the diameter of the shaft. The distance of a line of 
innermost sealing attachment of a stiffening ring about the outside of the 
disk from the surface of the shaft is 2 to 7 times the thickness of the 
disk. Preferably, a disk of a thickness of 0.4 to 0.6 mm has an inside 
diameter that is from about 3 to about 4 times its thickness less than the 
diameter of the shaft, the distance from the shaft of the innermost 
sealing-attachment line of the disk to the stiffening ring then being 3 to 
4 times the thickness of the disk. 
The disk sealing element of the invention is substantially smaller and more 
compact than the embodiment in accordance with the state of the art 
described above, yet it is entirely adequate in relation to the radial 
displacements of the shaft which occur in normal machine construction. In 
particular, shafts having a diameter of from 10 to 40 mm can be sealed in 
an excellent manner by using the seal according to the invention. 
The axial length of the dynamic sealing zone, where the disk is flexed 
about the shaft, is thus extraordinarily short. This increases assurance 
of necessary lubrication in this critical zone by the medium being sealed. 
Also of great importance in this regard is the use of non-elastomeric 
plastic, especially polytetrafluoroethylene (PTFE), for the production of 
the disk sealing element. 
Before the disk is installed, its inner margin can be rolled in the axial 
direction to form a cuff, for example by using a shaping tool. However, it 
is of decisive importance to avoid any heating of the disk material in 
this forming process, so as not to impair the shape-recovering tendency of 
the material that is used. As a result, the cuff of the disk is uniformly 
urged against the surface of the shaft as a dynamic sealing zone over long 
periods of time. This suffers no impairment during normal operation 
because the dynamic sealing zone is so extraordinarily short and, thus, 
well lubricated as to forestall any excessive heating. 
The disk has no hydrodynamically-acting back-pumping elements facing the 
shaft at its axially-turned end section or cuff. Its sealing effect is, 
thus, completely assured regardless of whether the shaft is at rest or 
rotating. Even changing directions or spaced shaft of rotation and 
alternating directions of pressure differential axially across the disk 
produce no adverse effect. When the disk is used as a crankshaft seal in a 
two-cycle engine, however, it has been found preferable to install it such 
that the inside margin of the disk is rolled, i.e., cold flexed, to extend 
in a direction away from the sealed chamber, i.e. the crankcase. This 
suits the seal above all for sealing the crankshaft of any two-cycle 
engine in its crankcase. 
In a preferred embodiment, the outside portion of the disk sealingly 
attached to the stiffening ring is approximately at a right angle to the 
shaft. Other orientations are easily possible, but the right-angled 
relationship achieves an especially-long useful life and good sealing 
results. 
The stiffening ring can have an annular projection on the side thereof 
opposite that toward which the disk is cold flexed to extend to the cuff. 
The annular projection can reach to the immediate vicinity of the shaft. 
This improves the axial support of the disk and, thereby, its cuff for 
producing the dynamic sealing action and makes the dynamically-sealing 
cuff less accessible to exterior dirt and dust. If the ring is installed 
so that the disk extends away from the sealed chamber, as preferred in 
sealing the crankshaft in the crankcase of a two-cycle engine, for 
example, this also improves the sealing action. This, latter result, might 
be due, in the exemplary case, to the fact that the pressure pulsations 
produced in operation of a two-cycle engine are substantially blocked from 
the dynamic sealing zone by the annular projection of the stiffening ring. 
Optimum results are achieved when the annular projection of the stiffening 
ring is brought very close to the shaft. The annular projection is best 
defined on its inside by a cylindrical surface having an axial length at 
least as great as its distance from the shaft, and preferably, at least 3 
and no more than 10 times as great. 
To reduce manufacturing costs, it has proven advantageous for the 
stiffening ring to be made of plastic injection-molded onto the outer 
margin of the disk. Thermosetting materials are preferred. Problems 
regarding the fastening and sealing together of the two parts can thus be 
reliably prevented, especially if the method of doing this is the one 
described in German Federal Patent publication No. 3 329 386.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
The seal embodiment illusrated in FIG. 1 is for sealing a crankshaft 3 of a 
two-cycle engine where it projects through a crankcase 5 of the engine 
(shown only by its crankcase 5). It consists of a stiffening ring 2 and 
disk 1. The stiffening ring 2 is made from glass fiber-reinforced phenolic 
resin that has been formed, i.e. sealingly attached, directly onto the 
outer margin of the disk 1 at an innermost line at L where the disk 1 
forms a right angle with the crankshaft 3. This can be done at low 
manufacturing cost. The disk 1 is annular and made of PTFE. It was planar 
before being cold flexed or rolled to extend away from the sealed chamber 
of the crankcase 5 to a cuff 1a engaging the surface of the thus-sealed 
shaft 3. 
For sealing a shaft 3 of 20 mm diameter, the disk 1 has an outside diameter 
of 27 mm, an inside diameter before cold flexing of 18.2 mm, and a 
thickness of 0.5 mm. The axial length of cuff contact with the shaft 
surface resulting after installation as shown in FIG. 1 amounts to 
approximately 0.3 mm and is, thus, extremely short. The seal is, 
therefore, easy to install. 
The seal shown in FIG. 2 is substantially the same as that shown in FIG. 1. 
Corresponding components which are identified in correspondence with FIG. 
1 will not be, therefore, re-described. 
The seal shown in FIG. 2 differs from that of FIG. 1 in an annular 
porjection 4 from the stiffening ring 2' toward the shaft 3' on the side 
the disk 1' opposite that toward which the disk is cold flexed to extend 
to cuff 1a'. When the shaft 3' is on the crankshaft of a two-cycle engine 
(not shown in FIG. 2, 5 in FIG. 1) the projection 4 gives the seal 
excellent sealing action through its entire life.