Note: Descriptions are shown in the official language in which they were submitted.
CA 02640477 2008-07-28
Shaft seal
The present invention relates to a shaft seal comprising an inner seating
element which is arranged on an axle shaft, and has at least one ring and an
outer sealing element which is arranged on a roller body having at least one
ring which is oriented essentially parallel to at least one ring of the inner
sealing element.
Furthermore, the present invention relates to a conveyor roller, in particular
a
belt or band conveyer comprising a hollow cylindrical roller body, and having
at least an axle shaft mounted in two bearings with a shaft seal provided for
each of the bearings.
Such shaft seals are commonly known and if the inner and outer sealing
elements do not touch each other they are commonly designated as labyrinth
seals. Principally, the sealing effect is generated between a rotating shaft
located in the enclosure by a suitable arrangement of the rings mounted
around the shaft which move in corresponding grooves of the stationary
housing. This extends the sealing land many times. The extension of the
sealing land causes pressure to release due to the friction imposed as the
medium flows through the sealing land which must not show or only slightly
show any pressure differential from the pressure existing outside the shaft
seal.
The sealing land comprises an array of alternating chambers which act as
orifices, where the chambers are interconnected through a labyrinth gap. The
pressure energy in the labyrinth gaps, which is partially transformed into
kinetic energy, is lowered in the adjoining chambers, partly by eddy formation
and partly by heat. The array of alternating chambers and labyrinth gaps thus
lowers the high pressure level which occurs in front of the shaft seal to a
low
pressure level behind the shaft seal.
Due to the high flow resistance which occurs in the long labyrinth gap only a
small and tolerable amount of gas or liquid can escape through the labyrinth
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CA 02640477 2011-09-19
seal. However, absolute gas- or water-tightness cannot be obtained using this
contact-free construction. The amount of the escaping liquid can be further
reduced
by providing a relatively narrow width of the labyrinth or sealing gap.
In the case of a stationary shaft, around which a roller body rotates, a
strain applied
to the roller by any transported material causes deformation and alteration of
the
geometry of both the roller body and the shaft seal. The outer sealing element
on top
of the axle shaft is bent towards the inner sealing element when a load and a
vertical
downward force are applied, such as the gravity of the transported material.
Below
the axle shaft the outer sealing element is bent away from the inner sealing
element.
The result is, that the sealing elements on top of the axle shaft will be in
contact and
rub against each other, while clearance below the axle shaft will be
increased. When
rotating this will generate a pump effect which can cause air moisture and/or
dust to
be sucked through the outermost labyrinth gap into the sealing land. The
alteration of
the clearance and in particular the contact on top of the axle shaft generates
a
braking effect and a friction locking which causes premature wear of the shaft
seal.
Against this background the function of the present invention is to provide an
improved shaft seal with a high reliability and a long durability even when
vertical
load is applied.
Certain exemplary embodiments can provide a conveyor roller, in particular a
belt or
band conveyor comprising a hollow cylindrical roller body, and having an axle
shaft
mounted in at least two bearings with each bearing having a shaft seal
comprising an
inner sealing element, which is arranged on an axle shaft and has at least one
ring
and an outer sealing element which is arranged on a roller body that at least
has one
ring, which is oriented essentially parallel to at least one ring of the inner
sealing
element, and wherein a ring of one of the two sealing elements provided with
an
annular projection facing a ring of the other sealing element; said annular
projection
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extending in an axial direction and being in contact with the ring of the
other sealing
element during the assembly of the shaft seal and being ground down, once the
axle
shaft and the roller body has started to rotate, until a narrow annular gap
between the
projection and the ring of the other sealing element is formed.
Certain exemplary embodiments can provide a shaft seal for use in for a
conveyor
roller, in particular a belt or band conveyor, comprising an inner sealing
element
arranged on an axle shaft, which at least has one ring and an outer sealing
element,
which is arranged on a roller body that at least has one ring which is
oriented
essentially parallel to at least one ring of the inner sealing element, and
wherein a
ring of one of the two sealing elements provided with an annular projection
facing a
ring of the other sealing element; said annular projection extending in an
axial
direction and being in contact with the ring of the other sealing element
during the
assembly of the shaft seal and being rubbed against, once the axle shaft and
the
roller body has started to rotate, until a narrow annular gap between the
projection
and the ring of the other sealing element is formed.
In other embodiments, in the case of a shaft seal of the kind described at the
outset,
a ring of one of the two sealing elements is provided with an annular
projection facing
a ring of the other sealing element. Furthermore, according to the invention,
said
projection shall be in contact with the ring of the other sealing element in
an
assembly of the shaft seal and being rubbed, once the axle shaft and/or the
roller
body has started to rotate, until a narrow annular gap between the projection
and the
ring of the other sealing element is formed.
The shaft seal initially functions as a contact seal, which has a nominal wear
point.
After a certain time of rotational movement this projection is partly rubbed
down.
Consequently, a narrow annular gap is formed between the projection and the
opposing ring which is contact-free to the full extent of the shaft seal -
hence, over
time this rotation leads to a contact-free labyrinth seal. The projection is
only be
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ground to such a degree as it is required for a contact-free operation as the
grinding
automatically stops upon reaching said degree. A comparable narrow annular gap
for
common shaft seals can be realized only when manufactured and assembled with
extreme accuracy which will considerably increase the costs for manufacturing
such
shaft seals.
The roller for a conveyor according to the invention is provided with a shaft
seal
according to the invention. As soon as the shaft seal is contact-free, smooth
rotation
of the conveyor roller is guaranteed. This reduces wear of the roller jacket
by a
significant degree, as due to the steady speed a lifting and the subsequent
dropping
of the belt (for a belt conveyor) is prevented.
Furthermore, in particular for conveyor systems which comprise multiple
rollers to
form a conveyor route up to many kilometers in length, it is important to
manufacture
rollers including seals at a reasonable price as well as to provide rollers
which have a
long service life.
In order to avoid a relatively strong braking effect, particularly at the
beginning of the
rotational movement, the projection is preferably tapered. The tapered
projection
which becomes gradually thinner towards its end and which is formed into a
blunt
cone due to the abrasion is a compromise between the low friction demanded and
an
adequate stability.
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According to the invention of the shaft seal a beneficial design would consist
of an axle shaft functioning as a stator and a roller body functioning as a
rotor.
Preferably the design provides a sealing land in the shaft seal in which one
of
the two opposing boundary walls is arranged to face the inner sealing element
with the other facing the outer seal.
To obtain an ideal extension of the sealing land, the rings of the inner
sealing
element and the rings of the outer sealing element are preferably arranged
alternately axial to the axle shaft.
Furthermore, in case of multiple projections, in a shaft seal according to the
invention, it is preferred that said projections are arranged only on one of
the
two sealing elements. Only then by selecting a suitable, different material
for
both sealing elements, can the rubbing down of the projections without
damage to the opposing rings can be secured.
For a simplified construction and a reliable assembly of the shaft seal, each
of
the inner and/or outer sealing elements is preferably provided as a one-piece
unit.
As the shaft seal according to the invention also works reliably when a load
is
imposed, it can preferably act as a roller for conveyors, in particular for
belt
and band conveyors.
In the following the invention will be further explained by means of a
detailed
description with reference to the attached drawing.
The single figure shows a longitudinal section through a typical embodiment
of a shaft seal according to the invention. The illustration shows a partial
longitudinal section in axial direction along a roller with an essentially
hollow
cylindrical roller body 1 and an axle shaft 2 which is affixed to the roller
body
1. The sketch depicts only one side of the roller and the opposite side in
general is symmetrically designed. The end of the roller body 1 is inward
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formed in order to provide a bearing seat 3 for the roller bearing 4. The
bearing area which includes the roller bearing 4 is enclosed by an annular
bearing cover 5 to separate it from the hollow space of the roller body 1. The
side of the bearing area which faces towards the opposite direction and/or the
periphery is enclosed by the shaft seal according to the invention.
The shaft seal has an inner sealing element which is arranged on the axle
shaft 2. The inner sealing element 6 consists of a first and a second inner
hollow cylinder with the latter connected to the outside of the first hollow
cylinder. A first inner ring 7 is arranged on the first hollow cylinder in
order to
form essentially an L-shaped longitudinal section or cross section on both
sides of the central axis 8. The second hollow cylinder comprises a second
inner ring 9 which is arranged in such a way that between the first inner ring
7
and the second inner ring 9 a groove-shaped intermediate gap 10 is formed.
A third inner ring 11 which is less thick than the second ring 9 is arranged
on
the second inner ring 9 in order to extend the same. Each free end of the
three inner rings 7, 9, 11 is radially outward-pointing. The side of the inner
sealing element which is pointing to the periphery contains a sealing cover
12.
In the example shown the first hollow cylinder is formed together with the
first
inner ring 7 as one piece. Also, the second hollow cylinder is formed from one
piece together with the second and third ring 9, 11. It is obvious as well
that
the whole inner sealing element 6 can be formed in one piece. The material
used for the inner sealing element 6 can be a fiberglass-reinforced polyamide,
for instance PA66 GF30.
An outer sealing element 13 is arranged on the roller body 1 and according to
the illustrated example of a roller conveyor it rotates together with the
roller
body relative to the axle shaft 2 and the inner sealing element 6. The outer
sealing element 13 consists of an annular element and a first outer ring 14
with the annular element showing an essentially Y-shaped profile in its
longitudinal section.
CA 02640477 2008-07-28
The first outer ring 14 is arranged on one arm of the Y and its free end is
radially inward-pointing. The leg of the Y is formed by a second outer ring
15.
The free end of the second outer ring 15 is also radially inward-pointing,
that
is, towards the central axis 8. The arm of the Y which is pointing towards the
periphery is formed by a third outer ring 16 which contrary to the first two
inner
rings 14 and 15 has a free end that is radially outward-pointing.
Each of the three outer rings 14, 15, and 16 has a projection 17, 18 and 19,
with the tapered projection 17 of the first outer ring 14 pointing towards the
first inner ring 17. The second projection 18 of the second outer ring 15
points
towards the second inner ring 9. Finally, the third projection 19 of the third
outer ring 16 is arranged in such a way that its cone end points towards the
third inner ring 11. In the illustrated example the inner sealing element 13
is
made of softer and/or more abrasive material than the one used for the inner
sealing element 6, for instance POM (Polyectal).
During the assembly of the shaft seal the projections 17, 18, and 19 are in
frictional contact with the first inner ring 7, the second inner ring 9, and
the
third inner ring 11. Once the roller body 1 has begun to rotate relative to
the
axle shaft 2, the outer sealing element 13 rubs against the inner sealing
element 6, causing the respective cone ends of the projections 17, 18, and 19
to be ground due to the friction, until a respective narrow gap has been
formed. Once the annular gaps have been formed the shaft seal is contact-
free. This "running-in" process, until a shaft seal that runs contact-free has
been formed takes a few days, and on the basis of experience in about two
days.
It is obvious that during the "rolling" process increased power is needed to
rotate the roller as the energy loss through friction has to be compensated
for.
As soon as the annular gap has been formed, the drive system will require
corresponding less power.
If a load is applied to the roller body 1, for instance by a transported
material,
the deformation of the roller causes the outer rings 14, 15, and 16 to tilt
and/or
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bend in relation to the inner rings 7, 9, and 11. In this case the projections
17,
18, and 19 will be further rubbed down until a contact-free state is once more
obtained. Then the change from a loaded roller 1 to an unloaded roller 1 only
causes an alteration of the annular gap, without generating a friction contact
-
thus preventing the so-called pump effect.
The labyrinth gap of the shaft seal is preferably filled with a lubricant
and/or a
sealing compound, for instance a grease. The rotation of the roller body 1
causes the grease to be centrifuged radially outward and then gathered, in
particular in the labyrinth gap around the section of the free end of the
inner
ring 7. That efficiently seals off the labyrinth gap from the first inner ring
7 and
the outer sealing element 13.
With an idle roller body 1 the centrifugal force which caused the grease to be
driven outwards does not occur. The preferred relatively viscous grease
settles and slowly "flows" down the shaft seal due to gravity. This means that
in the lower part of the shaft seal the grease seals off the labyrinth gap
around
the section of the free end of the inner ring 7, while in the upper part of
the
shaft seal the grease fills the section around the free end of the second
outer
ring 15 and seals off the labyrinth gap, respectively. Hereby the roller body
1
is protected from both dirt or moisture-laden air that could penetrate during
operation or at idle.
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