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1. Why special requirements on the floor
surface?
The picture below shows the function of
an air-cushion. Inside of the lifting chamber with the load
area “A” the overpressure has to be kept constantly
higher than the load.
For that reason there are no joints, gaps or cracks allowed,
which could cause a deaeration of the lifting chamber.
The gap between the bellow of the air-cushion
and the floor is very thin. Due to this fact the quality
of the floor surface is very important for the use of the
air-cushion technique, because it affects on the one hand
directly the air-consumption and on the other hand it decides
about the possibility to get an overpressure inside of the
lifting chamber.
2. Demands on the covered floor surface
and the covering material
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Demands |
Annotations |
| -Material |
Smooth, polished and sealed concrete
floor or smooth synthetic-resin coated floor (probably
shock resistant through additional glass fibre layer) |
The surface of the floor can affect
the air-consumption and wear due to friction between
the air-cushion bellow and the floor.n. |
-Maximum surface pressure
wheel/floor |
P < 25 N/mm² |
It is not allowed to detect any
indentations on the floor-coating.. |
- Minimum of static friction
shoe sole/floor |
µ = 0,4 (trocken/dry) |
According to VW-production facility
specifications |
- Even- and waveness
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According to DIN 18202, part 5, line
4 |
The slope affects directly the necessary
towing forces |
| - Surface roughness |
Rauhigkeit/roughness
Ra: 6,3µm - 12,5µm |
Güteklasse 2, Rauhigkeitsgrößen
nach DIN 4768, Teil 1, Oberfläche glatt wie Papier |
- Joints, cracks and steps |
None |
Airtight Finish |
| - Dehnungsfugen |
Airtight, infinitly finish
gap form: V-form (angle 90°)
|
-Material based on Polyurethane, shore-hardness
circa 80, with very high resilence capacity, without
volume expansion during compression. |
3. Evenness
The friction-coefficient of an active air-cushion
is nearly zero. Therefore even the smallest slope causes
a downward gliding of the load. This could create a dangerous
situation, because especially heavy loads could get uncontrollable.
The following table shows possible tolerances for the evenness
of the floor.
| Kind of execution |
Evenness tolerance in [mm]
depending on the distance between the measuring points |
|
0,1m |
1m |
4m |
10m |
15m |
Standard execution according
DIN 18202 part 5, line 3
|
2 |
4 |
10 |
12 |
15 |
Enhanced exactness according
DIN 18202 part 5, line 4 |
1 |
3 |
9 |
12 |
15 |
| Out levelled surface |
1 |
3 |
6 |
6 |
6 |
Out levelled surface with
enhanced exactness |
1 |
1 |
3 |
5 |
5 |
4. Steps
Steps and sharp edges are "cutting"
the pressure-cushion inside of the lifting chamber and the
system breaks down (Fig. 4.1).
The edge of the steps have to be chamfered to a ramp (Fig.
4.2).
When designing a ramp the resulting descending forces should
be taken into account. These have to be eliminated through
securing the system with winches (manual systems) or by
means of inbuilt drives (automatic systems) .
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Fig. 4.1:In case of steps or sharp
edges the pressure cushion inside of the lifting
chamber is breaking down.
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Fig.. 4.2: By means of a ramp it
is easy to drive over steps without disturbing the
air-film or damaging the air-cushion bellow.
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5. Cracks
The most available floor is polished concrete.
This kind of floor has one important disadvantage. It gets
very often cracks. Due to this fact it could happen that
the air below the air-cushion, inside of the lifting chamber,
flows away through the crack. The air-cushion breaks down
and it isn't possible to continue with the transport.
Cracks have to be filled with synthetic resin or something
similar (Fig. 5.1). If the height difference between both
sides of a crack is more than 0.5mm it has to be levelled
out (Fig. 5.2).
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Fig. 5.1: Airtight sealing of a
crack with synthetic resin. Height difference less
than 0.5 mm.
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Fig. 5.2: Height difference more
than 0.5mm, both sides have to be levelled out.
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6. Expansion Gaps
Very often, it is not possible to avoid
expansion gaps in the floor, because of structural demands.
The filling material should be based on Polyurethane with
a shore-hardness of approx. 80.
To avoid damages to the edges of the floor and to be sure
that the expansion gap will be rigid enough for the pressure,
it should be executed in an angle of 90°.
To keep the loss of air at a minimum, the surface of the
filling should be cambered (Fig. 6.1).
A concave surface curvature is not allowed, because the
air-cushion can’t affect to the surface and to much
air gets lost (Fig. 6.2).
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Fig. 6.1: Correct gap form, with
gentle convex curvature.
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Fig. . 6.2: Wrong gap form
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