218
March 2014
Article
Tracto-Technik GmbH & Co KG
GmbH & Co KG
imaginary Cartesian coordinate system, the risk of a looping
of the bending die by the bending component becomes
less, the smaller the angle between the limiting rays is. As a
consequence the bending component must be destroyed to
rescue the bending die. The loop is also named “overbending”
(figure 3).
Category 1 of the intercept factor includes bending parts with
bends in multiple directions. The bending angles are small.
There are neither undercuts nor returning bends.
Typical examples are S-shaped bending components or fitting
tubes for dashboards in automobiles. With rising angle between
the limiting rays increases the risk of overbending effects, so
the bending contour should be checked for the scheduled
bending method. The second category includes geometries
with a rearward profile. This usually includes still open complex
contours. The freedom of movement of the bending machine
must be tested before the start of the bending procedure. The
third category of the intercept factor includes multiple crossing
bending characteristics. The contours can be produced only
with difficulty and always require prior investigation regarding
their realisation. Furthermore category 3 is assigned when
there is a rearward bending and the centre points of bending
are continuously plotted over multiple bending planes.
In order to counteract the torsion during the bending process,
it might be necessary to change the design of the bending
part or to provide a moment which compensates the torsion.
In contrast to inadvertent torsion, there are bending parts, in
which torsion has to be specially applied and within a certain
area. For this purpose and for describing inadvertent torsion,
the torsion factor is very suitable. According to figure 3, this
parameter describes the existence of torsion and its direction
around the longitudinal axis of the bending part. The torsion
factor has to be considered particularly with open profiles,
because these parts generally have a very low moment of
resistance against torsion. At torsion factor 1, torsion is non-
existent; torsion factor 2 describes plastic torsion in one
direction around the longitudinal axis of the bending part;
and torsion factor 3 implies occurrence of torsion in both
circumferential directions (see figure 5).
The semi-finished part and its geometrical dimensions are
of significant influence on selecting and determining an
appropriate bending process. The main focus of the geometry
is on one hand the cross-sectional shape and on the other
hand the relation between wall thickness and outside diameter.
These two parameters of the semi-finished part have to be
considered particularly when selecting the appropriate bending
tools. Constructive restraint of the bending contour must be
ensured in order to avoid declining quality of the bending part.
According to the required geometrical arrangement of the
tooling the selection of eligible bending methods is limited due
to constructive restrictions of the corresponding machines.
Where tubes are concerned, the wall factor (W = H / s)
describes the quotient between the outside tube diameter (H)
and the wall thickness (s). Reflecting the common process
limitations the wall factor is classified into four categories.
Due to continuous further development of tube and profile
bending processes towards open, asymmetric and alterable
cross-sections
[8,p.288f]
, this also has to be particularly considered
when selecting an eligible bending process
[9, p.1ff]
. For this
reason the criterion cross-sectional form has been introduced,
which is of decisive importance for the selection of an eligible
bending method and the respective bending tools. The
criterion itself is categorised into closed and open profiles and
for both in symmetric and asymmetric cross-sectional forms.
In order to meet the increased requirements for lightweight
construction there will also be a rising demand for bending
parts with an alterable wall thickness distribution. This fact is
recognised by the additional introduction of the wall thickness
change criterion. A distinction is made between bending
parts with constant wall thickness, tube bending parts with
variable wall thickness above the longitudinal axis (so called
“tailored tubes”) and profile bending parts with variable wall
thickness above the longitudinal axis (“tailored profiles”). The
term “tailored” implies the development of parts, whose wall-
thickness is especially adapted to the expected operational
loading conditions to save material and weight.
In the simplest case of “tailored tubes” only the inside diameter
of the tube differs above the longitudinal axis, so that just the
inside tooling has to be extra manufactured or rather modified.
Intercept factor 1
Intercept factor 2
Intercept factor 3
Figure 3: The intercept factor describes the bending direction and the positions of the bending sections towards each other
[6]
Figure 2: Danger of collision
of bending parts with tools
and the bending part itself –
the “overbending” effect
[6]
