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S
traightening &
F
inishing
T
echnology
72
J
uly
/A
ugust
2007
Rotary straightening machines configured
with 3 pairs of vertical opposed rolls, all
rolls driven, have been the tube industry
standard for several decades. The 6-roll
setting process remains complex and
often the least understood process in a
tube making plant. This has been further
complicated by the introduction of machines
with 10 rolls.
Rotary tube straightening machines are
termed ‘rotary’ due to the tube revolving as
it passes through the machine. In the early
days these machines were designed with
a number of roll configurations. However,
the 6-roll machine became the standard for
modern tube straightening.
This machine features 3 pairs of vertically
opposed rolls, all rolls being driven. The
tube is fed into the machine, after which the
rolls grip the tube and rotate it while feeding
through the rolls until the whole length
has been straightened. While passing
through the machine, the tube is subjected
to two specific straightening forces:
pressure straightening and bend (or offset)
straightening.
During pressure straightening, each pair
of rolls can be adjusted so that the gap
between them is slightly smaller that the
outside diameter of the tube.
As the tube passes through this restricted
gap, it is subjected to pressure. If this
pressure is sufficient, it will cause the tube
walls to be strained past their elastic limit
thereby causing some straightening of the
tube. If this ability to squeeze the tube is
used correctly, the tube will be ‘rounded
up’ thus removing some or all of its ovality.
Ovality correction is an important ability of
multi-roll rotary straightening machinery.
In the process of bend or offset
straightening on the machine, one or more
pairs of rolls can be adjusted to cause the
tube to follow a curved path through the
machine. Bending (or offsetting) the tube in
this manner is the main straightening action
performed.
The amount of bend must be carefully
controlled so that it is sufficient to take
the tube past its elastic limit to achieve
straightness, but without resulting in
excessive work hardening of the tube
material. As the revolving tube passes
through the machine it is subject to
pressure and bend straightening along its
full length and in all axes.
As mentioned, there have been a number
of machine roll configurations over the
history of rotary straightening. Although
some staggered roll machines are still in
operation, the tube industry
has standardised in order to
use vertical opposed pairs of
rolls with all rolls being driven.
Machines with 3 pairs of rolls
(ie 6-roll machines) have
been in production for several
decades.
In the late 1970s the first
machines with 5 pairs of rolls
were introduced, specifically
where very high levels of
straightness were required. Six
roll machines require a high
level of operator understanding
to set the machine and achieve
good straightening results.
Ten roll machines are inevitably more
complex to set and only a few were
installed until computer-setting systems
were developed in the 1980s. A 6-roll
machine has a single bending moment, in
comparison with its 10-roll counterpart that
has two bending moments.
All straightening machines use specially
developed and carefully contoured
hardened steel rolls. The profile is
hyperbolical and is produced by a three
dimensional process. The roll profile is
specifically designed to enable a range of
tube diameters to be accommodated within
a single machine. The full range of tube
diameters is accommodated by adjusting
the angular position of the roll and the gaps
between each pair of rolls.
Rolls with worn profiles will not perform
correctly and cause poor straightness and
marks on the tube. However, rolls can be
reprofiled several times during their life. It is
important to understand that this generated
profile is not a radius to prevent the tube
from making contact with the roll along the
profile. The machine can only perform well
if the rolls are in good condition and have
the correct profile.
It is important to recognise roll problems,
as the roll profiles must be in good
condition for correct machine operation.
If the profile is worn, the tube will be
marked and good line contact will not be
achievable. Once the profile has worn ‘off
centre’, it will mark the tube and further
rapid wear will occur.
Understanding 6 and 10 roll rotary tube straightening machines
fi
As a tube is passed through the roll pairs, it is
subjected to pressure which leads to a degree of
straightening
›
In offset straightening, one or more pairs of rolls
can be adjusted to cause the tube to follow a
curved path
›
Roll configuration: the 6 roll machine
fi
Hyperbolic straightening rolls: the contact pattern on a simple
radius is displayed