Note: Descriptions are shown in the official language in which they were submitted.
~iL5358
1.
"Method of rolling a length of metal bar or wire and
apparatus for carrying out the method"
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention relates to a method of rolling a
length of metal bar or wire, especially hot-rolled bar or
wire, and is particularly concerned with the steps
necessary to keep such a metal length stretched at the
end of rolling when it passes through a cooling device
after the final roll stand. The invention also relates
to apparatus for carrying out the method. For ease of
description, the metal length will generally be called a
bar in the following description, but is not limited
thereto.
2. DESCRIPTION OF THE PRI~R ART
It is known to keep a "continuous" metal bar or
wire, which is passing through a finishing roll stand and
subsequently through a cooling device, stretched by means
of a set of bridle rolls (also known as pinch rolls)
located after the cooling device, between which the bar
is frictionally gripped. Typically the bridle rolls are
driven by independent identical motors with a rated
peripheral velocity exceeding the finishing rolling speed
of the bar. See Dutch patent 154129, which discloses use
` ~ s~s
of an adjustable parallelogram-shaped yoke in a plane at
right angles to the passage of the bar, the yoke being
supported by two fixed pivots above and below the rolls.
The rolls turn in bearings in the upstanding sides of the
parallelogram-shaped yoke. The motors are fitted in the
said upstanding sides.
In practice, it has been found that, possibly
owing to the large inertial mass involved, the first part
of the bar becomes twisted or buckled in the cooling
device. This is apparently caused by some difficulty of
entry of the leading end of the bar into the bridle
rolls, or some slippage in the grip of the bridle rolls,
and occurs notwithstanding the slightly higher speed of
the bridle rolls at entry of the bar. This higher speed
c 15 arises from the power input to the motors of the rolls
which is selected so that the rolls exert a drawing on
the bar to keep it under tension. Bar ends damaged in
this way have to be scrapped, which is costly.
SUMMARY OF THE INVENTION
The object of the present invention is to avoid
this disadvantage.
According to the invention, the power input to the
bridle rolls is temporarily increased, so that,
immediately before entry of the bar into the bridle
rolls, their periph~ral speed is greater than the
1153
peripheral speed corresponding to the normal power input
used during passage of the bar. For instance the power
input is briefly increased as the leading end of the bar
approaches, so that the bridle rolls are rotating at the
higher speed at the moment of entry. Thereafter, or even
before the moment of entry, the power input is reduced to
the level used while the bar is passing. It appears that
the extra peripheral speed of the bridle rolls causes a
considerably greater tensile force on the bar as the bar
begins to pass through them, and this greater force
virtually eliminates twisting or buckling of the head of
the bar.
Preferably the bridle rolls are driven
independently by permanently fitted motors by means of
Cardan shafts, but a single drive to the two rolls is
possible.
In order to obtain a good grip on the bae
material, the gap between the bridle rolls before entry
of the bar should preferably be about 2 mm smaller than
the thickness of the bar concerned.
Since a certain amount of time is required to
increase the speed of the bridle to the higher level, it
is preferred that the approach of the leading end of the
hot-rolled bar towards the finishing roll stand is
observed optically, to provide an electric signal which
1~L53~i~3S
4.
1,
is used to cause the temporary increase in power input.
BRIEF INTRODUCTION OF TE~E DRAWINGS
An embodiment of the invention will now be
described by way of example and with reference to the
accompanying drawing.
In the drawing,
Fig. 1 is a diagrammatic side view of a finishing
roll stand, cooling device, bridle rolls, shear and run-
out table, to which the method according to the invention
is applied;
Fig. 2 is a double graph to clarify the operation
of the method.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In Fig. 1 is shown part of a mill train for the
hot-rolling of bar and wire material WS. The finishing
roll stand W is followed by a cooling device K, and by a
pair of bridle rolls Rl and R2, between which the
ncontinuous" bar is frictionally gripped, relatively
lightly. S indicates a shear and U the run-out table or
cooling bank for the material which is cut to measure by
the shear.
The bridle rolls Rl and R2 are driven
independently through Cardan shafts CDl and CD2 by
permanently fitted electric motors Ml and M2, which are
supplied from a static frequency convertor ST. In one
1 1535f~5
practical embodiment, the motors Ml and M2 are squirrel-
cage rotor motors rated at 3.75 kVA each and the static
requency convertor has an output of 15 kVA with a yield
of 90~. The convertor ST receives slgnals from a photo
electric cell F which detects the presence of the hot bar
material in the mill train before the finishing stand W,
the cell being located above the mill train before the
stand W. The convertor also receives signals from a
tachogenerator TG which determines the finishing rolling
speed, being linked to the finishing stand '~. Data are
also entered, indicated by R in Fig. 1 relating to the
diameter correlation and the drawing adjustment for the
bar being rolled.
Fig. 1 shows, highly diagrammatically, that the
upper bridle roll Rl is fixed while the bottom bridle
roll R2 is movable since the frame in which the roll R2
is fitted can swing round a pivot located "upstream" with
respect to the bar travel. An adjustable stop A
determines the minimum gap between the two rolls Rl and
R2. This gap is set at about 2 mm less than the
thickness or diameter of the bar to be drawn. An air
cylinder C, supplied from a compressed air network
through a reducing valve V, normally presses the frame in
which the roll R2 is fitted against the stop A (whose
position is adjustable by means of a screw spindle). As
soon as the bar arrives between the rolls Rl and R2, the
frame leaves the stop and the air cylinder C ensures that
the bar is spring gripped between the two rolls.
The gripping surface of the two rolls is profiled,
e.g. by applying a pattern of recesses or holes and then
hardened. When changing the rolls, e.g. because of wear
or to change to another diameter of roll, the top roll Rl
can be placed in the correct position in the rolling line
by adjusting the frame carrying it.
Fig. 2 shows how a temporary accumulation of
energy can be used to keep the bar stretched when
entering the bridle rolls. Time is indicated along the
horizontal axis. In the upper graph, the rotational
speed n(rpm) of the bridle rolls is given on the vertical
axis, and in the lower graph the control signal to the
bridle roll motors is shown on the vertical axis. This
control signal indicates the period of time for which a
higher power input is applied. Power input can be
accurately controlled by means of the static frequency
convertor ST, using frequency control of the motors.
Fig. 2 thus shows that there is a temporary
increase in the power input over a time period tS which
leads (see the upper graph) to an acceleration of the
bridle rolls to a higher speed, this acceleration
occurring over a time period of tv. The initial speed of
s
the bridle rolls (glven as 800 rpm in this example) is
the idling speed of the rolls at the normal power input
which is used when the bar is actually passing; this
idling speed is 5 to 10% higher than the actual rolling
speed of the bar. When the bar is passing, the bridle
rolls must of course run at a peripheral speed equal to
the rolling speed of the final roll stand W, as indicated
by the slightly lower speed (e.g. 760 rpm) at the right
hand side of Fig. 2.
As ~entioned above, when the approach of the
leading end of the bar is signalled, the power input to
the bridle roll motors is increased, leading to a rise in
speed of about 25% in this case (to about 1000 rpm).
This rise is synchronised so that the leading end of the
bar arrives after the higher speed is reached. In this
case, the moment of arrival of the bar corresponds with
the end of the time period ts and is followed by the
decline in speed to the level oorresponding to the final
rolling speed. It is this initial tension applied to the
bar which minimises the damage to the head of the bar.
This leads to a considerable reduction in wastage of
material.
A 25% increase in speed is indicated above (to
1000 from 800). Suitably, this increase is in the range
15~ to 35%.
1:~53~
8,
The time ts during which the control signal is
present is not fixed, nor is the time tv over which the
rotating parts are speeded up. soth depend on, inter
alia the speed of the bar material. The time ts is
adjusted in dependence on the roll speed (via TG)
automatically in ST .
There is a linear correlation between the supply
frequency and the rotational speed n of the squirrel-cage
rotor motor under zero load. As the frequency is briefly
increased, the speed of the drive motors M1 and M2 rises
from, in this example, 800 to 1000 rpm. As a result,
more rotational energy is accumulated in the rotating
parts, i~e. the rotors of the motors Ml and M2, the
Cardan shafts CDl and CD2 and the rolls Rl and R2.
If the photo-electric cell F is located at a
distance of about 7 metres from the bridle rolls Rl, R2,
and the speed of the bar is about 14 metres per second,
0.5 seconds is available to increase the speed. On
entering the rolls Rl,R2, the head of the bar will be
subjected to a considerable tensile force between the
rolls owing to the difference in the speed between these
rolls and the stand W, so that the head of the bar will
remain straight.
Because also the gap between the rolls at the time
of entry of the bar is about 2 mm less than the diameter
~15;3~
of the bar, there ls less sllp on entry between the
bridle rolls and the bar material, so that there is an
additional advantage that the brldle rolls last longer.