[0001] The subject of the invention is a method of drawing ribs on tubes, especially ribs
of small pitches or ribs on tubes of low-plastic materials.
[0002] Hitherto, production of ribs on tubes by the drawing method, called also the rolling
method, has been carried out in heads comprising usually three symmetrically arranged
cylinders composed of a series of rolling disks. The axis of each cylinder forms with
the axis of rolling, which is the axis of the tube, a constant angle. The value of
the said angle results from the geometrical dependencies of the helical outline of
ribs drawn on the tube. Cylinders are driven with rotary motion, due to what, after
introducing the tube into the zone of the action thereof., the cylinders transmitting
the rotary motion to the tube draw the rib. Forming the ribs on the tube is the result
of producing in the zone of the action of the tool a corresponding state of stresses
in the material of the tube, causing a plastic strain. The material of the tube deforms
in the zone of the action of the tool in the axial and radial direction. Radial strain
manifests itself in a form of increasing the diameter of ribs, and axial strain manifests
itself in a form of elongation of the tube. Elongation of the tube in the process
of drawing the ribs is a disadvantageous phenomenon because the quantity of the elongation
is random variable for each segment of the tube. This characteristic causes the necessity
of applying technological allowances, and thus, production of wastes. Axial strains
of the tube are caused by corresponding axial stresses induced by the tool in the
zone of its operation. In the result of the action of the said stresses the tool is
subject to very high loads in the axial direction. The said loads cause bending out
the disks of which the tool consists. In the result thereof the pitch of ribs is not
uniform and always bigger from the pitch of the disks. The substantial problem in
the hitherto applied methods of drawing the ribs is a low durability of the tool whose
disks are destroyed in the result of the action of bending loads of a fatigue character.
[0003] The Polish patent specification no. 113419 presents the design of a tool with the
use of an additional disk stiffening the tool and increasing its durability. Such
a solution does not eliminate, however, the elongation of the tube and it is difficult
to draw ribs of very small pitches- on tubes of materials having big resistance of
the plastic strain because the increase of loads in the axial direction of the tool
is considerably bigger than the possibility of increasing the rigidity of the tool.
[0004] The US patent specification no. 3,876,593 presents a method of procuding bimetallic
ribbed tubes with the application of blocking the possibility of elongation of the
tube on which ribs are drawn. This blocking consists in clamping the end of the external
tube on the internal tube, and then the drawing process starts from the second, free
end of the tube. During drawing ribs by the said method different values of axial
stresses are observed along the length of the tube. In the initial stage, when starting
drawing the ribs from the free end of the tube, axial stresses in the non-deformed
part of the tube grow systematically till reaching a definite state of equilibrium
of stresses in the material of the external tube. At the same time, in the result
of increasing axial stresses, in the external tube, from the value "O" at the beginning
to the value corresponding to the state of equilibrium, the diameter of ribs increases
from a certain initial quantity to the required quantity. The segment of the tube
with different diamters of ribs usually does not meet the requirements imposed to
dimensional tolerances and is a waste material. A disadvantageous phenomenon in the
first stage of drawing is also the excessive bending load of the disks of the tool.
[0005] The essence of the method of drawing ribs on tubes according to the invention consists
in that to the material of the tube from which ribs are drawn initial compressive
stress is introduced, of the quantity smaller from the plasticity limit Re of the
material of the tube, and then in the zone of the action of disk tools, with maintenance
of the initial compressive stress in the material of the tube, ribs are formed by
pressing down to the tube driven rolling-disk tools. It is preferable when the quantity
of the initial compressive strength is ( 0.2 - 0.7 )

, where Fo denotes the size of the area of the section of the tube, and Fr denotes
the size of the area of the section of the core of the tube, after drawing the rib.
By introducing the initial state of compressive stresses to the material of deformed
tube, the tool is discharged from loads along the axis, bending out the rolling-form-disks,
practically already in the first stage of the contact of the tool with the material.
Another effect of introducing the initial state of compressive stresses is the avoidance
of the axial elongation of the tube. In these conditions the work or the tool resolves
itself to inducing plastic strains of the tube in the radial direction.
[0006] By realizing the production of ribbed tubes with the presented method tubes of a
constant geometry over the whole length are obtained, with maintenance of the high
service life of tools. By means of the tool discharged from harmful bending stresses
it is possible to draw ribs of very small pitches and big heights, from materials
having high plasticity limits.
[0007] The method of drawing ribs on tubes according to the invention is presented in more
detail in the drawings, wherein shows:
Fig. 1 the tube clamped with a screw and the diagram of compressive stresses of the
said tube,
Fig. 2 the axial section through the tube and the tool in the course of drawing ribs,
Fig. 3 the cross-section through the tube before the operation of drawing ribs, and
Fig. 4 the cross-section through the tube after drawing ribs.
[0008] The tube 1 before the operation of drawing ribs has been clamped with a screw 2,
thus inducing the origination of an initial compressive stress of an established quantity.
The quantity of the said stress is preferably (
0.2 -
0.7)

· Re, where Fo is the size of the area of the cross-section of the tube before the
drawing operation, Fr is the size of the area of the cross-section of the core of
the tube 3 after the operation of drawing the rib 4, and Re is the plasticity limit
of the material of the tube. After initial clamping the tube 1 with the screw 2, the
tube is introduced into the zone of the action of driven tools 5 provided with rolling-form-disks
6. During drawing of ribs 4 axial stresses practically do not act on the disks 6,
what enables obtaining very small pitches of ribs, and the material of the tube is
subject to plastic strains only in the radial direction.
1. A method of drawing ribs on tubes by means of tools driven with rotary motion and
comprising rolling-form-disks, characterized in that into the material of the tube
(3) from which ribs (4) are drawn an initial compressive stress is introduced with
a quantity being smaller than the plasticity limit Re of the material of the tube,
and that then in the zone of the action of disk tools (6) maintaining of the initial
compressive stress in the material of the tube, ribs are formed by pressing down the
driven rolling-form-disk tools to the tube.
2. A method of drawing ribs on tubes according to claim 1, characterized in that the
quantity of the initial compressive stress is (O.2 - 0.7)

, where Fo denotes the size of the area of the cross-section of the tube before drawing,
and Fr denotes the size of the area of the cross-section of the core of the tube (3)
after drawing the rib.