Field of Art
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The present invention relates to rolling practice and, more particularly, to the working stand of a rolling mill.
Prior Art
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At present, the quality of rolled stock, the service service life of bearing assemblies and of the working roll surfaces are insufficiently high.
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Each working stand of a rolling mill has clearances between the housings and the chocks of back-up rolls. The working rolls have their drive located at one side of the stand so that when metal is gripped by these rolls they become twisted. This leads to redistribution of the friction forces along the length of contact between the working and back-up rolls. The back-up rolls turn in the zone of said side clearances between their chocks and housings which results in their skewing relative to the working rolls and creates heavy axial loads on the working rolls. This cuts down the sevvice life of the bearing assemblies and of the working roll surfaces.
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As distinct from the working rolls, the back-up rolls are not twisted since they are not subjected to the rolling moment.
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Due to twisting of the working roll the peripheral velocities in the sections along its contact with the back-up roll are different while the peripheral velocities of the back-up roll in the sections along its contact with the working roll stay constant due to absence of twisting.
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As a result, a difference of peripheral velocities of the working and back-up rolls develops along the length of their contact.
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This results in slipping of the working roll relative to the back-up roll and in origination of variable forces of sliding friction in the zone of their contact.
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As a result, the contact of the working and back-up rolls gives rise to nonuniform wear of the roll surface along the length of their bodies which curtails the durability of the working and back-up rolls. This, in turn, disturbs the profile of the external surface of working rolls with resultant distortion of the correct shape of the roll-to-roll clearance and, consequently, of the profile of the rolled strip which results in longitudinal and transverse thickness variations of the strip and there fore, worsening the quality of the rolled stock.
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Known in the prior art is the working stand of a rolling mill (A.I. Tselikov et al "Machines and Mechanisms of Metallurgical Plants, 1981, Moscow, Metallurgiya, v. 3, p. 191-195, Fig. IV-33, IV-38) comprising two housings accommodating back-up and working rolls with chocks. The working roll drive is arranged on one side of the stand.
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The housings and back-up roll chocksa are provided with replaceable plates contacting with one another, their contacting planes being parallel to the axis of the back-up rolls and to the axis of the stand.
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Inasmuch as the working rolls are also parallel to the stand axis, there is no skewing between the working and back-up rolls which reduces substantially the axial loads on the working and back-up rolls and their bearing assemblies.
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However, in the course of stand operation the contact surfaces of the replaceable plates are gradually worn down thus creating side clearances between the back-up roll chocks and the housings. This leads to longitudinal displacements of the working rolls in the zone of said clearances as well as skewing of the working rolls relative to the back-up ones.
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While the rolls bite the strip, their twisting caused by one-sided installation of their drive on the stand gives rise to the nonuniform distribution of friction forces over the length of the contact zone between the working and back-up rolls so that the back-up rolls turn in the zone of said side clearances between their chocks and housings which results in skewing of the back-up rolls relative to the working rolls, development of heavy axial loads on the rolls and their bearings, and lower durability of the working surfaces of rolls and bearing assemblies.
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Besides, due to skewing of the working and back-up rolls, the resultant of the inter-foll pressure is displaced from the rolling axis which brings about nonuniform distribution of contact stresses along the length of the body of each roll in the centre of deformation and, as a result, development of transverse and longitudinal thickness variatons of the rolled strip. These thickness variations reduce the quality sof rolled stock considerably.
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Apart from that, the nonuniform distribution of friction forces in the contact zone between the working and back-up rolls brings about nonuniform wear of the working surfaces of said rolls along the length of their bodies which, as has been stated above, also reduces the durability of rolls and the quality of rolled stock.
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Known in the prior art is a rolling mill stand (SU, A, 1045965) comprising housings accommodating back-up rolls with chocks and working rolls with chocks and their rotation drive installed on one side of the stand. The housings and back-up roll chocks are provided with replaceable plates contacting with one another.
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The plates on the back-up roll chocks are wedge-shaped while those on the housings have parallel sides.
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The wedge-shaped plates are screwed down to the back-up roll chocks so that the worn plates can be re-installed and the clearances developed in service, eliminated.
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Such an arrangement reduces the skewing angles of the rolls relative to the stand axis, decreases the axial loads on the bearing assemblies of rolls and extends their service life. Reduction of skewing angles of the working and back-up rolls decreases the wall thickness variations of the rolled strips and improves the quality of rolled stock.
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However, reinstallment of plates calls for removing the rolls from the stand which causes downtime of the rolling mill and reduces its output.
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Besides, the contact surfaces of the plates become worn in service which gives rise to the development of side clearances between the back-up roll chocks and the housings.
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This increases the skewing of rolls relative to the stand axis and of working rolls relative to the back-up rolls, and causes longitudinal displacements of the working rolls.
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When the working rolls bite the strip, this causes, as has been stated above, nonuniform distribution of friction forces over the length of contact between the working and back-up rolls. This brings about the skewing of the tack-up rolls relative to the working rolls, development of heavy axial loads on the rolls and their bearing assemblies which diminishes their service life.
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Besides, skewing of the working and back-up rolls lendst to nonuniform distribution of contact stresses along the length of each roll in the centre of deformation and, as a result, to the development of transverse and longitudinal thickness variations of the rolled strip. These thickness variations worsen the quality of frolled stock considerably.
Disclosure of the Invention
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The main object of the invention lies in providing the working stand of a rolling wmill whose plates, by automatic elimination of side clearances between the back-up roll chock and housings, would extend the life of the surfaces of back-up and working rolls, their bearing assemblies and improve the quality of rolled stock.
Methods for Realization of Technical Problem
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This problem is realized by providing a working stand of rolling mill, comprising two housings accommodating back-up rolls with chocks and working rolls adapted for connection with their rotation drive installed on one side of the stand, the housings and back-up roll chocks being equipped with plates contacting one another wherein, according to the invention, the plates on the housings and back-up roll chocks have bevels forming a wedge joint of said chocks with the housings, the wedge point of the back-up roll plates facing the working roll drive.
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Beveling of the plates on the housings and back-up roll chock to form the wedge joint of the chocks with the housings and directing the wedge point towards the working roll rdrive ensures automatic elimination of the clearances between the back-up roll chocks and the housings.
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When the working rolls bite the strip, the location of the drive of said rolls on one side of the stand causes their twisting which produces axial loads which displace the back-up rolls with their chooks in the axial direction thus ruling out the development of clearances between the back-up roll chocks and housings.
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When the working rolls are twisted, the friction forces become redistributed along the length of their contact zone with the back-up rolls and the rolls are acted upon by the turning moments in a horizontal plane which cause their skewing and the development of the above-mentioned axial loads. Due to the one-sided location of the working roll drive the axial loads on the back-up rolls are always directed to one side, i.e. towards the working roll drive.
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As a result, the beveled surfaces of the plates of the back-up roll chocks are always pressed against the beveled surfaces of houwing plates so that clearances between them are nonexistent.
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Owing to this, the longitudinal dynamic loads (in the rolling direction) on the bearing assemblies of the tools are considerably smaller thus extending their durability.
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Skewing of the back-up rolls relative to the stand axis is ruled out and the skewing angles of the working rolls relative to the back-up rolls become considerably smaller as are the axial loads on the rolls and their bearing assemblies.
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This reduces wear of the active surfaces of working and back-up rolls due to reduced friction forces in the contact zone which extends the life of the rolls.
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Reduction of the skewing angles of rolls conduces to forming the inter-roll clearance of a correct geometrical shape and, consequently, to a reduction of the thickness variations of the rolled strips and improvement of their quality.
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It is practicable that the bevels on the plates of the back-up rolls and housing plates should be set at an angle of 1.5-20° to the axis of the back-up rolls.
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On the one hand, this rules out wedging of the back-up roll chocks in toe housings, permits adjusting the position of the upper back-up roll for height and free replacement of the back-up rolls because the bevel angles of the plates equal to, or more than, 1.5° are larger than the angle of friction between the plates on the chocks of the back-up rolls and the plates on the housings.
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On the other hand, at bevel angles of plates smaller than 20° the back-up roll chocks cannot be forced out of the housings in the direction away from the working roll drive by the longitudinal forces acting on the back-up rolls and their chocks in the transitional processes of rolling (biting of strips by the rolls, mismatching of roll velocities in adjacent stands, etc.).
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Thus, when the bevels on the plates of the back-up roll chocks and housings are made at an angle of 1.5-20°, this ensures both free adjustment for height of the upper back-up roll with chocks and replacement of rolls and their stable position in coaxiality with the working stand in which there are no clearances between the back-up roll chocks and the housings within the entire process of strip rolling.
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This, in turn, reduces the longitudinal dynamic loads on the rolls and their bearing assemblies, and step up their durability.
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The skewing angles of the working and back-up rolls and the axial loads originated in the roll contact zone and transmitted to the bearing assemblies are reduced. This diminishes the wear of the roll surfaces with the corresponding improvement of quality of the rolled stock.
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If the angle is smaller than 1.5°, the back-up roll chocks will get jammed in the stand because the wedge angle will be smaller than the angle of friction. If the bevel angle of the plates is larger than 20°, the longitudinal forces acting on the back-up roll during rolling may force the roll out of the stand thus causing break-down of the rolling mill.
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Thus, the use of the present invention increases the durability of the surfaces of back-up and working rolls, their bearing assemblies and improves the quality of rolled stock.
Brief Description of the Drawings
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Now the invention will be described by way of example with reference to the accompanying drawings in which:
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Fig. 1 is a schematic diagram of the working stand of a rolling mill according to the invention, horizontal section along the axis of the upper back-up roll;
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Fig. 2 is a section taken along line II-II in Fig. 1.
Best Mode of Carrying out the Invention
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The working stand of a rolling mill comprises two housings 1 (Fig. 1), working rolls 2 (Fig. 2) installed in the housings and adapted for connection with their rotation drive (not shown in the drawing), and back-up rolls 3 (Fig. 1) with chocks 4 and 5. The housings 1 and the chocks 4, 5 of the back-up rolls 3 are provided with replaceable plates 5,6, 7 installed, respectively, on the housing 1 and chock 4 of the back-up roll 3 at the side opposite to the location of the drive of the working rolls 2 (Fig. 2).
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The replaceable plates 6 (Fig. 1), 7, and 8, 9 have bevels whose surfaces contact with one another (plate 6 with plate 7, plate 8 with plate 9), forming a wedge joint of the chocks 4 and 5 with housings 1. The point of the wedte formed by plates 7 and 9 of the chocks 4 and 5 of the back-up rolls 3 is directed towards the working roll drive.
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The bevels on the plates 7, 9 of the chock 4, 5, respectively, of the back-up roll 3 and the bevels on the plates 6, 8 of the housings 1, respectively, are set at an angle ∝ = 1.5 - 20° to the axis 10 of the back-up rolls 3.
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This angle ∝ is selected on the condition of eliminating the wedging of the chocks 4, 5 of the back-up rolls 3 in the housings 1, ensuring the possibility of adjusting the position of the upper back-up roll 3 for height and free replacement of the back-up rolls 3, i.e. on the condition that the angle should be larger than the angle of friction between the plates 6,7 and 8,9.
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The angle ∝ should not exceed 20° otherwise the chocks 4 and 5 may be forced out of the housings 1 away from the location of the drive of the working rolls 2 (Fig. 2) under the effect of longitudinal loads transmitted to the back-up rolls 3 and their chocks 4 (Fig. 1) and 5 during the transitional rolling processes (biting the strip with rolls, mismatching of roll velocities in adjacent stands, etc.).
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When the horizontal forces P₁ are transmitted to the
chocks 4, 5 of the back-up roll 3, this originates a pushing-out force R on the wedge shaped surface in the direction away from the drive of the working rolls 2 (Fig. 2). This force R is equal to:
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The
chocks 4, 5 (Fig. 1) are held in the housings 1 by the forces of friction: F₁ in the zone of contact of one
chock 4 or 5 with the support of the cross-piece of housings 1 and F₂ in the zone of contact of the wedte-shaped surfaces. The expressions for these forces are as follows:
where P₁ = horizontal force;
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P₂ = rolling force;
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f₁, f₂ = coefficients of friction in the zone of contact of chocks 4 or 5 with the support of the cross-piece of housings 1 and in the zone of contact of the wedge-shaped surfaces, respectively.
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The horizontal forces P₁ are determined by the action of inervia forces and friction forces in the zone of contact of the working roll 2 (Fig. 2) and back-up roll 3 (Fig. 1) caused by their relative slipping due to the difference of peripheral velocities.
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Investigations have proved that the force P₁ transmitted to one
chock 4 or 5 is not larger than 10% of the rolling force, i.e. in the calculations it can be assumed that:
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The condition of holding the
chocks 4, 5 in the housings 1 at a maximum angle ∝
max can be expressed as follos:
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After substituting said values into this expression and transforming said expressions we get:
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The second term of the L.H. part of this expression (due to the smallness of f₂ ≦αµρ¨ 0.05) is considerably smaller than its R.H. part and can be disregarded.
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Besides, this admission goes for the reliability reserve of holding the chocks 4, 5 in the housings 1 (decreases ∝max).
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Then the expression for ∝
max takes the form:
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When f₁ = 0.1 (sliding friction of steel to steel), ∝max = 30°.
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The minimum bevel angle of plates corresponds to the angle of friction between them, i.e.
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Then the expression for the permissible bevel angle of platem can be written as follows:
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By substituting the values of f₂ and f₁ we get
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Taking in account the spread of values of the coefficients of friction f₂ and f₁ in the capacity of optimum limits of angle ∝ at which wedging is excluded and the
chocks 4, 5 are reliably held in the housings 1 we can take:
i.e. the angle ∝ shall be selected within 1.5 and 20°.
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Thus, when making the bevels on the plates 6, 7 and 8,9 at an angle varying from 1.5 to 20° this ensures both the possibility of free adjustment for height of the upper back-up roll 3 with chocks 4, 5 and replacement of the back-up rolls 3, and the steady position of the back-up rolls coaxial with the working stand in which there are no clearances between the chocks 4,5 of the back-up rolls 3 and the housings 1 within the entire period of rolling the strip.
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This, in turn, provides for a reduction of longitudinal dynamic loads on hthe back-up and working rolls and their bearing assemblies and for increased life of these units. This reduces the skewing angles of the working and back-up rolls and the axial loads in the zone of their contact transmitted to the bearing assemblies of the rolls. And this reduces the wear of the surfaces of the working and back-up rolls and steps up the quality of the rolled stock.
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The working stand of a rolling mill functions, as follows:
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As the working rolls 2 bite the metal, the onesided location of the drive and their twisting over the width of the strip being rolled gives rise to the nonuniformity of distribution of friction forces along the length of contact of the working and back-up rolls 2 and 3 because the peripheral velocities of the working rolls 2 change due to being twisting over the strip width while the peripheral velocities of the back-up rolls 3 stay constant throughout the length of their bodies because the back-up rolls 3 are not twisted.
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The resultant of the contact friction forces id displaced relative to the axis of rolling and creates turning moments which act on these rolls in a horizontal plane in the opposite, but aloways the same, directions. The back-up rolls 3 tend to turn and their chocks 4 at the drive side move against the direction of rolling and the chocks 5, at the no-driving side, along the direction of rolling.
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The working rolls 2 turn in the zone of clearances between their chocks and housings 1 in the opposite direction and are skewed relative to the back-up rolls 3 so that the axial loads acting on the back-up rolls 3 are always directed towards the drive.
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As a result, the chocks 4 and 5 of the back-up rolls 3 become always pressed against the housings 1 over the planes of the bevels of the plates 7, 9 and 6, 8 so that there are no clearances between the chocks 4 and 5 and the housings 1.
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Owing to elimination of these clearances the back-up rolls 3 take a stable position in the working stand, without skewing and without horizontal displacements.
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This reduces the longitudinal dynamic loads on the rolls 2 and 3 and their bearing assemblies and improves the durability of these units. In addition, the angles of skwwing of the working and back-up rolls 2 and 3 as well as the axial loads in the zone of their contact diminish which leads to smaller wear of the surfaces of the rolls 2 and 3, and, as a result, to a higher quality of the rolled stock.
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Thus, the use of the dis closed invention ensures a higher durability of the active surface of the rolls, their bearing assemblies and a higher quality of the rolled stock.
Industrial Applicability
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The present invention will be used to the best advantage in wide-strip hot- and cold-rolling mills handling thin strips 0.5 to 2.0 mm thick of high-quality steels.
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The use of the present invention reduces the thickness variations of the rolled strips by 20-30%, increases the durability of the rolls by 15-25% and reduces the consumption of costly roll bearings by 2 - 3 times.
