Method for designing refiner plates with equidistant curved bars

Abstract

The present invention discloses a method for designing a refiner plate with equidistant curved bars, comprising following steps of: designing a central bar are of center curved bar and defining the bar angle for the equidistant curved bar; designing circle arcs for curved bars on two sides of center curved bar of equidistant curved bar segment; when the whole refining segment is full of circle arcs, trimming lines of outer circle arcs of the refining segment to complete the design of equidistant circle arcs on the two sides; and if required, dividing the bars into zones. In the present invention, by the definition of the bar angle for the curved bars and the parametric design of the equidistant curved bars by using circle are equations, it is ensured that the flexibility in designing an equidistant curved bar refiner plate is improved.

Claims

1. A method for designing a refiner plate with equidistant curved bars, comprising the following steps: step 1) defining, on the basis of defining a bar angle of the curved bars, a center circle arc for the curved bars, and establishing an equation for the circle arc for the curved bars by establishing a polar coordinate system; wherein the step 1) comprises the following steps: substep 1 ): setting an inner diameter of the refining segment as R.sub.i ; setting an outer diameter of the refining segment as R.sub.o; setting a circle center of the refining segment as O; setting a center circle arc of the refining segment as MN; setting a radius of the center circle arc MN as (R.sub.i+R.sub.o)/2; defining a bisector of the refining segment as OB; and intersecting the center circle arc in the refining segment with OB at a point B, making BD passing through the point B at the top right of OB if the curved bars are right-hand bars and making BD passing through the point B at the top left of OB if the curved bars are left-hand bars; representing an included angle between BD and OB by α, selecting any point A from an inner circle in the refining segment as a starting point of the curve bars, connecting the points O and A, making a circle O.sub.1 passing through the points A and B by using BD as a tangent line, making a tangent line AE, passing through the point A, which is tangent to the circle O.sub.1 with an included angle between AE and OA represented by β, with a line perpendicular to the tangent line AE and a line perpendicular to BD intersecting at a point O.sub.1 and the radius of the circle O.sub.1 being measured as R.sub.1; and obtaining an intersected portion of the circle O.sub.1 with inner and outer circles in the refining segment as a center line for curved bars, and assuming that an included angle a between the tangent line BD that is tangent to the center line for curved bars at the point B and OB starting from the point B in the radius direction is the angle of inclination of the equidistant curved bars and an included angle βbetween the tangent line AE that is tangent to the circle O.sub.1 at the point A and OA is a starting angle of inclination of the equidistant curved bars; substep 2): designing a center circle arc AC for the equidistant curved bars by determining the points A and B and defining a bar angle a, wherein A is the starting point of the center arc for the bars, which can be expressed by (γ, r.sub.A), where γ is an included angle between the OA and the center line of the refining segment, r.sub.A is the radius of the circle where the starting point is located, and then the center arc AC for the equidistant curved bars can be determined by the point A and the bar angle α; and obtaining an equation for the circle O.sub.1 according to the polar coordinate system by: using the point O.sub.1 as a pole and drawing a horizontal ray O-x from the pole as a polar axis, using the clockwise direction as the positive direction, and representing an included angle between a connecting line from any one point on the circle O.sub.1 to the pole, and the polar axis as θ: $\begin{array}{cc}\left\{\begin{array}{c}x=R_1\cos \theta \\ y=R_1\times \sin \theta \end{array}\right\}& \left(1\right)\end{array}$ wherein the equation (1) is the equation for the circle of the center circle arc AC for the equidistant curved bars; substep 3): setting the width of the curved bars as b, and respectively representing equations for inner and outer circle arcs for the center bars as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}\right)\sin \theta \end{array}\right\}\mathrm{and}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}\right)\sin \theta \end{array}\right\};& \left(2\right)\end{array}$ step 2) establishing, in consideration of a bar width and a groove width, an equation for circle arcs for bars on two sides of the center circle arc; step 3) when the whole a refining segment is full of circle arcs of bars, trimming lines of outer circle arcs of the refining segment to complete the design of equidistant circle arcs on the two sides, wherein, according to refining process requirements, the refiner plate with equidistant curved bars is divided into zones along a predetermined standard line and then the curved bars are trimmed, a bar height is determined according to the specific process requirements; and step 4) machining such a refiner plate in accordance with methods for common refiner plates by casting which is applicable for industrial mass-production of refiner plates and milling which is applicable for experimental refiner plates, with casting including the following operations as main steps: design and development of a refining segment mold, manufacture of a cavity suitable for casting, alloy smelting and casting, opening the mold for the purpose of cleaning, initial machining, thermal treatment, finish machining, and inspection.

2. The method for designing the refiner plate with equidistant curved bars according to claim 1, wherein the step 2) comprises the following steps: substep 1: setting the groove width of curved bar plate as g, when the refining segment with equidistant curved bars are designed with right-hand curved bars, representing an equation for a circle arc for the first bar on the left side of the circle arc for the center bar as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}+g\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}+g\right)\sin \theta \end{array}\right\}& \left(3\right)\end{array}$ representing an equation for a circle arc for a 2n.sup.th bar on the left side as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}+n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}+n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(4\right)\end{array}$ representing an equation for a circle arc for a (2n+1).sup.th bar on the left side as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}+g+n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}+g+n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(5\right)\end{array}$ where, n≥1; when the refining segment with equidistant curved bars are designed with left-hand curved bars, equations for circle arcs for bars on the right side of the circle arc for the center bar are the same as equations for circle arcs for bars on the left side of the circle arc for the center bar in the case where the refining segment is designed with right-hand curved bars; substep 2: when the refining segment with equidistant curved bars are designed with right-hand curved bars, representing an equation for a circle arc for the first bar on the right side of the circle arc for the center bar as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}-g\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}-g\right)\sin \theta \end{array}\right\}& \left(6\right)\end{array}$ representing an equation for a circle arc for a 2n.sup.th bar on the right side as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}-n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}-n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(7\right)\end{array}$ representing an equation for a circle arc for a (2n+1).sup.th bar on the right side as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}-g-n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}-g-n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(8\right)\end{array}$ where, n≥1 and n is a positive integer; when the refining segment with equidistant curved bars are designed with left-hand curved bars, equations for circle arcs for bars on the left side of the circle arc for the center bar are the same as equations for circle arcs for bars on the right side of the circle arc for the center bar in the case where the refining segment is designed with right-hand curved bars.

3. The method for designing the refiner plate with equidistant curved bars according to claim 1, wherein the step 3) comprises the following steps: after the design of the refiner plate with equidistant curved bars and the design of the circle arcs are completed, if required, dividing the refiner plate into zones by concentric circle arcs, circle arcs or broken lines; wherein, during the division by concentric circle arcs, the refiner plate is divided into three stages: a breaking zone, a coarse refining zone and a fine refining zone, at a ratio of k.sub.1:k.sub.2:k.sub.3, and equations for circle arcs in the crushing zone and the coarse refining zone are represented as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left[\frac{k_1\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\cos \theta \\ y=\left[\frac{k_1\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\sin \theta \end{array}\right\}& \left(9\right)\end{array}$ equations for circle arcs in the coarse refining zone and the fine refining zone are represented as: $\begin{array}{cc}\left\{\begin{array}{c}x=\left[\frac{\left(k_1+k_2\right)\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\cos \theta \\ y=\left[\frac{\left(k_1+k_2\right)\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\sin \theta \end{array}\right\}& \left(10\right)\end{array}$ the methods for determining equations for concentric circle arcs in other zones are similar to equations (9) and (10); after the division, according to process requirements, the bars are optimized, and usually, the number of bars in the breaking zone, the coarse refining zone and the fine refining zone are successively increased.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of defining a center circle are of the bar angle of the equidistant curved bars according to the present invention;

(2) FIG. 2 is a schematic view of establishing a curve at the edge of the center curved bar of the equidistant curved bars according to the present invention;

(3) FIG. 3 is a schematic view of establishing curves for curved bars on two sides of the center curved bar according to the present invention;

(4) FIG. 4 is a schematic view of a refining segment with equidistant curved bars according to the present invention;

(5) FIG. 5 is a schematic view of a refining segment with equidistant curved bars, which is divided into two stages, according to the present invention;

(6) FIG. 6 is a schematic view of a refining segment with equidistant curved bars according to an embodiment of the present invention;

(7) FIG. 7 is a refiner plate with straight bars, having the same parameters as the refiner plate according to an embodiment of the present invention;

(8) FIG. 8 shows the influence on the freeness of pulp by the refining segment with equidistant curved bars according to an embodiment of the present invention and a refiner plate with straight bars, with same parameters; and

(9) FIG. 9 shows the influence on the average length of fibers by the refining segment with equidistant curved bars according to an embodiment of the present invention and a refiner plate with straight bars, with same parameters.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(10) The present invention will be further described below by specific embodiments. The description is merely provided for explaining the present invention, rather than limiting the present invention.

(11) A method for designing a refiner plate with equidistant curved bars is provided, comprising following steps:

(12) 1) designing a center circle are for the equidistant curved bars:

(13) defining, on the basis of defining a bar angle of the curved bars, a center circle are for the curved bars, and establishing an equation for the circle arc for the curved bars by establishing a polar coordinate system;

(14) 2) designing circle arcs for curved bars on two sides of a center bar of equidistant curved refining segment:

(15) establishing, in consideration of the bar width and groove width, an equation of circle arcs for bars on two sides of a center curved bar;

(16) 3) when the whole refining segment is full of circle arcs of curved bars, trimming lines of outer circle arcs of the refining segment to complete the design of equidistant circle arcs on the two sides, wherein, according to refining process requirements, the refiner plate with equidistant curved bars are divided into zones along a predetermined standard line and then trimmed, so far the design of a refining segment with equidistant curved bars is completed, and a refiner plate with equidistant curved bars is obtained.

(17) 1. The design of a center circle are for the equidistant curved bars specifically comprises following steps:

(18) step 1: designing a circle are for the bar angle of the equidistant curved bars:

(19) as shown in FIG. 1, given that the refining segment has an inner diameter R.sub.i, an outer diameter R.sub.o and a circle center O, the refining segment has a center circle are MN, the center circle arc MN in the refining segment has a radius (R.sub.i+R.sub.o)/2, OB is a bisector of the refining segment, and the center circle are in the refining segment intersects with OB at a point B, making BD passing through the point B at the top right of OB if the curved bars are right-hand bars and making BD passing through the point B at the top left of OB if the curved bars are left-hand bars, representing an included angle between BD and OB by a, selecting any point A from an inner circle in the refining segment as a starting point of the curve bars, and connecting the points O and A;

(20) making a circle O.sub.1 passing through the points A and B by using BD as a tangent line, making a tangent line AE, passing through the point A, which is tangent to the circle O.sub.1 with an included angle between AE and OA represented by β, with a line perpendicular to the tangent line AE and a line perpendicular to BD intersecting at a point O.sub.1 and the radius of the circle O.sub.1 being measured as R.sub.i; and obtaining an intersected portion of the circle O.sub.1 with inner and outer circles in the refining segment as a center line for curved bars, and assuming that an included angle α between the tangent line BD that is tangent to the center line for curved bars at the point B and OB starting from the point B in the radius direction is the bar angle of the equidistant curved bars and an included angle β between the tangent line AE that is tangent to the circle O.sub.1 at the point A and OA is a starting bar angle of the equidistant curved bars;

(21) step 2: designing an equation for the center circle are for the equidistant curved bars:

(22) designing a center circle are AC for the equidistant curved bars by determining the points A and B and defining a bar angle α of the bars, wherein A is the starting point of the center are for the bars, which can be expressed by (γ, r.sub.A), where γ is an included angle between the OA and the center line of the refining segment, r.sub.A is the radius of the circle where the starting point is located, and then the center are AC for the equidistant curved bars can be determined by the point A and the bar angle α; and obtaining an equation for the circle O.sub.1 according to the polar coordinate system by: using the point O.sub.1 as a pole and drawing a horizontal ray O-x from the pole as a polar axis, using the clockwise direction as the positive direction, and representing an included angle between a connecting line from any one point on the circle O.sub.1 to the pole, and the polar axis as θ:

(23) $\begin{array}{cc}\left\{\begin{array}{c}x=R_1\cos \theta \\ y=R_1\sin \theta \end{array}\right\}& \left(1\right)\end{array}$

(24) wherein the equation (1) is the equation for the circle of the center circle are AC for the equidistant curved bars;

(25) step 3: designing equations for circle arcs at the edges of the center bars:

(26) as shown in FIG. 2, given that the width of the equidistant curved bars is b, respectively representing equations for inner and outer circle arcs for the center bars as:

(27) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}\right)\sin \theta \end{array}\right\}\mathrm{and}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}\right)\sin \theta \end{array}\right\}& \left(2\right)\end{array}$

(28) 2. The design of circle arcs for curved bars on two sides of the equidistant curved refining segment specifically comprises following steps:

(29) step 1: designing curved bars on the left side of the equidistant curved refining segment:

(30) given that the groove width for the curved bar plate is g, representing an equation for a circle are for the first bar on the left side as:

(31) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}+g\right))\cos \theta \\ y=\left(R_1+\frac{b}{2}+g\right))\sin \theta \end{array}\right\}& \left(3\right)\end{array}$

(32) representing an equation for a circle are for a 2n.sup.th (n≥1) bar on the left side as:

(33) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}+n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}+n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(4\right)\end{array}$

(34) representing an equation for a circle arc for a (2n+1).sup.th (n≥1) bar on the left side as:

(35) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1+\frac{b}{2}+g+n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1+\frac{b}{2}+g+n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(5\right)\end{array}$

(36) By the arrangement of circle arcs, when the whole refining segment is full of circle arcs of bars, lines of outer circle arcs of the refining segment are trimmed to complete the design of equidistant circle arcs on the left side.

(37) Similarly, when the curved bars are left-hand bars, equations for circle arcs for bars on the right side are the same as equations in the step 1;

(38) step 2: designing curved bars on the right side of the equidistant curved refining segment:

(39) given that the groove width for the curved bar plate is g, representing an equation for a circle are for the first bar on the right side as:

(40) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}-g\right))\cos \theta \\ y=\left(R_1-\frac{b}{2}-g\right))\sin \theta \end{array}\right\}& \left(6\right)\end{array}$

(41) representing an equation for a circle are for a 2n.sup.th (n≥1) bar on the right side as:

(42) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}-n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}-n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(7\right)\end{array}$

(43) representing an equation for a circle arc for a (2n+1).sup.th (n≥1) bar on the right side as:

(44) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(R_1-\frac{b}{2}-g-n\left(g+b\right)\right)\cos \theta \\ y=\left(R_1-\frac{b}{2}-g-n\left(g+b\right)\right)\sin \theta \end{array}\right\}& \left(8\right)\end{array}$

(45) by the arrangement of circle arcs, when the whole refining segment is full of circle arcs, lines of outer circle arcs of the refining segment are trimmed to complete the design of equidistant circle arcs on the right side; so far, the design of the refining segment with equidistant curved bars is completed and a refiner plate as shown in FIG. 4 is obtained.

(46) Similarly, when the curved bars are right-hand bars, equations for circle arcs for bars on the left side are the same as equations in the step 2.

(47) 3. The division of the refiner plate with equidistant curved bars specifically comprises following steps:

(48) after the design of the refiner plate with equidistant curved bars and the design of the circle arcs are completed, if required, dividing the refiner plate into zones by concentric circle arcs, circle arcs or broken lines or the like.

(49) Taking the division by concentric circle arcs as example, the refiner plate is divided into three stages: a breaking zone, a coarse refining zone and a fine refining zone, at a ratio of k.sub.1:k.sub.2:k.sub.3, and equations for circle arcs in the breaking zone and the coarse refining zone are represented as:

(50) $\begin{array}{cc}\left\{\begin{array}{c}x=\left[\frac{k_1\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\cos \theta \\ y=\left[\frac{k_1\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\sin \theta \end{array}\right\}& \left(9\right)\end{array}$

(51) equations for circle arcs in the coarse refining zone and the fine refining zone are represented as:

(52) 0$\begin{array}{cc}\left\{\begin{array}{c}x=\left[\frac{\left(k_1+k_2\right)\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\cos \theta \\ y=\left[\frac{\left(k_1+k_2\right)\left(R_0-R_i\right)}{k_1+k_2+k_3}+R_i\right]\sin \theta \end{array}\right\}& \left(10\right)\end{array}$

(53) the methods for determining equations for concentric circle arcs in other zones are similar to equations (9) and (10).

(54) The refiner plate, which is divided into two stages and then trimmed, is as shown in FIG. 5.

(55) The specific embodiment will be described below.

(56) Papermaking plate refiners are important devices used in the pulping process. Now, it is required to design an experimental refining segment, which has an inner diameter of 82.5 mm and an outer diameter of 203 mm. The bar angle of the curved bars is 420, the starting angle of inclination is 34°, and the center angle of the refining segment is 40°. The bar width is 2 mm, the groove width is 3 mm, and the bar height is 4 mm.

(57) A pattern is established, as shown in FIG. 1. A point A (20°, 43 mm) is selected from the inner diameter of the refining segment as the starting point of the bars, with the center angle of the refining segment of 40°, R.sub.i=41.25 mm, R=101.5 mm. The center circle are in the refining zone has a radius of 71.375 mm, α=22°, β=34°. A circle O.sub.1 passing through the points A and B is made by using BD and AE as tangent lines.

(58) Then, R.sub.1=71.375 mm, an equation for the center circle are for the equidistant curved bars is represented as:

(59) $\begin{array}{cc}\left\{\begin{array}{c}x=71.375\times \cos \theta \\ y=71.375\times \sin \theta \end{array}\right\}& \left(11\right)\end{array}$

(60) As shown in FIG. 3, from the bar width and the groove width, equations for circle arcs O.sub.1 and O.sub.2 are represented as:

(61) $\begin{array}{cc}\left\{\begin{array}{c}x=72.375\times \cos \theta \\ y=72.375\times \sin \theta \end{array}\right\}\mathrm{and}\left\{\begin{array}{c}x=70.375\times \cos \theta \\ y=70.375\times \sin \theta \end{array}\right\}& \left(12\right)\end{array}$

(62) then: an equation for a circle are for the first bar on the left side is represented as:

(63) $\begin{array}{cc}\left\{\begin{array}{c}x=75.375\times \cos \theta \\ y=75.375\times \sin \theta \end{array}\right\}& \left(13\right)\end{array}$

(64) an equation for a circle are for a 2n.sup.th (n≥1) bar, for example, the second, fourth, sixth or eighth bar, on the left side is represented as:

(65) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(72.375+7n\right)\times \cos \theta \\ y=\left(72.375+7n\right)\times \sin \theta \end{array}\right\}& \left(14\right)\end{array}$

(66) an equation for a circle are for a (2n+1).sup.th (n≥1) bar, for example, the third, fifth, seventh or ninth bar, on the left side is represented as:

(67) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(75.375+7n\right)\times \cos \theta \\ y=\left(75.375+7n\right)\times \sin \theta \end{array}\right\}& \left(15\right)\end{array}$

(68) As shown in FIG. 3, an equation for a circle are for the first bar on the right side of the center circle arc for the equidistant curved bars is represented as:

(69) $\begin{array}{cc}\left\{\begin{array}{c}x=67.375\times \cos \theta \\ y=67.375\times \sin \theta \end{array}\right\}& \left(16\right)\end{array}$

(70) an equation for a circle are for a 2n.sup.th (n≥1) bar, for example, the second, fourth, sixth or eighth bar, on the right side is represented as:

(71) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(67.375-7n\right)\times \cos \theta \\ y=\left(67.375-7n\right)\times \sin \theta \end{array}\right\}& \left(17\right)\end{array}$

(72) an equation for a circle are for a (2n+1)±(n≥1) bar, for example, the third, fifth, seventh or ninth bar, on the left side is represented as:

(73) $\begin{array}{cc}\left\{\begin{array}{c}x=\left(64.375-7n\right)\times \cos \theta \\ y=\left(64.375-7n\right)\times \sin \theta \end{array}\right\}& \left(18\right)\end{array}$

(74) A refiner plate, as shown in FIG. 6, may be finally designed in a pattern, according to the equations (11)-(18) for circle arcs and the height of the bars of 4 mm.

(75) According to actual requirements, 2Cr13 is used as material for manufacturing the refining segment and the designed curved bar plate shown in FIG. 6 was machined. It is compared with a refiner plate with straight bars (as shown in FIG. 7 and the detailed parameters can be found in Table 1), with same parameters such as the bar angle, the bar width, the bar height and the groove width, by low consistency refining tests in which bleached sulfate eucalyptus pulp is used as the pulp for experiments and its consistency is controlled at 3%. Cyclic refining tests were carried out by a MD3000 single-plate refiner at a constant rotation speed (1460 rpm). It was found that the refiner plate with curved bars designed in the present invention has a refining intensity lower than that of the refiner plate with straight bars. The length of fibers is effectively maintained while keeping a same freeness. The average length of fibers is 20%-30% greater than that of pulp obtained by using the refiner plate with straight bars, as shown in FIGS. 8 and 9.

(76) TABLE-US-00001 TABLE 1 Straight bar Curved bar BEL 276.55 m/rev 327.58 m/rev A (20°, 55 mm) (20°, 43 mm) Bar Channel Bar Inner Outer Center angle Number width width height α radius radius of segment of bars Common bar 2 mm 3 mm 4 mm 42° 82.5 mm 203 mm 40° 117 parameters