Improve Coil- Performance by ert554898

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									               Improve Coil-
       Straightening
                 Performance
                 To enhance production runs on coil-fed stamping presses,
                 metalformers must focus efforts on feeding a properly
                 straightened coil into the die. Today, stock feed speeds can
                 reach 400 ft./min., but to reach these speeds, the coil must
                 be perfectly straight.                                           by Susheel N. Choubal


      he process of straightening, or      A Simple Explanation                        the line intersects the curve defines

 T    more precisely leveling, a sheet-
      metal coil requires passing it
 through a staggered arrangement of
                                               The basic principle of coil level-
                                           ing can be explained with a classical
                                           stress-strain curve for an elastic-plas-
                                                                                       the yield strength.
                                                                                           A test specimen loaded beyond
                                                                                       its yield strength, unloaded to show a
 rollers (Fig. 1). Roller straightening    tic material (Fig. 2). The elastic limit,   permanent strain of p, then reloaded
 bends the coil using the principle of       e, shown in the figure represents
 a rotating tool, producing alternate      the greatest stress that can be ap-
 bends in opposite directions during       plied to the test specimen without                Susheel N. Choubal is senior
 the process.                              causing permanent strain when that
                                                                                         manager in the production
    Although it sounds fairly basic,       stress is released. Stress beyond the
 shopfloor personnel need to clearly       elastic limit places the material in the      engineering department of the
 understand and appreciate this com-       plastic zone and causes permanent             Controlgear Division of the Elec-
 plex process and should therefore         deformation, shown by strain p on             trical Business Group of Larsen
 set the equipment based less on trial     release of stress.                            & Toubro Limited, Madh Works;
 and error and more on engineering             Because it’s difficult to accurately      e-mail snc-msw@msw.ltindia.
 principles.                               measure the elastic limit, engineers          com. Larsen & Toubro Limited is
    This article attempts to apply         express the start of plastic deformation      India’s largest multi-product
 basic engineering theory to explain       by using the yield strength, y. To            engineering conglomerate and
 the process of coil leveling, recom-      determine the yield strength, defined         its Electrical Business Group
 mend a procedure for leveler set-         as the stress producing a 0.2-percent         manufactures and markets a
 ting, set guidelines for leveler speci-   plastic strain, we draw a line parallel
                                                                                         wide range of low-tension
 fication and explain the coil-stock       to the straight portion of the stress-
 characteristics that result in optimum    strain curve and passing through the          switchgear products.
 leveling results.                         point of 0.2-percent strain. Where
18                                            www.metalformingmagazine.com                          MetalForming/January 2002
 will exhibit a higher yield strength,
                                                        Initial offset,
 indicated by y' in Fig. 2. This results
 from cold working, which increases
 the material yield point.

 Preset Curvature
     The process of coiling forms a
 preset curvature to the coil, which
                                                       Point A
 varies with coil cross-section and di-
                                                       (    0)
 ameter. In the example shown in                                          R
 Fig. 1, a typical point, such as point A,                                              Point B
 on the surface of the concave por-                                                     (     0)
 tion of the coil represents a maxi-
 mum value of strain within the mate-
 rial cross-section. This is the strain       Fig. 1—The coil-leveling process.
 induced during coiling, defined as
 negative (compressive) when the
 coil’s radius of curvature, R, points        the coil. The outermost fibers of the         depends on the correct estimation of i.
 upward, and positive (tensile) for           material are stressed into the plastic            Three observations are important
 point B, again on the lower portion          zone, as indicated by points 1 and 2          in order to understand the process
 of the coil, when its radius of curva-       in the figure. The springback strains         of coil leveling:
 ture points downward.                        from 1 and 2 are shown as positive,               • f is opposite in sign compared
     Referring again to point A in Fig. 1,    implying that the coil, at OD and ID,         to i;
 the maximum surface strain (as               emerges out of this set of rollers with           • Range of f is less than range of i;
 mentioned above) for a portion of            a downward curvature— f denotes                   •Range of f is closer to zero than
 the coil near the inside diameter (ID)       the new springback strain range.              range of i.
 will exceed the surface strain at a             A similar analysis made for the                These observations signify that,
 portion near the outside diameter            convex side of the coil must account          throughout the coil ID and OD, the
 (OD), since the ID undergoes more            for the residual stresses present in          first set of leveler rollers reverses the
 bending than does the OD.                    the coil (a topic beyond the scope of         direction of the preset coil curvature;
     In Fig. 3, the initial range of strain   this article). The two stress-strain          reduces the variation in curvature;
 in the coil before entry into the lev-       curves shown in Fig. 3 do not begin           and flattens the coil.
 eler—the difference between the              from zero stress due to differences in
 maximum values at the ID and OD              the residual stress at the coil ID and        What’s All of This Mean
 —is shown by i on the negative axis.         OD. The curves may start from the             in the Shop?
 The two stress-strain curves con-            two shaded circles whose position is             Several inferences can be made at
 structed to pass through the two ex-         indeterminate (Fig. 3).                       this point:
 tremes of this range i are similar in           Looking at the above complexities,            1) A leveler needs at least five
 form but horizontally shifted by the         plotting Fig. 3 poses a challenge and         rollers to function properly, al-
 distance i. The yield strength, y, at
 the ID is shown slightly higher than
 that at the OD, since the portion at
 the ID undergoes more cold working.
     Flattening a coil requires bending
 it in a direction opposite to the cur-           y'
 vature initially present. When the
 stresses release, the coil stays in a            y
 near-flat form. To reach this flatness,          e
 the straightener must bend the coil
 to stress the outermost fibers of the
 material cross-section beyond the yield
 strength and into the plastic region.
     As shown in Fig. 3, the first set of
 rollers in a straightener imposes a
 strain i at point A on the coil, de-
 fined earlier, opposite in sign to the
 strain initially present. The magni-
 tude of this strain depends on the
 geometry of the rollers, such as the
 center distance or pitch between the                                         y         p
                                                       0.002
 rollers, the vertical offset of the cen-
 ter roller, as well as the thickness of      Fig. 2—Typical stress-strain curve.
January 2002/MetalForming                       www.metalformingmagazine.com                                                        19
 Coil Straightening


 though if i could be varied con-
                                                                                                                   1
 stantly through some feedback con-
 trol system, to eliminate springback
 strain ( f ), a three-roller leveler
 would suffice. A five-roller leveler
 contains three sets of three rollers.                 y (ID)
 Bending and counter bending take                      y (OD)
 place in each set beyond the materi-                                                                              2
 al yield strength, and alternate sets            Surface strain
 tend to reduce the springback strain              at ID of coil
 and bring it towards zero until the
 coil emerges flat out of the last set of
 rollers.                                          Surface strain
                                                    at OD of coil
    2) The amount of bending is re-
 duced with each successive set of
 rollers, resulting in the coil inside
 the leveler physically taking the
 shape of a damping wave train. The
 damping wave train exhibits a grad-                                    i                       f              1
 ual reduction in the heights of the         Fig. 3—Effect of the first set of leveler rollers, as indicated on a stress-strain curve.
 peaks and valleys from the entry to-
 wards exit. Smaller offsets are neces-      beyond y at the outer fibers of coil         shape of a damping wave train as it
 sary for subsequent sets of rollers.        requires more bending for thin ma-           passes through the leveler. One pro-
 The gap between the upper and               terial than for thick material. The di-      cedure for accomplishing this:
 lower rollers in each set of rollers is     ameter of the straightening rolls also           Step 1: Set the gap between all
 adjusted to impart reducing spring-         needs to be smaller.                         sets of rollers wider than the coil
 back strains and decreasing its mag-           When working with thin materi-            thickness. Cut a piece of coil, prefer-
 nitude towards zero.                        als or when under stringent straight-        ably a portion nearer to the ID but
    3) The first bend must be in the         ening requirements, shops should             discarding the imperfections at the
 opposite direction from the initial         use more rollers (19 to 21) with a           tail (“coil breaks” that cannot be eas-
 bend. With a standard roller configu-       gradual decrease in offset to im-            ily removed), and insert one end be-
 ration (Fig. 1), insert the coil into the   prove control. There also is less            tween the first set of three rollers.
 leveling machine with its radius of         need for fine adjustments. The roller            Step 2: Adjust the first top roller to
 curvature pointing upward. If an op-        diameter, number of rollers and dis-         some prescribed offset ( ).
 erator inserts the coil with the radius     tance between them, with reference               Step 3: Lift the upper set of rollers
 of curvature pointing downward,             to material thickness and yield              and remove the coil to check for any
 the springback strain will increase in      strength, should be in specific pro-         slight kink being introduced in an
 the same direction as is initially pre-     portion for optimal results; this judg-      opposite direction of the initial cur-
 sent. This will increase the coil cur-      ment is best left to the equipment           vature of the coil. If the coil has a
 vature as it emerges from the first set     supplier.                                    kink, skip to Step 5; if not, proceed
 of rollers and reduce the effective-                                                     to Step 4.
 ness of the leveler.                        Leveler Setting                                  Step 4: Increase the offset and in-
    4) Materials with ultimate strength         All leveling machines include two         sert the coil again between the first
 close to the yield strength can prove       rows of leveling rollers, typically of       set of rollers. Lower the upper set of
 difficult to level, since stressing coil    the same diameter, arranged in off-          rollers and repeat Step 3.
 beyond the yield strength can cause         set formation relative to each other.            Step 5: Set subsequent top rollers
 surface cracks. In these cases, the         These upper and lower rollers gen-           with decreasing offsets. If top rollers
 metalformer must pay special atten-         erally arrange horizontally at some          are mounted on a swinging plate,
 tion when setting the first set of          fixed center distance. The upper             then only the first and last sets are
 rollers.                                    rollers, which sit precisely above the       adjustable. With a leveler containing
    5) Forming materials with a small        gaps between the lower rollers, are          19 to 21 rollers, adjust the gap be-
 modulus of elasticity, E, calls for use     adjustable in height. Each upper             tween the last set of rollers by an
 of a larger roller offset. A small value    roller can adjust separately, or all of      amount equal to coil thickness. In a
 of E results in a smaller slope of the      the upper rollers group together in a        five- to nine-roller leveler, the gap
 stress-strain curve, requiring a larger     block that can tilt and lower toward         between the last set of rollers can be
 strain to ensure proper stressing of        the lower rollers.                           narrower than the coil thickness.
 the material ( = /E).                          Proper setting of roller offsets en-          Step 6: Pass the coil through the
    6) Feeding thin material requires        sures that the coil will be stressed         leveler and observe it exiting. If the
 a larger offset and thinner roller di-      into the plastic region. As mentioned        coil curves up proceed to Step 7; if it
 ameter. Reaching a value of stress          previously, the coil should take the         curves down to Step 8; and if the
20                                              www.metalformingmagazine.com                              MetalForming/January 2002
 coil exits the leveler flat, the leveler          transducers to permit presetting the          5) Maximum vertical distance be-
 is properly set.                                  leveler to values known to be cor-         tween rollers
    Step 7: To adjust for the coil curv-           rect based on previous history. Most          6) Maximum material cross sec-
 ing upward: for a 19- to 21-roller lev-           precision roll levelers have dial indi-    tion at a specified yield strength
 eler, decrease the offset at the entry            cators used to set the exact depth of         7) Material thickness range
 side; for a five- to nine-roller leveler,         engagement of the upper leveling              8) Running-through speed range
 decrease the offset at the exit side.             rolls at the front and rear ends. This        9) Connected load
 Pass the coil further inside the level-           permits precise adjustment, repeata-          10) Equipment weight.
 er until a sufficient portion of it exits         bility and accuracy.                          As a good practice, the metal-
 from under the roller that has been                  Sometimes, the adjusting mecha-         former should follow the recom-
 adjusted. Repeat Step 6.                          nism may be power driven, which            mendations of the press and die
    Step 8: To adjust for the coil                 facilitates computerized control from      manufacturer in selecting a leveler.
 downward, increase the offset at the              a remote location. An arrangement
 entry side, for all types of levelers.            having linear position transducers         Coil Material Requirements
 Pass the coil further inside the level-           can permit remote readout of the               To optimize results with leveling
 er until the portion of it exits from             roll setting.                              equipment, strive to purchase material
 under the roller that has been adjust-                                                       with a constant cross-section, uniform
 ed. Repeat Step 6.                                Leveler Selection & Specification          properties and tight thickness toler-
    Near its end, the coil may curve                  Levelers come in many sizes,            ances. The metalformer can recommend
 downward, since this part of the coil             force capacities and degrees of pre-       a lower limit on the coil ID, Dmin,
 undergoes maximum strain. If this                 cision. They can straighten coiled         achieved during the coiling process.
 occurs, the metalformer must further              material to 1 in. thick, or more, and      The initial coil strain imposed during
 increase offset at the entry side. As             more than 3 ft. wide, as well as nar-      coiling should not significantly exceed
                                                                                              the elastic limit, e since this would
                                                                                              lead to plastic deformation and perma-
   Material                      Yield strength,                                              nent curvature in the coil. This im-
                                                   y (psi)   Modulus of elasticity, E (ksi)
                                                                                              plies that the maximum stress in the
                                                                                              coil near ID should not exceed e.
   EC grade Aluminum, (Annealed)         2,500                         10,000
                                                                                                  Since the more deterministic prop-
   ETP grade Copper, (1⁄2 hard)         36,000                         17,000                 erty—yield strength, y—has been
   5% Sn Phosper bronze, (1⁄2 hard)     55,000                         16,000                 used in place of e all through the
   Mild steel                           25,000                         30,000                 discussion, the relations y = (0.002 +
   Silicon steel                        42,000                         30,000                   y/E) and y = t/Dmin can be used to
   Brass, half hard                     62,000                         14,500                 determine Dmin. The two relations can
                                                                                              be equated to get Dmin = t/(0.002 +
   Spring steel                         67,200                         30,400
                                                                                                y /E). The coil ID should exceed
                                                                                              Dmin. As an example, for a 1⁄2-hard
 Fig. 4—Approximate yield strength and modulus of elasticity.                                 brass coil of 2 mm thickness, Dmin
                                                                                              computes to 320 mm.
 recommended in Step 1, cut a por-                 row strips of material less than 0.08          The above formula theoretically
 tion of the coil near the ID when set-            in. thick. Better control of the           excludes the possibility of forming a
 ting the leveler.                                 process results as the number of           preset curvature during coiling. In
    Avoid passing the same portion                 rollers increases from five to seven,      practice, many other complex phe-
 of the coil through the leveler more              nine, 11 and so on. Powered preci-         nomena that occur during coiling
 than twice during setting. Too many               sion levelers get the call for handling    should be considered, such as the
 passes of the same portion of coil                thin materials or tackling stringent       effects of tension applied during
 will influence the material properties            straightening requirements.                coiling, cold or hot rolling, and tem-
 and result in an incorrect setup.                    Before selecting a leveler, the         pering, which can cause residual
 Some motorized levelers can be run                metalformer should consider the coil       stresses and lead to preset curvature
 in reverse, for cleaning—don’t run                material, width and thickness ranges.      of the coil. Also, most steel mills use
 the machine in reverse during setup.              He should match the speed range of         a single mandrel for steel coils on
                                                   the leveler to the production rate of      the temper mill and cannot change
 Rapid Setup                                       the stamping press.                        the minimum ID once the equipment
    A number of leveler features                      Typical leveler specifications          is built—hence the need for coil lev-
 allow the metalformer to dramatical-              given in a manufacturer’s catalog          eling before subsequent processing.
 ly reduce setup time, by allowing                 will include:                                  Fig. 4 shows the reasonable esti-
 rapid adjustment of roll depth. Look                 1) Number of leveling rollers           mates of y and E for some common
 for levelers that have adjusting                     2) Diameter of rollers                  coil materials, for ready reference.
 mechanisms with built-in position                    3) Width of inlet                       Use them to calculate the minimum
 scales, turn counters or position                    4) Least count of machine adjustment    coil ID.                            MF
January 2002/MetalForming                            www.metalformingmagazine.com                                                   21

								
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