Abstract:
To enable safe coiling of very thin metals at high speed, the invention discloses a coiler comprising a mandrel of considerably smaller diameter then prior art mandrels, enabling said mandrel to be directly driven by an electric motor without using reduction gears. The coil outer diameter is also reduced, the result being a coiler having far lower polar moment of inertia than prior art coilers, therefore greatly reducing the incidence of tension errors and strip breaks. The mandrel is constructed specifically for operation with a spool upon which the coil is wound, and incorporates the novel feature of concentric expansion using a plurality of circumferentially oriented wedges, which ensures uniform support and grip of the spool bore, holding said spool and coil concentric with the axis of the mandrel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Prior art coilers, used for many years for coiling metal strip in cold rolling mill installations can generally be divided into two types—firstly, those having collapsible mandrels, and secondly, those having non-collapsible mandrels also known as solid block mandrels. These types are well known in the art. Collapsible block mandrels are designed to operate in the expanded position, where the outside diameter is fixed at a value which is somewhere in the range 400 mm (small, narrow rolling mill installations) up to 750 mm (large, wide rolling mill installations), the most common diameters being 508 mm (20 in.) and 610 mm (24 in.). After completion of rolling, the mandrel is collapsed to a smaller diameter, enabling the coil to be stripped, that is removed from the mandrel by transferring it in an axial direction until it clears the mandrel. The mandrel typically consists of internal pyramid shaft  61 , said shaft being keyed to segments  63 , which form the outside cylindrical shape of the mandrel. As is shown in  FIG. 1 , the mandrel is typically expanded and collapsed by axially shifting said pyramid shaft relative to said segments  63  using a hydraulic rotating cylinder  64 , mounted at the back end of the mandrel drive shaft. For narrow strip the mandrel is cantilevered out from a drive shaft, but for wider strip, a third bearing, known as the outboard bearing  62  is used to support the mandrel at its front end during coiling. The support structure for the outboard bearing can be retracted to enable the coil to be stripped when coiling is complete. 
         [0002]    When thin gauge strip (less than about 0.5 mm thick) is to be coiled on a collapsible mandrel, it is essential to use a spool (a hollow cylinder usually made of steel) inside the coil to avoid damaging the inside of the coil. The spool inside diameter must be just less than the expanded diameter of the mandrel so that the mandrel will grip the spool when the mandrel is expanded. Then the coil is wound on the outside of the spool. Eventually, at a subsequent process such as annealing or slitting, the coil must be unwound from the spool, enabling the spool to be returned to the rolling mill installation for use with another coil. 
         [0003]    Solid block mandrels, as the name implies, are made from a solid block of alloy steel. They consist of a cylindrical body with an integral reduced diameter portion (a neck portion) at each end, upon which radial bearings are mounted. At the outer end of each neck piece a half coupling is fixed. A grooved wheel is mounted on the outside of each radial bearing. 
         [0004]    In the working position of the coiler, the wheels are clamped in position upon a base and a sliding coupling is used to connect the mandrel to a drive shaft. The drive shaft is mounted on two radial bearings, similarly to the drive shaft of the collapsible bearing mandrel. Solid block body diameter range is similar to that for collapsible mandrels. 
         [0005]    Spools are never needed with solid blocks—even very thin strip can be coiled on a solid block. However, after coiling it&#39;s necessary to rewind the strip from the solid block to a collapsible mandrel, since rewinding is the only way to remove a coil from a solid block. Usually a mill installation having solid block coilers will include equipment for rewinding the coils, at low tension, to a collapsible rewinder mandrel, using a light spool at the rewinder in the case of thin strip. 
         [0006]    In virtually all cases, the drive to mandrel of both solid and collapsible varieties is via a variable speed electric motor  65 , traditionally D.C., but more commonly A.C. today, and through reduction gears  66 , the drive shaft described above being the output shaft of the reduction gear box, the input shaft  67  of said gear box being directly coupled to the drive motor through drive spindle  68 . The reduction gears, usually having a ratio of 3:1 or more, are needed to achieve the required coiling speeds and torques using a motor having standard speed range. 
         [0007]    In general, high strength materials such as stainless steels cannot be rolled to a strip thickness less than around 25 microns (0.001″) unless a very small Sendzimir mill having work rolls less than 25 mm in diameter is used. Such mills only exist in widths of around 250 mm or less. Because such materials must be rolled with very high tensions applied to the strip, extremely high compressive stresses are developed in the mandrel around which the strip is coiled, and the preferred solution is a solid block mandrel, which is best able to withstand such stresses. The strip subsequently has to be rewound at low tensions to remove it from the solid block mandrel. 
         [0008]    For coiling softer materials such as aluminum and copper at very thin gauges, it&#39;s possible to use collapsible mandrels using spools. As the strip tensions used when coiling these materials are relatively low, then spools made from plain carbon steel, relatively thin walled and so relatively inexpensive, are able to withstand the compressive forces applied by the coil as it is wound on the spool. The spools can even be used for shipping the coils so rewinding can be avoided in many cases. These materials can be rolled down to foil gauges of 25 micron and less using larger work rolls of 50 mm in diameter or more and so can be rolled on many standard Sendzimir mills at widths up to about 750 mm. 
         [0009]    The problem when rolling softer materials at very light gauges of 150 microns and below, is that the strip tension when coiling must be maintained at a very steady low value at all times yet the tension stress in the strip must be quite high to achieve good strip flatness and to get the desired reductions. This includes (a) during speed-up and slow-down of the mill at the ends of the coil and (b) during operation of the mill screwdown to adjust the thickness or elongation of the strip in the mill. The very light gauge strip is very fragile and prone to break if the tension is not held steady. Using the prior art coiler designs described above, there are several factors which can induce unacceptable tension fluctuations, and thus cause the strip to break.
       1. The high polar moment of inertia of all the rotating parts includes mandrel, drive shaft, rotating cylinder, driven gear, drive gear, input shaft, input coupling and drive motor armature. This inertia can give rise to large tension fluctuation as the speed changes. Note that the polar moment of inertia of the latter four items must be multiplied by the square of the gear ratio to find their equivalent values at the mandrel.   2. Because the mandrel is not perfectly round, and/or if the spool is not perfectly cylindrical, or there is some variation in spool wall thickness, some cyclic variation in strip speed will occur, and the resultant tension variation will be proportional to the inertia of the rotating parts.   3. Because the total weight of the rotating parts is high, even when using anti-friction bearings the friction in the bearings will cause significant tension errors in the strip.   4. Because of large sizes and large number of rotating elements, there will be strip tension errors due to windage, which will vary with the speed of rotation, and will therefore vary with both coiling speed (m/s or ft/sec) and coil diameter.       
 
       OBJECT OF THE INVENTION 
       [0014]    The object of the invention is to achieve a coiler design for very thin gauge metal strip which overcomes the drawbacks of prior art systems by (a) greatly reducing the polar moment of inertia of the rotating members, (b) gives a collapsible mandrel designed for operation with spools, which maintains a true circle outer diameter throughout the expand/collapse range in order to ensure concentricity of coil, spool and mandrel and (c) provides for measurement of strip tension without the use of pass line rollers. These improvements minimize strip tension variation during coiling, and thus enable coiling at higher speeds with reduced risk of strip breaks. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0015]      FIG. 1 : shows a prior art mandrel, gear reducer and drive motor in plan view, drawn with front end of the mandrel at the left side. 
           [0016]      FIG. 2 : shows one embodiment of a coiler consisting a mandrel, drive shaft and drive motor in plan view, according to the present invention, drawn with front end of the mandrel at the left side. 
           [0017]      FIG. 3 : shows partial side elevation and section of a mandrel according to the present invention, drawn with front end of the mandrel at the left side, and the back end of the mandrel at the right side. 
           [0018]      FIG. 4 : shows a partial end elevation section of a mandrel as viewed from the line  4 - 4  of  FIG. 3 . 
           [0019]      FIG. 5 : shows some scrap sections taken from  FIG. 4 . 
           [0020]      FIG. 6 : shows an enlarged view of the stave return spring assembly in  FIG. 3 . 
           [0021]      FIG. 7 : shows a longitudinal section of a mandrel according to the present invention, drawn with front end of the mandrel at the left side, and the back end of the mandrel at the right side. 
           [0022]      FIG. 8 : shows a transverse section of a mandrel according to the present invention. 
           [0023]      FIG. 9 : shows a longitudinal view of mandrel shaft, oil feed ring and axial retention ring and outboard support bearing of a mandrel according to the present invention, drawn with front end of the mandrel at the left side, and the back end of the mandrel at the right side. 
           [0024]      FIG. 10 : shows a view taken from the rear of the mandrel, showing the rear axial retention structure. 
           [0025]      FIG. 11 : shows a transverse section of a mandrel at the front end plate, with the mandrel in a partially expanded condition, gripping the inside diameter of a spool. 
           [0026]      FIG. 12 : shows a transverse section of a mandrel at the front end plate with the mandrel fully expanded. 
           [0027]      FIG. 13 : shows a transverse section of a mandrel at the front end plate, with the mandrel fully collapsed. 
           [0028]      FIG. 14 : shows a radial cross section of a mandrel having linear wedge surfaces replaced by curved wedge surfaces to give improved operation and extended life. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Because this coiler is designed for very light gauges in relatively soft materials only, spools  106  ( FIG. 1  and  FIG. 2 ) will always be used. In the embodiment shown in  FIG. 2 , the mandrel is designed to engage with a spool having an inside diameter of 12 in. or 305 mm. This is much smaller than the prior art diameter range of 508-610 mm, and thus can be directly driven by an electric motor having a standard speed range. The mandrel has to expand tightly against the inside of the (nominally cylindrical) spool. Depending upon the material of the spool and the tension stress applied to the strip as it is coiled, the spool outside diameter would be in the range 330-350 mm. To avoid the need for excessive drive torque and thus to enable a standard drive motor to be used, the maximum coil diameter is limited to around 600 mm. Even so, the lengths of light gauge coils at this diameter are sufficient to ensure high productivity. 
         [0030]    By using such small mandrel and coil outer diameters, ifs possible to use direct drive between motor and mandrel as shown in  FIG. 2 . The combination of these reduced diameters and elimination of reduction gears leads to an order of magnitude reduction in polar moment of inertia of mandrel, coil and drive relative to the prior art. It should be noted here that an outboard bearing  51  and retractable support structure according to the prior art may be used to support the front end of the mandrel. 
         [0031]    Since there is always a spool, and it&#39;s not necessary to support the inside diameter of the spool continuously around its circumference we support the spool only at a finite number of points. In the embodiment shown in the drawings, we support the spool by a total of 24 identical support members (staves)  14 , which are incorporated in the mandrel, and are equally spaced around the circumference of the mandrel, approximately 40 mm apart as seen in  FIG. 4 . This provides sufficient support for a spool 17.5 mm thick (outside diameter 340 mm). The staves are spaced apart and guided by cage  13  in which 24 equally spaced axial slots are machined, into each of which a stave  14  is mounted. Said slots constrain said staves to permit only movement in a radial direction relative to said cage. It is also possible to use fewer than 24 staves and slots. This simplifies and strengthens the structure, but may require a greater spool thickness because of the resulting increased spacing of the staves. In no case would fewer than 8 support members (staves) be adequate. 
         [0032]    An inner sleeve  12 , mounted on and keyed to drive shaft  11  by keys  16  and  17  ( FIG. 7 ), incorporates 24 identical wedges  33 , said wedges being equally spaced around the outside circumference of said inner sleeve. These wedges, unlike the expansion wedges of all prior art mandrels which are oriented in the axial direction, are oriented in a circumferential direction and form the outer surface of said inner sleeve. Return springs  15 , one mounted in each end of cage  13 , hold each stave  14  firmly inwards against its mating wedge  33 . Each of said return springs is secured in said cage using a special threaded retainer plug  40 , as seen in the enlarged view of  FIG. 6 . 
         [0033]    It is envisaged that standard construction materials would be used for the mandrel, such as steel for shaft  11 , end plates  18 , 18   a , oil feed ring  19  and axial retention ring  34 , inner sleeve  12  and cage  13 , and other materials such as bronze or ductile iron for the staves  14 . 
         [0000]    The expand/collapse mechanism is as follows: 
         [0034]    As can be seen from  FIG. 4 , if inner sleeve  12  is rotated in a counter-clockwise circumferential direction relative to cage  13  and staves  14 , said staves will ride up wedges  33  in a radial direction and increase the outer diameter of the mandrel, this diameter being that of the circle that joins or circumscribes the outer ends of the 24 staves  14 . This is the expansion mechanism. 
         [0035]    Because both staves and wedges are equally spaced around the mandrel axis, the radial movement of the staves will be synchronized such that said outer diameter is always concentric with the said mandrel axis, and will grip and support the inside diameter of said spool to hold said spool concentric with said mandrel axis. 
         [0036]    If inner sleeve  12  is now rotated in a clockwise circumferential direction relative to cage  13  and staves  14 , with return springs  15  holding staves  14  firmly down on wedges  33 , said staves will ride down said wedges and reduce said outer diameter. This is the collapse mechanism. 
         [0037]    Note that the total relative rotation of said inner sleeve relative to said cage and said staves, from fully collapsed to fully expanded condition is 10 degrees. 
         [0038]    In  FIG. 4 , and  FIGS. 11-13 , the expansion wedges are shown with a straight profile, necessitating a curved profile on the inner ends of the staves because the wedges are rotating relative to the staves, and thus the wedge angle relative to the staves changes as the mandrel is expanded. 
         [0039]    It&#39;s also possible to use a curved surface of the wedges and matching curvature of the inner ends of the staves, in order to improve the uniformity of pressure distribution over the contact area between each stave and its mating wedge, as is described later in this specification. 
         [0040]    If torque is applied in a clockwise direction to the cage and staves in the drawings, by the application of strip tension to the coil mounted on the spool which contacts the 24 staves, this tension torque will urge the staves to ride up the wedges and so increase the pressure between staves and spool inner bore, thus tightening the grip of the staves on the spool. The mandrel thus has a self tightening action. Such an action is new in the art. Since, on a coiler, the tension torque always acts in one direction, regardless of the direction of rotation (i.e. regardless whether the coil is being wound or unwound) this self-tightening action is always present. 
         [0041]    Clearly, for a reversing mill having a coiler at the left side and a coiler at the right side of the mill stand(s) in a symmetrical arrangement, the tension torque direction, either clockwise or counter-clockwise, on the left hand coiler will be opposite from that of the right hand coiler. The same major components can still be used for both coiler mandrels, because inner sleeve  12  and staves  14  will simply be assembled back to front in one mandrel relative to the other in order to reverse the orientation of the wedges and staves. In this way both mandrels will be self-tightening, even though the tension torque on one mandrel always acts in a clockwise direction, and the tension torque on the other mandrel always acts in a counter-clockwise direction, when viewed from the front of the mandrel in each case. It follows that the cross section shown in  FIG. 14  will be valid for one mandrel viewed from the front and for the other mandrel viewed from the back. 
         [0042]    The arrangement shown in the embodiments of  FIG. 4 ,  FIG. 8  and  FIGS. 11-14  is suitable where the tension torque direction is clockwise when viewed from the front (as is the case with these drawings). This would be the left coiler, if over-winding, or the right coiler, if under-winding. 
         [0043]    In fact, even though this mandrel is self-tightening, it may be necessary to provide additional force when expanding the mandrel, in order to apply sufficient radial force to the bore of the spool by the staves  14 , so that the spool will not slip on the staves while full strip tension is applied to the outside of the coil. It&#39;s also necessary to expand the mandrel until said mandrel grips the bore of the spool initially. The initial expansion and additional force are provided by hydraulic expand cylinders, as follows: 
         [0044]    At both front and back ends of the mandrel, respective front end plate  18  and back end plate  18   a  are provided, said end plates being attached to cage  13 , one at each end of said cage, by axial cap screws  30  as shown in  FIG. 3 . These end plates are mounted and keyed to shaft  11  and have the primary function of axially locating inner sleeve  12  relative to said cage, and locating said cage concentric with shaft  11  and inner sleeve  12 , and also of applying additional expansion force, which they apply by means of hydraulic pistons  21  which slide within hydraulic expand cylinders  22  bored into said end plates and bear against lugs  20  which project into recesses  28  in end plates as shown in  FIGS. 11-13 . Note that said hydraulic expand cylinders are enclosed by plug  41  screwed into end plate  18  or  18   a , hydraulic oil being supplied through feed holes  23 ,  24 ,  25 ,  26  at the front, and feed holes  38 ,  37  and  26  at the back, as shown in  FIG. 7 . When pressure is applied to said hydraulic expand cylinders, pistons  21  press against lugs  20 , which are formed at each end of inner sleeve  12  (two lugs at each end) in order to rotate said inner sleeve relative to said end plates and cage  13 . This causes the staves  14  to ride up the wedges on inner sleeve  12  and thus expand the mandrel and push the staves tightly against the bore of the spool on which the coil is to be wound, as shown in  FIG. 11 . 
         [0045]    Return springs  29  acting through plungers  39 , in line with said hydraulic expand cylinders, serve to collapse the mandrel and to retract said pistons prior to stripping the coil. This they do by pressing said plungers against lugs  20  thus rotating the inner sleeve to collapse the mandrel when pressure is released from said hydraulic expand cylinders, as shown in  FIG. 13 . (It is also possible to incorporate hydraulic collapse cylinders to collapse the mandrels, instead of using return springs). 
         [0046]    It should be noted that the outside diameter of the mandrel, formed by the circle circumscribing the outer ends of the staves, remains circular and concentric throughout the expand/collapse range of the mandrel, unlike the prior art mandrel of  FIG. 1  and all other known prior art mandrels, which are only circular at one diameter (usually the expanded diameter). 
         [0047]    Axial retention ring  34  is provided at the back to secure the mandrel assembly to the drive shaft  11  as shown in  FIGS. 7 and 9 . Said axial retention ring is bolted to the back end plate  18   a  by cap screws  31  which clamp the assembly on to shoulder  27  on said drive shaft. 
         [0048]    Oil feed ring  19  is provided at the front, attached to front end plate  18  by cap screws  31  and incorporates oil holes  23  and  24 , to feed hydraulic oil from the axial hole  26  in the drive shaft, via radial holes  25  in the drive shaft, to the hydraulic cylinders (2) in end plate  18 . O-ring seals  32  seal the oil as it passes from the shaft into oil feed ring  19 , and o-rings  36  seal the oil as it passes from oil feed ring  19  to front end plate  18 . Axial hole  26  ends in a port at one end of shaft  11 , the port being located at the end of said shaft from which said hole was drilled, either at front or back of said shaft and hydraulic oil is supplied to this port using a commercial rotating coupling of prior art form. 
         [0049]    At the back of the mandrel drive shaft  11  incorporates shoulder  27 , to which the back end plate is secured using axial retention ring  34 , bolted to back end plate  18   a  by screws  31  as described above. The hydraulic oil is delivered to the back cylinders (2) via axial hole  26 , radial holes  37  (2) and axial holes  38  which connect within back end plate  18   a  to the hydraulic cylinders similarly to the connection at the front, O-ring seal  36  in this case sealing the hydraulic oil as it passes from the shaft directly to end plate  18   a . It should be noted here that the back end plate  18   a  is essentially similar to front end plate  18 , but is to the opposite hand, since its internal hydraulic expand cylinder pistons  21  must rotate the cage in the same direction as the corresponding pistons in end plate  18 , when viewed from the front of the mill, which is the opposite direction when viewed from the back of the mandrel. 
         [0050]    As shown in  FIGS. 7 and 9 , drive shaft  11  and inner sleeve  12  are keyed together using axial keys  16  and  17 , these keys being a press fit in the keyways in shaft  11  and a slip fit in inner sleeve  12 . A clearance slot  40  is machined in the inner sleeve bore joining the keyways at each end to clear the keys at assembly. The keys  16  and  17  also pass through keyways  35  in end plates  18   a  and  18 . These keyways are extra wide and serve to limit the rotation of inner sleeve  12  and shaft  11  relative to end plates  18  and  18   a  and cages  13  as the mandrel is expanded and collapsed. This avoids the possibility of damage to the staves if they jam into the end faces of the wedges (if the mandrel is collapsed too much) or slide off the top of the wedges (if the mandrel is expanded too much). As shown in  FIG. 8  the side clearances between keys  16 ,  17  and keyways  35  limit the rotation to plus and minus five degrees, or a total of ten degrees between fully collapsed and fully expanded conditions. 
         [0051]    Normally, as shown in  FIG. 11 , when expanding the mandrel, the stroke of expand piston  21  will be limited by the staves stalling against the bore of spool  106 . However, if the spool bore is too large, or if the mandrel is expanded without a spool in place, keyways  35  provide a very useful function in limiting the expansion stroke, as described above. 
       Quick Change and Pre-assembly 
       [0052]    It can be seen from  FIG. 7  that, if the outboard bearing support  52  is retracted, then it&#39;s possible to remove the mandrel assembly from shaft  11  by releasing cap screws  31  at the back and sliding said mandrel assembly (except for axial retention ring  34 ) towards the front end and off said shaft. If a spare mandrel assembly is available, it can be slid from the front end onto shaft  11  and secured in place by replacing cap screws  31  at the back end. This enables quick change of mandrels. The structure also lends itself to pre-assembly, using a dummy shaft (which could be a length of steel pipe mounted with axis horizontal mounted on a welded steel stand) in the maintenance shop. After pre-assembly of all components, except axial retention ring  34  and back cap screws  31 , on to the dummy shaft, the assembly will be ready for quick installation at the mill. 
         [0053]    In  FIG. 2  the remainder of the coiler assembly is shown. At the back end of the mandrel the drive shaft  11  extends back through and is supported by two bearings  54  and  55  mounted in fixed pedestals  56  and  57  respectively. The rotating portion  60  of a torque meter is keyed to the back end of shaft  11 , and a coupling  58 , keyed to the back end of said rotating portion  60  of said torque meter and to the shaft of electric motor  59 , couples the said drive shaft to said motor. 
         [0054]    In  FIGS. 4 ,  5  and  8  and  11 - 13  the wedges  33  incorporated in inner sleeve  12  are shown with the classic linear form, for the sake of simplicity. This is not the preferred form, as  FIG. 5  illustrates.  FIG. 5  shows two views of top stave  14 , a wedge  33  and parts of cage  13 , which can be compared with the corresponding view of these parts in  FIG. 4 . In the left view, the inner sleeve and wedge have been rotated clockwise relative to cage and stave in order to collapse the mandrel fully. This rotation causes the effective angle of the wedge to drop from 10 degrees to 4.26 degrees (rotation angle=10−4.26=5.74 degrees). In the right view, the inner sleeve and wedge have been rotated counter-clockwise relative to cage and stave in order to expand the mandrel fully, and this rotation causes the effective angle of the wedge to increase from 10 degrees to 15.30 degrees (rotation angle=15.30−10=5.30 degrees). Since the bottom surface of the stave is machined at the 10 degree angle, full area contact between wedge and stave will only be achieved at the mid-stroke position as shown in  FIG. 4 . In this situation it would be advisable to machine a convex crown on the inner end face of the staves to avoid point contact. Even so, only partial contact would be achieved so stress in the contact area between wedge and stave would be high, and wear on these parts would be high and uneven. 
         [0055]    Furthermore, the ability of the mandrel to grip the spool would depend upon the inside diameter of spool  106 . If said inside diameter was such that the staves gripped the spool at mid-stroke, the effective angle of the wedge would be 10 degrees. However, if said inside diameter were larger, such that the staves gripped the spool close to the end of the expansion stroke, the effective wedge angle would be close to the maximum of 15.3 degrees. This would not be conducive to obtaining a tight grip between mandrel staves and the spool, because this angle is too high. 
         [0056]    The preferred form of the wedge surfaces is shown in  FIG. 14 . This figure is a transverse section of the mandrel, showing cage  13 , staves  14 , inner sleeve  12  and wedges  33 . In this figure the topmost stave and its mating wedge is examined as before. Although the nominal wedge angle is set to 10 degrees as before, the wedge surface is made cylindrical and convex in form, the cylindrical profile having its center located on the diameter line normal to the radius at the centerline of the stave, as shown. The inner (bottom) surface of the top stave  14  is machined with a matching concave cylindrical surface, this profile also having its center in the same location as the center of the wedge profile. In this case, as the inner sleeve is rotated relative to said cage and said end plates to raise and lower the wedge profile (and hence expand and collapse the mandrel), the motion of the wedge profile, if small, will be close to a pure radial (vertical) motion. Since the motion of the stave is radial due to its constraint by the radial slot in cage  13 , virtually 100% area contact between wedge and stave is achieved as the inner sleeve is rotated relative to said cage and said end plates. Of course the top stave examined is typical of all 24 staves, which are arrayed around the mandrel as shown, with equal 15 degree spacing. Note that said relative rotation is limited to plus and minus 5 degrees (10 degrees total) by the clearance provided between keys  16 ,  17  and keyways  35  in end plates, as shown in  FIG. 8 . 
         [0057]    In fact, during the initial part of the expansion stroke, before the outer ends of the staves contact the bore of spool  106 , there is negligible pressure and therefore zero wear on stave and wedge contacting surfaces, even if 100% contact is not achieved. For this reason, the mandrel is designed so that, at the point where the stave outer ends contact the spool bore, the inner sleeve has rotated about 50% of its expansion stroke as shown in  FIG. 11 . At this point, the radial forces will be the highest, both before and during winding of the coil. This is the point at which the centers of curvature of each wedge and mating stave lie exactly on the diameter line normal to the stave center line, as shown in  FIG. 14 . Therefore for small excursions of rotation of inner sleeve and wedges about this point, the direction of motion of both wedge profile and stave is purely radial, and 100% contact between each wedge and its mating stave is maintained. In service such excursions will be tiny, and will be outwards in case of outward elastic deflection of spool when mandrel is expanded inside an empty spool and in case of radial wear of spool bore, and inwards in case of inward elastic deflection of spool when coil is fully wound. 
         [0058]    It should also be noted here that, by using mating cylindrical surfaces on wedges and staves as described above, the effective wedge angle remains at 10 degrees throughout the working expand/collapse stroke, and does not vary as was the case with the linear wedge profile. 
         [0059]    It should be further noted that the ten degree wedge profile angle that we have used in these embodiments was selected purely for the sake of clarity. It&#39;s possible to use greater or smaller angles depending upon the specific application. A larger angle gives a bigger radial expansion stroke for a given rotation stroke of the inner sleeve, but less radial force on the spool bore, and therefore less ability for the staves to grip the spool bore when high strip tensions are used. A smaller angle gives less radial expansion stroke (and thus less radial clearance between spool and mandrel when mandrel is collapsed when mounting spool or removing spool, and coil) but enables higher grip forces to be generated between staves and spool bore. 
         [0060]    To achieve high accuracy in tension control, the motor should be designed for minimum moment of inertia and can be either A.C. or D.C. type, but must be driven at variable speed in order to maintain the correct tension in the strip as the coil builds up (during winding) or gets smaller (during unwinding) and the torque regulation system must be very fast acting. The motor bearings should be low friction type, either hydrostatic or ball or roller bearing type and should be sealed using labyrinths rather than rubbing seals. Similarly, drive shaft support bearings  54  and  55  and outboard bearing  51  should be sealed using labyrinths to avoid friction drag losses. 
         [0061]    It is possible to use either conventional strip tensiometers, which measure strip tension by measuring the force on a deflector roll around the strip passes as said strip travels between rolling mill and coiler, or to use a torque meter of the bearingless variety, such as the MRCT 86000V series manufactured by the S. Himmelstein and Company of Hoffman Estates, Ill. to measure the torque applied by the drive motor to the coiler mandrel, from which the strip tension can be calculated. This is shown in  FIG. 2 . Said torque meter consists of shaft mounted portion  60  and a stationary portion mounted underneath said shaft-mounted portion, and so hidden from view in  FIG. 2 . 
         [0062]    Other devices such as an encoder mounted at the back of the drive motor to measure motor speed and to count the number of wraps on the coil would be according to prior art.