Hitherto, one known method of providing tension is to press against opposite sides of the strip by the use of drag pads in the form of wooden blocks or other rigid support structure wrapped in felt, carpet or other replaceable friction material. A disadvantage of this known method is that dirt particles accumulate on the pads and these sometimes cause scratch marks in the finished strip. In addition, the pressure of the friction pads is often sufficiently high to cause marking or buffing of the strip surface. The foregoing are particularly serious disadvantages because of the modern trend to provide material direct from the mill having a high quality ornamental finish.
Further disadvantages of this known method are that maintenance and running costs are high. More specifically, the friction material has to be replaced frequently, cleaning of the friction material takes up to 20% of total operating time, noise levels are extreme when certain materials are processed, and lubricating oil is used in a not wholly successful attempt to alleviate the noise and buffing problems.
Another known method of providing tension uses treaded bridle-rolls but this method also involves contact with both sides of the strip and can result in colour imprinting from one coil to the next.
Further disadvantages of this method are that it has a high capital cost and the time taken to remove, clean and replace the treaded rolls is of the order of an hour and represents up to 20% of total operating time.
Yet another known method uses the linear motor principle to exert a drag force. This method is particularly appropriate for non-ferrous metals, but has been applied to tensioning steel. Some of the disadvantages of this method are that tension depends strongly on strip speed, and, if the line stops, the strip immediately over the drag stand will heat up quickly to a temperature well in excess of 150.degree. C.
Finally, a method disclosed in U.S. Pat. No. 2,433,014 (RENDEL) uses an electromagnet with a drag facing to provide a small leading tension for a drag stand which consists essentially of a bridle roll system. This method has the major disadvantage of not being able to tension each strand separately, and so is completely unsuitable for tensioning slit strips. Further disadvantages are that the rolls touch both sides of the strip, about 3 m of line length would typically be taken up, and cleaning the rolls would be time consuming.
The tensioning methods of the present invention, as described hereinafter, can be advantageously applied for the coiling of strip where no slitting step is involved but embodiments of the invention are especially advantageous where the more demanding requirements of coiling slit strips exists.
For the coiling of metal strip in a commercial plant, it is known that a drag force of about 3 kN/m width of strip is required to back tension the strip. The tension must be sufficient to ensure that the coils, which are normally wound onto a mandrel, do not collapse during subsequent handling after the mandrel has been withdrawn from the bore of the complete coil. Coils are normally handled by inserting a hook or similar device into the bore. If the coil has been formed without sufficient back tension, then the bore can partially collapse by changing shape from the initial desirable cylindrical shape to an elliptical or kidney shape and then, to permit further normal handling of the coil, it must be restored to the desired shape. This is a difficult, time consuming and a costly task. The alternative is the even more costly option of scrapping the coil.
Coil collapse due to inadequate coiling tension is thought to be due to the fact that the wraps of the coil are not tightly bound to each other by frictional forces and, consequentially, the wraps can slip under the action of the inter-wrap forces generated by the weight of the coil, with the result that the coil will have an elliptical bore. It is considered that each wrap has to support itself in isolation from the other wraps, but since normally the strip is too thin to have sufficient stiffness for a wrap of typical diameter to maintain an approximately cylindrical shape, it is necessary to rely on the friction forces generated when the coil is formed to retain the cylindrical shape. Thus, acute problems arise when the coiling tension is insufficient.
However, if the coiling tension is too high, then a different form of coil collapse can occur. If the coiling tension is too high, then the accumulated pressure exerted by the outer wraps will cause the inner wraps to be forced into compression. If the compressive forces are too high, then the inner wraps will not be able to resist the tendency to buckle inwards. Buckling may be initiated at inhomogeneities in the bore, caused by the recoiling mandrel deviating from a perfect cylinder, or initiation may be due to impact forces which occur during coil handling. In either case, the buckle gradually creeps through the wraps of the coil, resulting in a kidney shaped bore.
Coiling tension must be high enough to prevent the first form of collapse, but not so high as to cause the second form.