This invention relates to an apparatus and method for forming pipe from steel plates and sheets, and more particularly relates to an efficient relatively low cost apparatus for forming long sections of large diameter steel pipe from relatively thick gauge steel plate.
The pyramid roll forming process is one of the earliest commercial methods for forming steel plates into large diameter pipes. The U-O process is a frequently used method for forming large diameter pipe. The pyramid method has serious limitations by way of size and product quality, and the U-O-E method, although excellent in many respects, has the disadvantage of high equipment cost.
In the pyramid method flat plate to be formed into a circular section is brought into position between the top and bottom rolls with one edge of the plate resting on the bottom roll further away from the center of the plate. The top roll is then moved downward by screws or the like which impart a downward force on the steel plate causing the plate to bend between the two bottom rolls. As the bottom rolls are rotated, bending of the plate commences and continues from one edge of the plate to the other as the plate is wrapped around the top roll and is formed into a circular shape.
Since bending of the plate occurs only under the top roll, the edges of the plate between the bottom roll and the top roll at the beginning and end of forming remain flat even though the plate has been formed into a circular section. To overcome this problem, the plate edges are usually preformed or crimped before forming into a pipe section in the pyramid roll former. Alternatively, the pyramid roll forming method may be practiced by curving the flat edges after the pipe is formed in the pyramid rolls. Other variations of pyramid roll forming have been devised to minimize the flat edge problem. These include offsetting the top roll closer to one of the bottom rolls. This is referred to as the pyramid "pinch" roll method.
The major disadvantage of the various forms of pyramid roll forming are that pipe lengths are limited because the upward deflection of the top roll, which is caused by the upwardly directed forces exerted on the top roll, results in inaccurate forming throughout the pipe length and particularly at the center. The longer the pipe the greater the undesired deflection. Further, the minimum pipe diameter is limited by the top roll diameter. When thick walled pipes are formed the possible pipe lengths are further reduced because greater forming pressures are used with consequent greater upward forces on the top roll. The foregoing disadvantages also limit the production rates of pyramid roll equipment even with skilled operators.
The disadvantages of forming pipes by the pyramid roll forming method have been overcome by the U and O method. This process utilizes heavy press forming as the means of shaping the plate into a circular form. This is done in two separate operations after initially preforming the plate edges a short distance in from each edge to the exact curvature of the finished pipe. First, the plate is forced downward by a punch between two side rolls or dies to form a `U` shape in a press called a U-Press. After the plate has been so formed in the U-Press, it is transferred to an O-Press which contains large hydraulic cylinders or rams that act vertically upon semi-circular dies in the press that force the U into an O-shape. In the O-press a lower semi-circular die is mounted rigidly in the bottom of the press, and an upper semi-circular die is attached to the bottom of the ram so that when the two halves of the dies are brought together, they form a circle the exact size of the pipe. In carrying out the forming operation the top half circular die is raised a sufficient distance and the U shape is conveyed longitudinally into and between the two die halves. When in this position, the hydraulic ram descends with the top half die toward the bottom half die until both die halves come together under immense force to form the U into an O shape.
The U-O method of forming pipes has virtually no limitations as to pipe diameters, lengths and wall thicknesses except for the design limits of the presses. For example, U and O presses have been designed that can produce pipes from 16 inches to 64 inches diameter and up to 60 feet long from 2 inch thick high strength steel plates. Such presses may have hydraulic ram forces totalling over 100,000 tons in order to properly form the steel plate beyond its yield strength.
The U and O method of pipe forming is a highly useful and advantageous pipe forming method and is accepted as a fast and efficient method. However, the forming presses and other required machinery in a U and O pipe mill are extremely expensive. The high capital expense for such equipment can only be justified where there is a market for very large quantities of pipe on the order of several hundreds of thousands of tons of pipe per year. If such tonnage production is not needed the equipment is not economically feasible. Thus the U-O method and mills are not practical where the projected market for pipe, particularly large size wide diameter pipe, is too small to justify the cost of the U and O mill.
Another method for roll forming plate into wide diameter pipe is disclosed in U.S. Pat. Nos. 3,879,994 and 4,428,215 to Hume. This method requires a drum type mandrel which includes a `Tee` shaped bar that is mounted along the length of the mandrel and projects radially out beyond the cylindrical surface thereof. The Tee bar is so mounted that the space between the Tee bar and the mandrel is slightly greater than the plate thickness to be formed into a pipe. The plate edge is inserted into this space and as the mandrel is rotated upwardly, the plate edge is gripped as the plate bends between the mandrel and the bottom forming roll. The mandrel is rotated at least 180 degrees then reversed for a short distance to release the plate edge. The plate is then conveyed across the machine and the opposite plate edge inserted into the space on the opposite side of the Tee bar. The mandrel is then rotated in the opposite direction until the plate is completely formed. The mandrel is again reversed slightly to release the grip on the plate edge. After forming the section is conveyed lengthwise off the mandrel.
The main disadvantage of this method is that the plate edge entering the space between the Tee bar and the mandrel cannot be preformed to shape because the plate edge must be flat in order to properly grip the edge as the plate starts to bend. Another disadvantage is that the bending forces acting against the mandrel particularly on smaller pipe sizes of standard 40 foot lengths, cause the mandrel to deflect upward which can cause the plate edge to become disengaged with resultant inaccurate forming. Therefore, this method is not satisfactory for producing all pipe sizes, thicknesses, and grades as generally required by the many industries using such products, and particularly the gas and petroleum industries.
The serious problems encountered in using conventional pyramid rolls, that is, linear deflection of the top mandrel, is also encountered in both of the patents. The mandrel or top roll in such installations tends to deflect or spring in a direction away from the workpiece or plate. As the length of the apparatus increases due to the length of the plate to be formed into pipe the deflection becomes more serious and affects the performance of the equipment in properly shaping the pipe. Thus, the length of the pipe to be formed is limited. The present invention is intended to overcome these problems as hereinafter described.