1. Field of the Invention
The present invention relates to an apparatus and a method of steel pipe bending.
2. Brief Description of the Related Art
FIG. 10 shows a conventional steel bending apparatus. A bending procedure according to this apparatus is carried out as follows:
(1) As shown in FIG. 10 a steel pipe 51 to be bent is placed between support rollers 52 and rear end of the pipe facing a pusher 54 is held by a tail-stock 53. The front end of the pipe is held by an arm clamp 57 attached to a pivotal arm 56 which revolve the front end of the steel pipe 51 around a pivot 55. PA1 (2) Power is supplied to a heating coil 60 via a heating unit 62. Then as shown in FIG. 11, the steel pipe 51 is pushed through a pair of guide rollers 58 and 59 disposed right and left sides of the pipe by the pusher 54 in the direction of an axis of the steel pipe, is transferred toward heating coil 60 and is passed through the coil 60. In this way, the pushed steel pipe 51 is successively heated with induced current from the heating coil 60. Since the coil has a circular shape, the periphery of the steel pipe 51 is heated circularly around the axis of the pipe. As shown in FIG. 12, since the front side of the heated portion of the pipe is cooled successively by water 62 spouted from a plurality of holes h formed circularly on a circular channel 60a of the coil 60 so as to obtain cooled circular portion c of the steel pipe 51, only portion t having a width W, of the steel pipe 51, virtually remains in a heated state. The locally heated potion t successively transfers toward the rear end of the pipe as the steel pipe 51 transfers forward. The temperature of the locally heated portion t is kept over a crystallization temperature of the pipe. In the case of a carbon steel pipe, for example, the heated zone having width W in the direction of the steel pipe axis is kept between 760.degree. C. to 900.degree. C. The front end of the steel pipe 51 is transferred forward by a successive pushing force from the pusher 54, but since it is fixed by the arm clamp 57 attached to the pivotal arm 56, the steel pipe is forced to bend successively at the locally heated portion t. PA1 (1) Since enough rigidity is necessary to cope with a bending moment of the steel pipe and with an applied restraining force to the steel pipe via the pivotal arm, a massive and huge pipe bending apparatus is required. Therefore, because of the inferior portability of the apparatus, a large lot of pipes have to be bent at bending plants situated far from construction sites. Which is inevitably accompanied with the following drawbacks. PA1 (2) According to the conventional method, since a compression force is imposed in the direction of the axis of the steel pipe due to the restraining force to the steel pipe moving forward via the pivotal arm, thinning a thickness of the pipe is prevented to a certain extent, but which is not satisfactory yet. In order to compensate such thinning thickness of the pipe, a one gage thicker pipe compared with a straight pipe to be connected with the bent pipe, is employed as the pipe for bending. PA1 (2) An apparatus of steel pipe bending comprises; a heating means to heat the steel pipe circularly around an center axis of the pipe, a cooling means to cool the heated portion of the pipe circularly around the center axis of the pipe, a tensile force applying means to apply the tensile force on points of application which are located in the opposite directions from the circularly heated portion, a variable controlling means to control the tensile force variably, a transfer means to transfer relatively the steel pipe and the heating means and the cooling means in a direction of the axis of the steel pipe, a controlling means to control the relative transfer velocity and a scale to measure bent values stepwise according to a predetermined bending schedule (Hereinafter referred as "the second apparatus"). PA1 (1) A method of steel pipe bending comprises; forming a locally heated circular portion around a center axis of the steel pipe, relatively transferring the locally heated portion and the steel pipe in a direction of the center axis of the steel pipe and controlling the relative transfer velocity of the heated portion and the steel pipe during a bending procedure by applying a tensile force between two points of application which are located in the opposite directions from the heated portion along an eccentric axis of the steel pipe (Herein after referred as "the first method"). PA1 (2) A method of steel pipe bending comprises; forming a locally heated circular portion around a center axis of the steel pipe, relatively transferring the locally heated portion and the steel pipe in a direction of the center axis of the steel pipe, measuring actual bent values stepwise during a successive bending procedure according to a bending schedule where bent values are predetermined stepwise and controlling the relative transfer velocity of the heated portion and the steel pipe during the bending procedure by applying a tensile force between two points of application which are located in the opposite directions from the heated portion along an eccentric axis of the steel pipe according to a difference between the predetermined bent value and the actual bent value (Hereinafter referred as "the second method").
However, there are the following problems in the conventional steel pipe bending apparatus.
1 At first, straight pipes are transported to a pipe bending plant and bent pipes are transported to construction sites by trucks or ships. However, bent pipes occupy more voluminous space, namely, higher transportation costs are inevitable. PA2 2 It is difficult to adjust pipe bending schedules flexibly according to modified schedules or designs or additional orders which often occur at sites such as plants and pipelines construction sites etc.