The pipe bending machine is a versatile tool capable of both pipe bending and jacking operations. It boasts numerous advantages—such as a rational structure, safety, ease of use, reasonable pricing, quick setup/dismantling, and multi-functionality—allowing it to capture a significant share of the domestic market. Before operation, the machine should be run without a load to verify proper functioning; only then should the bending die be fitted and the pipe positioned according to the required diameter. Non-operating or non-assisting personnel must not loiter near the machine during operation. After work is completed, the power supply must be cut off, the electrical control box locked, and routine maintenance performed.
The initial distance between the machine's clamping jaws is set slightly larger than the sum of the post-upsetting jaw distance, the upsetting stroke, and the flashing allowance. Generally, a greater steel strip thickness or width requires a larger initial jaw distance. The post-upsetting jaw distance is selected based on the strip thickness. An excessive distance can lead to lap welding and twisting, while an insufficient distance results in a significant temperature gradient across the weld cross-section. The thinner the steel strip, the smaller the post-upsetting jaw distance should be; for steel strips 2–4.5 mm thick, the post-upsetting jaw distance is typically set between 12 mm and 18 mm.
The upsetting stroke is also determined by the steel strip thickness. If the upsetting stroke is too short, the upsetting pressure is reduced, preventing the expulsion of slag and metal oxides (which remain in the weld joint) and failing to fully close the craters formed during flashing, resulting in a weak weld. Conversely, if the upsetting stroke is too long, the upsetting pressure becomes excessive, squeezing out all the hot metal from the joint and reducing weld strength. The upsetting stroke is generally set at 1 to 1.8 times the steel strip thickness; a greater strip thickness requires a longer upsetting stroke. Factors affecting the quality of pipe bending include the deformation dynamics during pure bending: under the influence of an external bending moment (M), the outer wall of the pipe (relative to the neutral axis) undergoes tensile stress and thins, while the inner wall undergoes compressive stress and thickens. The resulting forces (N1 and N2) cause a change in the pipe's cross-sectional shape. Consequently, the primary factors influencing bending quality are the bending radius ratio (RX) and the wall thinning ratio (SX). To ensure quality, the standard GBJ235-82 explicitly specifies the required RX values and maximum allowable outer wall thinning for various pressure ratings, thereby controlling these critical parameters.
As previously mentioned, the material experiences tension on the outer side and compression on the inner side during bending, with the position of the neutral axis varying according to the bending method employed. In push-bending (compression bending), the neutral axis is located approximately one-third of the way from the outer wall; in rotary draw bending, it is located two-thirds of the way from the outer wall. Therefore, the rotary draw bending method is advantageous for bending thin-walled pipes.
The precision of the bending die is another factor affecting bending quality. During die manufacturing, dimensions must be controlled within specific tolerances; additionally, users must select the appropriate die based on the pipe diameter. The inherent bendability of the pipe material and the condition of its surface (such as corrosion) can also impact the quality of the bend. During on-site operations, the operator must understand the material properties and workability of the pipe and assess its surface condition.
