Zhuojin Pipe Fitting Co., LtdOverall Design Parameters and Drive Scheme of Elbow Pushing Machine
Literature Overview
The paper by Yang Weifan, Liu Chaoji, and Wang Lianhe (1989), published in Petroleum Machinery, addresses the fundamental engineering design of a mandrel-drawn elbow pushing machine. This work originates from Daqing Petroleum Institute and Daqing Automobile Repair Plant, reflecting the strong industrial-academic collaboration that characterized Chinese petroleum equipment development during the late 1980s. The authors systematically discuss three critical process parameters — pushing temperature, pushing force, and pushing speed — and propose an overall machine layout and transmission scheme tailored to the specific working conditions of elbow push-forming operations.
Core Technical Parameters
Pushing Temperature
The pushing temperature is arguably the most sensitive parameter in elbow push-forming. The workpiece must be heated to a temperature within the hot-working window of the selected steel grade, typically between 950°C and 1150°C for carbon and low-alloy steels such as 20#, 16Mn, and X70 equivalents. The authors emphasize that uniform heating is essential; temperature gradients across the cross-section lead to differential plastic flow, resulting in non-uniform wall thinning and ovality defects. In practice, the furnace exit temperature should be verified by optical pyrometers or infrared sensors at multiple positions along the pipe length.
Pushing Force
The pushing force is determined by the plastic deformation resistance of the material at the forming temperature, the reduction ratio, and the friction between the pipe and the pusher plate. The authors present a calculation framework based on the flow stress at elevated temperature and the geometry of the die. For a typical elbow with an outer diameter of 325 mm, wall thickness of 12 mm, and a bend radius of 1.5D, the required pushing force can reach 200–350 kN. The authors recommend that the machine design capacity be set at 1.2 to 1.5 times the calculated maximum force to accommodate variations in material properties and die wear.
Pushing Speed
Pushing speed directly affects the strain rate and consequently the flow stress. Excessive speed leads to adiabatic heating, which can push the local temperature beyond the recommended range, causing grain coarsening or even incipient melting at the die contact zone. Too low a speed results in excessive heat loss, requiring higher pushing forces and reducing productivity. The authors suggest an optimal speed range of 5–20 mm/min for medium-diameter elbows, with the specific value adjusted based on the steel grade, wall thickness, and die geometry.
Machine Layout and Transmission Scheme
The authors propose a horizontal layout with the furnace, pusher, and die station arranged in a linear configuration. This arrangement minimizes the transfer distance between heating and forming, reducing heat loss and ensuring consistent forming temperatures. The transmission scheme is analyzed with particular attention to the force transmission efficiency and the ability to maintain constant pushing speed under varying load conditions.
| Design Aspect | Recommended Configuration | Rationale |
|---|---|---|
| Machine layout | Horizontal linear | Minimizes heat loss during transfer |
| Pusher drive | Hydraulic cylinder or screw drive | Provides high force at low speed |
| Speed control | Variable displacement pump or motor with reducer | Enables real-time adjustment |
| Die station | Fixed with hydraulic clamping | Ensures alignment and repeatability |
| Cooling | Induction or air cooling post-form | Controls cooling rate to avoid HAZ issues |
Engineering Practice Reflections
From my decades of experience in pipe fitting manufacturing, I recognize that the parameter framework presented in this 1989 paper remains fundamentally valid, though modern implementations incorporate significant refinements. The determination of pushing force through simplified plasticity theory is still used as a first approximation, but finite element simulation now allows for more accurate prediction of force evolution during the forming cycle. The concept of a 1.2–1.5 safety factor on the calculated force is consistent with current ASME and API design philosophies for forming equipment.
One critical insight from this paper that deserves emphasis is the interdependence of the three parameters. Temperature, force, and speed cannot be optimized independently; they form a coupled system. In my own workshop experience, I have observed that a 50°C deviation in pushing temperature can shift the required pushing force by 15–20% and alter the optimal speed by a similar margin. This coupling necessitates a systematic approach to process qualification, which the authors implicitly advocate through their emphasis on both calculation and experimental testing.
Key Questions and Study Insights
The paper raises an important question that remains relevant today: how to balance productivity with quality consistency in push-forming operations. The authors advocate for a moderate pushing speed that allows sufficient time for uniform deformation, which aligns with the modern philosophy of "slower is better" in hot-forming operations where microstructure control is paramount. I would add that the paper does not explicitly address the die design and material selection, which in practice are equally critical to forming quality. A comprehensive process development program should integrate die geometry optimization, material selection for the die (typically H13 or equivalent hot-work steel with appropriate coating), and the three process parameters into a unified optimization framework.
This study serves as a valuable historical reference for understanding the foundational engineering principles of elbow push-forming equipment design, and its systematic approach to parameter determination remains a sound methodology for new equipment development.