This final of three articles discusses a new method of producing hollow shafts for power transmission. The forming technique is based on the differential heating of a tubular billet. For quick adaptation to industry, the method is conceived such that the process can be carried out using conventional forging equipment. The heavy-duty rear-axle shaft and pinion-gear shaft are used to demonstrate how the differential-heating-based forging process may be applied to create hollow shafts with flange or cone features.
Due to the growing scrutiny into the fuel consumption and emissions of class-7 and -8 heavy-duty vehicles (HDVs), reducing vehicle weight is becoming increasingly important to HDV manufacturers. One of the most effective ways is to create lightweight power transmission components because the powertrain accounts for 48% of HDV weight. Major strides in this direction can be made by simply switching from the conventional, solid powertrain shafts in HDV to hollow components.[2,3] However, the switch to hollow shafts is expensive mainly because of the specialized equipment and operations.
This article is the third in a series on weight reduction. An article on weight reduction for heavy-duty power transmission shafts that appeared in December 2016 focused on a parametric study where different tube sizes were studied for potential application in full- and semi-float axles for HDV. In the October 2017 article, the potential of reducing weight of power transmission components via innovative heat-treatment schemes was discussed. This article will discuss an innovative forging method to produce a hollow shaft from differential-heating-based forging concepts.
Available Manufacturing Techniques
The currently available manufacturing techniques for producing hollow power transmission shafts include rotary swaging, flow forming, spinning extrusion combined with cross rolling, tube hydroforming, extrusion combined with deep drilling, and extrusion followed by friction welding. Each of these has its strengths and weaknesses, but they generally suffer from drawbacks, such as low production rate, numerous operations and costly equipment. For example, rotary swaging and axial radial forming are incremental cold-forming processes that can be used to produce hollow shafts by shaping the outside diameter of a workpiece using radially oscillating dies.
These processes provide improved mechanical properties, tight tolerances, good surface finishes and high flexibility of shaping the exterior and the wall thickness. However, high equipment cost and low production rates compared to conventional forging limit their application.[6-8]
Flow forming deforms a rotating metal tube into a stepped axisymmetric part using mandrels and rollers. Because the exterior shape of the workpiece is dictated by the kinematic motion of rollers, flow forming is capable of producing shafts with several steps at various axial positions with simple tool design.[9,10] However, it is difficult to produce flanges with large aspect ratios.
Spinning extrusion is another process that can be used to produce hollow and stepped axisymmetric parts from a solid billet with a spinning roller shaping the exterior and a punch forming the interior. The difficulty with this process is producing products with deep features and small diameters.[11,13]
Stepped hollow shafts can also be produced by a combination of extrusion and deep drilling, where an extrusion operation is used to shape outer geometries and deep drilling to create inner geometries. However, material waste and tool wear are two main disadvantages of the process.
Another technique that has been used to produce hollow shafts is based on a combination of extrusion and friction welding. In this process, backward-extruded cups are joined by friction welding to produce a shaft. The major limitation of this process is the fact that it requires numerous operations on different equipment.[7,14]
Forging Process for Hollow Axle Shafts
The axle shaft itself is a narrow, pole-like part with a flanged end that connects a wheel to the gears in the differential. It transmits torque from the differential to the wheel and, in some cases, bears the weight of the vehicle. Unlike the conventional solid axle shaft, the targeted hollow axle shaft geometry is composed of a solid flange and a tubular body (Fig. 1).
The conventional solid shaft is forged by first heating the portion of the shaft that is to be deformed with induction coils followed by one or more upsetting blows carried out in a specialized press. The proposed forging process uses a tubular workpiece as the feedstock and is composed of four operations: induction heating, upsetting, coning and flanging. Figure 2 shows the steps of the proposed forging process and the evolution of workpiece geometry.
In the induction heating operation, a portion of workpiece is heated by induction coils while the rest of the workpiece is held out of the coils. Appropriate design of the induction heating process leaves the workpiece with a non-uniform temperature distribution that can be divided into hot, cold and warm regions.
The upsetting operation is used to force hot metal to flow inward and fill the central hole. This is accomplished by using a ram to compress the workpiece, a die case to restrict outward radial flow and the cold material to constrain the downward metal flow. The cold metal acts as a pseudo die, undergoing much less deformation than hot metal due to its high flow stress. A rod-like solid structure is created from the hot material at the end of upsetting operation, and the cold material remains hollow.
The coning operation is then used to further reduce the height of the hot solid section and create a conical head. This stage is important because it prevents buckling and aids the material to flow concentrically. In the flanging operation, the hot metal is further upset and flows radially to form the flange. In order to further enhance dimensional accuracy and mechanical performance of the part, a final machining and heat treatment may be needed at the end of the forging process.
Evaluation of Process Feasibility for Hollow Axle Shafts
An initial evaluation of the feasibility of the proposed forging process was carried out using the finite-element method (FEM). This gives the process designer insight into the strain distribution, forming loads and final geometry of the forged component. In this study, FEM simulations were carried out with the commercial FEA software DEFORM2D. As shown in Figure 3, the overall simulation is divided into four operations: heating, upsetting, coning and flanging.
The induction heating module is used in the first operation. The deformation mode is activated for the rest of the operations, and it is assumed that there is no heat transfer between the workpiece and the die. In order to consider thermal and deformation history, the mesh and nodal values are copied from the final step of one operation to the initial step of the subsequent operation.
The initial tube simulated is 1,600 mm long, with an inner diameter of 25.4 mm and an outer diameter of 50.8 mm. AISI 1045 steel is used as the workpiece material, and AISI H-13 tool steel is selected as the die material due to its excellent hot hardness, heat-checking resistance, shock resistance and abrasion resistance.
The workpiece was initially meshed with 9,000 elements. The frequency and the input current to the coil of the induction heater are 60 kHz and 600 A. Because the contact time is short, the workpiece is assumed to be adiabatic. Therefore, there is no heat transfer between the die and the workpiece. The contact time of the forging operations is limited to 4 seconds. In the modeling of the upsetting operation, the workpiece is modeled as rigid plastic, and the dies are considered rigid. Remeshing is done every five steps or when the interference depth exceeds 0.2 mm. Contacts between the workpiece and the die and self-contact of the workpiece are specified by setting the shear friction factor as 0.3. Similar modeling settings are used for the coning and flanging operations.
Finite-Element Simulation Results and Analysis
The final strain distribution in the workpiece is shown in Figure 4, where it can be seen that a maximum strain of 6.5 is exhibited at the center of the flange. Figure 4 also shows a concentrated strain band in the flange, which may result in internal fracture. While a strain of 6.53 is achievable in hot forming of steels, the possibility of defects created by the concentrated strain band in the center of the flange should be further examined by damage model-based simulations and microstructure analysis of formed parts.
Strain concentration can be addressed by optimizing the metal flow pattern and improving the heating scheme. It should be noted that the material at the center of the flange will be drilled out as part of the finishing operations (Fig. 5).
The forming load also plays an important role in the feasibility of the process. The maximum forming loads for the upsetting, coning and flanging operations are 130 tons, 138 tons and 380 tons, respectively. The proposed forging process is also able to maintain a uniform wall thickness in the axial direction. Simulation results show 92% of the tubular body has a wall-thickness deviation less than 0.8 mm, which is 5% of the nominal wall thickness.
Application of the Proposed Process to Pinion-Gear Shaft
The proposed process can be combined with the hollow extrusion to manufacture a hollow pinion-gear shaft that has a solid conical head and a stepped tubular body. The strain distribution of the pinion-gear shaft is shown in Figure 6. The maximum strain of 5.3 occurs at the center of the head. After the center hole is drilled, the maximum strain is reduced to 4.4 (Fig. 7).
This article describes a cost-effective method for the mass production of hollow axle and other power transmission shafts. The proposed process uses tubular workpieces as the feedstock and consists of four main operations: selective heating of a tubular workpiece, upsetting of the workpiece to cause the hot material to fill the hole in the center of the part, coning and flanging.
The feasibility of producing an axle shaft and a pinion-gear shaft was investigated with the aid of the finite-element method. The study has shown that by using a differential-heating-based forging concept it is feasible to produce a hollow axle shaft and other power transmission shafts using conventional forging equipment.
The study showed that the cold tubular part, which supports the hot material during the forging process, does not undergo any significant deformation. The vast majority of tube wall thickness of the cold section stays within 5% of its original geometry. The study also revealed that a shear strain band is induced at the center of the flange. To reduce the strain gradient occurring at this location, optimization of the material flow during upsetting and flanging operations should be carried out. The improvement of material flow to reduce strain localization could be done by manipulating the billet temperature field and possibly changing die geometric features.
The Advanced Metal Forming and Tribology Laboratory (AMTL) at NCSU is currently working with a forging company to test the proposed concept.
The authors would like to express gratitude for the financial support given by the Forging Industry Educational and Research Foundation (FIERF) and the American Iron and Steel Institute (AISI).
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Co-author Dr. Gracious Ngaile is Professor of Mechanical and Aerospace Engineering, Advanced Metal Forming and Tribology Laboratory at North Carolina State University. He may be reached at 919-515-5222 or firstname.lastname@example.org. Co-author James Lowrie is a Ph.D. candidate at North Carolina State University. He may be reached at 919-515-0685 or email@example.com. Co-author Hao Pang is also a Ph. D. candidate at North Carolina State University. He may be reached at 919-515-0685 or firstname.lastname@example.org.