A cylindrical metal workpiece lends itself to induction heat treatment by axial rotation of the workpiece around a stationary arcuate inductor coil having a curvature somewhat greater than the outer radial curvature of the workpiece. Metal workpieces that include substantially cylindrical, non-concentric components, such as crankshafts used in internal combustion engines, pumps, compressors and the like, are more difficult to effectively harden and temper by induction heating. A crankshaft, typically cast or forged in one piece, comprises a series of crankpins (pins) and main journals (mains) interconnected by webs. While all pins (and mains) are in the shape of substantially right circular cylinders, each web is individually shaped to serve as a balance weight for a particular crankshaft design. In alternate crankshaft configurations, double-width (common) pins, which are twice the axial length of a standard pin, may be used. Also two pins may be connected to each other (rather than separated by a web) with offset pin axes in a configuration known as split pin. Consequently, the shape of each web can deviate substantially from that of a solid right circular cylinder. The cylindrical surfaces of the pins and mains are the seating surfaces for the connecting-rod bearings and main bearings, respectively, and are referred to as the pin and main bearing surfaces. All mains have a common axis of rotation, which is referred to as the main axis. The axis of each pin, or pin axis, is offset radially from the main axis. Skewed passages are drilled through adjacent mains, webs and pins to provide a lubricating fluid path from the main bearing surfaces to the pin bearing surfaces. The passages terminate in skewed openings in the bearing surface of the mains and pins. Additionally, one or more crankshaft terminating components, such as oil seals, flywheel attachments and crank noses, are typically attached to either end of the crankshaft. The transitional area defined by the circumferential boundary between each pin and main and its adjacent webs is typically called the fillet. Pin and main bearing surfaces are hardened and tempered by heat treatment to achieve a hard high wear and seizure resistant surface. The fillets are hardened and tempered by heat treatment or mechanical roll hardening to improve crankshaft performance since the fillets are subjected to high stresses due to bending moments during use of the crankshaft. Heat treatment of only pin and main bearing surfaces, or those surfaces along with the fillets associated with the surfaces, is selected based upon a particular crankshaft design.
Each pin and main can be induction heat treated by bringing a generally conformal U-shaped inductor close to the pin or main bearing surface while the crankshaft is rotated about its main axis. Since the pin axis is radially offset from the main axis, the pin will orbit around the main axis. Consequently, the U-shaped inductor must travel with the orbital motion of the pin for 360-degrees heat treatment of the pin. U.S. Pat. Nos. 3,188,440 and 5,680,693 disclose an inductor and process for this type of heat treatment. The major disadvantage of this approach is the requirement that the inductor moves in a plane perpendicular to the direction of the pin axis during the heat treatment process. Consequently, complex and massive components for the inductor assembly, inductor motion control, and power supplies are required for coordinating the rotation of the pin with the inductor. Maintaining an optimum spatial relationship between the pin and inductor is inherently difficult due to the dual motion of the pin and inductor during the heat treatment process. Carbide guides extending beyond the face of the U-shaped inductor ride on the pin as it is heat-treated. The carbide wears during the heat treatment process and results in deviations from the optimum air gap between the face of the inductor coil and the pin or main. Deviation from the optimum air gap results in uneven heating. Essentially, wear of carbide guides results in premature coil failure. Since the U-shaped inductor assemblies must move during the heat treatment process it is preferred that they be light weight and small. However, these design constraints compromise the strength and robustness of the process and coil assembly. Circumferential variation of web shapes, and the existence of holes in the pins and mains, requires a complex control system that applies varying induction power levels over a heat treatment cycle. Moreover, rotation of the crankshaft (and the associated holes and webs) during heat treatment makes the control of induction power levels around those features virtually impossible.
Therefore, there exists the need for a relatively simple method of heat treating crankshafts and other metal workpieces that include multiple generally cylindrical components whose axes are parallel to and offset from a common workpiece axis, and compact apparatus to achieve improved hardening and tempering of such metal workpieces.
In the present invention, during heat treatment, the crankshaft and inductor are stationary at one or more heat treatment stations during the hardening and tempering processes. The crankshaft is transferred between each station with a workpiece transport system and properly positioned at each station to heat a pre-selected number of workpiece components. The apparatus and process results in a significant reduction in the complexity of the heat treatment process that can be accomplished with compact and unitized components while maintaining comparable production rates achieved with rotation of the crankshaft during heat treatment. The crankshaft is more accurately hardened and tempered with improved mechanical properties.