Fig. 1. Process development procedure
Over 75 years ago, Ajax Tocco Magnethermic developed and introduced induction hardening of crankshafts to the industry. In that time they have shipped hundreds of crankshaft hardening systems. ATM has also supplied high-production automotive-crankshaft hardening systems incorporating the critical requirement of fillet hardening. This concept originated in Europe approximately 50 years ago, and the Ajax Tocco supply of high-production crankshaft hardening systems was in cooperation with a European partner.
As the automotive market and the automotive equipment-supply marketplace went global and became more competitive, and as the market demanded an improved solution, ATM needed to return to its roots and produce an entire system that met the market requirements.
This was facilitated when in 2004 a major truck manufacturer was searching for a supplier to provide equipment to harden a new V-8 crankshaft to be used in an all-new diesel engine.
The specifications were daunting:
- TIR – eliminate or minimize straightening
- Auto-temper integral to the process cycle
- Coil life maximized
- Shoe wear minimized
- Cpk of the fillet-hardened case depth
- Successful equipment delivery with a proven process in less than one year
Benchmarking
A conventional crankshaft hardening machine would not be a viable re-entry into the market nor would it meet the defined requirements. Starting with a blank sheet of paper with respect to both machine design and process, all approaches were considered.The existing global crankshaft hardening systems, fillet-hardening coil designs and transformer designs were benchmarked to establish a baseline starting point. Design improvements for all aspects of the systems and detailed component designs were brainstormed to determine possible improvements. Where several options appeared attractive, both or several were included in an experimental unit to test and evaluate the alternatives to optimize the final equipment.
Crankshaft hardening is the most challenging of the induction heat-treating applications. Not only are there circumferential mass changes, changes in journal configurations and oil holes, but each part also has multiple heat-treated sections that all must be processed properly to meet the metallurgical and durability requirements. Conversely, the individual feature variability and multiple features greatly increase the probability of a rejected part.
The inductor design and the basic process have changed very little from the early days of the development. There have been several attempts with non-rotational and noncontact approaches, but they have not been successful for fillet-hardening applications.
The non-rotational approach, which recently was revived from the early developments, was ruled out because of the need to fillet harden. The non-rotational approach does not lend itself to fillet hardening. Another deterrent is that coil development is long and arduous, and the process is very inflexible, i.e. it is sensitive to crankshaft dimensional considerations in production and coil life can be an issue.
The noncontact approach does not follow the crankshaft distortion during heating, requiring generous clearances – leading to even more heating and distortion – that prevent the optimization of the pattern. Therefore, it was also eliminated from consideration.
This analysis definitely established that the rotational concept provided the unique flexibility to precisely program and profile the heating and quenching rate circumferentially to yield the optimum result and to adjust to variations that may occur in production.
Establishing "Best in Class"
ATM determined that the control and accuracy of the “conventional crankshaft hardening process” could be substantially improved with the advancement in control technology in servos and valves, process modeling and solid-state IGBT power supply responsiveness. In essence, crankshaft hardening could be advanced from an “experience-based technology” to a “science-based technology,” going from an art to a science.Many of the existing equipment approaches utilizing the conventional technology are difficult to maintain and have coil- and shoe-life issues. To avoid or minimize those effects, the following guidelines were established for the production system:
1. The uptime of the crankshaft machine prototype/concept must meet the requirements of the associated in-line machine tools.
2. Accessibility for maintenance – a major challenge on a crankshaft machine – must be improved from existing concepts.
3. Inductor replacement must be simple, easily accessible and allow for inspection after installation.
4. Inductor-life issues must be primary, and all factors affecting inductor life must be optimized from the cooling system through coil design, inductor alignment and motion-generated forces.
5. Inductor shoe life must be improved.
6. Distortion and dimensional control to meet the stringent straightness requirements, thereby avoiding straightening, requires a flexible machine with process sequence and self-correcting options.
7. Integral auto-temper must be implemented, which eliminates the tempering oven, reduces floor space and reduces operating costs. All contributing variables must be controllable.
8. The visibility of the process to the operator is essential for monitoring the operation and the refinement, fine-tuning and maintenance of the crankshaft hardening process.
9. Assured quality monitoring – the system must be capable of absolutely assuring that each part is processed properly or be rejected if it is not.
10. Process monitoring profiles – all process variables must be profiled to assist in optimizing the metallurgy and TIR.
The machine concept was three individual stations to allow three individual processing steps. All three stations were capable of hardening up to five journals each with any station capable of hardening any journal or group of journals. This allows for future adjustments in the sequence of hardening journals to correct for distortion resulting from a prior process or a crankshaft design change.
The system included a TIR station between the last two stations, allowing for process adjustments at the final station to minimize distortion if required and for the final measurement of TIR as a basis of reject and/or overall process correction to control distortion.
The customer considered robots to be the most reliable and flexible material-handling technique in their facility. The robotic handling also allowed the machines to be accessible for process monitoring, maintenance and coil exchange when required.
Accessibility, maintainability and operational reliability were the primary criteria for the layout of the machine. Accessibility to the stations was allowed in the front load area through the use of the robots. Rear access was gained by spacing the power supplies and control panels away from the machine. Transparent access doors for the machine allowed the power supply HMI (Human Machine Interface) panels to be viewed along with the process.
Fig. 2. Typical pattern
Prototype Lab Development
Recognizing the interrelationship of the machine design elements and the process-control requirements dictated a comprehensive prototype test facility. This was implemented to verify the process, equipment and inductor design. A laboratory development unit capable of hardening all of the journals of one class (either pins or mains) was designed and manufactured.The laboratory unit was equipped with a servo rotational axis; circumferentially variable power, pneumatic counterbalance cylinders and valves; and quench-control valves. Signature analysis was used for coil voltage, coil current, coil power and quench flow. It includes a TIR gauge to measure TIR on all five mains of a V8 crankshaft simultaneously, including TIR magnitude and direction.
After simulation analysis, the crankshaft development became an application of the scientific approach to develop the optimum process results as shown in Fig. 1.
The fillet-hardening pattern was developed first for both mains and pins in the as-hardened condition as shown in Fig. 2. This involved not only selecting the coil copper and lamination configuration but also establishing the power pulsing menu to properly profile the hardened pattern to accommodate the circumferential mass variations.
Fig. 3. Typical voltage profile
In parallel with the hardening development, techniques were developed to vary the counterbalance pneumatic pressure to control the force on the shoes for minimizing shoe wear. The challenge was to find valves and a pneumatic system that could respond to the system dynamics, as the crankshaft rotates at 30 rpm. Satisfactory components were located, installed and experimentally verified.
Fig. 4. Typical TIR station-to-station movement vectors
The customer requested parts with both low distortion and high distortion to evaluate the effects of straightening. Two groups of crankshafts were produced. The low-distortion group averaged less than 0.015-inch TIR that did not require straightening. By reversing the process sequence, a group was produced that averaged 0.030 inch for straightening.
Auto-temper is a technique where the part is hardened to full hardness by quenching the hardened region below the Mf (martensite finish) temperature but leaving sufficient residual energy that restores the hardened region back to the tempering temperature. This requires very precise energy input and very precise cooling. The mass variations in a crankshaft – especially a pin – are substantial, varying from the counterbalance portion of the pins adjacent to the mains to the TDC (top dead center) location that have little mass.
The temper specification was based upon an oven temper result, which was to be attained with the auto-temper process. Again, quench pulsing was studied, and it was determined that precisely varying the quench circumferentially was required to attain the desired result. The same profiling technique used on the voltage, current and power was employed for the quench. The quench flow was profiled, and the response time of pneumatic valves and the pneumatic system was studied with respect to the input signal and the resulting hardness. This was done to optimize the signal and allow for lags in the system. So, the required flow was delivered to the desired region to increase or decrease the hardness.
The production of parts for fatigue testing and verification of the process specification completed the development project. The results obtained dictated the final design specifications for the production system.
Fig. 5. Crankshaft system isometric
Production System
With the criteria for the production system defined and with the specifications finalized, the production machine was completed.To allow the pantograph hanging assemblies to track the part, the cooling and plumbing must enter from above the crankshaft assembly. In conventional machines this makes a relatively high-maintenance area inaccessible. This problem was alleviated by creating a balcony above the power supplies (Fig. 5) that would allow access to the hanger assemblies for adjustment and maintenance and to the quench valves, flow meters and water-cooling manifolds. No moving parts or valves are over 30 inches from direct accessibility on the entire machine. The concept is very user and maintenance friendly.
Coil inspection is extremely convenient. The coil insertion and clamping adjustment are less than 12 inches inside the machine and at an elevation of approximately 54 inches. This allows for simple coil inspection and replacement if required. The process can be continuously viewed from the rear of the machine during operation, allowing for direct feedback, for placement of the coils and knowledge of the process variables.
The IGBT power supplies are “state of the art” and specifically adapted for a rapid response to allow for power adjustments. They are capable of operation from 10 to 30 kHz, so any reasonable frequency can be utilized. The specific frequency selected provided the optimum balance between metallurgical factors, coil life and mass geometry considerations.
A key to the successful process is the control that includes an HMI (Human Machine Interface) – an independent computer for process monitoring and data collection – and servo axes for part rotation in the three processing stations and on the TIR gauge.
The controlled process variables are power, counterbalance pressure and quench flow. The monitored and profiled variables are coil voltage, coil current and quench flow. Coil power is integrated and presented as energy for each cycle. The control and monitoring are key process-development inputs that allow the metallurgical results to be compared and controlled in minute steps to optimize metallurgical properties and distortion. Most conventional crankshaft machines are controlled and monitored in 30-degree increments, which is insufficient for control of auto-temper and distortion.
The profiles and TIR are stored for every processing step on every crankshaft for quality assurance and to determine the cause of any variation that may be observed when subsequently processing the crankshaft. Rejects and faults are also stored with a record of the contributing variable or malfunction.
SPC is included and can be monitored by individual parts or by grouping up to 50 parts for X-bar, trending analysis and display.
Conclusion
Distortion and metallurgical requirements were met. Depending upon the desired fillet-hardening depth, TIR averaged 0.010 inch to 0.011 inch with no TIR exceeding 0.020 inch during the runoff of the first machine. The distortion results were very robust and actually proved to be independent of the prior processing. The original trial parts were lathe-turned, and the final production parts were milled. There was very little difference in distortion between the two processes. The as-processed distortion also had no correlation to incoming distortion. Both of these distortion observations are contrary to conventional wisdom for crankshaft hardening.The significance of this observation is that experienced-based knowledge is no longer adequate. The heat-treating industry must change to use simulation-based engineering and a scientific approach to drive progress and improved solutions. These concepts as outlined and implemented in this development are the basis for the success of this project.IH
For more information:Contact Ronald R. Akers, vice president research and development, Ajax Tocco Magnethermic Corporation, 1506 Industrial Blvd., Boaz, AL 35957; tel: 256-840-2334; fax: 256-593-4735; e-mail: RAkers@ajaxtocco.com; web: www.ajaxtocco. com. George D. Pfaffmann is vice president technology, and Richard H. McKelvey is senior applications specialist.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: crankshaft hardening, non-rotational induction, inductor, human machine interface
