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Temperature control boosts push-rod quality.

Induction heating is a fast and cost-effective way to harden components, particularly high-volume parts for the auto industry. The process is readily engineered to the point where the only operator involvement is manual loading.

However, one function that remains difficult to automate is the measurement of part temperatures before quenching. Moving parts, oil mist, and powerful RF fields render most forms of temperature measurement ineffective.

SPS Technologies, a British manufacturer of automotive push rods, recently began using Ircon fiberoptic infrared thermometers to measure push-rod temperatures during induction heating, and gained significant quality improvement through greater statistical control of this process.

Controlling hardness

The push rods are made from steel rod cut to length, forged to produce cup and ball ends, induction hardened, and oil quenched. Lab tests of final hardness and a statistical analysis in January 1989 identified a quality risk that required measuring push-rod heating and quench temperatures.

A thermocouple was installed to measure quench temperature, but several factors eliminated that approach for rod-end temperature. The rods travel along a track and are heated at both ends by induction coils. The correct temperature is a function of the mass of the rod end, inductor energy output, and the time the rod is in the induction field. A thermocouple cannot be used because the rods are: moving, varying in position, in a high-energy field, surrounded by oil mist from the quench tank, and radiating too much heat (up to 200 F) for even a noncontact sensor.

Stray electromagnetic fields ruled out an infrared thermometer at the sides of the track. An attempt to mount the IR thermometer above the machine housing failed because rising oil mist attenuated the rods' IR radiation, and their varying positioning on the track prevented close control of the measurement point.

The solution was positioning a reimaging lens at the side of the track accurately focused on the rod ends and connected to the IR sensor via a 3-ft fiberoptic cable. The lens can stand temperatures to 200 C (400 F) and is fitted with an air-purge unit to maintain positive pressure in front of the lens to prevent mist particles from settling on it.

Hitting a moving target

The continuously moving rod ends are a challenge for the IR sensor. The sensor system overcomes this with a peak-picker function that provides rapid response (10 msec) to rising temperature combined with slow drop-off to bridge the measurement discontinuities.

Output of the two IR thermometers, quench temperature, and track speed are all recorded for future analysis. Immediate security is provided by alarms that respond to any deviation from defined tolerances.

The system has helped SPS achieve significant improvement. Hardness samples showed a 100% improvement over those taken before installation of the IR thermometer. In addition to a more consistent product, cost savings resulted from major reductions in rework because warnings now come in seconds, before significant numbers of bad parts can be made.

These results have encouraged SPS to explore installing similar equipment in other production areas where temperature is a critical parameter, but measurement had not been previously considered.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Quality in Production
Publication:Tooling & Production
Date:Jan 1, 1991
Words:513
Previous Article:Machine vision: reaching for maturity.
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