Un réchauffeur à circulation à huile contrôle l’expansion des moyeux des avions à réaction et élimine les rejets

Le défi
L’une des opérations de production les plus importantes dans cette usine est le meulage de finition des moyeux et arbres des réacteurs J57 et J75. Il existe 66 pièces différentes et elles ont une valeur pouvant atteindre 1 000 $. Toutes les tolérances sont extrêmement faibles sur les pièces finies -- d’un maximum de ±0,01 po. pour les diamètres d’environ 4 po., à ±0,0001 po. pour les diamètres de 19 po. Lorsqu’il effectue de légers meulages de finition, l’opérateur de machine stoppe à intervalles réguliers la meuleuse pour vérifier la pièce avec une jauge à barres, préréglée sur un calibre étalon.

Un nombre trop élevé de moyeux et d’arbres, environ 25 %, ayant passé le calibrage de contrôle de l’opérateur de machine ont été rejetés à l’inspection finale. Certaines pièces rejetées étaient trop petites et devaient être plaquées alors que les pièces trop grandes devaient être rebroyées. La difficulté à maintenir les tolérances était due à la différence de température entre le calibre étalon à l’emplacement de la meuleuse et la pièce, quand celle-ci était mesurée. La température de la pièce était déterminée essentiellement par la température du liquide de refroidissement de la meuleuse, un mélange d’un volume d’huile pour vingt volumes d’eau. Le calibre étalon était à température ambiante.

The air temperature of the building varied seasonably from 65 ° F, to 100 ° F and the coolant temperature range was from 54°F. to 90°F. Because of the small amount of stock removed in this operation, very little heat was generated to raise the coolant temperature. With large diameter wheels whipping the coolant, sufficient evaporation took place so that the part was 10-12°F cooler than the master gauge. The temperature differential of the coolant and master gauge was not constant. In the early morning, the coolant temperature was higher than the master gauge. The size change of many of the parts, with a 10 ° F temperature differential, was greater than the allowable tolerance. Since the differences in temperature between the part and master gauge was by no means constant, and with normal grinding variations, the occurrence of rejects were unpredictable.

The Solution
It was necessary to make the temperature of the part, when it was being measured, equal to .that of the master gauge. Immersion heaters were not installed in the coolant tanks because they would interfere with tank cleaning and require power for heating all of the coolant. A Chromalox Circulation Heater was located in the coolant line, between the tank and the grinder outlet. The heater, mounted in the coolant line, heats up quickly and warms only the actual coolant flow to the work. The heater has a rating of 6 kW and a surface watt density of 20 WPSI. The temperature sensor for the PID process temperature controller was located in the coolant line, just above the outlet. The circulation heater and  the temperature controller are mounted on a special frame along side the machine.

The coolant temperature is precisely maintained several degrees above the temperature of the master gauge. This is accomplished with Chromalox PID change in temperature controller and SCR power controller. Tight process control is required so that with evaporation, the part temperature remains equal to that of the master gauge. The master gauge temperature is checked hourly. Then, the feed controller at the grinder machine is reset accordingly. Adjustments in the coolant temperature control level are made until the master gauge and part temperature are equal at the time of measurement. These adjustments take into account evaporation and changes in part temperature while the machine is shut down and parts are being measured against the master gauge. From March 1957, when the heaters were installed, up to the present time, there has not been one piece that had to be reworked due to gauging errors because of temperature differences.

Benefits

  • Elimination of rejects due to expansion because of temperature differential between parts and master gauge.
  • Easy installation; Chromalox Circulating Oil Heater and PID temperature controller are easily mounted into the coolant line beside the machine.
  • Fast, automatic response of the circulation heater is managed by a SCR power controller.
  • Reduced operating cost
  • Many Circulation Oil Heaters, PID temperature and SCR power controllers are standard and immediately available from Chromalox stock.