Case Studies

Aircraft

Oil Circulation Heater Contols Expansion of Jet Hubs, Eliminates Rejects

Oil Circulation Heater Contols Expansion of Jet Hubs, Eliminates Rejects

The Challenge
One of the most critical production operations at this plant is the finish-grinding of the hubs and shafts for J57 and J75 jet engines. There are 66 different parts and range in value up to $1,000. All tolerances are extremely close on finished parts . . . from a maximum of ±.01" on diameters around 4", to ±.0001" on 19" diameters. In making light finish grinds, the machine operator periodically stops the grinder to check the part with a bar-type gage, pre-set on a master gage.

An excessively high number of hubs and shafts, about 25%, which had passed the machine operator's control gaging were rejected in the final inspection. Some of the rejected parts were undersize and had to be plated, while oversize parts had be be reground. The difficulty in maintaining the tolerances was due to the temperature differential between the master gage at the grinder location and the part when it was measured. The temperature of the part was determined essentially by the temperature of the grinder coolant, a mixture of one part oil to twenty parts water. The master gage was at room air temperature.

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 wipping the coolant, sufficient evaporation took place so that the part was 10-12°F. cooler than the master gage. The temperature differential of the coolant and master gage was not constant. In the early morning, the coolant temperature was higher than the master gage. 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 gage was by no means constant, and with normal grinding variations, the occurance 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 gage. 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 watts per square inch. 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 gage. 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 gage. The master gage 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 gage 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 gage. 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 gaging errors because of temperature differences.

Benefits

  • Elimination of rejects due to expansion because of temperature differential between parts and master gage.
  • 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.