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FLEXURE FACTS

Solved Application Story # 34

COMMERCIAL AVIATION - Fuel Control

THE CHALLENGE

On the common twin engine turbo-prop commuter aircraft, a fuel control system utilizing a ground end wire wound spring was used. The spring and surrounding assembly required servicing after 50 hours of usage. The airline industry demanded an assembly with a longer service life. 

THE SOLUTION

The alternative design developed by Helical's Application Group basically was the same envelope (OD/length). The ID was a little smaller and some integrated attachments were added. It was found that the existing wound spring had its first mode frequency excited at normal engine running speeds, this action was a side to side motion. When excited at this mode, the spring would swing back and forth and wear out a diaphragm surrounding the spring. After extensive analysis at Helical, an alternative was found that exhibited its first mode excitation frequencies outside normal engine speeds. With this configuration, service intervals have been extended by 10 times and tests suggest that 100 times may be possible. This explains why the customer was willing to purchase a machined spring over a wire wound spring even with a price increase exceeding 4 times.

THE ANSWER

HELI-CAL^® Flexure Machined" Spring # 2345