Engineer Tells a ‘Moving’ APU Story

Ronald Maty, a principal engineer for Honeywell Aerospace, tells this story …

Back in the late 1980s, I was given the task to design a driven compressor for a new auxiliary power unit (APU) for my company. It was to be a larger version of an existing unit already in production, with many hours of service. Airline maintenance people using the older engine disclosed that the largest reason for engine removal was the inner rod end on the actuator for the radial inlet guide vane system. One representative said he waited to remove the engine until he could move the rod back and forth an eighth of an inch.

The older engine had a lever attached to one of the vanes that moved the vane thru a 90-degree rotation and drove the rest of the vanes via a ring gear. The rod end was attached to the lever with an axial bolt. The only way to remove that bolt was to remove the engine and separate the cases.

In my redesign, I attached the actuator directly to the ring gear which only rotated about 10 degrees, reducing the rotation of the ball in the rod end by nearly 90 percent and increasing the life of the rod end by a factor of nine. I then positioned the attachment bolt in a radial orientation and placed an external access hole and cover in line with the bolt. During initial engine assembly, the guide vanes are placed in their closed position and the link rod is adjusted with right- and left-hand threaded rod ends as a turnbuckle to adjust the position of the vanes to the actuator stroke end. To eliminate the need to re-adjust the system when the rod end is replaced, I had a close-tolerance flat machined on the end of the rod end relative to the spherical ball hole centerline. When a worn link rod is removed, the mechanic measures the length from flat-to-flat, replaces the rod ends, re-adjusts to the same measurement and reinstalls the rod. No need to remove the engine or separate the cases to re-rig the guide vanes. At the same time, we replaced bronze bushings at the vane pivot points with plastic bushings to eliminate another major wear point. A comparison of the force needed to move the new system (almost none required) relative to the old one (the system barely moves) is like night and day.

This same basic system has since been used with success in several new APU’s in the intervening years.