1667. Considerations for the Design of Spin and Static Balance Fixtures

Paper

Abstract

This paper discusses several practical matters relating to the measurement of mass properties which are often overlooked in the literature. These are:
1. Making certain the test article has the necessary precision interfaces so it can be fixtured accurately
2. Defining the datum points on the test article
3. Designing the ballast system on the test article so all anticipated conditions of unbalance can be corrected for without rework of the test article
4. Fixture design
5. Fixture accuracy verification
6. Selection of a balancing machine
This paper emphasizes the need for the mass properties engineer to become intimately involved with the design of the test article from the early stages to the final design, and to warn management if the mass properties requirements are unrealistic in sight of manufacturing tolerances on the fixture interface. Traditionally the mass properties engineer often is called upon to balance a test article which is almost impossible to fixture accurately and which does not contain adequate locations for balance correction weights (ballast). It is our hope to start a trend which will change that situation.
Balancing machine technology has reached the point where the limiting factor in the accuracy of mass properties measurement is the accuracy of fixturing rather than the accuracy of the balancing machine. Surprisingly, very little effort appears to be underway to improve fixturing accuracy. Other than Space Electronics, very few balancing machine companies offer precision fixtures as standard products. Furthermore, fixturing is often handled as an afterthought, the design either being delegated to a junior draftsman, or the available funding for the design woefully inadequate. This paper lists the factors to consider when designing custom fixtures. Several types of self-adjusting fixtures are illustrated.