Current design methodologies as applied to rotorcraft transmissions yield reliable, safe, and weight effective systems. In
general. weight and performance (?.e., torque transmission per pound of transmission weight) are the primary active
variables while safety is addressed by monitoring the design so that conventional criteria, such as calculated stresses, are
kept within established allowable levels. This procedure is almost universally applied and has been quite successful both
at Boeing and at other major rotary wing firms. It is possible, however, to raise these already good safety characteristics to
ultra safe levels through the use of certain design approaches. UltraSafe gear system design will require a basic change in
the overall philosophy of design as it is practiced today. This change must be revolutionary rather than evolutionary.
In order to develop an UltraSafe transmission a basic paradigm shift will be required. In addition to addressing the design
process from the typica1”prevent a failure from occurring” point of view, it will also be necessary to view the system from
the point of view of ”what happens when a failure occurs.” Thisdesign philosophy was presented in 1997.l) The currently
reported effort describes single tooth bending fatigue testing which was accomplished to test the viability of certain
UltraSafe design features.
A dual rim gear blank configuration was developed to be compatible with existing single tooth bending fatigue test
equipment. Gears were manufactured and seeded faults were implanted to simulate unexpected defects in various
portions of the highly loaded gear tooth and rim sections. High loads were applied to initiate cracks at the seeded faults
after which the loads were reduced to allow the cracks to propagate. Crack propagation was monitored by measuring
effective mesh stiffness and applied loading. This paper presents the results of this testing which demonstrate the
feasibility of creating redundant structural load paths in a portion of a typical gear blank. This redundancy provides both
warning of an impending failure and a reasonable period of safe operation after initiation of a failure during which a safe
landing may be affected.