Old Engineer
I liked your analogy as to the spline wear being the "canary in the coal mine".
Here is why:
Spline coupling allows a shaft to be easily dismantled while providing a high-torque capacity for minimum size. In addition, it can allow relative axial and radial motions between the coupled shafts. Stability conditions can change should the alignment state between the driver and the driven machines (rotors) changes. In one manner, angular misalignment can greatly increase the slip, relative slip of small amplitude can occur between the contacting tooth surfaces. This can give rise to fretting damage which may limit the life of the coupling and has the potential to compromise integrity. This has been a particular concern in aircraft mechanical systems. In another manner, misalignment can cause high vibrations with different symptoms that sometimes cannot be explained. For a real system, there are two kinds of misalignments: static misalignment and dynamic misalignment (dynamic vibration displacement). In a real rotor-spline coupling system, all splines meshing tightly by transmitting large torque cause a deformation of each spline. At the same time, for coupling nodes vibrating with the system, there is relative displacement between splines in two half-couplings, which cause another deformation of each spline; both the deformations generate meshing force.
When the static misalignment appears and keeps constant, the following holds true:
(a) Meshing force changes linearly with dynamic vibration displacement, approximately, but there is an inflexion.
(b) The slope coefficient of meshing force curve reflects the stiffness of coupling, consequently, during the vibration of system, the stiffness of coupling is not a constant, it relates to dynamic vibration displacement and depends on static misalignment.
The dynamics of rotor-spline coupling system shows the following: 1X-rotating speed is the main response frequency when there is no misalignment; while 2X-rotating speed appears when misalignment is present; with the increase of misalignment, shaft orbit departs from original orbit, the magnitudes of all frequencies increase, and 2X-rotating speed increases rapidly. Thus, misalignment makes the vibration of a system more complicated.
This information is from:
Research Article
Meshing Force of Misaligned Spline Coupling and the Influence on Rotor System
Guang Zhao, Zhansheng Liu, and Feng Chen
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
Could this be a viable explanation as to increased loading on the bearings and bearing structures at high thrust levels, e.g. takeoff?
Turbine D