1. Tuned Mass Damper Investigation for Apache Struts
RAM 8 Workshop
November 3 & 4, 2015
Paul Riehle
Roush Industries

2. Overview Tuned Mass Damper (TMD) Background Apache MRGB Strut Dynamics
MRGB Strut TMD Application

3. Rubber Applications for NVH Control
Isolation Systems
Tuned Mass Dampers
Free Layer Damping
Constrained Layer Damping
Damping Links
from RAM 5 Workshop - October, 2012

4. Rubber Applications for NVH Control
Tuned Mass Dampers
mass ratio is important
stiffness and damping are important
frequency tuning is critical
rubber elements typically used for TMD damping and stiffness
TMD
Base
System

5. Tuned Mass Damper Examples
Large Torsional
Damper
Planer Dampers
Tuned to
Two Frequencies
Transfer Case
Tuned Damper Heaven!!
Tuned
Damper

6. Mounting bracket resonance issue controlled with TMD
Transmission Mount Bracket TMD
Mounting bracket resonance issue controlled with TMD
Gear Whine
due to
Bracket
Resonance

7. Rubber Property Nonlinearities
Temperature
Frequency
Static preload
Dynamic strain amplitude
Important Rubber property nonlinearities
will cause TMD performance
nonlinearities.

8. Roush TMD Design Process

9. Apache MRGB Strut TMD Application
Objectives
Reduce noise and vibration that passes from the main rotor gearbox (MRGB) through the gearbox mounting struts to the Apache airframe resulting in improved component durability and reduced aircrew fatigue.
Approach
Use modeling, component testing, and rig testing to develop concept TMDs for MRGB struts.

10. Apache MRGB Strut Dynamics
Testing by Roush on a “Blue Rig” identified the MRGB mounting resonance frequencies, damping, and mode shapes.
Results indicated that strut structural dynamics create amplifications of MRGB operating vibration and forces.
Important resonances groupings occur in the Hz and the Hz frequency range where the struts exhibit 1st and 2nd bending modes, respectively.
Apache MRGB primary gear mesh excitation forces align with 2nd bending modes.
“Blue Rig” Test Stand
Struts
MRGB

11. Strut Bending Modes 1st Bending Modes 130 – 326 Hz 2nd Bending Modes

12. Modal Testing Measurement Locations
Single Strut Test Fixture
Modal Testing Measurement Locations
Strut:14
Strut:11
All individual strut testing was performed using a bedplate fixture that was designed to mimic the Blue Rig approximation.
Initial modal testing was performed using the measurement locations shown to the right.
This was done to attain a correlation between the physical test and FE model.
Strut:3

13. TMD Simulation – Lateral Mode Tuning
Strut Point 3 - Lateral
Figure 3
Performed curve fit on FRF data for 408 Hz Lateral Mode
Participating Inertia of strut is ~3 Kg.
Very low damping indicates that tuned damper will be very effective.
~22 dB
Figure 4
Damper simulation with 0.1 kg (~¼ Lb) inertia mass shows high attenuation of the response peak

14. FE: Single Strut – TMD
18 dB
8 dB
The TMD proved successful in damping the chosen mode by 18 dB in the lateral direction and subsequently reducing the vertical second order mode by 8 dB

15. Testing: Single Strut TMD
~20 dB
~5 dB
Baseline Strut (No TMD) Strut with Second TMD Tuning Strut with Large Ring TMD Tuning

16. FE : Blue Rig
TMDs

17. FE: Blue Rig – TMD Global Z Direction
Significant response reduction in the frequency ranges of the 2nd strut bending
Reduction of response amplitude for the second order bending modes ( Hz) is up to 18 dB

18. Testing: Blue Rig – TMD Installation
Blue Rig Measurement Locations
Blue Rig TMD Installation Locations*
Impact FRF measurements were made with and without the TMDs installed.
The TMD installation locations were chosen based on the individual strut testing installation location.
The TMDs were installed using the same local coordinate system that was used for the individual strut testing.

19. Testing: TMD Results, Blue Rig Struts Base
Point 6 Excitation
Bedplate: 4 Response
X Response Direction
~20-25dB of FRF amplitude reduction are seen at ~ Hz.
Y Response Direction
Z Response Direction
Baseline
TMDs on All Struts

20. Testing: TMD Results, Blue Rig Struts
Based on testing
Frequency response of the Blue Rig system was damped very well
It was seen that similar benefits could be achieved using fewer dampers that are strategically placed

21. Summary
TMDs can be very effective at controlling structural resonant and forced responses. It is critical to optimize the TMD mass, damping ratio and tuning to maximize the performance for a given application.
Rubber materials are recommended to provide the required TMD damping and stiffness. Material selection criteria should consider the environment and operating conditions such as frequency, temperature, dynamic strain, and static pre-strain. The nonlinear behavior of the TMD must be understood.
TMDs were designed for the Apache MRGB mounting struts to reduce the amplification of gear mesh vibration transmitted to the airframe. The TMD development process required the use of several analysis tools and test setups. The TMDs produced a 10 to 20 dB drop in MRGB strut forced response levels.

22. Questions?