Elastic Network Vibration Isolators

open_access
Open Access
Author:
Rai, George
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 05, 2022
Committee Members:
Daniel Haworth, Professor in Charge/Director of Graduate Studies
Stephen Hambric, Outside Unit & Field Member
Joseph Najem, Major Field Member
Chris Rahn, Chair & Dissertation Advisor
Edward Smith, Co-Chair & Dissertation Advisor
Keywords:
Vibration Isolation
Vibration Control
Structural Dynamics
Band Gap Isolation
Abstract:
Passive vibration isolators are widely used in engineering applications where the operating conditions are known, such as helicopter gearbox isolation, engine mounts, or sensitive laboratory equipment isolation. Their passive nature makes them favorable over more complex active-control type vibration isolation techniques. The main challenge, however, is that passive isolators often trade a simplistic design and excellent isolation behavior for a larger footprint and a large mass-penalty. Fluid-based passive isolators trade improved isolation performance and footprint over strictly mechanical designs for a more complex manufacturing process and an increase in temperature sensitivity. Moreover, passive isolators are isolation frequency dependent, and sensitive to system disturbances that change the operational frequency. This thesis introduces elastic network vibration isolators, defined as a system of flexible elements that are designed and connected in a way to reduce the vibrations across the system from input to output. This class of isolators is based on the principle of antiresonance, where inertial forces cancel stiffness forces, leading to isolation at a tuned frequency. Two stacking methods for flexible elements are explored, namely, parallel and series stacking. It is shown that parallel-stacked beam network isolators overcome some of the basic challenges of passive isolation, namely High-Static-Low-Dynamic-Stiffness (HSLDS) behavior combined with low frequency isolation. Due to their continuous system characteristic, beam network isolators exhibit multiple interesting features, such as, multimode isolation frequencies, and multi-isolation frequency clustering, even at low frequencies. Using a beam-network design, a modified Dynamic Antiresonant Vibration Isolator (DAVI) with a flexible lever is introduced. An analytical model predicts that a compliant lever design can provide the same isolation frequency as that of a rigid DAVI but with half the auxiliary mass. The analytical model also shows that for the same auxiliary mass, compliant levers can provide lower isolation frequencies compared to DAVIs with rigid levers. The proposed isolator includes monolithic compliant features that are realizable with additive manufacturing, which reduces friction and wearing losses compared to traditional isolators. Two fabricated and tested devices validate the model and demonstrate the improved performance of a flexible design. The validated analytical model is used to design an isolator with elastomeric features to meet industry standard specifications. A proof-of-concept isolator is fabricated, and it is experimentally shown that the rubber-based isolator can provide 93\% reduction in stiffness at the tuned isolation frequency, in-line with theoretical predictions. Series stacking is explored for beam and plate elements. This thesis introduces nodal beam stack isolators, consisting of symmetric stacks of beams with tip masses connected in series via compliant hinges near nodal points. An analytical model is derived to predict the vibration behavior of a beam stack. It is shown that nodal beam stacks are capable of generating multiple bandgaps, (isolation over a frequency range as opposed to single frequency isolation) even at low frequencies. An isolator with four beams is designed to provide a band gap of 32 Hz with a center frequency at 72 Hz. A Polylactic acid polymer prototype is 3D printed and the frequency response of input force to transmitted force is experimentally measured showing a large 41 Hz band gap with a center frequency also around 72 Hz. The experimentally validated model is used to design a 10 beam isolator with a low frequency band gap from 10 to 86 Hz, and a high frequency band gap from 202 to 543 Hz. An experimental study shows that band gap effectiveness can be affected by rotational modes that are induced by load path misalignments, which reflects on the sensitivity of real life band gaps to small load path disturbances, as opposed to analytical band gaps that are generally deeper. Moreover, this thesis introduces plate stack isolators, consisting of axisymmetric stepped plates stacked in series and connected using compliant hinges. A Kirchoff-Love Plate model predicts the frequency response, and it is shown that these isolators are also capable of broadband vibration isolation over multiple frequency ranges. Additively manufactured polymer 4-plate and 2-plate isolators, show that multiple bandgaps can be achieved by model based tuning of plate geometries. The experimentally validated model predicts that metal isolators manufactured with Laser Powder-Bed fusion can achieve low frequency bandgaps and other design metrics of commercially available vibration isolators. The use of compliant hinges allows for a monolithic design of elastic network isolators (both beams and plates), which eliminates friction, leading to deeper stop bands and larger cycle lives. Elastic network isolators have a fairly complex structural design which could not be manufactured with traditional manufacturing methods. This thesis shows that additive manufacturing can be used to realize complex structures with a favorable dynamic performance.