EFFECTIVE SOLUTIONS FOR SHOCK AND VIBRATION CONTROL Part 1 Alan Klembczyk TAYLOR DEVICES, INC. North Tonawanda, NY Part 2 Herb LeKuch Shocktech / 901D Monsey, NY SAVIAC Tutorial 2009
Part 1 OUTLINE Introduction - Brief Description Isolation - Types of Isolation Systems - Photos - Case Study of Effective Shock and Vibration Isolation Damping - Types of Damping Systems - Pictures and Videos of Damping Applications Shock Absorbing - Key Points - Photos - Videos
Shock and Vibration Isolators Shock Absorbers Dampers
Isolators, Shock Absorbers and Dampers all remove harmful energy from a dynamic system Isolators: Dampers: Absorbers: De-couple, or isolate to some degree, the input energy from a protected mass or structure. Some energy does get through, but isolator parameters are optimized to reduce the response to acceptable levels. Doesn t necessarily absorb a maximum amount of energy, but attempts to remove the input as much as possible. Continuously remove energy from a moving system to control its response. Absorb a maximum amount of kinetic energy, and bring a moving mass to a stop with minimal force.
The design of a system subjected to shock and vibration can be greatly improved by the addition of isolators, dampers, or shock absorbers Improvement Areas Include: - Reduced Response Acceleration - Reduced Deflection and Stress - Reduced Weight - Improved Bio-dynamics - Longer Fatigue Life - Architectural Enhancement - Reduced Cost
Isolation
TRANSIENT SHOCK RESPONSE Potential improvements from added isolation... Combining springs and dampers into a practical Combining springs and dampers into a practical shock isolator can often reduce stress and deflection by up to 95%, provided that sufficient rattle space is provided.
ISOLATION SYSTEM DEFINITIONS Passive System ~ Isolation system component parameters do not change with varying input. Adaptive ~ Component parameters can be adjusted manually or electronically to adjust the passive parameters. Also include systems that mechanically react to varying input. Semi-Active ~ Parameters change based on input from a sensing element but do not drive the system with internal power. Fully Active ~ React to varying input, change parameters accordingly, and drive the isolated mass with an internal power system according to control algorithms.
SHOCK ISOLATORS ~ SPRING ELEMENTS Mechanical-Coil, Leaf, Wire Rope ~ Advantages: Low cost, long life Disadvantages: Bulky, large sizes unavailable Elastomer-Tube, Block, Shear, Strap ~ Advantages: Low cost, moderate life Disadvantages: Temperature sensitive, not manufactured in large sizes Pneumatic ~ Advantages: Disadvantages: Compact, moderate life Temperature sensitive, difficult to seal in large sizes
SHOCK ISOLATORS ~ SPRING ELEMENTS CONTINUED Liquid ~ Advantages: Disadvantages: Very compact, moderate life, easy to incorporate damping Temperature sensitive, requires high-strength steel, somewhat expensive Machined-Helical, Cantilever ~ Advantages: Disadvantages: Very compact, can be made in any size, has almost zero damping Expensive
MECHANICAL ARRANGMENT OF SPRING ELEMENTS Un-Centered ~ Displacement changes with load like an auto suspension Soft Centering ~ One spring element is used to precompress a second spring element to mid-stroke Hard Centering ~ Spring is loaded by a mechanism to provide a re-centering force in either direction from center. The hard centering force is usually 2-4 Gs
FOR TRANSIENT SHOCKS If pulse is well known, minimum G s or deflection results from large amounts of damping more than 40% critical. If pulse is not well known, the least risk design will have 25% critical.
FOR TRANSIENT SHOCKS Oftentimes requires a full shock analysis and optimization procedure for defining isolation system parameters. Most optimization is iterative. Optimization will define isolation attributes such as stroke (travel), frequency (spring rate), damping level, and preload, if any.
FOR HIGH FREQUENCY VIBRATIONS Specialty damper designs exist that roll-off their output from a true linear damper. In some simple cases, just mounting the damper with soft mounts can provide some roll-off.
Good Vibration Isolation System Bad Vibration Isolation System
No System Friction Analytical Predictions Small Amount of Friction
Useful Equations PSD Power Spectral Density G rms ( PSD )( Frequency ) db logtr(20) TR ( transmissibility ) G response G input
Vibration Isolation Mount
Case Study of Effective Shock and Vibration Isolation
Mobile Launch Platform (MLP)
DYNAMIC SYSTEM BREAKDOWN Three main phases: 1. Define the dynamic environment that exists. 2. Define the fragility level of the equipment. 3. Determine whether or not mitigation through isolation or other means is necessary. Isolation system parameters required to be defined through analysis or test as necessary.
DYNAMIC ENVIRONMENT Defined by direct measurement. - Response during launch of the floor of the MLP was captured using accelerometers in the vertical direction during a recent Shuttle mission. - This response was then converted into the form of a Power Spectral Density, PSD. - This data was to be used as the input to which p the system was to be analyzed.
ANALYTICAL MODEL Provided preliminary estimate. PSD Input. - Recorded input in vertical direction only. - Assumed equivalent in all three orthogonal directions. PSD Input 1 0.1 g 2 /Hz 0.01 Original Scaled 0.001 0.0001 1 10 100 1000 Frequency (Hz)
Switchgear Analytical Response Switchgear Response at Center of Gravity: Y-Direction g 2 /Hz) PSD ( 1.00E+00 1.00E-01 1.00E-02 1.00E-03 100E-04 1.00E 04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1 10 100 1000 MOD TRANS Frequency (Hz)
Damper Spring Assembly
STS-115 Shuttle Launch September 9, 2006
STS-117 Shuttle Launch June 8, 2007
Z-Direction STS-117 Launch Maximum G-Force in Z-direction 6 4 2 Side A Base G-Forc ce 0-2 -4-6 -8 Z Maximum Change in G Force Percentage Maximum Minimum 71.32% 70.35% 73.44% 77.17% Minimum Side A Struct. t Side B Base Side B Struct.
Input / Response Summary X Y Z Input Response Change in G-Force Percentage Maximum Minimum Maximum Minimum Maximum Minimum 8.7153-8.1549 3.0363-2.2252 65.16% 72.71% 11.282-10.119 2.5143-2.6058 77.71% 74.25% 2.5935-1.8545 0.82451-0.68862 68.21% 62.87% 2.2127-2.4847 1.222-1.1423 44.77% 54.03% 6.0495-5.5254 1.7348-1.6384 71.32% 70.35% 6.0845-7.7605 1.6158-1.7715 73.44% 77.17%
Damping Systems
TYPES OF DAMPING DEVICES SELECTION CRITERIA When utilizing ing a damping device, one must have the following: 1. The exact output function of the damper over the entire anticipated translational velocity range 2. All environmental aspects of the application, and how these will affect damper performance 3. A software code that can accurately model the anticipated non-linearities and environmental performance shifts of the damping device
HOW MUCH DAMPING CAN BE USED? 1. Most structures have inherent damping of 1% 5% of critical 2. Automotive suspensions have fluid dampers of 20% 25% critical 3. Truck suspensions have fluid dampers of 30% 40% critical 4. Damping of 50% critical will prevent amplification in a structure subjected to forced resonance 5 Military applications often use damping up to 2000% of critical to 5. Military applications often use damping up to 2000% of critical to suppress weapons shock when used in an isolation system
TYPES OF DAMPING DEVICES 1. Structural 2. Coulomb Friction 3. Elastomer 4. Active Drivers 5. Passive Hydraulic 6. Semi-Active Hydraulic 7. Tuned Mass Dampers
Frequently Used Equations C cr 2 ω m ω natural frequency ( ω 2 π ( frequency) C cr 2 km k m 2 x acceleration rad sec )
Structural Damping ~ 1. Inherent in a structure, not inherent in a mechanism 2. Magnitude varies widely with the design of the structure and construction tolerances thereof 3. Can be as low as 0.5% critical for a rigid structure 4. Can be as high as 10% critical for massive structures having lightweight construction and complex joints
Passive Hydraulic Damping ~ Modern dampers achieve much different outputs t than the classical case. These outputs are optimized for performance in systems subjected to a highly variable pulse field, those that are real world in nature. In general, an ideal damper has an output that is completely out of phase with structural bending and shear stresses. This allows the damper to reduce both stress and deflection, simultaneously
CURRENT TYPES ~ PASSIVE HYDRAULIC DAMPERS 1. Fluidic Uses specifically shaped orifice to achieve output characteristics ranging from: F = CV^0.4 to F = CV^2 2. Pressure Responsive Valve Uses Multiple spring loaded poppet valves: F = CV^0.2 to F = CV^1.8 3. Metering Tube Uses a piston which progressively covers a series of ports output is: F = CV^2*f(x) This design is effective only when tuned for a specific pulse signature 4 Metering Pin Similar to metering tube but orifice is 4. Metering Pin Similar to metering tube, but orifice is continuously varied
TEST RESULTS Let s consider some tests by MCEER with a complex seismic input into a structure, with added dampers. In this case, the seismic pulse field indicated that a linear damper, F = CV, was a best fit
1-Story, No Dampers, El Centro 33.33% Total Damping = 2%
1-Story, 2 Dampers, El Centro 100% Total Damping = 22%
Tuned Mass Damping ~ A suspended mass set near the natural frequency of the structure that oscillates out of phase with the structure to effectively damp the response of the structure to external influences Advantages: Disadvantages: Relatively easy to incorporate and install into one location within a structure Works only at one frequency and provides only limited damping
Tuned Mass Damper
Effective Damping from TMD
Performed a Fourier Transform to convert Performed a Fourier Transform to convert data from time based to frequency based
Shock Absorbers (Kinetic Energy Killers)
Squaring the curve technique for the worst motion of a transient F Ideal Constant Force Response Damping Force Spring Force a. Determine how damping must vary with displacement during this discrete cycle b. Obtain data for velocity across damper at various values of X c. You now have an idealized damping function, where: F 0 =f f(v) X
Useful Equations mv KE Energy Kinetic 1 ) ( 2 x F DE Energy Driving gy ) ( ) ( ) ( 2 ) ( TE Force Absorber DE KE TE Energy Total ) ( Force Absorber G s Factor Efficiency Stroke Force Absorber ' ) )( ( Mass of Weight s G '
The most efficient shock absorber reacts with constant t force over its entire stroke for a given input. For a system with varying velocity input, a V 2 damper works best as long as the F vs. X curve remains square. Why? Force should vary with velocity the same way that energy does. Machines and mechanical systems are usually optimized with minimal acceleration. However,... people like minimal jerk (X). People have an efficient active/semi-active isolation system in how they react to impulses.
Typical Output Curve
CONCLUSIONS 1. Isolation, damping and shock absorbing systems are very useful tools in the world of shock and vibration. 2. These are complex tools and no manuals are available. Thus, a close relationship with suppliers should be established for optimal use of the technology. 3. It is essential that the analyst and system engineer be equipped with a full database of isolation system attributes that have been proven effective and reliable in a wide variety of shock and vibration isolation applications in the past.