Vibration Testing
Random Vibration Testing
The legitimacy of random vibration as an effective tool of screening workmanship defects came about during the first manned spacecraft programs. Up until that point, limited Hertz sine vibration was applied during reliability testing. Pure sinusoidal vibration is composed of a single frequency at any given time. Comparison tests revealed that to equal the effectiveness of random vibration, the test items will have to be subjected to many sine frequencies over a longer period of time, and may unintentionally fatigue the test item. Random vibration uncovers defects faster.
Real world simulation
Most vibration in the real world is random. For example, vehicle traveling over roads experiences random vibration from the roads irregularities. A ground-launched rocket vehicle experiences non-stationary vibration during its flight - the motor ignites, the rocket travels through the atmosphere, the motor burn ends, and so forth. Even a wing, when subjected to turbulent airflow, undergoes a random vibration response. Among items typically tested with random vibration are motorcycle component, jet engines, cruise missiles, catalytic converters and any products that will see transportation.
Random vibration is composed of multitude of, a continuous spectrum of, frequencies. Motion varies randomly with time. It can be presented in the frequency domain by a power spectral density function [G2/Hz where G is GRMS]. If real world data exists, NTS can use these measurements to develop a profile and implement testing. With just general predictions, NTS can estimate PSD with a formula PSD = g2/Hz or PSD = g2/(freqend freqstart).
NTS Random Vibe Capabilities
NTS is capable of generating up to 70,000 force pounds with tandem shakers and up to 45,000 force pounds on a single shaker. NTS has performed testing in excess of 200 GRMS in a single band. and can hook up in excess of 100 data channels. Our engineers understand proper placement of accelerometers and force limiting instrumentation. Need high number of data channels? Our systems can accommodate 60 or more data channels and still give you the data right at the end of the test with a formal report following in less than two weeks.
Random on Random Vibration Testing
NTS performs a wide variety of tests designed to evaluate equipment and materials when subjected to various types of noise/sound.
Structure borne
Undesired vibration in (or of) solid bodies such as machinery, foundations, ship structure, or cavitations caused by variation in fluid pressure are classified as structure borne noise. Noise that interferes with peoples' ability to perform their functions properly, especially when such tasks are in a critical environment, such as a naval ship during conflict, must be mitigated.
Airborne
When structure borne noise is coupled to a surface capable of radiating this noise into the atmosphere, the noise becomes what test standards describe as airborne noise. From shipboard structures and jet engines to concert halls and subway stations, noise measurement and abatement is a huge part of testing, design, and retrofit segments of the industry.
Self-generated
Noise created by rotor tips (either in turbofan engines or in helicopters) is deemed self-generated as it stems from the rotor's leading edge shocks, rotor-wake/stator interaction, tip-clearance vortex stator interaction, and tip-leakage induced noise, among others.
Testing and noise control consulting
NTS operates noise testing facilities capable of noise assessment per MIL-STD-740, ANSI standard NEBS requirements, and a compliment of other standards. All our measuring equipment is calibrated per the relevant standards and we have facilities equipped with very small, as well as very large, noise generators.
In addition to performing testing for all grades and types of equipment, NTS will also help you look for possible solutions and ways to reduce your broadband and narrowband noise problems, be it with suggesting the right density of damping isolators for machinery, expansion chambers to ameliorate low frequency noise, smooth pressure transition piping to prevent cavitations, interference mufflers tuned to self-cancel each other's noise, or the use of empty spaces as absorbers of sound in noise buildings and structures.
Sine on Random Vibration Testing
Vibration sine on random testing is performed by superimposing a sine wave on top of a random environment.
A sine on random vibration test duplicates the combined environment of a spinning helicopter blade with its distinct resonant levels and the rest of the aircraft which generates random engine and aerodynamic induced vibration. Gunfire on board an aircraft causes sine vibration while the rest of the aircraft generates random excitations. These types of tests are duplicating vibration characterized by dominant peaks (sinusoids) superimposed on a broadband background.
Another variation would be a swept sine on random test. The testing lab performs a sine frequency sweep such as 5Hz to 20 Hz which passes below and beyond an established resonance frequency level (such as 10Hz) to additionally stress the test material.
NTS can taylor your vibe testing to duplicate real life data or help you develop a valid simulation. We can utilize accelerometers, force limiting transducers and/or strain gauges, with over 60 data channels real time download possible. NTS facilities have safe, non-obstructed access for high worth hardware, and our personnel have an outstanding record of testing and handling delicate space equipment for over 45 years.
Sinusoidal Vibration Testing
Dynamic deflections of materials caused by vibration can cause a host of problems and malfunctions including failed electrical components, deformed seals, optical and mechanical misalignment, cracked or broken structures, excessive electrical noise, electrical shorts, chafed wiring. Because sine vibration is basically a certain fundamental frequency and the harmonics of that fundamental, in its pure state, this type of vibration is generated by a limited but significant number of sources. Expressed as amplitude versus frequency, sine vibration is the type of vibration generated in the field by sources such as engine rotational speeds, propeller and turbine blade passage frequencies, rotor blade passage and launch vehicles.
While much of "real world" vibration is random, sine vibration testing accomplishes several important goals in product qualification and testing.
Much material and finished product was modeled on some type of sine vibe signature. A sine sweep of frequencies will determine whether the assumptions were correct and if the deviations are significant enough to cause design changes. In other words, sweep will establish if the anticipated frequency has been met and/or discovers the test item fundamental frequency.
Similarly, a sweep will help identify the test subject resonance frequencies, which may be the points at which the item experiences particularly stressful deflections. By dwelling at those frequencies in subsequent tests, premature failures due to the properties of the material may come to light before the items sees field use. Some of the following tests include fixed frequency at higher levels of the controlling variable (displacement, velocity, acceleration), and random vibration. Per customer request, NTS will run sweeps in one direction, decreasing, increasing or bi-directionally and can change frequency logarithmically or linearly.
Another typical sinusoidal vibration test, sine burst such as the teardrop, goes rapidly to peak pulse and then decays at lower rate (to prevent damage to the unit). The burst test puts a maximum load into an article at a rapid rate and particularly stresses joints and seams to identify workmanship and design issues.
NTS understand the uses of sine vibration and the parameters required to help you develop and perform a valid, useful test. We have extensive experience working with designers and manufacturers from all industries, including automotive, transportation, nuclear, aerospace and consumer.
NTS has extensive experience in vibe test fixturing, understanding how to locate and take into effect the test item's center of gravity and its placement in relationship to the driving mass of the armature and preventing side load issues.
Our engineers understand proper placement of accelerometers and force limiting instrumentation. Need high number of data channels? Our systems can accommodate 60 or more data channels and still give you the data right at the end of the test with a formal report following in less than two weeks. We're capable of frequencies from 0 Hz to 200 Hz on hydraulic shakers, and up to 5000Hz on electrodynamic shakers. and due to a large number of advanced, well maintained electrodynamic as well as hydrodynamic vibration equipment, can develop the right combination of frequency, displacement and acceleration to satisfy your vibe test requirements.
