From sub-atomic particles all the way up to skyscrapers, internal movements and motions resulting from the absorption of energy make all objects vibrate to some degree. This fact means that in a world filled with energy and movement, vibrations — or the oscillating responses of objects when moved from a position of rest — are the norm.
Some vibrations are expected and even needed for products to function as expected. As a perfect example, think of traditional speakers that turn energy into vibrations, which ultimately allows music lovers to hear their favorite singers and musicians. Another example is the tightly stretched diaphragm contained in the chest piece of a stethoscope, which, when excited by sound waves, allows a physician to listen to a patient’s heartbeat and/or breathing.
Of course, not all objects vibrate in a way that’s helpful or even intended. For instance, there probably isn’t a civil engineer alive who doesn’t know the story of the Tacoma Narrows Bridge and how 40-mile-per-hour winds induced its collapse due to structural vibration. As for the rest of us, we know of the bridge’s final, fateful moments on November 7, 1940 thanks to the frequently viewed footage captured by camera store owner Barney Elliott. The film shows the bridge going into violent wavelike motion before breaking up and falling into Washington State’s Puget Sound below.
A more recent example of unintended vibration is the now famous June 10, 2000 opening day of London’s Millennium Footbridge. The combined synchronous movements of pedestrians caused what’s known as positive feedback — a swaying motion emanating from the natural human instinct to remain balanced while walking. The effect resulted in Londoners dubbing the structure the “Wobbly Bridge.”
Fortunately for manufacturers and consumers alike, the materials and products we rely on today in everything from airplane wings to suspension bridges are made stronger and more reliable thanks in large part to vibration testing.
What Is Vibration Testing?
The applications of vibration testing form a major part of quality assurance for everything from printed circuit boards to steel girders. With specifically designed equipment — such as mechanical, electro-hydraulic and electromagnetic shakers — test samples are typically subjected to varying degrees of controlled vibration stresses.
Of course, it takes both precision test equipment and a highly trained laboratory staff to consistently produce vibration testing results that industries ranging from consumer electronics manufacturers to military contractors can count on. Fortunately, with an ISO/IEC 17205 certification from the American Association for Laboratory Accreditation, NTS comprises one the nation’s largest networks of commercial testing labs with the experienced engineers and technical specialists to tackle even the most complex vibration test projects.
To determine the physical limits of a test specimen, a controlled lab test can subject a material sample to sine vibration testing, random vibration testing or both. For products whose intended applications are not of a high-risk nature, sine — or sinusoidal — testing may be sufficient. In sine testing, the specimen is subjected to an even rate of vibration for the duration of the test.
In contrast, for more robust vibration testing, random vibration testing subjects specimens to unpredictable and random rates of vibration — like an automobile running over a potholed road’s surface or a boat’s hull being struck by a series of rogue waves.
With a full line of vibration testing services available, NTS can perform both random and sine vibration testing for a diverse selection of clients including automotive, aerospace, aviation, consumer goods, electronics, medical devices and military equipment manufacturers. Anticipating how a product will react to the shocks and vibrations it encounters during shipping or everyday use is a critical part of testing and quality control. With shaker tables in a variety of sizes, including dual shakers that can meet 70,000 force pounds, our vibration testing labs are fully equipped to perform a full selection of tests that will allow you to make informed decisions and comply with your industry’s regulatory requirements — from MIL-STD-810 to FAA regulations.
Real World Simulation Through Random Vibration Testing
Most vibration in the real world is random. For example, a vehicle traveling over roads experiences random vibration from the road’s irregularities. A ground-launched rocket 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.
For these reasons, it is essential that any vibration testing protocol accurately reproduces the diversity of forces encountered by an object over the course of its operational life. Random vibration testing to failure can identify weaknesses and other design issues, allowing you to further refine your product. Among the many items typically tested with random vibration are motorcycle components, jet engines, cruise missiles, catalytic converters and any products that will see transportation.
Types of Vibration Testing
The key to vibration testing is first and foremost to know the exact levels of stress you expect the device under testing — or DUT — to either endure or not tolerate. To accomplish this, testing labs must be able to precisely control both the frequency and amplitude of the vibrations they’ll be subjecting the DUT to.
Put simply, the frequency of vibrations is a measurement of how many peaks the DUT will face over a given period. In similar fashion to other wave-like energy such as light and electricity, vibration frequency is measured in Hertz — most commonly in kilohertz representing 1,000 peaks per second.
In addition to frequency, amplitude is a measurement of the maximum amount of vibrational force exerted on the DUT from its position of equilibrium. Amplitude can be highly varied to match the changes in peaks — or energy — that manufacturers expect their products to face in their real-life applications. Like frequency, vibration amplitude is also measured according to its peak value employing the Hertz scale.
Manufacturers typically employ vibration testing for two distinct reasons. The first involves the preliminary testing of a product — before it goes into production — in order to assess its design and durability. It’s at this crucial point that manufacturers of everything from aviation parts and military equipment to consumer electronics and medical devices can still make changes to a product before it’s released for use. In addition, this type of vibration testing assures makers that their products meet the standards of their respective industries.
Of course, to better predict failures and maintenance scheduling, as well as more accurately estimate the lifecycle of product that’s already in use, companies also employ vibration testing in secondary evaluations. Especially when it comes to mechanical devices like engines and turbines that continually produce vibrations throughout their natural lifecycles, the benefits of secondary evaluation vibration testing are invaluable when a catastrophic failure can be avoided.
In many examples of vibration testing, the mechanism used by accredited testing labs like NTS is commonly referred to as a shaker table or shaker. While the name may sound simplistic, the machinery is anything but simple. Shakers are actually highly precise pieces of testing equipment that can subject specimens ranging from a couple of ounces to thousands of pounds to incredible vibrational forces. This function is in part why the proper running of shakers requires skilled operators, like the ones at NTS, who are highly experienced in producing vibration testing results that industry leaders can count on.
By now, it should be clear that to get any value out of vibration testing, you must always be aware and in control of the exact frequency and amplitude of the vibrations the DUT will be exposed to as well as whether they’ll be sine or random vibrations. A specimen must first be firmly fixed to a shaker table by means of a standard spring-activated clamping device, or in many instances with highly complex test pieces, by means of custom-built test fixtures. NTS has a great deal of experience building these types of fixtures to meet our clients’ specific testing needs.
Naturally, not all test specimens need to tolerate the same levels of stress and vibration, so not all shakers are the same. In general, shaker testing can be classified by the mechanisms driving the vibrations in a given type of shaker.
The three main categories of shaker testing are generally grouped according to the type of vibration testing instrument employed:
- Mechanical vibration shakers: For the most affordable form of vibration testing, mechanical shakers are highly reliable. However, due to their limited range of speed and oscillation, mechanical shakers are not always able to meet the testing requirements of heavy-duty test products.
- Electro-hydraulic shakers: Electro-hydraulic shakers employ a source of electricity to activate a hydraulically controlled shaker table. This form of shaker testing is costlier than mechanical shakers, but it produces a wider range of vibrations for products that require more robust testing.
- Electromagnetic shakers: To create some of the highest levels of vibrational stress — for example, for products for the aerospace and aviation industries — electromagnetic shakers have the ability to be fine-tuned while generating vibration rates beyond what any other type of shaker can produce. They’re the costliest of the three types of shaker testing.
Vibration Testing Standards
Since all products are subject to some amount of vibration in their daily use, numerous associations and agencies produce vibration testing standards that certain industries must adhere to. Here’s a concise selection of some of the most recognized vibration testing standards and guidelines:
- AC 156 Seismic: For manufacturers of equipment used in areas that are prone to earthquakes, seismic testing provides validation that their products can withstand a certain amount of seismic energy.
- ASTM Standards: The American Society for Testing and Materials (ASTM) International publishes various standards related to vibration levels as they pertain to the manufacture and transport of a wide variety of goods — from biomedical and military equipment to aviation landing gear and automotive parts.
- ISTA Methods: The International Safe Transit Association publishes several testing methods that are aimed at ensuring that transport stresses — including vibration — don’t adversely impact the safety of the equipment used in the industry.
- ISO Directives: The International Organization for Standardization issues several directives, including directive 10816 that’s aimed at setting requirements particular to vibration testing, as well as directive 18436-2, which allows qualified personnel to be certified in vibration condition monitoring.
- MIL-STD-202: Issued by the Department of Defense, MIL-STD-202 is concerned with the quality of electronics and electrical components that contractors supply to the military — including products’ abilities to withstand certain levels of vibration.
- MIL-STD-810: This military standard sets out rigorous testing methods — including vibration testing — in order to simulate the harsh environmental conditions under which products and devices will be expected to perform in the field.
NTS Vibration Testing 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 vibration 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 a 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.
NTS performs a wide variety of tests designed to evaluate equipment and materials when subjected to various types of noise/sound. Contact us today for more information about how we can assist with your specific vibration testing requirements.
NTS vibration testing labs have been carefully calibrated to compensate for the impact of ambient vibrations on testing processes. All buildings have some degree of undesirable vibration, often aggravated by nearby equipment. Minimizing the transfer of vibrations between testing equipment and the facility floor is essential — this is typically done through the use of air bearing pads, high-mass bases and other system components.
At NTS, our facilities have been specifically set up in accordance with current best practices for reaction mass, isolation mass and inertial mass design. This allows us to produce the most accurate results when performing vibration tests on sensitive components.
50 Years of Vibration Testing Expertise
Advanced facilities are only one of the reasons why NTS has been a valued vibration testing partner to manufacturers and contractors for more than 50 years. Our engineers are available to assess your project requirements, determine the best sequence of tests to prepare your product for the market, or meet military, NASA, ISO or other standards. To request a quote or further information about our vibration testing services, contact us by phone or email.