A natural frequency is the frequency at which a system will oscillate after an external force is applied and then removed.  All objects have a natural frequency at which they vibrate.  Most products have many natural frequencies.

During vibration testing using vibration shakers, a sinusoidal vibration resonance scan or resonance search is used to study resonances in products.  When performing a resonance scan, the vibration table exposes the product to controlled forced vibrations through a range of frequencies in an effort to study the product response vibrations.  The forced vibration levels are low (typically ½ G) and are considered non-damaging.  The product response accelerations are compared to the controlled table accelerations.  Any amplification equal or greater than 2 to 1 is typically considered a resonance.  The frequency at which this occurs is called a resonance frequency.  DES has seen very severe resonance peaks greater than 20 to 1!  A typical resonance scan plot is shown below.   As an example, the product (DUT) response acceleration is 2.288 G’s at 688.1 Hz.  The table accelerations (Control) are ½ G at 688.1 Hz.  Thus, 688.1 Hz is considered a resonance frequency because the product to table acceleration ratio is 2.288G/0.5G = 4.576, which is greater than 2 to 1.

Some random vibration test specifications such as RTCA DO-160 (Environmental Conditions and Test Procedures for Airborne Equipment) require a sinusoidal vibration resonance scan before and after the test along each axis.  Any significant change in the resonance scan results could be sign of damage or product failure.

The most famous failure due to resonance was the catastrophic Tacoma Narrows Bridge collapse as seen in our blog article Vibration Response of Products.  When products are exposed to vibrations near their natural frequencies, fatigue failures from a vibration resonance can occur.  Compressors and motors are examples of equipment that could generate significant vibrations.   Products installed near such equipment should not have natural frequencies near their running speed.

In order to prevent early fatigue failures in your product, DES is capable of testing and analyzing the vibrational response of your product using their wealth of knowledge and vibration analysis software.

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See Sinusoidal and Random Vibration Testing Primer for a more technical explanation of sinusoidal vibration testing.

See Sinusoidal Vibration Testing to learn more about the different types of sinusoidal vibration testing.

Structural vibration testing and analysis contributes to all of the progress in many industries, including aerospace, automobile manufacturing , tool manufacturing, wood and paper production, power generation, defense, electronics, telecommunications and transportation. The most common application is identification and the suppression of unwanted vibration to improve the quality of the company’s product.

Vibration Analysis

Vibration is divided into three sub-categories of:

• Sinusoidal vs. random vibration, and
• Linear vs. rotation-induced vibration
• Free vs. Forced

Free vibration

Structure reacts to being impacted or displaced. What happens next is completely, absolutely determined by the structure properties, and the vibration understood by examining the mechanical properties.   When you strike a guitar string, it vibrates at the tuned frequency and gives you the desired tone.  Frequencies of tone is a function of the tension in the instrument’s string, and unrelated to the striking or strumming.

Forced vibration

Forced vibration means the structure’s response to repetitive forcing function causing vibration of the structure at the excitation frequency.  Your rear view mirror in an auto always vibrates at the frequency paired with the engine’s RPM.  In forced vibration a relationship exists between amplitude of the forcing and the vibration level that corresponds.

Sinusoidal Vibration

This vibration is a special class. The structure excites by the forcing with a pure acceleration and one frequency.  Sinusoidal vibration is not common in nature by any means but always provides engineering that permits understanding complex vibrations when broken down into one tone vibration.

The motion of any structure point can be described as a sinusoidal function of Time.

Random Vibration

With this vibration which you always feel when driving a car results from a complex combination of the rough road surface, engine vibration, and outside wind buffeting the car. They are usually described using statistical parameters. Random vibration quantifies the vibration over time across a frequency spectrum.

Vibration Fatigue

Engineers and managers should keep firmly in mind that the term, Vibration, describes the repeating motions which are measured in a structure.  Any unwanted vibration causes fatigue and degrades performance of the structure.  Metal fatigue in airplanes is a paramount cause of air disasters, and can be avoided with proper and correct maintenance to fight metal fatigue from plane vibrations.

Therefore it is desirable to eliminate or reduce the effects of vibration. In other cases, vibration is unavoidable and even desirable. The goal may be to understand the effects of vibration on the structure’s compounds, to try to control or modify the vibration, or isolating vibration from the structure and minimize the structural response in a disastrous manner.

Structural Vibration

Structural vibration can be very complex.  Therefore it is best for the novice to begin with a simple model to gain basic concepts and build up to advanced systems. The simplest vibration model is:

• the single-degree-of-freedom or
• mass-spring-damper model.

It consists of a simple mass suspended by an ideal spring with known stiffness and a dashpot damper from support. A dashpot damper is like an automobile shock absorber and produces an opposing force proportional to the velocity of the mass.

Mass Matters

Imagine for a moment that your choices in configuring the vibration effect on are a bowling ball and an automotive spring.   Surely the heavy weight of the sizeable bowling ball would be more resistant than the coiled automotive spring.  These two factors will have utmost importance as engineers endeavor to calculate the speed, velocity and resistance factors of the two test objects vs. the vibration strength to which they are subjected in your experiment.

It is important for our engineers, scientists, statisticians and manufacturers of our automobiles to airplanes to possess the educated abilities to assess how a simple vibration in one seemingly insignificant part can cause a disaster of great proportions.  Moreover, it explains to the professionals how that insignificant part can become significant and prevent the disaster.

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To keep up with increasing vibration and shock testing demand, DES added a brand new Unholtz-Dickie Electro Dynamic (ED) Shaker Test System. The shaker is a model SAI30F-S452/ST system with slip table to perform vibration and shock testing along 3 axes. This gives DES additional vibration and shock testing capability and also will help us turn your projects around faster.

The Unholtz-Dickie SAI30F-S452/ST specifications are:

• 6,000 lbf sine force
• 5,500 lbf random force
• 100G peak vibration acceleration
• 200G peak shock acceleration
• 1 to 3,000 Hz frequency range
• 2 inch stroke
• Vibration: Sine, Random, Sine on Random, Random on Random
• Replication of measured field data
• Gunfire vibration

DES can perform the most complicated shock and vibration test projects with our:

• Two (2) ED Shaker vibration and shock test systems
• Two (2) AGREE temperature cycling chambers to perform combined vibration and temperature testing.
• State of the art controllers
• Many high speed data acquisition channels

What sets us apart from other labs is our in depth experience and technical capability to understand and reproduce the most complicated vibration profiles. We also continually invest in new, high-quality equipment to ensure that vibration and shock testing are as accurate as possible. We have performed vibration and shock test on complex products used in automotive manufacture, space applications, rocketry, and medical and military environments.

The post DES Adds New Unholtz-Dickie Vibration Test System appeared first on Delserro Engineering Solutions.

Vibration testing can be a complicated process. We have created this questionnaire to help make communication between the vibration test lab and customer more efficient. The questionnaire allows us to capture all the pertinent facts about your test requirements. Providing the information below will help us provide an accurate quotation and to perform a successful vibration test. We can help you with answering the questions if needed. Many engineers, not familiar with vibration and shock testing, a subject not taught by universities, may wish to further their education.  May we respectfully suggest that you visit http://equipment-reliability.com/training-calendar/vibration-and-shock-testing/  for vibration and shock training courses.

1: What are the vibration test specifications?

A: Please list any required test specifications such as MIL-STD-810 or RTCA DO-160 or IEC?

B: What is the type of vibration (random, sinusoidal, etc.)?

C: What is the acceleration level in G’s or m/s²?

D: What is the frequency range?

E: How many axes to be tested?

F: What is the duration per axis?

G: Please provide other known details such as sinusoidal sweep rate, number of sweeps, random vibration PSD profile

There are many different vibration test specifications. Many times the questions above are clearly answered in a test specification such as MIL-STD-810G. However sometimes they are not and the customer will need to provide further details. A good vibration test lab can help you answer the questions above.

2: Is combined temperature and vibration required? If yes, please answer the sub questions. If no skip to the next question.

A: What are the maximum or minimum temperatures?

B: Are the temperatures constant or cyclic?

C: If cyclic, please define the temperature cycle parameters such as number of cycles, temperature transition rates and the dwell times at each maximum and minimum temperature.

3: Please provide information on the products or Devices Under Test (DUT’s).

A: What are the dimensions of the DUT and what is the mounting foot print?
(A drawing or sketch or picture with some overall dimensions is usually helpful)

B: How much does the DUT weigh?

C: How many DUT’s will need to be vibration tested?

This information is required to assess vibration test fixtures and also to determine if the DUT will fit on the vibration shaker table. Remember vibration shakers are rated for maximum force. The maximum force is determined from the formula

Force = Mass (or Weight) x Acceleration

The units for Mass are usually pounds or kilograms and acceleration is usually given in G’s or m/s².

It is important to note that the Mass is not only from the DUT’s but is:

MASS or Weight = weight of shaker armature + shaker table (slip table or head expander or cube or angle fixtures) + DUT fixture + DUT’s + weight of any other adapter fixtures or significant added moving weight.

The customer only needs to provide the weights of components they are supplying such as the products to be tested, the fixture weight if they are supplying the fixture, and any other significant moving weights attached to the shaker such as heavy cables.

4: Who will supply the DUT fixture? (Customer or Test Lab)

The DUT will have to be attached to the vibration table by some means. Typically clamps or a flat aluminum adapter plate are used. The adapter plate is bolted to the vibration table and the DUT is bolted to the adapter plate. Vibration test fixtures can be simple or elaborate depending upon the DUT and test requirements. A good fixture design is important for a successful vibration test. It is important to have good communication with the vibration test lab about vibration test fixtures so that no surprises occur before the start of the vibration test.

5: Does the DUT need to be powered and monitored during the test? If yes, please answer the sub questions. If no skip to the next question.

A: What are the power requirements

   i: What is the voltage
ii: Is it AC or DC?
iii: What is the power or current required?
iv: Is it single or 3 phase?
v: If AC current, what is the frequency (Hz) required for the electrical power source?
vi: Any special electrical loads required?

B: Who will provide the power source if a special source is required? (customer or test lab)

C: What are the monitoring requirements to verify that the DUT is operating properly during the test? (This can be as simple as visual observations of LEDs or continuity measurements or comprehensive electrical measurements)

D: Who will provide the necessary equipment required to monitor the DUT? (customer or test lab)

6: Do you need response accelerometers to measure any DUT resonances? If yes, please answer the sub questions. If no skip to the next question.

A: How many response locations do you need?

B: Do you need single or three axis accelerometers?

7: List any other unique requirements needed for the vibration test

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Sine Sweep Vibration Testing traverses or sweeps between a low and high frequency. The G levels and displacements can be constant or variable. It is useful for identifying resonances by comparing response vibrations of the product to the vibrations on the shaker table. Also many test specifications use Sine Sweep Vibration Testing to demonstrate the endurance of the devices under test by requiring many sweeps.

Sine Dwell Vibration Testing involves vibrating at a specific frequency and G level amplitude. Typically the dwells are performed at resonances found in the product under test. However the sine dwells can be at the frequencies corresponding to equipment running at specific speeds such as reciprocating compressors.

Sine-on-Random Vibration Testing consists of superimposing sinusoidal spikes over a random vibration profile. Sine-on-Random Vibration Testing simulates environments for propeller driven airplanes and helicopters. Sometimes Sine-on-Random Vibration Testing is referred to as Mixed Mode Vibration Testing.

Some common test standards that have specifications for Sinusoidal Vibration Testing are:

• ASTM D999 Standard Test Methods for Vibration Testing of Shipping Containers
• ASTM D3580 Standard Test Methods for Vibration (Vertical Linear Motion) Test of Products
• IEC 60068-2-6 Environmental testing Part 2-6: Tests –Test Fc: Vibration (sinusoidal)
• MIL-STD 167 Department of Defense Test Method Standard – Mechanical Vibrations of Shipboard Equipment
• MIL-STD-202 Department of Defense Test Method Standard for Electronic and Electrical Component Parts
• MIL-STD-750 Test Methods for Semiconductor Devices
• MIL-STD-810 Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests
• MIL-STD-883 Department of Defense Test Method Standard for Microcircuits
• RTCA DO-160 Environmental Conditions and Test Procedures for Airborne Equipment

The post Sinusoidal Vibration Testing appeared first on Delserro Engineering Solutions.

See Sinusoidal Vibration Basics to learn more about vibration fundamentals.

See Sinusoidal Vibration Testing to learn more about the different types of sinusoidal vibration testing.

Examples of vibration test videos can be found on our YouTube page.

Sinusoidal or Sine Vibration Testing

Sinusoidal or Sine Vibration has the shape of a sine wave as seen in Figure 1.  The parameters used to define sinusoidal vibration testing are amplitude (usually acceleration or displacement), frequency, sweep rate and number of sweeps.

A typical sinusoidal vibration test profile is shown in Figure 2.  The amplitude is defined over a range of frequencies.  The amplitude can be constant or variable.  During a sine vibration test, the vibration wave forms are swept through a range of frequencies, however they are of discrete amplitude, frequency and phase at any instant in time.  An important note is that the displacement increases as the frequency decreases for a given acceleration.  At low frequencies, the displacement could exceed the limits of the test equipment.  That is why some specifications use displacement for amplitude in the low frequency range.

Sine vibration is not usually found in the real world unless your product is attached to equipment such as a motor or reciprocating compressor running at a fixed frequency.  Why is it done?  It is good to find resonances (amplitude magnification in the device under test), it is a simple motion and it also produces a constant acceleration vs. frequency.  Also, it is probably carried over from old test methods prior to digital computer controllers.  However sine vibration does not correlate to a field life unless the product is exposed only to fixed frequencies over its life.

Some parameters and definitions of a sine vibration test are:

Amplitude:  The amplitude for a sine vibration test is usually specified as displacement or acceleration.  In DES’s experience, velocity is rarely used in a specification.  As seen in Figure 1, amplitude can be expressed as peak or peak-to-peak.  When displacement is used to define amplitude, it is defined in either peak units of inSA (mmSA) or peak-to-peak units of inDA (mmDA).  SA stands for Single Amplitude (peak) and DA stands for Double Amplitude (peak-to-peak).  When acceleration is used to define amplitude, its units are usually G’s or millimeter per second squared (mm/sec^2) or meter per second squared (m/sec^2).

Frequency:  Frequency is defined as cycles per second.  Its’ units are Hertz (Hz).  Frequency is equal to the reciprocal of the period.

G:  One G is equal to the acceleration produced by earth’s gravity and is equal to 386.1 inches/sec^2 or 9.8 m/sec^2.

Octave:  The interval between one frequency and another differing by 2:1.

Period:  The time it takes to complete 1 cycle.  Its’ units are seconds.  Period is not typically used in the definition of a sine test.  It is listed here because of its relation to frequency.  Period is the reciprocal of the frequency.

Resonance:  A frequency at which an amplitude magnification occurs in the device under test when compared to the vibration table amplitude.  Usually a resonance is defined as a 2:1 or greater magnification.

Sweep and Sweep Cycles:  A sweep is defined as a traverse from one frequency to another.  A sweep cycle varies from one frequency to another and then back to the staring frequency.  For instance in Figure 2, a sweep could be a traverse from either 5 to 500 Hz or from 500 to 5 Hz.  A sweep cycle would traverse from 5 Hz to 500 Hz, then traverse back to 5 Hz.  Some specifications require sweeps while others require sweep cycles which causes confusion.

Sweep Rate:  The rate at which the frequency range is traversed.  The units for sweep rate are usually Octave/minute or Hz/minute.  Octave per minute is a logarithmic sweep rate while Hz/minute is a linear sweep rate.

Random Vibration Testing

Random Vibration is a varying waveform.  It’s intensity is defined using a Power Spectral Density (PSD) spectrum.  Whereas sinusoidal vibration occurs at distinct frequencies, random vibration contains all frequencies simultaneously.  Also phase changes occur over time with random vibration.  Sine and random vibration testing cannot be equated.

Real world vibrations are usually of the random type.  Vibrations from automobiles, aircraft, rockets are all random.  A random vibration test can be correlated to a service life if the field vibrations are known.  Since random vibration contains all frequencies simultaneously, all product resonances will be excited simultaneously which could be worse than exciting them individually as in sine testing.

A typical random vibration test PSD is shown in Figure 3.  The PSD is defined over a range of frequencies.  The square root of the area under the PSD curves yields the Grms.  Specifying Grms only is not sufficient because a wide variety of spectra can result in the same Grms.

Some parameters and definitions of a random vibration test are:

Averages:  Since random vibration constantly changes over time (that is why it is called random), the controller takes samples or snap shots of the vibration data over time.  Successive samples are averaged.  The averaging occurs in each band of resolution.

Average Weighting Factor:  An exponential weighting factor that defines how fast the controller reacts to changes.  The controller reacts faster for small weighting factors vs. slower for larger weighting factors.

Statistical Degrees Of Freedom (SDOF):  The number of independent values (measurements) used to obtain a PSD estimate at a particular frequency.  A higher SDOF means that more measurements are taken.

• For a measurement channel, SDOF = 2*K,
• For control channels, SDOF = 2*K*(2*N – 1) * n.
• K = Number of averages per control loop
• N = Averaging weighting factor
• n = Number of control channels

Grms:  Grms is used to define the overall energy or acceleration level of random vibration.  Grms (root-mean-square) is calculated by taking the square root of the area under the PSD curve.

Kurtosis:  Fourth moment of the Probability Density Function (PDF).  It measures the high G content of the signal.  The kurtosis of a Gaussian PDF is 3.

Number of Lines:  The frequency range of the test divided by the band resolution equals the Number of Lines.  The vibration controller or spectrum analyzer will perform its calculations for each narrow band.

Open Loop/Closed Loop:  Closed loop means the controller will continuously adjust the drive signal to account for changes in the response of the device under test that are fed back from the control accelerometers.  Open loop means the drive signal will be fixed or the controller will stop adjusting the drive signal regardless of changes in the response of the device under test.

PDF:  A statistical Probability Density Function.  A histogram showing the probability of occurrence and the distribution of data.

Power Spectral Density (PSD) or Acceleration Spectral Density (ASD):  Defines the intensity of the random vibration signal vs. frequency.  Its units are usually G^2/Hz or (m/s^2)^2/Hz.

Sigma (σ) & Sigma Clipping:  Sigma is the standard deviation of a statistical PDF.  A Gaussian PDF distribution is assumed for random vibration which takes the shape of a bell shaped curve.  Since the amplitude or intensity of the random vibration will change over time, the time spent at different amplitude excursions is measured using a PDF.  Figure 4 shows a Gaussian PDF.  The vertical axis would be 1/G, the horizontal axis would be sigma and µ is the mean which is equal to zero for a shaker control.  For a Gaussian distribution, 68.2% of the peak G excursions occur between ± 1 sigma, 95.4% between ± 2 sigma, 99.7% between ± 3 sigma.

Vibration controllers allow you to clip peak amplitude excursions using Sigma Clipping.  Many specifications allow the clipping to be set at ± 3 sigma.

Frequency, G, octave, period and resonance are the same as defined under sinusoidal testing.

For more information on Vibration Shock Testing or other testing services contact DES or call 610.253.6637.

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