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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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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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 together which could be worse than exciting them individually as in sine testing. Sometimes random vibrations are mixed with sine vibrations in Sine-on-Random Vibration Testing. Also, a low level of broad band random vibration can be mixed with additional high levels of narrow band random vibrations in Random-on-Random Vibration Testing.

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

• ASTM D4169 Standard Practice for Performance Testing of Shipping Containers and Systems
• ASTM D4728 Standard Test Method for Random Vibration Testing of Shipping Containers
• GMW 3172 General Motors Specification for Electrical/Electronic Components – Environmental/Durability
• IEC 60068-2-64 Environmental testing Part 2-64: Tests Fh: Vibration, Broadband Random and Guidance
• ISTA 2A and 3A Procedures for Testing Packaged Products
• MIL-STD-202 Department of Defense Test Method Standard for Electronic and Electrical Component Parts
• 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
• SAE J1455 Recommended Environmental Practices for Electronic Equipment Design in Heavy-Duty Vehicle Applications

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