If you have not done so yet, please check out Part 1 – Defining your project and  Obtaining a Quote.

You obtained quotes from a couple of vibration test labs.  Your next task is to select a vibration test lab to perform your vibration test.  Your selection should not be based upon price alone.  Factors that should affect which lab you choose are capability, cost, timing, location, quality, and other special requirements.

Most importantly, the test lab must be capable of running your test.  Running a successful vibration test is not easy.  A good vibration test lab will make it look easy, but running a vibration test requires significant knowledge of vibration theory, fixture design, testing skills, and a skilled operator to run the vibration test equipment.

Of course, cost and timing should be a significant part of your decision.  If one quote is considerably lower than the other quotes, then you should question that lab to see if they misunderstood the project requirements.

Location can be important if you need to haul bulky support equipment to the test lab.  It can also be important if witnesses need to travel and be present during the test.

The lab should have up to date calibrated vibration test equipment.  If you have never used them before and this is a large project, then it would be a good idea to schedule a visit to review their equipment and capabilities.  If your company has special quality or other requirements, then review them with the lab.  Special requirements could be handling, lifting or Electro Static Discharge (ESD).

Planning & scheduling your test

After you have selected a lab, it is time to schedule your vibration test.  First contact the test lab to inquire about their vibration test schedule.  The test lab may be backed up with large projects and may need some time to schedule your test.  DES has multiple shakers with a goal to reduce scheduling time as much as possible.

If any special support equipment is required for the test, then it may take time to gather this equipment and to set it up.  If rental equipment is required, then the time to ship the equipment from the rental company must be considered.  Most importantly, significant time may be required to design and manufacture special fixtures.

Much or all of the planning could fall on the test labs shoulders if they are supplying everything.  Expediting schedules is always an option, but you will likely have to pay more.  It is important to know what each party is responsible to supply.  For instance if assembly is required, who will supply the bolts?  It is a lousy situation for all involved, to plan a test for weeks ahead and then to find that you are missing something during the test setup.  Good communication with the test lab is very important!

Fixture Design and Manufacture

Most vibration test labs can design and make fixtures.  Sometime you may have in-house fabrication capabilities.  However, you may not have the experience to design good vibration test fixtures.  Designing good vibration test fixtures requires specialized knowledge.

The vibration test fixtures should be stiff and light weight.  They should have a high natural frequency.  It is preferable that all resonances fall above the test frequency range, but many times that is not possible especially when testing to high frequencies of 2,000 to 3,000 Hz.  A good rule of thumb is to make natural frequencies as high as possible without adding too much weight.  Easier said than done!  When designing vibration test fixtures, many more bolts are required than for static loading to securely join the fixture to the vibration table and increase the fixture resonances.  Good materials for vibrating test fixtures are aluminum and magnesium.

Figure 1.  Fixture Cube Designed by DES

Computer analysis software such as Finite Element Analysis (FEA) is a great tool for estimating fixture resonances during fixture design.  If the lab is designing your fixture, inquire if they have FEA capability.

Test setup

Setting up the vibration test on the shaker is the responsibility of the test lab.  If the requirements are for your product to be operationally functional tested, then that may fall on your shoulders.  It is very common for DES’s customers to bring support equipment to functional test their specialized products during the test.  Many labs including DES, can also provide support equipment, but it if your product is very specialized, then it probably makes more sense for you to be responsible for this task.  Again, good communication with the test lab is very important to determine the responsibilities for each party.

The test setup may also involve a trial run with the fixtures alone to scan for severe resonances.  This is a good idea especially when testing to high frequencies of 2,000 to 3,000 Hz.  Control problems could occur if the fixtures have a severe resonance in the frequency range of the test causing the shaker controller to abort.  Sometimes (but not always) a large resonance can be “notched out” meaning that the test acceleration levels are reduced significantly at frequencies a little higher or lower than the resonance.  Note that some test specifications will not allow for notching.

Running the vibration test

Finally you are ready to run your vibration test.  Before starting the shaker, all support equipment and the DUT should be operated to verify that it is working acceptably.

Before the vibration testing begins, the shaker controller will first go through a “self-check” to make sure that the amplifier and the vibration accelerometers are connected and producing signal.  It is looking for a “closed loop” between the controller, the amplifier and the control vibration accelerometers.  Once the “self-check” is successful, the operator will start the vibration shaker.  A skilled operator is taught to start up at low vibration levels.  This is typically -12 (≈1/4 of full level) to -6dB (≈1/2 of full level) below the full vibration level (which is 0 dB).  Thus if anything such as a loose bolt was missed during the setup, it can be caught before reaching full level.  So it may take a couple of minutes to ramp up to full level even for an error free setup.  Once at full level, the shaker controller will be constantly adjusting the vibration table to meet the specification.  This is called running in “closed loop”.  The definition of “closed loop” control is contained below.  (Note, in some special cases the shaker can be run in “open loop”, meaning that the controller will not adjust the shaker, but that is a subject for another discussion).  There are abort limits in the controller software to automatically stop the test in case an accelerometer or cable breaks.  Also, if a failure occurs in the product or fixture, it will probably cause an abrupt change in the control accelerometer outputs causing the controller to abort the vibration test.  In addition, a skilled operator knows that a change in the audible sound coming from the vibration test could be a sign of an imminent failure.

Now the shaker starts moving.  What do you need to worry about?  At this point, if everyone did their homework, then the only thing to worry about would be if vibration fatigue causes a failure in your product or if your product does not operate properly.

Most vibration tests are performed along three orthogonal axes, but only one axis at a time.   Time will be needed when changing from one axis to another especially when switching from a horizontal to vertical axis.  Fixtures will need to be unbolted from the table and the lab may need to disconnect from the horizontal slip table, rotate the shaker vertically and then install a vertical head expander.  So if your vibration test is 8 hours in total duration, plan for extra time to switch axes which could mean that your test takes more than 1 day.  The time required to switch to a different axis could be less than 1 hour to 1 day depending upon the size of the product, the complexity of the test fixtures and the support equipment.

Hurray!  Your test completed, your product operated smoothly and no structural failures occurred.  Now it is time to clean up and for the lab to provide a test report.

The definition of running in closed loop is the following:  The vibration controller sends a drive signal to the amplifier.  The amplifier powers the shaker using the drive signal causing it to vibrate.  The vibration levels are sensed by the control accelerometers.  The output from these accelerometers is fed back to the controller.  The controller then adjusts the drive signal to make the vibration levels match the test specification. 

Test report

The vibration test lab should provide a thorough, accurate, well written test report with prompt delivery as a final step.  The test report should include:

1. Photographs of test set-up
2. Location of accelerometers
3. List of test equipment used and calibration status
4. Summary of the test procedure
5. Plots of vibration data
6. Visible observations of the product after the test and summary of the product operational test results (if product was operational during the test)
7. Photographs of any product failures

In conclusion, running a successful vibration test is difficult.  A good vibration test lab will make it look easy, but there are many specialized skills and knowledge required.  We hope that this two part article will help you gain perspective from a vibration test lab’s point of view and to understand what information that you need to provide to perform a successful vibration test.  Most importantly, good communications is essential for a positive experience.  DES looks forward to performing your next vibration test!

The post Choosing A Vibration Test Lab Part 2 appeared first on Delserro Engineering Solutions.

Part 1 – Defining your project and Obtaining a Quote

Vibration testing is a very specialized field, not very well understood by many.  There are different types of vibration and there are an enormous number of vibration test specifications.  Vibration testing equipment is very expensive to purchase forcing many companies to hire a vibration test lab to fulfill their vibration testing requirements.  So what should you expect when Choosing A Vibration Test Lab?

You contact a lab to obtain a quote for vibration testing.  The lab replies with many questions.  You ask yourself, why does the lab ask so many questions just to prepare a quote?  The answer is that details matter and can significantly affect the vibration test time and cost.  This article is written from a vibration test lab’s point of view to help explain what information is needed to perform a successful vibration test.

Before contacting the lab, it might help to ask yourself, what is the purpose of your test? Do you need to comply with customer qualification requirements, improve your product reliability, estimate life expectancy, perform package testing, etc.?  It is more effective to contact the lab after your goals are clear.  It is also helpful if you can prepare a list of requirements or a statement of work or a test plan.  The following are questions that a vibration test lab may ask you:

1. What are the vibration test specifications?
2. What is the size and weight of the product or Device Under Test (DUT), what does it look like and how many samples need to be tested?
3. How will the DUT be mounted?
4. Does the DUT need to be powered and monitored during the test?
5. Do you need response accelerometers on the DUT?
6. Are combined environments (such as temperature) and vibration required?
7. Are any other unique requirements needed for the vibration test?

1. What are the vibration test specifications?

There are many different vibration test specifications and test parameters that need to be defined.  A few common test standards are: MIL-STD-810, MIL-STD-883, MIL-STD 167, RTCA DO-160, IEC 60068-2-64, and IEC 60068-2-6.  However there are many more.  The types of vibration can be random, sinusoidal (sine), sine dwell, sine on random, random on random, sine on random on random.  The most common types of vibration testing services conducted by vibration test labs are sinusoidal and random.  A decision will have to be made as to what type of vibration is needed and if a test specification applies.  Some other test parameters that need to be defined are: acceleration or G level, frequency range, how many axes to be tested, duration per axis, sinusoidal sweep rate and number of sweeps for sine testing, random vibration PSD profile or sine vibration curve.

Many times all of the test parameters above are well defined in a test specification such as MIL-STD-810.  However sometimes they are not and then you will need to provide these details to the test lab.  The reason is that the details do matter and can significantly affect the test time and cost.  For instance a sine sweep from 10 to 2,000 Hz at a rate of 1 octave per minute takes approximately 7.5 minutes.  Performing the same sweep at a rate of 60 Hz/minute will take approximately 33 minutes.  So if you need to perform 10 sweeps per axis x 3 axes, the time to complete each test will be approximately 225 minutes (3.75 hours) at a rate of 1 octave/minute vs. 990 minutes (16.5 hours) at a rate of 60 Hz/minute.  Obviously a test that takes 16.5 hours to complete will cost more than one that takes only 3.75 hours.

Test requirements such as G level and random vibration PSD profile greatly affect the overall difficulty of the test.  Testing at high G levels for larger, heavier products may be beyond the capabilities of some vibration test equipment.  The frequency range of the test will affect the design of the vibration test fixtures.

2. What is the size and weight of the product or Device Under Test (DUT), what does it look like and how many samples need to be tested?

The lab is going to ask you to provide the dimensions of the DUT and the mounting foot print.  This information is required to assess vibration test fixtures and also to determine if the DUT will fit on the vibration shaker table.  A drawing or sketch or picture with some overall dimensions is very helpful.  Ideally the DUT should be mounted to a rigid vibration table or fixture.  It is not a good idea for the product to hang over the sides of the table because that could affect its vibration response.  The size and weight will also affect how many products can fit on the table simultaneously or if multiple groups need to be tested.

That leads into the question of how much does the DUT weigh?  This is important because it may be easier for a vibration shaker to test a 200 pound product at 1 G than a 10 pound product at 20 G’s, (neglecting handling!!).  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 units are usually G’s or m/s².  Someone inexperienced with vibration testing might think that the same amount of force is required to test a 200 pound DUT at 1G and 10 pound DUT at 20G because the forces are equal using F=MA.  It is important to note that the Mass is not only from the DUT’s but is also from the moving masses of the shaker and fixtures per the formula below:

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

For example, let’s say the 200 and 10 pound products will use the same test fixtures and will be used on the same vibration shaker.  Using the assumed weights in the table below, the total moving weight is 425 pounds for the 200 pound product and 235 pounds for the 10 pound product.

Don’t stress too much.  You will only need to provide the weights of components that you are supplying such as the product to be tested, the DUT fixture weight if you are supplying this fixture, and any other significant moving weights that you are supplying such as heavy cables.  The test lab will calculate the required shaker force.The total force required for the shaker is 425 pounds for the 200 pound product and 4,700 pounds for the 10 pound product.  The required shaker force is more than 10 times greater for a product weighing 10 times less!!  The 4,700 pounds of force required for the 10 pound product may overload some small to medium shakers.

Let’s briefly explain the parts involved.  The vibration shaker armature is part of the electrodynamic shaker.  Typically a magnesium head expander is bolted to the armature when testing in a vertical configuration, Figure 1.  A magnesium slip table is bolted to the armature when testing in horizontal configurations, Figure 2.  (Note, we will refer to the head expander or slip table as a vibration table.) These tables have a generic hole pattern.  Usually an adapter plate is used to adapt the mounting pattern of the product to the magnesium shaker tables.

3. How will the DUT be mounted?

The mounting hole pattern of your product will probably not match the hole pattern of the vibration test lab’s tables.  So an adapter plate or DUT fixture will be required as seen in Figures 1 and 2.  The DUT fixture will need to be bolted to the test lab’s vibration table.  Then the product will be mounted to the DUT fixture.  Test labs will not drill holes in their vibration tables because they are expensive specialized fixtures usually made with magnesium.  The DUT fixture is typically an aluminum plate with two sets of hole patterns.  One pattern matches the vibration table while the other pattern matches the product mounting.  To reduce cost and lead-time, DES has generic aluminum adapter plates available and will add holes to fit your product.  Bars and threaded rods can be used to attach products to the vibration shaker for products that do not have mounting holes such as a cell phone.  So the DUT fixture is a cost that will have to be accounted for by you or the test lab.  This cost is usually non-recurring.

4. Does the DUT need to be powered and monitored during the test?

If the DUT needs to be powered and monitored during the test, then clearly this will require support equipment.  This can be a simple or very complicated task.  One complicated setup for a past test took DES months of planning.  It required 3 phase electric, cooling water flowing through the product, and many measurements to be made during the vibration test.

Power can be simply provided from 120 alternating current (AC) single phase wall outlets or it can require complex high voltage 3 phase power from the facility or power supplies. Sometimes AC power requires special frequencies such as 400 Hz for some military equipment.  This is provided from special AC power supplies.  Direct Current (DC) power can be provided from DC power supplies or batteries.

Another important consideration will be if the DUT needs to be electrically loaded during the test.  This can be resistive loading from components such as heaters.  The loading can be inductive from AC motors.  DES has previously built a 6 foot tall test rack of resistors to draw a high load through a product.  On another test, DES had large blowers on the floor to load an air conditioning unit for a rail car.

Monitoring can be from simple visual observations or it can require complex measurements.  Typical monitoring equipment are data loggers, digital multi-meters and oscilloscopes.  Different kinds of sensors can be used to provide monitoring data such as thermocouples or speed sensors.

The big question is who will provide this equipment.  Test labs can typically provide most needed equipment at added cost.  Nobody has every piece of test equipment, but there are test equipment rental companies that provide calibrated test and measurement equipment.

5. Do you need response accelerometers on the DUT?

Some test specifications such as RTCA DO-160 require a resonance scan on the DUT.  This involves installing response accelerometers on the DUT, then performing a resonance scan with low level accelerations (usually 0.5 or 1G) over a frequency range such as 10 to 2,000 Hz.  Or perhaps you are interested in examining the response of your product under vibration loading.  Response accelerometers are usually small and are typically attached to the product with an adhesive such as superglue, Figure 3.  They are not used for control.  Control accelerometers are typically mounted in the DUT fixture.

If you want or need response accelerometers, it is helpful to specify this up front.  While installing response accelerometers on the DUT is typically not a big task (however it can be!), there may be some cost added for this effort.  It does take extra time to install response accelerometers, take pictures for documentation in the report, setup extra data acquisition channels and process a resonance search plot for inclusion in a test report, Figure 4.

6. Are combined environments (such as temperature) and vibration required?

Obviously vibration testing in combined environments is much harder and will be more costly than testing at room temperature.  For now, let’s consider combined temperature and vibration testing.  First, there is some time and effort involved in setting up the chamber over the shaker, Figure 5.  While it varies from lab to lab, the vibration table will be inside the chamber or the table will be flush with the floor of the chamber.  The chamber will be fixed.  There will be space between the chamber and the vibration table.  A rubber membrane will be used to seal the chamber to the vibration table.  The vibrations will occur inside the chamber in a confined workspace.

Many chambers only fit over the shaker when it is oriented vertically and do not fit over a horizontal slip table.  If that is the case, then different types of fixtures are required since testing in 3 axes requires rotating the fixtures, not moving the fixtures from a vertically mounted table to a horizontal slip table, Figure 6.

Also, special charge mode accelerometers are needed for working in extreme temperature environments because standard accelerometers will drift under these conditions.  Accelerometers should be mechanically mounted because many adhesives do not stand up to extreme temperatures.

7. Are any other unique requirements needed for the vibration test?

Unique requirements can be very challenging.  Unique requirements could be mechanical loading, internal pressure, fluid flow, etc.  For mechanical loading, it is preferred to use a spring or bolt mechanism to apply a steady load to keep weight to a minimum.  DES has also performed vibration tests with internal pressure applied to products and fluid flowing through heat exchanger products.  Adding extra weight to vibration tests can demand much more force from the shaker.  Testing with fluids may cause sloshing and extra risk of spillage onto expensive shakers.

Vibration test labs are more than happy to help you with your vibration test requirements.  But please do understand that some effort is needed on your part to define your vibration test requirements.  Good communications is essential for a positive experience.  DES looks forward to hearing from you!

The post Choosing A Vibration Test Lab Part 1 appeared first on Delserro Engineering Solutions.

Mixed Mode Vibration Testing is less common than Sinusoidal and Random Vibration Testing.  However, it does have a special purpose for simulating specialized helicopter vibration or vibration from tracked vehicles such as tanks.

The three mixed modes of vibration testing are:

• Sine-on-Random (SoR)
• Random-on-Random (RoR)
• Sine-on-Random-on-Random (SoRoR)

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

• MIL-STD-810 Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests
• RTCA DO-160 Environmental Conditions and Test Procedures for Airborne Equipment

Sine-on-Random (SoR) Vibration Testing

Sine-on-Random (SoR) vibration testing contains sine tones that are superimposed on a low level of broadband random vibration.  The sine tones can be fixed frequency or sweeping.  If they are sweeping, they are normally very narrow frequency bands.  Some examples of SoR vibration are from helicopters, propeller driven airplanes and aircraft rapid gun-fire events.

All aircraft have some levels of random vibration.  In helicopters and propeller driven airplanes, the sine tones are produced by the main rotary components.  In addition, sine tones can come from rapid gun fire events.  Figure 1 shows a typical SoR helicopter vibration test profile from MIL-STD-810G.

Random-on-Random (RoR) Vibration Testing

Random-on-Random (RoR) vibration testing has narrow band random peaks that are superimposed on a low level of broadband random vibration.  Typical applications that contain RoR vibration are tracked vehicles such as tanks or a truck changing speed while driving over a rough road.  For example, the pitch of the tracks produces rhythmic random vibration peaks at specific frequencies versus speed of the vehicle.  Figure 2 shows a typical tracked vehicle vibration test profile from MIL-STD-810G.

Sine-on-Random-on-Random (SoRoR) Vibration Testing

Sine-on-Random-on-Random (SoRoR) vibration testing contains both sine tones and narrow band random peaks superimposed on broadband random vibration.  Applications that contain SoRoR vibration could be a rapid gun-fire event on a tracked vehicle or components mounted near a turbine engine with various rotating machinery elements and background random vibration produced by air turbulence.

Whether your vibration testing needs are complex or simple, DES has the experience and knowledge to perform your test.  For more information on Vibration Testing or other testing services contact DES or call 610.253.6637.

The post Mixed Mode: Sine on Random Vibration Testing, RoR, SoRoR appeared first on Delserro Engineering Solutions.