High Energy Vibration testing

| Environmental Testing

Comet and laser vibration controllers

Jonathan Newell talks to Brüel and Kjaer about applications for high energy vibration test systems and the factors to be considered when purchasing them.

Investing in test equipment for verifying a product’s ability to withstand mechanical stresses such as shock and vibration can result in a lot of head-scratching and detailed analysis of specifications and equipment capabilities as well as how such testing is going to be combined with other product assurance requirements, such as climatic testing.

To find out more about what must be considered when purchasing high value, high energy vibration test systems, I spoke to Miguel Marous, the European Vibration Test System Sales Manager for Brüel and Kjaer.

Having supplied over 10,000 systems since its formation half a century ago, the company now produces around 900 shaker tables a year with peak force ratings ranging from just 9N to a mighty 289 kN. Brüel and Kjaer also supplies vibration controllers, analysis platforms and sensors as well as acoustic analysers, enabling customers to perform the complete measurement cycle for vibration and acoustic tests and assessments.

Choosing high energy shakers

The main focus on specifying the kind of table needed is the force requirements which relate to the payload mass and the test acceleration. This is the easiest parameter to specify and is often made more complex by the need to specify the amplifier, which can limit the test profile.  In this respect, specifying certain voltages at specific frequencies might not achieve the required forces.

A 10g RMS random vibration profile might have some limitations depending upon the armature size and the amplifier limitations, something which Brüel and Kjaer always points out during the product specification stage to suggest alternatives and make sure that the customer test expectations can be met.

Some industry users have very predictable requirements with a known product range to test, standard specifications against which they’ll be tested and known size, asymmetry and fixturing needs, all of which can be taken into account when the shaker table is specified.

Test houses

Supplying shaker tables to test houses requires a more generalised approach to specifications since they perform testing as a service to a range of clients from multiple industries.

In this respect, Brüel and Kjaer provides a detailed parametric profile to accompany the table so that the test house has a very clear picture of the limitations and constraints. For such generalised operations, the shakers need to be placed into the hands of expert users so that problems such as strange payload resonances can be identified and prevented so they don’t damage the shaker table.

Other specification factors

Suppliers such as Brüel and Kjaer need to know as much detail as possible about the payload during the specification phase so that all factors are taken into account. Such factors include:

Orientation – Shaker tables are supplied on trunnions so they can be oriented in either vertical or horizontal positions. In the horizontal position, they are supplied with a slip table to facilitate the X-Y axis motion.

Test environment – For inline production testing environments, cycle times are important and Brüel and Kjaer is able to supply tables with trunnions that enable the expander head (which sits on the shaker actuator to support the payload) to be lifted clear allowing the shaker to be swivelled swiftly to be attached to the slip table for horizontal testing. This is not usually necessary in research and development or product accreditation environments.

Payload size and complexity – It’s important to understand how the payload is to be attached to the shaker table and what the characteristics of the payload will be, including whether it will have a high centre of gravity or whether it has any significant asymmetries. For payloads that have large dimensions, which are greater than the load-bearing area of the shaker, a head expander can be used. A head expander is a table installed between the shaker and the payload to expand the armature to the payload size and shape. An important factor when adding a head expander is that its mass should be added to the overall payload mass, the combined total of which shouldn’t exceed the rating for the shaker armature.

Fixturing – Often used in the aerospace industry, fixtures are used in order to deliver the shaker force to a specific point in the payload and are used to replicate the mounting of a component within a sub-assembly. By matching the shape and positioning of the mounting, the fixture can deliver the same force vectors as will be seen in normal operational service. An example is the refuelling nozzle on a military aircraft, which has a contoured, offset mounting at one end of the assembly. Some fixtures can be substantial and their mass needs to be calculated as part of the payload mass.

Combined testing – If a number of tests need to be performed simultaneously, these should be included as part of the specification to the shaker manufacturer and can often be accommodated. An example quoted by Brüel and Kjaer is altitude simulation where a specially adapted shaker table is sealed to the floor of a vacuum and temperature chamber and is equipped with a vacuum compensation extender system which resists the vacuum effects on armature position to keep it in a central position and enable it to maintain its full dynamic range.

Large shaker applications

Apart from in test houses, large shaker systems are used in a number of applications including pack testing, aerospace, defence, automotive and ground transport simulation. Some installations for defence and ground transport are multi-shaker configurations, which can be coupled or operated independently of each other, depending on the test or simulation being carried out.

The aviation, defence and space exploration industries have very stringent environmental testing requirements which combine shock, vibration, temperature and vacuum testing as well as assessing resistance to harsh marine environments.

Automotive industry changes

The automotive industry is going through significant changes at the moment, particularly in the field of electrification. These changes are having an effect on the way vehicles are undergoing mechanical endurance testing.

Hybrid and all-electric cars are considerably quieter than older vehicles and consumer expectations of the cabin environment are becoming more stringent. Noise has therefore become more critical and so smaller sounds need to be removed. This drives a requirement for higher frequency testing.

The automotive industry is paying more attention to the way internal trim is fitted in cars and the durability of the fittings so that they don’t loosen and become noisy over time.

Brüel and Kjaer is supplying the industry with low load vibration equipment without shaker cooling to enable engineers to make subjective assessments of squeaks and rattles. Internal sub-assemblies can also be put through accelerated endurance testing to see whether or not they’re able to stand up to a decade or more of travel on bumpy roads.

Within the automotive industry, there’s also a need to test the car battery module and pack, a very large device typically weighing up to 200kg. Since its placement is as part of the floor pan of the vehicle and near to the wheels, it is subject to severe mechanical loads and needs to be verified using 100g shock tests as well as random and sine vibration tests.

With the payload for such tests being so large, it needs a 2m x 2m head expander and slip table.

Jonathan Newell
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