Structural Vibration Testing that saves you time and effort
Introduction to vibration testing
As a mechanical engineering and acoustics consultancy we are experienced in mechanical vibration testing, assessments, surveys, measurements, and development. And are constantly approached to investigate and improve products for their environments.
Why perform vibration testing at all?
In many industries vibration testing is used as a 'tick in the box' exercise; proof that the product meets the environmental design requirements.
For product design and improvement, we have performed vibration testing of products, equipment and instrumentation to ensure that they will withstand their environmental conditions and climatic elements to operate resiliently and reliably.
When do we get involved?
However, our involvement in projects begins with the design requirements and the design of the structure of the product. The initial vibration testing is used to validate and verify the analysis model. This is performed to ensure the constraints, loads and structural behaviour are correct.
On this page you will read basic guidance to vibration testing.
For information on vibration test fixture design, click on the test fixture button
Key benefits of using Environmentally Sound Limited
Experienced engineers dedicated to structural dynamic testing.
Broad range of expertise in all areas of structural dynamic testing for electronic and mechanical product development, testing and certification
Mobile engineering services for some testing
Vibration testing facilities
Vibration testing is basically exposing an item to vibration levels and frequencies similar to what it will experience in its field of operation.
There are different shakers that can be used for testing, but only two main types that can be controlled with accelerometer feedback.
Vibration testing equipment
Hydraulic actuator vibration testing
hydraulic actuators are used for high amplitude and low frequency testing. And cannot reach high frequencies as electrodynamic shakers. As they are limited to hydraulic fluid flow rates and servo valve capacity. To increase the frequency performance, use a three stage servo-valve configuration.
An example of hydraulic testing is tire burst testing on vehicles and plane. Where the frequencies are low and the displacements are high.
Electrodynamic / acoustic shakers
Electrodynamic shakers are used for high frequency low amplitude vibration testing. The shaker itself has a moving armature driven by an electric coil which induces a magnetic field. This moves the armature vertically up and down.
When and how to use electrodynamic shakers
Electro dynamic shakers have typical frequency ranges from 5Hz to 3000Hz.
Some limitations of electro dynamic shakers are:
displacement of the armature
the payload which is also dependent on
the vibration signal, i.e. high vibration levels need more acceleration and thus the payload reduces.
For large items there are other configuration, if you need more advice, please call.
For long test items, like actuators of several metres long, two shakers can operate in tandem at the same time, with ether ends of the actuator positioned on each shaker. There are shaker manufacturers in the UK that can help with the drive signal for tests like these.
Once again, if you have would like to take this approach, or you are having problems with this, please get in touch.
When is a slip table required?
An electro dynamic shaker is available as a stand-alone unit or with a slip table. The slip table is used to drive vibration into the test unit in its lateral and longitudinal axes.
vibration testing is usually performed in three main orthogonal axes, i.e. x, y and z axes. Where the z axis is the vertical axis in this explanation. These axes are all at right angles to each other.
When the controller controls the shaker movement, it uses feedback from the accelerometers. Whether it is the vertical axis, lateral or longitudinal axes, it controls the accelerometer on the shaker to the profile o nte controller, no matter what the direction.
Because of this, small items can be tested on shakers using in the vertical direction, and using a bookend fixture (A fixture to mount the test unit at right angles to the direction of excitation, avoiding the use of a slip table) for the other two orthogonal axes.
Except when the test unit contains fluids that must be representative of installation. Then a shaker with a slip table is required.
Running a Vibration Test
Typical vibration testing profiles
Three main vibration test profiles are
These signals are representative of the operating environments in which the products must function robustly and reliably.
The random profile is usually a histogram of the vibration content as expected on its designated vehicle, i.e. statistical probability for a similar type vehicle.
Depending on the standard, sweep rate are usually below 2 octaves per minute. However, the slower the sweep rate, the more visible the resonance peaks will be.
Resonance dwells are usually performed using a sinusoidal wave form. It is what it states, a dwell at resonances.
Typically, a number of resonances will be identified after the sine sweep. Usually three to five of the resonance peaks will be selected for dwell testing.
The resonance dwells are performed for a specific amount of cycles. Thus, the time durations will decrease as the resonance frequencies increase.
Unfortunately, the resonances drop in frequency during the resonance dwells, and it is therefore advised to use a controller that has software capabilities to track the resonance. In ensures that the shaker is exciting the resonance itself.
Control accelerometers position
The control accelerometer or accelerometers are positioned at the mounting points of the unit under test to the fixture. This is the ideal position, as the vibration profile is prescribed for the unit itself.
Position the accelerometers at locations in which you are interested, i.e. where you require more information to update your analytical model, not arbitrary locations.
Remember:  You are not just performing this one test. You are gathering information for your next design so that you pass the next vibration test at the first attempt.
Typical shock testing includes three pulses in the positive and negative direction in all three orthogonal axes.
The shock wave profile is a sawtooth curve with an amplitude of +/-3g, +/-6g and +/-9g.
Of all the products that we have subjected to vibration testing, we seldom see products fail shock testing.
However, most shock test reports are not accepted, as the shock wave trace of the control accelerometer is outside the stipulated boundaries of the profile.
So be sure to inspect the shock wave trace before taking the test item off the shaker.
Design for vibration (extreme) environments
There are many components and assemblies that we can model in design software. But there are times when the complexity of the products far outweighs practicality.
This is where our experience in designing products to pass vibration testing is most valuable to you.
Our experience is not just in performing vibration testing, but when the test article fails, we improve the design to pass the vibration testing satisfactorily.