Shock and Vibration Testing: Shakers and other Equipment

Shakers, transducers and similar vibration- and shock-testing equipment generate force in order to produce vibrations shock or modal excitation for testing and analysis. Using shakers, test engineers may:

• determine product or component performance under vibration or shock loads
• measure structural fatigue of a system or material
• detect flaws through modal analysis
• verify product designs
• simulate the shock or vibration conditions encountered in aerospace, defense, transportation, etc.

Shakers may operate using several different physical principles:

• Mechanical -- these shakers use a motor with an eccentric on the shaft to generate vibration.
• Electrodynamic -- these shaker-amplifier systems use an electromagnet to create force and vibration.
• Hydraulic -- these systems use pressurized fluid and hydraulic "motors" to generate vibration. Hydraulic shakers are used when large-force amplitudes are needed (e.g. testing large aerospace or marine structures or when the magnetic fields of electrodynamic generators are a problem).
• Pneumatic systems (or: air-hammer tables) -- use pressurized (compressed) air to drive a table.
• Piezoelectric shakers -- operate via application of an electrical charge and voltage to a sensitive piezoelectric crystal or ceramic element. This phenomenon deforms the crystal resulting in vibration.

Features common to most shakers are:

• active suspension -- compensates for environmental or floating platform variations.
• an integral slip table -- allows horizontal or both horizontal and vertical testing of samples. The slip table is a large flat plate that rests on an oil film placed on a granite slab or other stable base.

Important specifications for shakers include:

• peak sinusoidal force
• frequency range
• displacement
• peak acceleration and peak velocity

Note: These rating specs may be without a load, since shaker manufacturers may not always be able to predict how their shakers will be utilized.

The three main test modes shakers can have are:

• random vibration-- force and velocity of the table and test sample will vary randomly over time
• sine wave vibration -- sinusoidally varies the force and velocity of the table and test sample over time
• shock or pulse mode -- test sample is subject to high-amplitude pulses of force

Typical specifications of shakers:

• Sine Force -- may range from 4.5 lbs force peak (20 N) up to 500 lbs force peak (2,220 N) for large systems
• Random Force -- may range from 3.0 lbf rms random (13 N) to 350 lbf rms random (1,550 N) for large systems
• Shock Force -- may range from 9.5 lbf peak shock (42 N) to 1,000 lbf peak shock (4,440 N) for large systems
• Frequency Range -- may range from (DC to 11,000 Hz) to (DC to 4,500 Hz) for large systems

If you can only run one test, either sine or random, which should it be -- which is the most severe?

Some general assumptions:

• failures due to vibration are caused at the peak G level seen by the product
• most products have resonances at one or more frequencies
• at these resonances the vibration levels applied to the product are amplified by the Q factor of the resonance

The relative severity of a sine test and a random test will vary depending on a product's resonant frequencies and Qs (amplification factor). In general, when sine and random tests have the same peak vibration levels at the control point, the product will see higher vibration levels with a sine test than with a random test due to the resonances in the product.

Nevertheless, one must remember that a sine test only excites a single product resonance at a time; hence, a sine test will not test the interaction between two resonances in a product or component. Because a random test excites the full frequency spectrum all at once, it can be used to find problems resulting from the interaction between two resonances.

NEXT: Electrodynamic Shakers and Shaker-Amplifier Systems

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