The purpose of this Recommended Practice (RP) is to define an orderly approach that addresses the major issues involved in selecting a shaker for dynamic testing. In some cases, this selection process is associated with deciding which shaker system to procure. In other cases, the user will already have shaker systems available (in-house or at commercial test laboratories) and must decide which one is best suited for the task at hand. The process is the same in either case.

Shakers are generally used for sine vibration, random vibration, and many types of shock tests.

It is the intent of this document to cover the following applications for shakers: simulation of operating conditions, design qualification tests, transportation simulation, and vibration for environmental stress screening (ESS).

All major types of shaker systems will be included in the discussions, namely: electrodynamic (E-D), servo-hydraulic (hydraulic), mechanical, and pneumatic impactor. In the context of this Recommended Practice, a shaker system includes the power supply (electronic, hydraulic, or pneumatic) as well as the cooling system required for the shaker system. Closed-loop control systems (required to run tests on E-D and hydraulic shakers) are addressed in a different RP.

BACKGROUND AND PURPOSE

Those charged with the responsibility of selecting a shaker are faced with an opportunity that is challenging and concerning. The challenge results from the fact that selecting a shaker is not a straight forward or mundane task. The concern, be it mild or severe, centers around the nagging question: "What if I make the wrong selection?"

It is sometimes said that a shaker cannot be too powerful—and this is true as long as there is an inexhaustible supply of money! It's not uncommon, however, for an in-house test system to be stillborn (and the test work farmed out) because the technical people involved in defining what was required either over-specified a system that exceeded the available budget and therefore couldn't buy anything, or bought one that was undersized.

This Recommended Practice presents a structured approach to selecting a shaker that involves seven categories. If these factors are addressed properly, then the selection will have been made using the best available information. These seven categories are:

1) Frequency range and waveforms,

2) Displacement and velocity,

3) Number of axes to be tested,

4) Payloads and vibration and shock levels,

5) Force rating,

6) Dynamics of the shaker system,

7) Defining target tests.

Fixturing is usually extremely important in the shaker selection process. Sometimes it is a critical factor. Note that in this context, everything (except the unit under test and armature) is defined as fixturing.

Fixturing can be divided into two categories: a) special purpose fixturing used to attach the UUT to the shaker system, and b) general purpose fixturing such as slip tables, head expanders, cubes, T-plates, and so forth. In many cases, the actual test article is only a small part of the total weight that has to be vibrated or shocked. This is especially true in situations where a slip table or head expander is involved. In such cases, poor assumptions made about the fixturing will badly taint the shaker selection process.

If the fixturing ends up being a lot heavier than was assumed, the shaker may not be able to achieve the desired vibration levels. If, on the other hand, too much safety factor is built into the assumptions of fixturing weight, the shaker specified will be unnecessarily large. Lastly, if the fixturing has powerful, lightly damped resonances and anti-resonances in the test bandwidth, then it is entirely possible that even a drastically oversized shaker will not be able to run a desired test.