Shaker Selection Criteria

Unholtz-Dickie offers over 85 different vibration system configurations. Many of these systems cover "mainstream" testing applications, but a significant number are supplied for more specialized requirements involving extreme shock & vibration levels, high velocity, long stroke and other more demanding performance conditions.

UD has a staff of technically competent application engineers who are available during the system selection process and after the system has been installed and is in service. These application engineers are a key ingredient in UD's commitment to supplying the proper system for the job, with no surprises. Take advantage of this benefit.

Some of the fundamental technical considerations for system selection are reviewed below.

Force = Mass x Acceleration (F = MA)

In general, the selection of a vibration system requires application of Newton’s Second Law of Motion: Force = Mass x Acceleration (F = MA). But this basic formula is only a starting point. It may be instructive to describe one example of an incorrect approach to estimating the required system force rating.

Example: “I have a 50 lb test specimen that must be tested @ 10 g. Therefore, I need a vibration system rated at 50 x 10 = 500 lbs force.”

After reviewing the following technical discussion you will have a better understanding of why this estimate of system force is incorrect.

System Force Rating

Vibration systems have output force ratings typically expressed in terms of:

Sine force: lbs (kN) pk
Random Force: lbs (kN) rms
Shock Force: lbs (kN) pk

The stated random force rating assumes a certain spectrum shape (usually flat) over a given frequency range (usually 20 Hz – 2,000 Hz) and a certain minimum mass load on the shaker. If these conditions are different from your test requirements, a new calculation of the effective random force rating may be necessary. These output force ratings provide a starting point for the Force (F) parameter in F = MA.

Mass

The mass value (M) in the initial F = MA force calculation must include all of the moving masses involved in the actual test setup: (shaker armature + test specimen + test fixture + any specialized adapters such as a Head Expander or a Slip Plate with its Driver Bar). Getting the right mass (M) value for the initial F = MA force calculation may be different for vertical axis testing compared to horizontal axis testing. Ask a UD applications engineer to confirm that the mass figure used accounts for all of the various components needed for your test.

As the frequency range of the vibration test increases, it is safe to assume that some or all of the mass components included in the estimate of (M) above do not actually behave as “masses.” Every mechanical structure has a resonant frequency, which may result in a significant dynamic force absorber effect at certain frequencies. This effect, which must be taken into account as part of the overall force estimating process, increases the F = MA calculated value of force.

Acceleration

The maximum Acceleration figure for the F = MA estimate is taken from the test specification, for sine vibration (g pk), for random vibration (g rms) and for classical shock pulses (g pk).

Two important factors related to the (g) level given in the test specification include the resulting maximum velocity and maximum displacement (stroke) required for your test. The velocity and stroke requirements are sometimes overlooked, leading to a poor system choice. Ask a UD applications engineer to review these important velocity and stroke calculations.

Streamlined Selection Process

The preceding review of factors that determine required System Force Output helps to illustrate that many variables are involved in the final, correct selection. As part of your discussions with a UD applications engineer, some or all of the following information will be helpful in order to streamline the selection process:

Product Details

Product description
Product test weight
Product dimensions
Product center of gravity (CG)
Product mounting considerations

Fixtures

Do they exist or will they require design and fabrication?
Approximate weight (estimate if necessary)
Approximate dimensions (estimate if necessary)
Mounting issues
Sizing issues
Head expanders

Test Requirements

Test types:
Sine
Resonance dwell
Random
Shock
SRS shock
Mixed mode (Sine on Random, Random on Random)
Time replication
Test magnitude
Test frequency range
Test duration
Three axis testing requirement
- Product orientation with respect to gravity

Combined Environment Testing

Ambient only
Combined vibration / thermal testing conditions

Future Considerations

Change in product line
Requirement changes

Expandability

Higher level test requirements
Multiple product testing for reduced test time