Measurement Testing

Mjr · Site Admin Joined: 13 Jul 2005 Posts: 294

From basic physics recall Force = Mass x Acceleration (F = MA)

Normally, selecting a Vibration System – such as a shaker table – needs only Newton's Second Law of Motion: Force = Mass x Acceleration
(F = MA). This basic formula is only a beginning. Let’s see an example of a wrong approach to estimating the required system-force rating.

Ex.: "One has 100 lb test specimen which must be tested @ 10 g. Hence, one needs a shaker rated at 100 lbs x 10g = 1000 lbs force."

Read thru the technical discussion below. We think you will walk away with better understanding of why this estimate of system force is, at best, insufficient or, at worst, inaccurate.

System Force Rating
Vibration systems, such as shakers and shaker tables, have output force ratings normally given in these units:
Sine force: lbs (kN) peak
Random Force: lbs (kN) rms
Shock Force: lbs (kN) peak

The specified random-force rating assumes a certain spectrum shape (usually flat) over a given frequency range (normally 20 Hz – 2,000 Hz) and a certain minimum mass load on the shaker table. If these conditions are different from one’s test requirements, another calculation of the effective random-force rating may be needed. Output force ratings give us 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 present 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). Obtaining the correct mass (M) value for the initial F = MA force calculation might be different for vertical axis testing compared to horizontal axis testing. Ask your test 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’s safe to assume that some or all of the mass components included in the estimate of (M) above don’t really 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 comprehensive force-estimating process, increases the F = MA calculated value of force.

Acceleration
The maximum Acceleration value for the F = MA estimate is taken from the test specification, for sine vibration (g peak), for random vibration (g rms) and for classical shock pulses (g peak). "g" is the acceleration due to gravity on Earth 9.8m/s^2 or 32 ft/sec^2.

Two important factors related to the (g) level given in the test specification include the resulting maximum velocity and maximum displacement (stroke) required for one’s test. Velocity and stroke requirements are often overlooked, leading to poor system choice.

Streamlined Selection Process
The preceding review of factors that determine required System Force Output helps to show that several variables are involved in the final, correct selection. When discussing you applications with a vibration engineer, some or all of the following criteria may help to streamline the selection process:

Product Details:
Product Description
Product Test Weight
Product Dimensions
Product Center of Gravity (CG)
Product Mounting Considerations

Mounting Fixtures:
Are they pre-existing, or will they require design and fabrication?
Approximate Weight (may be estimated)
Approximate Dimensions (may be estimated)
Mounting Issues
Sizing Issues
Head Expanders

Test Requirements:
Test Types

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