Mixed signals provide Shock ‘n Shake

| Environmental Testing

Smaller shaker tables for high force shock testing

Jonathan Newell finds out how two-in-one mechanical stress screening is possible when combining test procedures on one electrodynamic shaker table.

For companies wanting to perform in-line production stress screening, the prospect of investing in separate shock and vibration tables can stretch budgets to the limit. In such cases, the option of outsourcing the process is often not viable as it adds lead time into time-sensitive production operations.

Another alternative is to specify a single table that meets the high shock force specification, which means it will be over-engineered for the lower force (sine and random) vibration testing requirements. This unnecessarily adds cost to the equipment purchase.

Combined function shakers

One way of overcoming this dilemma has now been made available from environmental and vibration testing equipment specialist, Thermotron. The company is offering a service whereby it supplies a modified shaker using different inverter configurations to balance the requirement to accommodate high shock without increasing sine and random specifications..

An example given by Thermotron is the ability to replace the need for a 20,000 force-pound water cooled shaker with a modified 8,000 lbf air cooled shaker to perform a 100g, 11ms half-sine shock test. The engineering model used for achieving this managed to halve the cost for customers requiring the system, one which is finding wide acceptance particularly in the automotive industry.

For fast moving goods manufacturers, capital cost is just one part of the equation. They also need to work within constrained space limitations in premium manufacturing facilities and so by using the smaller single shaker system, valuable floor space is saved as well as utilities such as electricity and cooling. The degree of structural alterations needed to accommodate the shaker table is also minimised.

Commenting on the positive effect of combined shock and vibration tables on the budgets of production companies, Ben Shank of Thermotron told us, “An electrodynamic table is a major capital investment and the cost of running and maintaining it is of serious concern.  Because we make all of our components in-house anyway, it is natural for us to consider the final intended test conditions during the initial specification process.  That way, the end product does everything required of it without becoming an excessive drain on capital and utility budgets.”

Optimising electrical characteristics

The amplifier that drives an electrodynamic shaker needs to supply enough current at a sufficient voltage to run the desired test.  For high-impulse shock pulses, the voltage requirement usually represents the strongest limit on shaker performance.  The impact of this limitation can be minimised by considering the specific shock pulses to be run in the construction of the shaker unit.

Different inverter configurations optimise how the shaker can be used. On those that can run at a lower voltage, more inverters have to be added to increase the available current.  For example, Thermotron’s most common 8000 lbf shaker model needs three inverters, each with an 85A RMS rating.  The enhanced shock shaker needs five inverters to achieve the same sine and random force rating, but can do the 11ms half-sine shock pulse test with 200 pounds of fixturing and product instead of only 100 pounds for the three inverter model.  During sine and random tests, the five inverter model does not use any more power than the three inverter model.  It just sends more current through the armature at a lower voltage.

Jonathan Newell
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