Interpreting Shock Data to Estimate Drop Height Levels During Handling

Abstract:

During distribution, consignments undergo numerous handling processes both mechanized and manual. These operations are known to produce drops and impacts of varying severity which have a potential to cause damage to the product. These shocks are the main parameters required for the optimum design of protective packaging systems. The severity of the shocks is often described in terms of their effective (free-fall equivalent) drop height (EDH) and impact orientation, in order to facilitate the laboratory testing conducted on a free-fall apparatus. The preferred approach is to survey the shocks with self-contained tri-axial shock recorders and process the data in such a way that statistical distributions of expected drop heights and orientations are obtained. On the other hand the Real Drop Height (RDH) method, based on the measurement of free fall time, is also used, mainly to discriminate between free-fall events and more commonly occurring complex causes of shocks, primarily for the quality control of distribution environment. The focus of the paper is on the EDH method and on the use of characteristic parameters of the tri-axial acceleration shock pulse to determine the EDH. An accurate estimate of the coefficient of restitution between the instrumented test package and the impact surface must be known and this poses a problem as it cannot always be established for every event during distribution. Consequently, the adopted approach is to calibrate an instrumented test package and obtain an estimate of the coefficient of restitution between the package and a test impact surface which is generally assumed to be hard relative to the cushioned package. The paper addresses the pitfalls and investigates various algorithms of determining the EDH from recorded shock data. It presents an analysis of the influence and errors associated with various methods used to estimate velocity change from characteristic parameters of a shock pulse such as the pulse width, the peak acceleration and its temporal location. The effects of analyzing the orthogonal acceleration vectors separately, as opposed to the resultant vector, are discussed. The results of a number of free-fall experiments, undertaken in controlled conditions, are used to validate and calibrate the proposed method for determining the EDH for free-fall drops on hard surfaces.

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