Shock indicator

A shock indicator is a mechanical device that detects and records mechanical-shocks experienced by it. It is a common practice to attach shock indicators to expensive goods, that are to be shipped[1]. This way, the magnitude and the time of shock is recorded and ensured that the delicate and expensive instruments are not mishandled during shipping. The shock indicator usually has a visual indicator when the shock-threshold is reached or exceeded, in the form of a broken spring or a digital display.

Contents

Introduction

The shock indicator was invented to ensure safe shipment of expensive goods and to hold the right people liable for any damage caused during the process. As any other invention, this one too has undergone various stages of development. The earliest shock indicators were simple mechanical devices with a spring-load assembly. The newer indicators, however, come integrated with a complex set of electronic circuits that detect, measure and record the shocks. There are many companies that make these indicators and these indicators are still a matter of active research.

Spring-Load assembly

The spring-load shock indicators involved a simple extensible spring with a load attached to it at one end. The spring constant was carefully chosen, based on the rigidity of the shipment material and the forces involved during the shocks. When these forces exceeded the limits, as set by the spring constant, the spring would either be broken or extended beyond its elastic limit. The law governing this relation is called the Hooke's law of elasticity. Mathematically, Hooke's law states that

\mathbf{F}=-k\mathbf{x},

where

x is the displacement of the end of the spring from its equilibrium position (in SI units: "m");
F is the restoring force exerted by the material (in SI units: "N"); and
k is the force constant (or spring constant) (in SI units: "N·m-1" or "kgs-2").

When this holds, the behavior is said to be linear. If shown on a graph, the line should show a direct variation. There is a negative sign on the right hand side of the equation because the restoring force always acts in the opposite direction of the displacement (for example, when a spring is stretched to the left, it pulls back to the right).

Modern indicators

The modern indicators that rely heavily on their supporting electronics[2], are much more complex than their mechanical counterparts[3]. They can not only detect and measure the shock, but also record the entire spectrum of shocks experienced by them over a period of several months. Their power consumption is rather low and are fueled by simple batteries. See also Accelerometer.

References

  1. ^ http://www.shockwatch.com/shipping_handling_monitors/shipping_video.php
  2. ^ http://www.agmcontainer.com/shock_indicators/shock_indicators_products.htm
  3. ^ http://www.shockwatch.com/shipping_handling_monitors/impact_indicator/technical_briefs.php