Vibrational Forces in Packaging

Vibration is the movement of an object in regard to a static reference position. The distance the pack moves is the amplitude of the vibration and the time taken for the movement is its frequency.

All methods of transportation have their own frequency and amplitude of vibration. Most of these, that we will encounter in packaging are in the 30 to 100 hertz range.

Vibration can come from many different sources, such as the load and the suspension system of a vehicle and the vibration is increased by any lumps and bumps in the road or railway track the vehicle is on. Even things such as uneven tire pressures can increase vibration in the load by reducing the uniformity of the ride. An uneven load or poorly stacked unitizer will also contribute to a vibration problem.

When a pack is vibrating it is under acceleration and deceleration, so this means that "G" level (Please see Drops and Shock Forces in Packaging) is an element to be taken into account and will naturally be greater in relation to the larger amplitudes of movement that the pack encounters.

Problems Encountered due to Vibration

Vibration can have numerous effects on a pack and its product from the minor to the fatal. Some of these are:

The three conditions for vibration that can occur in a sprung load are:

Direct coupling (output = input)
Resonance (output > input)
Isolation

 (output < input)

Resonance

Resonance is where the vibration felt by a pack and / or its product are amplified. All sprung loads, as encountered in, for instance, a unitized group of packs on a truck, have a unique frequency to which they will resonate, so resonance occurs when the vibration frequency is the same as the natural frequency of the pack / product. It is good practise to check a product and pack for resonance frequencies in the previously mentioned 30 to 100 hertz range. All three axis should be checked and all resonance points found and noted. Please see BS EN 28318 for further details.

A good dampening material will have a natural resonance of less than half that of the product it is required to protect. Such materials are usually similar to cushioning materials, but it should be remembered that ALL materials have a natural resonance and so a poor choice of cushioning material may lead to an worsening of vibration damage. For high cost and sensitive products it is sometimes necessary to use special dampening suspension systems within the pack.

A phenomenon that is particularly destructive is when a whole load goes into resonance. This induces a "feed back loop", where all of the packs in the load start to move. The consequence can be server damage to both the packs and the products they protect. This is why careful checks for vibration and resonance, plus a general general gathering of empirical data during transportation tests is vital.