There are formulas for success when selecting any isolator. Although many factors can influence the dynamic response of a shipping container system, one of the key factors to consider is energy being imposed on the system. This energy must be stored or dissipated. The energy stored in the mounts must then be released back to the system in a controlled manner such that the peak forces transmitted are below the critical level (fragility) for the mounted equipment.
The basic equations for initial estimates of shock isolation systems are fairly simple. They involve the input to the system and the characteristics of the mounted mass and the shock mounts. In general, the shock to the system is modeled as an instantaneous velocity change for most shipping container applications. The stiffness of the shipping container mount
determines the dynamic response of the support system. This mount stiffness depends on the geometry of the mount and the properties of the elastomer.
The “spring” portion of typical shipping container mountings is an elastomer (rubber) specially compounded and processed to provide certain stiffness characteristics. The standard line of our shipping container mounts uses a specially compounded synthetic elastomer which is called SPE® I. This material has high strength, medium damping and good low temperature flexibility – all of which are important to shipping container use.
With a given weight and geometry for the mounted equipment, the dynamic stiffness of the shock mounts is the adjustable factor at the designer’s disposal to provide the desired protection. This stiffness determines the mounted system natural frequency that, in turn, controls the rate at which the energy is returned to the system and the maximum forces that will be imposed on the equipment. Yet another factor is the energy input to the system over a particular time period (pulse length Ʈ), calculating the maximum force level (Fo) it reaches.
If the shock mounts are selected correctly to protect the mounted equipment, the response through the mounts will be such that the energy (assuming no dissipation) will be transmitted to the mounted mass over a longer time period than that at which it entered the mounts. With this longer time period, the peak force will be lower than that imposed at the outside of the container. Conversely, if mounts are incorrectly selected, they could result in amplifying the peak forces seen by the mounted equipment. Again, the energy is assumed equivalent to the original energy entering the container. It should be noted that shock amplification can occur in a number of ways including incorrect mount stiffness, non-linear mount stiffness in the necessary deflection range, as well as insufficient sway space available within the shipping container. Therefore, it is important to accurately define system parameters, select appropriate shock mounts and design the shipping container with the mounting system in mind.
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