Protective cases are typically used when something delicate needs to be transported from point A to point B. And, you want to ensure it arrives intact. To many, a protective case is nothing more than a case with an inch or two of foam lining the interior. This is an overly simplistic perspective. Protective case designs are quite diverse. Often, they reflect the unique characteristics of the items they are developed to protect.
Over the next couple of postings, I’ll provide an overview of protective case design fundamentals, starting with today’s posting on Fragility.
Before you can decide the right approach to protecting equipment, you need to first determine the equipment’s fragility. In packaging terms, fragility is the maximum acceleration (shock) a piece of equipment can withstand without damage. In aerospace and defense circles, fragility is typically expressed as multiples of acceleration due to gravity. Known as g-force, the unit of measure is g.
Free-falling objects continue to accelerate. That is, they pick up speed (velocity) as they travel towards the ground – right up until the point of impact. All free-falling objects have the same rate of acceleration due to gravity, regardless of size or mass. Assuming the trajectory is vertical, and excluding factors such as wind resistance, the rate of acceleration due to gravity is a constant 32.17 ft/s2 (9.81 m/s2). This is also known as 1 g.
With the above in mind, an item with a fragility rating of 10 g means it can withstand a rate of acceleration up to (but no greater than) 10 times that due to gravity without being damaged.
If you have purchased a piece of equipment, the original equipment manufacturer (OEM) should be able to provide some guidance. If you are the OEM, chances are someone within your engineering department can provide this information. For reference, the following is a table of estimated fragility ratings (excerpted from MIL-HDBK-304C):
TYPICAL FRAGILITY RATINGS (ESTIMATED)
|15 – 24 g||Missile Guidance Systems
Precision Aligned test Equipment
Inertial Guidance Platforms
|25 – 39 g||Mechanically Shock-Mounted Instruments
Digital Electronics Equipment
Airborne radar Antennas
|40 – 59 g||Aircraft Accessories
Most Solid State Electronics Equipment
|60 – 84 g||TV Receivers
|85 – 110 g||Refrigerators
|> 110 g||Aircraft Structural Parts (i.e. Landing Gear)
Once you determine your equipment’s fragility rating, the next step is to understand the manner in which your equipment will be transported. In my next blog, I discuss how transport methods affect shock attenuation (cushioning) design.
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