Survivability

Survivability

Survivability

Introduction

The notes relate to simple examples of shock absorber/ isolation systems provided to minimise the risk of damage from impacts e.g. to prevent damage if a support screw fails and a load is accidentally released. The detailed analysis of shock absorber/isolation systems is very complicated and involves assessment of the dynamic response of the equipment to different types of activating energy inputs. The notes below relate only to illustrating the benefits of using shock absorbers to reduce the forces experienced by equipment to impacts. The more complicated scenarios involving systems continuously operating and withstanding sudden changes of loading and acceleration e.g car suspension systems and aircraft landing gear, are outside of the scope of this work(Huenecke 1997).

Discussion

Moving objects have kinetic energy related to their velocity and their mass. If the velocity of an object is significantly changed in a short time span e.g. it impacts on a stationary body, then high forces result. These forces can be useful e.g a forging press using the kinetic energy to form metal. However real life impact forces (shock loads) are generally very destructive and are avoided.

Kinetic energy increases in a direct ratio to the mass and to the velocity squared. The heavier the object, or the faster it travels, the more energy it has. Methods of energy absorption include rubber buffers, metal springs, air springs, and hydraulic shock absorbers. When the systems have to continuously operate under the influence of shock loads the shock isolation system generally include spring-dashpot isolation systems.  For simple shock absorber applications required to mitigate the effect of a single events then viscous dampers which dissipate the energy, as heat rise of a fluid, are often preferred.

In normal everyday life simple examples of shock absorber systems include crash helmets, steel toe caps in industrial boots, collapsible bumpers on cars, motor way barriers(Hurt 1995).

The notes below are general in nature provided to show the benefits of using shock absorbers. For more detailed information - links are provided to shock absorber suppliers.

Shock Absorber types There are a number of different methods of converting an impact /collision into relatively smooth cushioned contact..

Metal Spring

Rubber Buffer

Hydraulic Dashpot

Collapsing safety Shock Absorbers

Pneumatic Cylinders

Self compensating Hydraulic

Simply locating metal springs to absorb the impact loads are a low cost method of reducing the collision speed and reducing the shock loading. They are able to operate in very arduous conditions under a wide range of temperatures. These devices have high stopping forces at end of stroke. Metal springs store energy rather than dissipating it. If metal sprint type shock absorbers are used then measures should be provided to limit oscillations Metal spring are often used with viscous dampers .

There are a number of different types of metal springs includine helical springs, bellville washers(cone-springs), leaf springs, ring springs, mesh springs etc etc. Each spring type has its own operating characteristics(Hurt 1995).Elastomeric Shock absorbers

These are a low cost options for reducing the collision speed and reducing the shock loading and providing system ...