In elastomer design development, we try to optimize the best properties for the intended service and minimize the interference of the undesired properties.
Analysis of Application Requirements
The key to successful design and application starts with analysis of the application and prioritizing those properties needed for best performance. In some cases, one property may be at the top of the list. In another, it may be at the bottom. It depends entirely on the application. The next step is the selection of candidate urethane elastomers that have a high potential for success. Then comes molding or machining of the elastomer to component design. Finally, the all-important prototype test.
Material Selection for Impact Bumper
A good example of the importance of the proper material selection is the choice that was made in the material for a particular impact bumper. This real-life application shows the interdependence of design/engineering selection.
A pre-loaded support pad capable of absorbing high impact force was required. In this case the bumper was to remain under a large static pre-load for years before a possible huge impact might be encountered.
It had to service the northern European climate with a fairly broad range of ambient temperature and humidity. It would come in contact with minor amounts of oil but no chemicals or solvents. Adhesion to metal was not needed and the pre-load would prevent oils or grease from getting into the interface between urethane and steel.
Resistance to compression set (or stress relaxation) was critical but resistance to fatigue from repeated cycles was not. Age resistance to weather, oxidation and ozone was important so that no loss in properties would be noted over a 25 year service life. Screening from ultraviolet light would be achieved by pigmenting the elastomer black.
The priority list for this bumper would read, in order:
- Compression set resistance to 42 kips preload
- High impact load-deflection properties to 84 kips at 4 inch travel
- Relatively stable stress-strain curve from – 40°F to 160°F (-40°C to 71°C)
- Oil and hydrolysis resistance, oxidation and ozone resistance
Given the list of parameters above, it is logical to select a compound that meets those requirements and proceed through the design process. Let’s select GC 1090 as our starting compound. GC 1090 has a limit of 10% for long term static deflection.
By performing several iterations of different shapes at 10% deflection, we are able to arrive at an unbonded stack of sixteen GC 1090 pads (each 9-3/4″ diameter x 1-3/16″ thick) separated by fifteen 1/4″ thick steel plates. By using the urethane pads in series we are able to use the load bearing capacity of one pad combined with the total deflection of sixteen. A dynamic load of 84 kips would produce a deflection of approximately 4 inches. Drop hammer tests confirmed that the calculated deflections were within the allowable tolerance and several hundred assemblies are now in service.