Background and Test Rationale
The purpose of this test was to compare the durability of two foam wheelchair cushions to withstand repeated compressions simulating two years of typical use.  One test article was from Free Wheelchair Mission and the other was from JARIK. 

Both cushions utilize urethane foam.  Urethane foam is open cell structure and is highly elastic, meaning that, when compressed the foam springs back immediately to its original thickness when the load is removed.  Each foam cushion has a “spring rate” which defines the pressure required to depress the foam a certain distance.  One measure of durability is the ability of the foam to maintain its original spring rate after many repeated compressions.  If the foam spring rate becomes less stiff due to repeated compressions, then the foam has degraded and may “bottom out” or completely compress under normal sitting pressures.   Bottoming out results in significantly higher pressures on the skin which in turn will lead to user discomfort, or possibly skin breakdown or pressure sores.

Normal sitting pressures at the ischial tuberosities (seat bones) are about 2-3 pounds per square inch (psi).   During a two year period, the user might, on average, exert a compression every 60 seconds by shifting his or her weight or propelling the wheelchair.  Using an estimate of 11.4 hours of sitting per day, 365 days per year would result in an estimated 500,000 compressions in a two year period.  Active users might stress the foam considerably more than this.

 Therefore the test was designed to simulate normal use for a two year period, measuring spring rates of each foam cushion before and after 500,000 repeated compressions to assess the durability of each test article.

Test Procedure
The test article was placed under a pneumatic cylinder in fixed position directly above the foam surface.  The cylinder impacted upon a rigid plastic disc 4 inches in diameter which was placed on the foam surface of the test article.  The force of the cylinder was adjusted to 2.5 psi test article contact pressure resulting in compression of approximately 80% of the original foam thickness for the Free Wheelchair Mission cushion.  Then the cylinder was continuously cycled at a rate of 1 cycle every 6 seconds until 500,000 cycles were complete. 

The same cylinder force was used to apply 2.5 psi test article contact pressure for the 500,000 cycles of the JARIK cushion; however the % compression of original foam thickness was approximately 50% due to the higher spring rate of the JARIK cushion.

Following repeated compressions, the spring rate of each cushion was measured in two locations – an uncompressed area of the cushion (to give spring rate as originally manufactured) and in the region of the 500,000 repeated compressions (to give spring rate after two years of simulated use).  The pressure to compress the foam at each 0.1 inch increment from zero to 0.9 inches was recorded and plotted on a graph for comparison.

Additionally, the percent compression of total cushion thickness at 2 psi was measured for each cushion, before and after 500,000 compressions.  The JARIK cushion was only 30% compressed at 2 psi both before and after repeated cycling.  The Free Wheelchair Mission cushion was at 65% compression at 2 psi before repeated cycling and 90% compression after repeated cycling.  The 90% compression at 2 psi indicates bottoming out under normal sitting pressures for the Free Wheelchair Mission test article following 500,000 repeated compressions.

Test Results
The graphs for each of the test article are below.

Note that for the Free Wheelchair Mission test article, the spring rate significantly declined (became less stiff) due to repeated compressions.

Note that the spring rate for the JARIK test article did not significantly decline following repeated compressions.

Conclusions
The test showed that the JARIK test article was significantly more durable than the Free Wheelchair Mission test article when subjected to 500,000 repeated compressions, simulating two years of normal use.  The Free Wheelchair Mission test article bottomed out under normal sitting pressures after being subjected to 500,000 repeated compressions.

In addition, this test showed that there was no visible foam degradation after 2 years' simulated use.  It can be reasonably anticipated that the foam last 5 years or more under normal use conditions.

Test conducted by
Richard Runkles, PE
Brock International