We are not an engineering firm. The ratings above are based on 10,000 hours of life in a continuous duty application @ 1800 RPMs using meatball math and 100 years of applied experience. They also assume greasing intervals of 40-50 hours and parallel installation angles less than 3 degrees.
The fact is, the shafts listed above will transmit much more torque than shown. So if you need a shorter life span, you may use a smaller shaft. -- please contact an engineer for precise figures.