</font><font color="blue" class="small">( can you give me some idea of the thickness of the steel that the fork brackets rest on? The fork brackets actually have a taper opening, and I'm not sure what thickness would hold best. Thanks! )</font>
The two primary design issues are those of supporting the
weight of the loaded forks and the forward torque of the
tractor when the forks hit an immovable object.
The former requires a beam at the top of the carriage
sufficient to withstand a vertical load equal to the lift
capacity of the loader and a horizontal load which is
dependent upon fork length/load. This horizontal load
will also be seen by the carriage bottom beam but acting
in the opposite direction.
The more important consideration for the bottom
beam is to withstand horizontal loading generated
by the tractor torque when it encounters an
immovable object. This will dwarf the force generated
by the forks/load and should be designed for a worst
case of both forks lying side-by-side in the center
of the carriage so as to point load the beam.
You probably want to keep stresses in mild steel around
20Kpsi to give yourself a safety margin in for impact
loads, etc.. Attached is a first cut of my particular design
which may better help explain the above.