vRigger can calculate the forces on almost every rigging system. And vRigger understands the complexity of friction, ratchets, and winches, the subtleties between raising and lowering a load, and the effects of rope angles.
You can specify the minimum safety factor, the weight of individual pieces of gear (even a single carabiner if you are so inclined), the gear's friction (including pulleys, carabiners, brake bar racks—even the edge of an I-beam), and whether forces are displayed in pounds, kilograms, or kilonewtons.
You can display the forces being applied to each piece of gear, the rope angles (the angles are displayed in red if they exceed the critical angle of 120°), and the system's overall mechanical advantage.
You can have warning symbols displayed on any component in the system when the forces exceed your safety factor. Failure symbols can be displayed on gear if the forces exceed the gear's breaking strength.