Merco manufactures a couple different hinge systems for wood stringer ramps. The first type is a stronger system for use on custom made wood ramps, we reccommend this hardware for ramps 20' in length or less. 
The hardware is 3/16", uses 3/8" carriage bolts, and is hot dipped galvanized. Can be used on any width ramp. Use 2" x 6" or larger stringers: