In reply to Alex Mill’s question at http://www.chiefdelphi.com/forums/showpost.php?p=1397708&postcount=16.
This is a sketch of how we define the belt pulley positions for our serpentine timing belt in our shooter. (The intake gearbox, which Alex asked about, is done in a similar manner, however this one does a better job of demonstrating the concept).
Before we laid out the timing belt, for this gearbox we chose to place the output gear and most of the motor positions. Obviously more of that positioning could be brought into this sketch to give greater flexibility in space optimization (that is what our intake roller gearbox does), but that comes at the cost of making the sketch more complicated.
We start with drawing a path that is the pitch line of the timing belt. We then create a set of linear lines with equal number of segments as we have arcs and lines in the belt. These two are then related together so that they have the same path length (by the blue circled dimensions and relations). The arcs are made equal length to the line segments by putting a driven dimension on the arc-lengths, and then creating an equation for the line dimension that sets it equal to this driven dimension. The idler pulley position and slot position is constrained to 1 degree of freedom by the parallel relationship and the distance from top line (circled red). The total length of the straight path is then dimensioned to be the pitch length of the belt, which then fully constrains the sketch.
It looks like SolidWorks 2014 should greatly simplify this process with path length dimensions. I am looking forward to trying them out next year. http://help.solidworks.com/2014/English/SolidWorks/sldworks/t_create_path_length_dimensions.htmrelationships