Voyager, It is not a broken pin or key but the rapid-traverse overrunning clutch,
that with time is no longer able to engage tightly enough - this is why you can stop the handle with your hand- to drive the table. The vertical output shaft that goes out the top of the gearbox is driven by an overrunning clutch so that when you use the rapid it can rotate faster, then when you release the rapid it takes over again and normal feeds are driven. There are some rollers (look like 1/4" dowel pins) that cam out against the inside of a hub to drive the hub which is part of the output shaft assy. Sometimes the surface of the hub ID is so smooth that the rollers can not get a good bite and they just slip around inside. There are also small springs that help the cam action by pushing on the rollers to help them engage but sometimes they become weak and are usless. Two things you can do are 1. roughup the inside of the hub (We heavy glass bead blast the inside) and 2. put in stronger springs. If you are able to dissassemble and reassemble mechanical units like this then you can do this fix yourself. If not then you should seek a competent repair person to assist you. Having a copy of the parts manual would help much.