The same principle as grizzly bars, but instead of clearing the rock, the grid holds the stone in place so that it can be broken down to a passable size with a hydraulic hammer. Vibrating Grizzly Feeder. A vibrating grizzly feeder (VGF) is located at the bottom of the hopper and directly feeds into the jaw crusher. As its name would suggest, the VGF vibrates and this agitation causes the material to separate