The variety of gear can be  the cylindrical gear, bevel gear , non-circular gear , gear  rack, worm rod & worm wheel ; according to their  external forms ; They can also be straight  gear , helical gear , herring bone gear , curve gear  according to the shapes of  toothed portion ; they also can be the external gear , internal gear , according to the surface on which the gear  is located ; they also can be the   gear  made by casting ,  the gear made by cutting , the gear  by rolling , and the gear by fritting  according to the methods of gear making .