1.The dynamical contact  of the gear driving

The gear  dynamical contact  means  the  contact status  of the gears  during  their   running ,at this moment  the  gears  meshing  with  each other  are actually   a double  involutes purely rolling on the  whole  tooth  width. Here  the pure rolling  means the  rolling  without  sliding . the stress point   is of linearity(the whole tooth width ) and   it is evenly  moving on the involutes  and make it possible  for the  whole  involutes  to be  weighted  at a certain  moment , in another word, the stress point  is changing uniformly on the  involutes  and  does  not  remain at a certain point .

2.The static  contact of the gear driving 

The gear static contact means the gear remains in the static  status  when the  gear is dragging  the load . for an example : when the  lift is stopping at a certain floor , the  load imposing on the gear  remains  the static status , the gear  remaining  the  static contact  is actually  a  couple of involutes  wheel gear which remains  a  whole tooth width   load  of linearity mutually( the stress point on the weighted surface of involutes  is actually a line  bearing   the load ),to the  tooth flank  of the involutes , this kind of load  is a kind of  static pressure .