About shearing simulation

Dear all,
I am running a shearing simulation with lammps. The model is shown below. The lower is the fixed group and the upper is the loaded group. I want the upper to move in x direction with constant speed and move in y direction automatically under the action of interaction between upper and middle. At first I set each component of force on each atom in group upper to 0.0, NULL, NULL, and set the velocities of these atoms to 0.0025, NULL, NULL. But the upper didn’t move in y direction. I wonder if there are any other commands can be used to achieve my goal.
May I have your suggestions ? Many thanks for your reply .

You can look at the scripts in examples/shear.