When building your robot's chassis or a gear box it is important to make a rigid frame. The image on the right shows a poorly built frame. Because there is not enough rigid cross support, the frame is skewed. This is especially bad for axles because the axles will rub against the side of the holes in the beam.
This shows how to fix this problem. By using cross plates that are twice as wide, the box is made nice and square, and axles should spin freely.
See also: Axle Support