This is a cylinder which allowed you to fine tune different settings in skeinforge but you can also use it to block your two ball bearings in the Y axis by changing the position of your ball bearings in the other side of your platform and putting the part between the two bearings as we can see in the two pictures. You can help you with a key to maintain the cylinder at the right place between the two baerings while you are putting the axis.You can remove the ears in the openscad file and use a raft when you are going to print the part with Replicator G it is may be easier to remove from the part.