old school maps

I have this old school map in great shape. However, if you look, you can see that the maps are supposed to spin inside the case to make all of them available for use. they are dated 1921 from Peerless Maps in Chicago. It has world maps and US maps. They pull down individually like window shades. The roller is frozen with rust and I want to fix it up so it is perfect. I've had this for over 30 years and picked it out of a school demolition in Gettysburg, Pa. It is really cool. Any suggestions that won't damage the maps themselves? I am not handy and am afraid to disassemble the mechanism. thanks.