I just installed a replacement jack on a Zieman, and as luck would have it I still had the directions lying on the garage floor...
It says in essence that you can protect it's finish with a coat of wax, and lubercation is needed if the jack ever feels rough or produces a grinding noise.
It says you take weight off the jack, remove the plastic cap on the end, remove a pin in there that holds the handle in place, remove the handle, and one of the miter gears, turn the jack upside down and shake out the other miter gear. Then there is a roll pin you have to align with a slot in a washer, so you can then tap the top of the jack screw to remove the inner tube. It says do not remove the screw from the inner tube, clean everything and work grease into the entire thread, grease up the bevel gears, and reassemble. It also says when you put it back together to make sure the roll pin is captured by the miter gear using the slot with closed ends...
Dutton-Lainson makes a bunch of trailer jacks.
Here's a page that shows an exploded view.
Exploded view typical tounge jack