If you are a trailer owner it is very good to learn how to replace a bearing in case you have no option but to do it on the road. It may be hours or days before road service or a mechanic is available. If you can repack a bearing then you can replace it. Of course you need to have the tools to do it. I sometimes do my own and sometimes have a tire shop do it if their price is reasonable.
You will have to purchase a new grease seal for each hub because when you pry the old ones out to get at the inner bearing it will bend the seal which is made out or relatively thin sheet metal with a rubber or plastic part that seals against the spindle. They come in many different sizes so you will have to find the part number which may be etched on one or just pry one out and take it to the auto parts store and they can match it. While you are at it take get at least one set of replacement bearings to have with you in case of a bearing failure. They are relatively inexpensive. I always try to keep a few extra seals and bearings that fit my trailer in my tool box. When replacing the seal just tap it in gently around the edges keeping it even as it seats. Stop it is flush with the hub.
It is very important not to get grease on the brake shoes. It will ruin them. If you get some on the inside of the metal brake drum wipe it out very thoroughly with a clean rag or paper towel. I usually use an entire roll of paper towels when packing bearings because it is a messy job.
It is very important not to get dirt, especially sand, in the bearing grease and that is very easy to do. If you drop a freshly greased bearing on the ground or is sand blows on it reclean it.
The large nut with the notches for a cotter pin is called a castle nut because it looks like a miniature castle tower. When you put the hub back on tighten this nut until it is snug THEN BACK OFF (LOOSEN IT) ONE OR TWO NOTCHES BEFORE INSERTING THE COTTER PIN. If you leave it too tight it puts a tremendous amount of pressure on the bearing assembly and the bearings will soon fail.
When you are on the road and make a stop after driving always feel the trailer wheels near or on the hub to make sure they are not hot. They will be warm but should not be so hot you can't keep your hand on them. After you check it a few times you will know what is normal warmth and what is not normal. If you find one getting hot either the bearing is failing or a brake is stuck. Also if a tire is under inflated it will get warmer than normal. You generally use the brakes
to get slowed down to exit the highway and the drums may be hot. That is normal. Do not stick your fingers through the air holes in the wheel and touch the drum because it may be very hot.
It is a similar topic but sometimes brake shoes wear away and need to be adjusted out so they will contact the drum when applied. Many of them have an automatic slack adjuster to adjust them but it may not be working. I will leave it to another thread about how to do that but a good time to check the brake adjustment is after you repack the bearings and replace the wheel. When the wheel is in the air you should be able to turn it freely. You adjust the brakes
out until they contact the drum then back off just enough so the wheel can turn freely. That brake is then correctly adjusted. This is real common in tractor trailer trucks and that is why you see brake check areas for trucks in the mountains. Going down over one mountain applying truck brakes
heavily can wear enough of the shoes off that the brakes do not work or loose part of their strength.