The most common cause of trailer wheel bearing failure is the castle nut being tightened too much which makes the wheel bearings harder to turn.
You want to tighten the castle nut so that the wheel bearings are not too lose but also not to tight.
When tightening the wheel bearing castle nut spin the hub as you tighten the castle nut until the hub is hard to turn and then back the castle nut off a little bit at a time until the trailer wheel bearing hub spins freely.
Then install the cotter pin.
Other cause of wheel bearing failure on trailers is the trailer wheel bearing not being packed with enough grease or not packed properly.
When packing trailer wheel bearings you want to force the grease through the bearing until it comes out the sides and front of it.
Simply Smearing the wheel bearing with grease is not the way to pack it.
Also make sure the wheel bearing has plenty of grease and the cap is on the wheel bearing hub to keep dust and debris out.
Signs that your trailer wheel bearings are bad or are going bad are grinding noises, roaring noises etc when the trailer is going down the road.
However it can be hard to hear those noises but if you hear something similar when driving and it only happens with the trailer then you should check the trailer wheel bearings.
Other signs of trailer wheel bearing failure are the trailers wheels are hard to spin and have resistance and the trailer wheel bearings when bad can create heat and your trailer wheel bearings could become hot and the trailer wheel bearing hub will get hot.
The trailer wheel bearing and hub should not be warm or hot to the touch and if it is then that's a sign of the trailer wheel bearings going bad.
When a trailer wheel bearing fails completely then the wheel will stop moving and the wheel could come off of the trailer and leave your trailer tire and wheel rolling down the highway and off into oncoming traffic.
I've seen that happen before on a road trip.
I was behind a trailer and I started hearing something grinding in front of me and then I saw some smoke coming from the trailer wheel.
A little bit later I saw the trailer wheel lock up and then the wheel came off the trailer and almost hit me.
I stopped to help the guy and the wheel bearing had sized and the hub came off the trailer spindle.
I helped him put a new hub and bearing set in after I went and picked a new wheel bearing and hub set up for him.
Before venturing out with your trailer you should inspect the wheel bearings and jack the trailer up off the ground so the wheel can spin freely.
Then with your hands spin the tire and wheel and see if there's any resistance and if there is too much resistance and the wheel does not spin freely then your wheel bearings are either going bad or they are too tight.
Pull the cap and also check for grease as there should be enough grease packed into the wheel bearings.
Work the wheel back and forth as well and if there's play in the wheel then the bearings either need tightening down or the bearings are going bad.
Usually when a trailer wheel bearing fails it will have a grinding or roaring noise to it that gets worse as the trailer goes down the road faster.
That is a sign you need to inspect and replace the wheel bearings.
Properly greased wheel bearings on a trailer should last 100,000 miles or more but it's recommended you repack the trailer wheel bearings every 50,000 or so miles to make sure everything is okay so you don't have a wheel bearing failure on the highway.