When it comes to keeping the old axle. I'm a dinosaur from the last century, I always think. "I might have a use for that someday". So I keep stuff around for years before I throw it away. Then after I throw it out I find that I could have used it.LOL
I am also of the philosophy "If it ain't broke don't fix it". The way I look at it is; and there are pluses and minus' to this. First not knowing how many miles the trailer has on it, maybe very few, maybe a lot. Also the fact that any new axle I buy may not be constucted as well as the original, ie Chinese Steel and other parts. But again I'm from the last century.
The springs are attached to the frame via a device known as a "perch", which is a "U" shaped device where the spring is attached with a bolt thru the perch and the "eye" (a loop or hole on the end of the spring). Inside the eye of the spring there is usually a bushing, kind of like a piece of pipe that the bolt goes thru. The other end of the spring is attached to the perch with what is called "shackles". On this end of the spring there are two bolts and their associated bushings; The eye of the spring is attached to one end of the shackle and the other end of the shackle is attached to the perch.
If the mobile or free end of the springs don't have shackles, they will be held up by a simple rectangular loop where the longest leaf of the spring can just slide back and forth within the loop as the length changes. This system has even fewer points that wear. Only one bolt and bushing in the eye of the spring at the opposite end.
As you know the springs have an arch in them and as weight is added and removed or the trailer bounces the arc flattens out causing the spring to get longer and shorter. The shackle is there to accomodate this changing length.
Springs are made from hardened steel and rarely are effected by any rust, which is mostly surface rust. If the springs have been immobile for some time; in your case I think you said it sat for 17 years. It would be a good idea to give them a good dose of penetrating oil just to losen the rust in between the leaves and allow them to slide more freely, because they do rub against each other as they change length.
You can check for wear a couple of ways. Either by removing the bolt(s) at each end of the spring and lowering the eye of the spring down to where you can see the eye and its bushing. If worn severely the bushing will have a hole thru it that is really egg shaped and out of round and the bolts shank will be worn and out of shape. If the bushing is not worn too severely it could just be a matter of replacing the bolt with a new one.
Of course you will have to raise the trailer and remove the tire (just to get weight off of the suspension).
The other way is to raise the trailer and block it so it won't fall and then use another jack, like a hydraulic bottle jack, first under one end of the spring. Push on it just enough to see if there is any significant movement of the eye of the spring relative to the perch or shackle. If not the bushings are probably OK.
With either method make sure the trailer is blocked to prevent it from falling. I'm sure you did this when you checked out the brakes.
If the brakes become inoperative sometime down the road and since parts are no longer available from what I understand. Then it might be time to replace the axle. Even if this happens I would search out new back plate assemblies and drums that would bolt to the end of the original axle before I replaced the entire axle assembly.
I would think that parts like bearings and seals would be available at most auto parts stores.
I would bet that with all of the "Canned Ham" trailers on the road today most of which are probably running the same axle as your trailer that there are still bearings and seals that will fit.
Some of my farm equipment is over 70 years old and I can find bearings and seals at most auto parts stores.
So for what it's worth.
How's the dinette coming along?
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