I have two of the cheapo $50 Harbor Freight welding carts. They are NOT built to the same standards as their tool boxes. They do work mostly. I have a full size AHP TIG 200 on one with a tall (210CF?) argon bottle. The other came from my dad's shop with the Weldmark 187 (Hobart/Miller) MIG machine and a middle size C25 bottle. The wheels broke on mine.
I didn't have to unload it to fix it though. I just happened to have a couple wheels I saved off of something on the shelf that were the exact same size. One of my Vevor toe jacks lifted it up so I could replace the wheels without unloading the cart. Done.
The cart that came from my dad's shop had loose casters. By loose I mean most of the bolts fell out, and the casters were hanging on by a thread. Literally.
Okay, after fixing the one welding cart I decided to deal with the loose casters on the other one.
I have some parts trays I bought years ago with hardware assortments in them. My thought was they were cheaper than the same/similar parts trays/racks empty. I figured even if I threw the hardware away the bins were worth it, and that was my plan. Rather than throw it all away though I doubled up the hardware from two tray racks into one. This left me with usable parts racks, and all the hardware. Sometimes I even use some of the hardware.
Today I grabbed some 1/4-20 bolts, and some 1/4-20 nylocks out of the trays to replace the hardware on those welding cart casters. The nylock nuts take a nice standard 7/16 socket, but the bolt heads are 10mm. No. Not a sloppy inch measurement that a 10mm works on. The heads are a nice fit in a 10mm wrench.
Here are some of the parts bins. I have 6 of these racks all together. They are every bit as durable as the Durham/Klein boxes they resemble, and they were a fraction of the price including the metrinch hardware.
Anyway, both welding carts are good to go again.