The hub is falling off or the axle shaft is sliding out?

If the hub then the two large lock nuts, the spindle threads, and the locking washer/tabs are in question.

If only the axle shaft slides out and if the big end looks like a gear then it is held in with a small domed steel cover and a few small bolts. Or the big end of the axle shaft has holes in the flange and studs on the end of the hub and nuts hold it in, sometimes with cone washers, or sometimes with just bolts.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-