rittz:
Yes, you certainly can calculate the cap’y. of a screw jack. You need to know all the material properties of the screw and the collar or nut, and the stand. You need to know the type of screw thread form; Acme, Square, Modified Square, etc. and all the dimensions of the screw and nut. Then it is primarily a problem of shear strength at the root of the thread, with a few other factors thrown in for complication of the problem.
As mentioned by others, I would look for a cap’y. cast into the base; I would look for a name or manuf’rs. letters cast into the base and ask them. You could take a quick cut at a cap’y. calc. and then have a testing lab actually load it and watch its deflection rate, or just use it at half the calc’ed. cap’y., at your own risk. Look at the cap’y. of a current day jack with the same screw form and size for an estimate of your jack cap’y. All-in-all it might be cheaper and less time consuming just to buy a new one.
The "capacity" of an (old, rusty, not-maintained) screw jack will be significantly degraded by the friction increase from rusty (male) thread to rusty (female) nut: Thus the turning force to twist the screwsjack = breaking and "re-machining" and cutting "new" threads force PLUS the "lifting" force you desire PLUS the "ordinary" friction force you'd expect with a new (lubricated) jack and (new) threads.
Check the jack stand too: Might be rusted on non-structurally strong also.
Depending on how much (what weight) you are going to lift might make using the old jack not worthwhile. What is your cost-per-hour of the project if this costs you 4 extra hours?
