EngineerRed:
A spring rate of 50 kips/in., 3” travel free to solid, and made of 3.9” dia. rod sounds pretty impractical to me, although I’ve not put any numbers to the problem at this point. That sounds like a pretty nasty sized spring, to say nothing of how it would be supported, confined, and how its ends would be finished, ground flat, or what? I assume you could find someone make such a spring for enough money. EdStainless suggested the most practical solution and everyone just waltzed right past it. Railcar truck springs are readly available, practical, economical, and of good quality. A spring group of 9 – D3 inners and outers would come pretty close to meeting the OP’s requirements; about 5.73 kips/inch per spring set, by 9 = 51.6 kips/inch, with 2.5” travel, free to solid. A std. commodity item on the American railroads. All in a package of about 18” sq., by 12” high. The Op’er. would have to design and fab the cage and load bearing system for this 3x3 spring package. And, I suspect that will be easier than finding a maker for the single large spring.