Ed, you're right of the need for a 5" or 6" cylinder if the desire is for a 25 to 30 ton press. BUT - for those folks like me where a 12 to 16 ton press will do everything needed for forging, a 4" cylinder works and is a good bit less expensive. Heck, even that 8 ton I got from Coal Iron was pretty good by using small 1.5" wide dies. Their claimed 12 ton with a 1.5" wide die is putting more pressure (per sq in) on a billet than a 25 ton press with 4" wide dies. Isn't that a valid statement - what counts is pressure per sq in.
If the max pressure "safe" is considered to be 2500 psi a 4" cylinder gives right at 16 ton. 1900 psi (and should be very safe) will give right at 12 ton.
While tons is easy to calculate with (cylinder area X psi)/2,000, this site is an easy tonnage calculator:
https://www.baumhydraulics.com/image...s/cyl_calc.htm
These hydraulic systems that run 10,000 psi just plain scare me to death! The only reason I'd want the 4500 psi rated hydraulic hoses is for the safety margin because I ain't planning to run anywhere near that kind of pressure.
Ken H>