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I really don�t have much to add to what has already been said but I thought I would list a few things to be considered as you design your press 1) The �tonnage� or force is proportional to the net square inch area of the cylinder multiplied by the working pressure. 2) Increased force is accomplished by increasing working pressure cylinder diameter or both. 3) Many porta-powers or shop presses use 10,000 PSIG in order to utilize a smaller diameter cylinder. 4) Working pressures exceeding 3000 PSIG usually call for specially built components to handle the pressure (read that additional $). 5) The cost of hydraulic cylinders is related to the diameter, the stroke length, and the pressure rating, increasing any of these factors increases the cost of the cylinder. (use the smallest shortest cylinder with the lowest pressure rating you can get by with) Additional comments/questions. 24 inches of stroke seems to be unusually long for a shop press, any specific application you have in mind that needs that much stroke? With your power pack limited to 3500 PSIG and a desire to build a 30 Ton press, you will need around a 5� diameter cylinder, if the rating on any of your other components limits your pressure settings to 3,000 PSIG or under then a 5� won�t do it. (You can do the math on this real easy)
Some other things to consider, I have worked on press with ratings up to 1,000 tons. One of these presses used a 16� 10,000 PSIG single acting Cylinder as the main push Ram and had two 3� double acting jack rams that aided to the push tonnage and provided return lift for the movable platen.
You are not limited to using a single cylinder, although extremely large diameter or multiple cylinders usually complicate the mechanical design and the Ram (or Rams) usually end up pushing a movable platen (with a movable push block) and that platen usually has guide rods and bushings at all 4 corners. If you are working on a budget you will probably find what you can scrounge up and work the design around that, good luck. Sharpen up the pencil and get creative.
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