what role is played by water to multiply the force in this hydraulic press?
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The role of the fluid is to transmit its pressure. To create a very large force you take advantage of the area of the cross section of the hydraulic cylinder that moves the press platen. So, using a positive displacement hydraulic pump, for example, with a piston cross section of say, 10 sq inches and developing 2000 psi, you can exert a force of over two million pounds if your press has a 36 inch piston.
An everyday example of the force multiplier might be the use of a hydraulic car jack. Say that it is a lever style rather than a rotary, and the lever is one foot between the grip and the plunger, with the pivot being a further 3 inches away and with the plunger piston being 0.25 inch square in cross sectional area. You press the plunger down with 100 lb of force, this develops 2000 psi in the hydraulic fluid. ( 100lb x 15″/3″ geometry / 0.25 sq in). If the cross sectional area of the jack cylinder is 2.5 sq in you will be able to lift 2000psi x 2.5sq-in = 5000 lbs. Your 100 lb force has been multiplied to 5000 lbs of lifting capacity. Like a lever however, you will have to exert your 100 lb force over a considerably greater distance than the load (car) will move. In the simplification of assumption of no friction losses or leakage, to raise the car 8 inches you will have to crank the grip through 400 inches. (5000lb/100lb x 8 inches). A rotary drive hydraulic car jack functions by the same principles, with the rotating motion advancing a small diameter plunger into the fluid, raising the pressure of the fluid which acts on the larger diameter lifting cylinder. The ratio of the square of the two diameters is the inverse of the relative stroke distance of the two cylinders.
Note that typical hydraulic systems use hydraulic oils, not water. This enables the use of strong and inexpensive steel for the tubing and fittings, as well as the wetted parts of cylinders and pumps, without raising the problem of corrosion. Corrosion is doubly troublesome by generating contaminants which erode and plug the system and cause leaks. Water is not a very good lubricant either.
An everyday example of the force multiplier might be the use of a hydraulic car jack. Say that it is a lever style rather than a rotary, and the lever is one foot between the grip and the plunger, with the pivot being a further 3 inches away and with the plunger piston being 0.25 inch square in cross sectional area. You press the plunger down with 100 lb of force, this develops 2000 psi in the hydraulic fluid. ( 100lb x 15″/3″ geometry / 0.25 sq in). If the cross sectional area of the jack cylinder is 2.5 sq in you will be able to lift 2000psi x 2.5sq-in = 5000 lbs. Your 100 lb force has been multiplied to 5000 lbs of lifting capacity. Like a lever however, you will have to exert your 100 lb force over a considerably greater distance than the load (car) will move. In the simplification of assumption of no friction losses or leakage, to raise the car 8 inches you will have to crank the grip through 400 inches. (5000lb/100lb x 8 inches). A rotary drive hydraulic car jack functions by the same principles, with the rotating motion advancing a small diameter plunger into the fluid, raising the pressure of the fluid which acts on the larger diameter lifting cylinder. The ratio of the square of the two diameters is the inverse of the relative stroke distance of the two cylinders.
Note that typical hydraulic systems use hydraulic oils, not water. This enables the use of strong and inexpensive steel for the tubing and fittings, as well as the wetted parts of cylinders and pumps, without raising the problem of corrosion. Corrosion is doubly troublesome by generating contaminants which erode and plug the system and cause leaks. Water is not a very good lubricant either.
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