Hydraulic press

 

 

 

A hydraulic press is a machine (see machine press) using a hydraulic cylinder to generate a compressive force. It uses the hydraulic equivalent of a mechanical lever, and was also known as a Bramah press after the inventor, Joseph Bramah, of England. He invented and was issued a patent on this press in 1795. As Bramah (who is also known for his development of the flush  toiletinstalled toilets, he studied the existing literature on the motion of fluids and put this knowledge into the development of the press.

Principle

The hydraulic press depends on Pascal's principle: the pressurethroughout a closed system is constant. One part of the system is a piston acting as a pump, with a modest mechanical force acting on a small cross-sectional area; the other part is a piston with a larger area which generates a correspondingly large mechanical force. Only small-diameter tubing (which more easily resists pressure) is needed if the pump is separated from the press cylinder.
Pascal's law: Pressure on a confined fluid is transmitted undiminished and acts with equal force on equal areas and at 90 degrees to the container wall.
A fluid, such as oil, is displaced when either piston is pushed inward. The small piston, for a given distance of movement, displaces a smaller amount of volume than the large piston, which is proportional to the ratio of areas of the heads of the pistons. Therefore, the small piston must be moved a large distance to get the large piston to move significantly. The distance the large piston will move is the distance that the small piston is moved divided by the ratio of the areas of the heads of the pistons. This is how energy, in the form of work in this case, is conserved and the Law of Conservation of Energy is satisfied. Work is force times distance, and since the force is increased on the larger piston, the distance the force is applied over must be decreased.

                                                   

       

Figure 26.1 Three-part hydraulic press.

1 head, 2 pump, 3 side column, 4 press cylinder, 5 ram, 6 guide, 7 cross-section of side column,

8 tie rod, 9 bottom platen

                                

                            

Figure 26.2 Highly simplified drive layout of a Hydraulic Press

 

Applications

Hydraulic presses are commonly used for forging, moulding, blanking, punching, deep drawing, and metal forming operations. Some of the largest in the world were built by the Heavy Press Program.

Factors That May Favor the Use of a Hydraulic Press

Factors that may favor the use of hydraulic presses over their mechanical counterparts may include the following:

1. Depending on the application, a hydraulic press may cost less than an equivalent mechanical press.

2. In small lot production where hand feeding and single stroking occurs, production rates equal to mechanical presses are achieved.

3. Single stroking does not result in additional press wear.

4. Die shut heights variations do not change the force applied.

5. There is no tonnage curve derating factor.

6. Forming and drawing speeds can be accurately controlled throughout the stroke.

7. Hydraulic presses with double actions and or hydraulic die cushions are capable of forming and drawing operations that would not be possible in a mechanical