About Hydraulic Presses

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The use of hydraulic presses over the traditional mechanical press has increased dramatically over the last 30 years. Until recent history, the mechanical press was the common machine for production press work. The mechanical press utilizes a rack and pinion as well as a crankshaft to achieve significant tonnage without an external power source. It wasn&#;t until the early s that hydraulic presses became the more popular option.

Hydraulic Presses utilize hydraulic cylinders, single-direction mechanic actuators, to create compressive force. Invented in by Joseph Bramah, hydraulic systems were conceptualized by studying the movement of fluids through early toilets, which were later replaced by toilets created by Bramah.

Hydraulics depend on Pascal's law, which basically states that when an incompressible fluid is given pressure, the pressure is applied evenly in every direction. This law allows the force generated from compressing a fluid to be utilized in moving mechanics like a piston. It was also discovered that using a smaller piston to pressurize the fluid, through which moves a larger piston, generates greater forces than using two equally-sized pistons. This is called a force multiplier and is crucial when creating a hydraulic system.

Needs

Before seeking out the best prices, you should consider the role a hydraulic press will have in your personal shop or business. Writing a full list of tasks you&#;d like a press to accomplish can provide a better scope of the strength, size, type, model, etc. of press you will need. Be sure to understand what a hydraulic press can do; the range of abilities are extensive. Bending, blanking, drawing, forming, molding, punching, and stamping are just some of abilities of hydraulic presses. Not all machines are capable of these actions and should be taken into consideration.

Tonnage

After considering the uses for your press, the next important question to ask is what tonnage will be required for the jobs.  If you are switching from a mechanical press to a hydraulic press, the tonnage requirements will not change. Do consider, however, when choosing the right hydraulic press, you only need to know the maximum tonnage you will require for any given job. Opposed to a mechanical press, most hydraulic presses can be tuned down to a lower tonnage depending on the task.

The required tonnage to punch material (or the punching force) varies depending on three factors: perimeter of the punch, shear strength of the material, and material thickness. The tonnage can be calculated with the following equation:

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Shear strength is a measurement of the force needed to produce a sliding failure on the material along a parallel plane to the direction of the force; punching a hole in paper is an example of paper failing in shear. Shear strength is commonly measured in kilonewtons per square millimeter (kN/mm2) or pounds per square inch (PSI)­. All materials shear fail at different levels of force, so it is important to understand the specifications of the material you are working with. The material specifications also include the thickness of the material being handled.

Though punching is a common use for a hydraulic press, other functions also require specific tonnage in order to be effective. It is encouraged to understand the tonnage requirements for any job you plan to utilize a hydraulic press for.

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Selecting a Hydraulic Press

• Introduction
• Hydraulic Presses - Fast, Reliable, Productive, and Flexible
• 10 Advantages of Hydraulic Presses
• Glossary
• Formulas and Data
• Selecting a Hydraulic Press
• Hydraulic Press Limitations

1. Tonnage. Is the tonnage required to do a job the same for a hydraulic press as it is for a mechanical press? The answer is yes. There is no real difference. The same formulae are used to determine tonnage. The tooling is usually interchangeable. There may be certain applications such as deep drawing where the full power stroke characteristic of a hydraulic press reduces the tonnage, but there are no known instances where using a hydraulic press requires more tonnage.

   Selecting press tonnage in the typical press room is often little more than guesswork. If, for example, a job is successful on a 100-ton mechanical press, it tends to stay there for the life of that job. The job may never have been tried at 75 tons or at 50 tons.

   With a hydraulic press, however, you can adjust tonnage quickly and easily, tuning the press to precisely the right tonnage for each specific job.

2. The action of the machine. Even though the tonnage question might be settled, the question of the effect of the stroke on the work is often asked. Is it the same as with a mechanical press?

   The answer, again, is yes in most cases. There are some specific limitations. Drop hammers and some mechanical presses seem to do a better job on soft jewelry pieces and impact jobs. The coining action seems sharper if the impact is there.

   In deep drawing, however, the full power stroke of a hydraulic press produces significantly better results.

   Otherwise there are very few examples where the application of 100 tons of hydraulic force produces any significant difference in the character of the part given the same tooling.

   Shear in the dies will reduce blanking tonnage for hydraulic presses in the same way it does for mechanical presses.

3. Type of press selection. Open-gap presses provide easy access from three sides. 4-column presses insure even pressure distribution. Straight-side presses offer the rigidity required for off-center loading in progressive die applications.

   The more critical the work and the more demanding the tolerances, the greater the reserve tonnage capacity should be.

4. Accessories. Most hydraulic press builders offer a wide array of accessories. These commonly include:
   • Distance reversal limit switches
   • Pressure reversal hydraulic switches
   • Automatic (continuous) cycling
   • Dwell timers
   • Sliding bolsters and rotary index tables
   • Die cushions
   • Ejection cylinders or knockouts
   • Electronic light curtains and other devices
   • Touch screen controls
   • Servo system feedback for precise, consistent, repeatable stroke control
5. Quality. The industry offers various levels of quality. There are light-duty presses that are capable of "spanking" the work momentarily and reversing, and there are heavy-duty machines designed for general purpose metalworking applications.

   Here are just a few construction points that will provide a basis for comparison of one machine with another:

   1. Frame. Look at frame construction-rigidity, bolster thickness, dimensional capacity, and other factors.
   2. Cylinder. What diameter is it? How is it constructed? Who makes it? How serviceable is it?
   3. Maximum system pressure. At what psi does the press develop full tonnage? The most common range for industrial presses is to psi.
   4. Horsepower. The duration, length, and speed of the pressing stroke determines the horsepower required. Compare horsepower ratings.
   5. Speed. See page 5 to determine the speed of a hydraulic press.

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