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May 03, 2024

Stamping 101: How does a hydraulic press work?

An examination of hydraulic press technology and the press’s anatomy, as well as its benefits, limitations, structure, frame types, terms, tips for selection, automation options, and best applications

An examination of hydraulic press technology and the press’s anatomy, as well as its benefits, limitations, structure, frame types, terms, tips for selection, automation options, and best applications can net the best utility from a press. Images: Greenerd

Presses of all types—mechanical, pneumatic, servo, and hydraulic–have their place and offer unique advantages. However, over the past 50 years, hydraulic presses have trended toward greater utility. Today's hydraulic presses are faster and more reliable than ever, and the technology has gone through significant changes and refinements. Improvements in seals, pumps, hoses, and couplings have nearly eliminated leaks and minimized maintenance.

An examination of hydraulic press technology and the press’s anatomy, as well as its benefits, limitations, tips for selection, and automation options, can net the best utility from a press.

Press Structure. These are the major components and features of the hydraulic press structure:

1. Cylinder. The cylinder assembly comprises a cylinder, piston, ram, packing, and seals. Piston diameter and oil pressure determine the force (tonnage) that a press can deliver.

2. Frame. The frame is the main structure of the press containing the cylinders and the working surfaces.

3. Stroke Control. Stroke length can be set for any distance within the stroke limits of the cylinder in the stroke control. Adjustments include the top of stroke, pre-slowdown point, and bottom of stroke.

5. Daylight. This is the vertical clearance from the top of the bolster to the underside of the ram in its maximum up position. This term is sometimes confused with the mechanical press term shut height. Shut height is the clearance over the bed with the ram fully down. Daylight describes the maximum vertical die capacity of the press.

An examination of the anatomy of a hydraulic press—1) cylinder, 2) frame, 3) stroke control, 4) throat clearance, 5) daylight, 6) bed, 7) bolster, 8) dual palm button controls, and 9) work height—allows for an understanding of its form and function. This is a C-frame press.

6. Bed. The bed is the flat, stationary machined surface that supports the bolster and dies.

7. Bolster. This is a plate or structure mounted on the bed that the tooling is mounted and attached to. Most hydraulic presses are constructed so that the bolster is removable.

8. Dual Palm Button Controls. This is a common method of actuating hydraulic presses. Both buttons must be depressed at the same time to bring the ram down, requiring the operator to use both hands. Control circuits include nonrepeat and anti-tiedown features.

9. Work Height. The distance from the floor to the top of the bolster is the work height.

Type of Frames. Many of these hydraulic frame types are common with mechanical press frames:

Four-post. This heavy-duty production press is built for continuous operation. It uses four heavy-duty, large-diameter, shouldered columns to tie the frame together. These large-diameter columns precision-guide the moving platen or press slide. This ensures exceptionally consistent upper and lower die alignment, minimizes deflection, and eliminates "lean-back." Any deflection that does occur in this style of press is straight up and down. It also enables loading of the press from any of the four sides, permitting easy integration into production lines and manufacturing cells.

Hydraulic presses can be built with larger table areas and beds or smaller beds than are standard. This 900-ton four-post double-acting hydraulic press is equipped with a main ram with 700-ton cushion ram and a 60-in. by 60-in. bed.

Being familiar with these terms will enhance your understanding of a hydraulic press:

This straight-side press is gib-guided to counter the effects of off-center loading.

Also, programmable logic controllers (PLCs) and other electronic-based controls have improved a hydraulic press’s speed and flexibility. With new computer interfaces and monitoring, hydraulic presses can be used in advanced computer-integrated manufacturing systems.

The built-in overload protection applies to the tools too. If they are built to withstand a certain load, there is no danger of damaging them because of overloading. Tools can be sized to withstand the load of a particular job, not a particular press. The pressure of the press can be set to suit the job. The lack of impact, shock, and vibration promotes longer tool life.

This transfer line with three stations for draw, redraw, and punch was built for an automotive company.

Open-gap presses provide easy access from three sides. Four-column presses ensure even pressure distribution. Straight-side presses offer the rigidity required for off-center loading in progressive die applications. One important thing to keep in mind is that the more critical the work and the more demanding the tolerances, the greater the reserve tonnage capacity should be.

This customized, complex, double-moving-platen deep-draw hydraulic press was integrated with two 6-axis robots that not only load and unload, but also prompt the press to perform multiple operations--draw, several redraws, and an ironing operation--required to produce a heavy-duty, high-pressure cylinder tank. To retrieve the newly drawn part out of the press, a FANUC robot moves between the double platens, and when in position, instructs the press when and at what velocity to extract the part.

Once the basics are determined, the next consideration is to select options. Most hydraulic press builders offer an array of accessories such as:

Note that the hydraulic circuit for a press is determined mainly by the application. In long-stroke applications such as deep-drawing, a dual-pump circuit with regeneration is typical. This allows the press ram to move quickly down to the work and out of the work while enabling a smooth draw speed.

However, when you are stamping on a hydraulic press, it is best to minimize how many valves you use during what is typically a very short stroke. Most presses used for stamping employ only a single hydraulic pump because of the short stroke required. This setup allows for fewer “valve shifts,” which reduces cycle time for a complete stroke and allows for many more strokes per minute.

Quality can vary greatly from press to press. Some light-duty presses are capable of "spanking" the work momentarily and reversing, and there are heavy-duty machines designed for general-purpose metalworking applications.

A few constructive points can be used to compare one machine with another:

Frame: Look at frame construction—rigidity, bolster thickness, dimensional capacity, and other factors. Cylinder: What diameter is it? How is it constructed? Who makes it? How serviceable is it? Maximum system pressure: At what PSI does the press develop full tonnage? The most common range for industrial presses is 1,000 to 3,000 PSI. Horsepower: The duration, length, and speed of the pressing stroke determine the horsepower required. Compare horsepower ratings.

Speed: Determine the speed each hydraulic press offers.

Press Tonnage. The tonnage required to do a job and the formulas to determine it is the same for hydraulic presses and mechanical presses. The tooling usually is interchangeable. There may be certain applications such as deep drawing in which the full power stroke characteristic of a hydraulic press reduces the tonnage, but there are no known instances of a hydraulic press requiring more tonnage.

An air-over-oil press is suitable for applications that require a force on the piece for long periods of time.

Selecting press tonnage in the typical pressroom often is little more than guesswork.

For example, if a job is successful on a 100-ton mechanical press, tonnage tends to stay the same for the life of that job. The job may never have been tried at 75 or 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.

How the Press Affects the Job. In most cases, the effect of stroke is the same on both hydraulic and mechanical presses. However, 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.

There are many ways and reasons to automate a hydraulic press, from simple to the very complex. Because press functionality is at the core of automation, it is advantageous to partner with a press manufacturer that is also an authorized system integrator and has a long history of providing integrated press automation systems for a range of applications.

Automation may be integrated during an initial press installation, or it can be added to an existing press setup. For the latter, most of the time automation is added not because the press is running slowly, but as a result of many other factors, some of them surprising:

The challenge is to identify the ”right size” for the automation system to address production needs for today and the future, while keeping within the project budget. As such, tailoring automation offerings to fit a range of applications is important. This includes everything from basic press automation (such as electrically integrating a material feeder to the press) to large multipress, multirobot autonomous working cells.

Hydraulic presses range in size from small , such as this relatively small bench press, to very large, such as this 500-ton.