Hydraulic presses: 101

Today’s hydraulic presses are faster and more reliable than ever and can do a wide variety of jobs within their tonnage range to provide excellent versatility.

Greenerd designed and manufactured a custom, double-moving platen deep-draw hydraulic press, fully integrated with two 6-axis Fanuc robots that in addition to loading and unloading, prompt the press to perform multiple operations to produce a heavy-duty high-pressure cylinder tank.

Today’s hydraulic presses are faster and more reliable than ever and can do a wide variety of jobs within their tonnage range to provide excellent versatility. Hydraulic presses are also uncomplicated and can provide significant cost advantages over mechanical presses in comparable sizes.

Moving parts are few, and they are fully lubricated in a flow of pressurized oil. These parts are usually standard, affordable, off-the-shelf components that are also relatively easy to replace. This means more uptime and lower maintenance costs.

Hydraulic presses also provide easy tonnage adjustment and more tonnage control throughout the stroke, expanding your application possibilities. For anyone considering going the hydraulic route, here are some key factors to consider when selecting and buying your new press.

Selecting tonnage and stroke

This C-frame bench press is a good example of the various styles of presses available from Greenerd.

One of the first things to consider when purchasing a hydraulic press is selecting the tonnage. The tonnage required to do a job is the same for a hydraulic press as it is for a mechanical press. There is no real difference.

In fact, the same formulas are used to determine tonnage for both types of press. And, the tooling is typically 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 tonnage in the typical pressroom 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 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.

Once the tonnage question is settled, it’s time to determine the effect of the stroke on the work. Is it the same as with a mechanical press? The answer 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. 

For 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.

Best press type

The Greenerd 1,000-ton, eight-point, gib-guided hydraulic press was designed for coining in the manufacturing of heavy-duty, aftermarket brake pads.

Now it’s time to determine which type of hydraulic press is best for your particular application. 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: The more critical the work and the more demanding the tolerances, the greater the reserve tonnage capacity should be.

Once the basics are determined, the next thing to consider is options. Most hydraulic press builders offer an 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
Touchscreen controls
Servo system feedback for precise, consistent, repeatable stroke control

Note the hydraulic circuit for a press is determined mainly by the application for which the press will be used. In long stroke applications, such as deep-draw work, a dual-pump circuit with regeneration is typical, allowing 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 are using during what is typically a very short stroke. Most presses used for stamping only use a single hydraulic pump due to 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. 

Next, determine what type of quality you need to get the job done. Quality can vary greatly from hydraulic press to press. 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. 

A few construction points can be used to compare one hydraulic press with another:

Frame: Look at frame construction rigidity, bolster thickness, dimensional capacity and other factors.

Cylinder: What is the diameter? How is it constructed? Who makes it? How serviceable is it?

Maximum system pressure: Look at the psi at which the press develops full tonnage. The most common range for industrial presses is 1,000 to 3,000 psi.

Horsepower: Compare horsepower ratings. The duration, length and speed of the stroke determines the horsepower required.

Speed: Determine the speed each hydraulic press offers.

Buyer beware

There are many potential pitfalls to take into account when selecting your hydraulic press. Things to watch out for include:

Speed: There are no hydraulic presses today that are as fast as the fastest mechanical presses. If the sole requirement is speed, the application is fixed and the material feed stroke is relatively short, the mechanical press remains the best solution.

Stroke depth: If a limit switch is used to determine the bottom, the stroke depth is not likely to be controlled much closer than 0.020 in. Many hydraulic presses can be set to reverse at a preselected pressure, which usually results in uniform parts. Generally, if absolute stroke depth accuracy is required, “kiss” blocks must be provided in the tooling. 

Automatic feeding equipment: Hydraulic presses require some external or auxiliary power to feed stock. The feeder must have its own power and must be integrated with the press control system. There is, however, an increasing selection of self-powered feeding systems available such as roll feeds, hitch feeds and air feeds.

Shock after breakthrough in blanking: Both mechanical and hydraulic presses experience this problem. But, the hydraulic system of a hydraulic press must also be isolated from the shock associated with decompression. If the hydraulic system does not contain an anti-shock feature, this shock can affect the lines and fittings.

As with any major purchase, it’s important to do your homework. Once you determine you want to go hydraulic, take the time to consider all the issues to make sure you get the best solution for your specific application. Strongly consider selecting a vendor you can easily partner with. Key things to look for include excellent communication and support, engineering expertise and custom-build capability, all of which inevitably provide pressing success.

Greenerd Press & Machine Co. Inc.

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