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Guide to Press Productivity
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Greenerd Hydraulics
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Guide to Press Productivity

Dotted Line

Jul 14, 2008

Today's hydraulic presses are faster, 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 have 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 and are also relatively easy to replace. This means more up-time and lower maintenance costs.  Hydraulic presses also provide easy tonnage adjustment and more tonnage control throughout the presses’ stroke, expanding your application possibilities.  If you decide to go hydraulic, here are some key factors to consider when selecting and buying your press.

Selecting Your Press Tonnage
One of the first things to consider when purchasing a hydraulic press is selecting the 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 for both types of presses. 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.

How The Press Affects the 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, 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.

What Type of Press is Best for You

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. 4-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 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
Then you need to determine what type of quality you need to get the job done.  Quality can vary greatly from 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 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 1000 to 3000 psi.

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

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 speed is the sole requirement, the application is fixed and the material feed stroke is relatively short, the mechanical press remains the best selection.

Stroke depth. If a limit switch is used to determine the bottom, the stroke depth is not likely to be controlled much closer than .020". 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.

Do your Homework
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 the issues reviewed above to make sure you get the best solution for your specific application.  And 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 press success.

Greenerd Press & Machine Company, Inc.