How to select a hydraulic press ?
- 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.
- 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.
- 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.
- 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
- 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:
- 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. See page 5 to determine the speed of a hydraulic press.
Ten advantages of hydraulic press
- Full power stroke – The full power of a hydraulic press can be delivered at any point in the stroke. Not only at the very bottom, as is the case with mechanical presses. Advantages? No allowances for reduced tonnage at the top of the stroke. In drawing operations, for example, you have the full power of the press available at the top of the stroke. You don’t have to buy a 200-ton press to get 100 tons throughout the stroke. Other advantages are faster set-ups and no time consuming job of adjusting the stroke nut on the slide to accommodate different dies.
- Built-in overload protection – A 100-ton hydraulic press will exert only 100 tons of pressure (or less, if you have set it for less) no matter what mistakes you make in set-up. You needn’t worry about overloading or breaking the press or smashing a die. When a hydraulic press reaches its set pressure, that’s all the pressure there is. The relief valve opens at that limit and there is no danger of overload.
- Much lower original cost and operating costs – Hydraulic presses are relatively simple, and you may be surprised at the significant cost advantage over mechanical presses in comparable sizes. The numbers of moving parts are few, and these are fully lubricated in a flow of pressurized oil. Breakdowns, when they occur, are usually minor; not, for example, like a broken crankshaft. Replacements of packing, solenoid coils, and occasionally a valve, are typical maintenance items. Not only are these parts inexpensive, but also they are easily replaced without tearing the machine apart. This means more up-time and lower maintenance costs.
- Larger capacities at lower cost – It is easier and less expensive to buy certain kinds of capacity in hydraulic presses. Stroke lengths of 12, 18, and 24 inches are common. Extra stroke length is easy to provide. Open gap (daylight), too, can be added without much additional cost. Similarly, larger table areas and small presses with big bed areas can be provided. Large 200-ton presses with relatively small beds are available; tonnage of the press doesn’t dictate what the bed size will be.
- More control flexibility – Hydraulic press power is always under control. The ram force, the direction, the speed, the release of force, the duration of pressure dwell, all can be adjusted to fit a particular job. Jobs with light dies can be done with the pressure turned down. The ram can be made to approach the work rapidly, then shifted to a slower speed before contacting the work. Tool life is thus prolonged. Timers, feeders, heaters, coolers, and a variety of auxiliary functions can be brought into the sequence to suit the job. Hydraulic presses can do far more than just go up and down, up and down.
- Greater versatility – A single hydraulic press can do a wide variety of jobs within its tonnage range. Commonly seen are deep draws, shell reductions, urethane bulging, forming, blank and pierce, stake, punch, press fits, straightening, and assembly. They are also used for powered metal forming, abrasive wheel forming, bonding, broaching, ball sizing, plastic and rubber compression, and transfer molding.
- Quiet – Fewer moving parts and the elimination of a flywheel reduce the overall noise level of hydraulic presses compared to mechanical presses. Properly sized and properly mounted pumping units meet and exceed current Federal standards for noise, even with the pump under full pressure.
Because each phase of the ram movement can be controlled, noise levels can also be controlled. A hydraulic ram can be controlled to pass through the work slowly and quietly.
- More compact – A typical 20-ton hydraulic press is eight feet high, six feet deep, and two feet wide. A 200-ton press is only ten feet high, nine feet deep, and a little over three feet wide. At ten times the capacity, the 200-ton press only takes up 50 percent more floor space. Hydraulic presses become less and less expensive compared to mechanical presses.
- Lower tool costs – the built-in overload protection (see advantage 2) goes for 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 down to suit the job. The lack of impact, shock, and vibration promotes longer tool life.
- Safety – No manufacturer will (or should) claim that hydraulic presses are safer than mechanical presses. Both types of machines are designed and built to be safe if the controls and safety features built in are used properly.Improperly used, all machines are potentially dangerous. But the factor of control of the ram movements makes hydraulic presses easy to make safe. Non-tie down, anti-repeat, dual palm button controls are used. The interlocking of guards, as well as other safety devices, is relatively easy because of the nature of a hydraulic press control system.