How does a automatic hydraulic press work?

• Platen: A plate, sometimes heated, attached to a moving or stationary press member.

• Pressure Reversal Switch: An adjustable switch to set the pressure at which the ram reverses.

Hydraulic Press Advantages

• Time Savings During Setup, Changeovers. Because the full power of a hydraulic press can be delivered at any point in the stroke, there is no need to determine the exact location of maximum tonnage.

• Flexibility for a Range of Applications. One hydraulic press can do many jobs within its tonnage range, including high-volume production. Common applications are deep draws, shell reductions, urethane bulging, forming, blanking, piercing, staking, punching, straightening, assembly, and press fits. They are also used for powdered metal forming, abrasive wheel forming, bonding, broaching, ball sizing, plastic and rubber compression, and transfer molding. A press fit is used for pushing a bearing onto a shaft without any mechanical fasteners to keep it in place. Typically, the hole in the bearing is several thousandths of an inch smaller than the diameter of the shaft that it is pressed onto, allowing for a nonmoving or rotating press fit.

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.

• Built-in Overload Protection. This eliminates worries 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.

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.

• Lower Operating Costs, Increased Uptime. Hydraulic presses are relatively simple with just a few moving parts, and these are fully lubricated in a flow of pressurized oil. Breakdowns are infrequent, but when they do occur, they usually are minor. Typical routine maintenance items include replacements of packing, solenoid coils, and occasionally a valve. Not only are these parts inexpensive, but they are also easily replaced without having to disassemble the entire machine.

• Larger Capacities for Minimal Cost. Compared to a typical mechanical press, it is easier and less expensive to buy certain kinds of capacity in a hydraulic press with common stroke lengths of 12, 18, and 24 in., and it easily can be equipped with extra stroke length. Open gap (daylight) can be added as well for a minimal cost. Similarly, hydraulic presses can be built with larger table areas and beds, or smaller beds than are standard. The press tonnage doesn't dictate what the bed size will be.

• Great Control. With a hydraulic press, the ram force, direction, speed, release of force, and duration of pressure dwell can all be adjusted to fit a particular job.

• Low Noise Level. Few moving parts and the absence of a flywheel equate to an overall low noise level. Because each phase of the ram movement can be controlled, noise levels also can be controlled. A hydraulic ram can be programmed to pass through the work slowly and quietly.

• Safety. Because ram movements can be controlled, it’s easy to make movements safe. Non-tiedown, anti-repeat, and dual-palm button controls are used to enhance safety. The interlocking of guards, as well as other safety devices, is relatively easy because of the nature of a hydraulic press control system.

Hydraulic Press Limitations

• No hydraulic press today is as fast as the fastest mechanical press. If speed is the sole requirement and the material feed stroke is relatively short, a mechanical press remains the best choice.

• Stroke depth is a key factor to consider. 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. However, some hydraulic presses are now available with an accurate built-in method of limiting the downstroke. New closed-loop servo-hydraulic systems dramatically improve stroke depth control, guaranteeing consistent, repeatable results. In many applications, this system eliminates the need for kiss blocks

• A hydraulic press requires some external or auxiliary power to feed stock. The feeder must have its own power and be integrated with the press control system. However, an increasing selection of self-powered feeding systems are available as roll feeds, hitch feeds, and air feeds.

• Both mechanical and hydraulic presses experience shock after breakthrough during blanking. But the hydraulics of a hydraulic press also must be isolated from the shock associated with decompression. If the hydraulic system does not contain an antishock feature, this shock can affect the lines and fittings.

Which Type of Hydraulic Press Is Best for My 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 is that 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 consideration is to select options. Most hydraulic press builders offer an array of accessories such as:

• 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 safety devices.

• Touchscreen controls.

• Servo system feedback for precise, consistent, repeatable stroke control.

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.

Designing the Press: Selection Tips

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.

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.

Automation Considerations

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:

• Parts demand has increased beyond the capacity or capability of manual labor to keep up.

• Personnel resources in the geographic area are not adequate or available.

• Press operators are required in other areas of the facility.

• It’s more cost-effective to automate an underutilized press than to purchase another new or used press.

• More consistent product quality and quantity are needed.

• Parts may have sharp edges, be high in temperature, be delicate, or are difficult to handle.

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.