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. Hot Foil Impressions Machine
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:
Bench. Compact and economical, a bench press is fast and easy to set up. It is used in a range of jobs, from single-cycle operations to automated cell components. Generally, its tonnage capacity is from 3 to 50. C-frame. An open-gap, C-frame press is built for long service in multishift industrial production applications in 10- to 500-ton capacities. It is called a C-frame because of its C shape. This type of frame tends to “yawn.” Applications that cannot withstand this yawning characteristic should be moved to another style of press, such as a four-post or straight-side (see lead image).
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.
Straight-side. Designed to meet the most demanding application requirements, a straight-side press has a superior frame construction. Generally, the press is built with a crown, slide, and bed weldment that is bolted, welded, or tie-rodded to large slabs of steel, called side plates. Unlike a four-post construction in which the frame is built entirely of the four columns, a straight-side press is supported by the two steel slab side plates and tie rods to reduce the amount of stretch in the frame under load. Heavy-duty blanking typically requires the tie-rodded construction to withstand the severe shock at material breakthrough. The press can run at high speeds and is capable of millions of cycles per year.
Gib-guided. A gib-guided press is designed to counter the effects of off-center loading. Rigid steel frames with gib-guided slides maintain parallelism under severe loading conditions.
Air-over-oil. An air-over-oil press has a four-post construction. It applies force in a vise-like grip for seconds, minutes, or hours, maintaining tonnage for nearly any length of time with the help of an air compressor. It is used mainly for lamination work. It is also suitable for applications that require a force on the product for long periods of time. It is not suitable for typical stamping applications as it has a single-acting cylinder and not conducive for applications that require any pullout/stripping tonnage. It is also upacting, which usually is not very useful for most conventional applications. With a low initial price and minimal operating costs, this press has the force of a conventional hydraulic press at a fraction of the cost.
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.
Custom. When a standard press design does not adequately meet your needs, a press tailored to your application may be in order.
Hydraulic Press Common Terms and Features
Being familiar with these terms will enhance your understanding of a hydraulic press:
Blank Holder: A controlled force to hold the edges of the blank in a deep-drawing operation, similar to a die cushion.
Die Cushion: A hydraulic or air cylinder positioned below the bolster and bed, providing uniform blank holding in deep drawing. Cushions also strip finished parts from the punch or die.
Distance Reversal Switch: An adjustable limit switch to set the depth of stroke at which the ram reverses.
Dwell Timer: An adjustable timer to set the length of dwell at the bottom of the stroke. The timer may be used for other functions such as timing a sequence of press movements.
Heat Exchanger: A device attached to the oil reservoir to circulate water or air to keep oil at the proper operating temperature.
Knockout: A device that strips the part from the punch or die.
Platen: A plate, sometimes heated, attached to a moving or stationary press member.
This straight-side press is gib-guided to counter the effects of off-center loading.
Pressure Reversal Switch: An adjustable switch to set the pressure at which the ram reverses.
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.
This transfer line with three stations for draw, redraw, and punch was built for an automotive company.
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.
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.
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:
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.
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:
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.
Hydraulic presses range in size from small , such as this relatively small bench press, to very large, such as this 500-ton.
See More by Greenerd Press & Machine Co.
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