Pairing the right hydraulic press with your application

Whether you stamp metal or composite components, choosing the right hydraulic press for your application is crucial. Identifying the press capabilities you need upfront can eliminate additional costs and start-up delays.

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All presses, mechanical or hydraulic, share the same common operating principle: Their rams extend and then retract. Where they diverge is in how they get there.

Most mechanical presses function via flywheel motion, with a top dead center and a bottom dead center. Full ram extension and full ram retraction are always at the same points. The retract position and shut height position are somewhat fixed.

Hydraulic presses allow you to adjust the retract position of the ram. Also, they can be configured to return at a given position or a given force. These capabilities are hydraulic presses' greatest advantages. This flexibility enables you to configure the press stroke for your application.

The most critical point to consider in selecting a hydraulic press is its ability to match the application requirements. Four primary hydraulic press capabilities are:

• Return on position
• Return on pressure
• Return on pressure with dwell
• Combinations of all of the above

Return on Position

Return on position is perhaps the most commonly requested but least utilized capability.

Return on position is a press cycle in which the ram lowers and closes the die to a repeatable depth, penetrates or forms the material, and then returns to the full up, or up limit position.

The majority of dies designed for mechanical presses are engineered for return on position. Often fixed stops have not been incorporated into these dies because, inherently, a mechanical press cannot overstroke. When these dies are then installed in a hydraulic press, they are expected to be run in a return-on-position mode also. This can cause problems in many standard hydraulic presses.

While punching and stamping, basic hydraulic presses experience what is known as breakthrough shock. This is caused when the ram encounters resistance at the point of contact with the work material and then builds or intensifies pressure to develop the needed working force to form or stamp the part. Once the part is stamped, the ram resistance ceases, and the press ram wants to continue downward. This, coupled with the varying response times of standard hydraulic valves, can make the repeatability performance of the down limit position somewhat erratic. Most basic systems offer repeatability of perhaps ±0.020 to 0.030 inch. For many applications, this may be suitable.

Other applications may require the much tighter tolerances that higher-performance hydraulic circuits can provide. Certain powder compaction and R&D applications require tolerances to ±0.001 total indicated runout (TIR). With proportional or servo valving incorporated into the hydraulic circuits, presses can easily meet the most challenging needs, but these needs must be presented to the machine builder early in the design process.

Return on Pressure

The most common stroke capability in hydraulic presses is return on pressure. This allows the press ram to advance until an adjustable pressure setting is achieved and then retract to the up position.

A hydraulic press's capability to apply full tonnage anywhere in the stroke provides inherent flexibility. You can run dies with different shut heights on the same press and with minimal setup (see Figure 1).

In addition, dies and applications that are designed for return on pressure benefit from absolute repeatability. Even traditional punch tooling can be run in return-on-pressure mode with fixed stops incorporated into the tooling or press system. Dies that have a predetermined target shut height can use stop blocks to ensure that the die is closed to this position every cycle (see Figure 2). In this mode, the ram lowers and extends to a preset target force and then returns to the up limit position.

Today most dies are designed either with stop blocks built into them or so that the die itself can be closed to bottom out and support the force of the ram. In this case, you can set the target return force set point slightly higher than the necessary force required to stamp the part. When the ram completes the work and bottoms out on the stop blocks or bottoms out the die, the target pressure is achieved, and the ram retracts to the up limit position, completing the cycle.

Because the height of the stop blocks or die never changes, the press closes to the same position every time. In this manner, you are using pressure as the target, but gaining position as the performance criterion. This can be achieved with even the most basic hydraulic press systems.

Even basic return-on-pressure systems are equipped with overforce or overload protection. For instance, if your application requires 25 tons and you inadvertently load two blanks into the die, the press ram will lower and develop the target force of 25 tons and then return. If the press were set up for return on position (or if it were a mechanical press), the ram would attempt to overcome the double blank, reach the full extension or target position, and subsequently cause die damage. The hydraulic press configured to return on pressure would apply only the desired force and then retract the ram, most likely protecting the die from damage.

For those applications that truly require a target force to be applied, the hydraulic circuit can be designed to provide for different levels of force accuracy. Basic hydraulic press circuits can deliver ±10 percent to ±15 percent, depending on the valve shift time and pressure-sensing device. Tighter-tolerance systems also are available that can provide repeatability greater than ±1 percent, again using proportional or servo control valves.

Another key process requirement often overlooked is overshoot. Compaction processes, bulge forming, and impression forming applications may not be able to tolerate overshooting the target force. Hydraulic press systems can be designed with the right combination of electrical controls and hydraulic valves so that the potential for overshooting the target force is nearly eliminated. To ensure you obtain the press capabilities you need, it is recommended that you communicate these needs to the machine builder upfront.

Return on Pressure With Dwell

Frequently the return-on-pressure feature is paired with dwell capability&#;the ability to achieve a target force and then maintain that target force for a period of time.

This is a quite common need for a number of applications, including laminating, coining, heated and nonheated composite forming, powder compaction, hydroforming, and molding.

While pressure-holding, or dwell, capability is very common on hydraulic presses, many systems are available, each with different benefits. Choosing the right system is crucial, as your choice will affect cost and performance.

Simple dwell systems consist of a pressure lock valve and small accumulator for maintaining pressure over small periods of time&#;typically up to 10 minutes (seeFigure 3). The concern in any dwell cycle is the amount and rate of pressure bleed-off. Most hydraulic valves have nominal leakage. Pressurizing a system with a fixed amount of fluid results in a steady drop in pressure over time. Basic systems, while economical, usually experience some type of pressure bleed-down. For some applications, this may be acceptable.

For those applications that require constant pressure, other circuit components can be integrated to provide the desired results. It is important to identify the process needs so that the right circuit is designed. Utilizing variable-volume pump systems can sustain holding force for any length of time. For longer dwell periods (multiple hours), variable-volume pump systems with accumulators can be used to turn the motor and pump off and on periodically to maintain the desired dwell or clamping force.

By integrating the proper electrical controls, you can configure these systems to vary the dwell force throughout the process. Varying the dwell force during heating and cooling stages within the press cycle can be beneficial and potentially shorten the cycle time.

All Three Returns

Most hydraulic presses can incorporate a combination of these features into a single machine. The complexity of the press largely depends on the application. Basic systems with return-on-position and return-on-pressure capabilities are not expensive; however, the performance must meet the demand for the application.

In a perfect world, every hydraulic press would be equipped with all of these features. Unfortunately, higher performance usually equals higher upfront costs. Efficient manufacturing requires efficiency throughout the process, especially when capital equipment is concerned.

Identifying your application needs and then finding the hydraulic press with the right capabilities for them is paramount. Discussing these needs upfront with your hydraulic press builder will net you the right tool to do the job.

Jay Douglas Hartzell is the engineering manager at Beckwood Press Co., 889 Horan Drive, St. Louis, MO -, 800-737- or 636-343-, , www.beckwoodpress.com.

Here is a quick primer on how to purchase the best hydraulic press solution for your application and what factors to consider to avoid any pitfalls.

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. These presses are also uncomplicated and can have significant cost advantages over mechanical presses of comparable sizes.

The company is the world’s best automated hydraulic press supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

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. All of this equates to more uptime and lower maintenance costs. Hydraulic presses also provide easy tonnage adjustment and more tonnage control throughout the press stroke ? which expands 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.
? Generally speaking, to compute tonnage requirements when pressure per square inch (psi) is known:
? psi x area of work/ =
? 2 tons of ram force required
? Example: Where it is known that 100 psi is needed to do a job on a 5 in x 8 in wide piece, then
? 100 x 5 in x 8 in/ = 2 tons

On a press fit, to determine the force required to press fit two round pieces together such as a shaft pressed into a bushing, use the following formula:
? F = D x π x L x I x P/2
? Where: F = force required in tons
? D = diameter of the part to be pressed in inches
? L = length of part to be pressed in inches (Note: the length of the interference fit only.)
? I = interference in inches (usually .002 in to .006 in)
? P = pressure factor (see table)

When punching, a quick guide to determine tonnage requirements for punching steel is:
? Diameter x thickness x 80 = tons
? Where: 80 is constant for steel. Use 65 for brass.
? Example: A 3 in hole punched through .250 in stock:
? 3 in x .250 in x 80 = 60 tons

For noncircular holes, instead of the diameter, use 1/3 of the total length of cut. Example: A 4 in x 6 in rectangular hole in .250 in stock: (4 in + 6 in + 4 in + 6 in/3) x .250 in x 80 = 133.3 tons
When deep drawing, these calculations can be complex because the press, dies, material, radius, and part shape all have bearing. For drawing round shells, the following formula is a simple guide:
? C x T x Ts = tons
? Where: C = circumference of the finished part;
? T = material thickness in inches
? Ts = tensile strength of the material
? Example: To draw a 5 in diameter cup of .040 in stock with a tensile strength of 46,000 psi would require the following tonnage:
? (5 x 3.) x .040 x (/) = 14.44 tons
? (A 20-ton press would be recommended)

For straightening, the pressure required to straighten a piece of metal depends on its shape. Below is an approximate formula with a further definition for different shapes:
? F = 6UZ/L
? Where: F is the ram force in tons
? 6 is a constant
? U is ultimate strength of the material in psi
? Z is the section modulus (see below)
? L is the distance between the straightening blocks in inches.

Example: A 2 in diameter shaft, 18 in between the blocks, 100,000 psi ultimate strength.

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. For example, 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 available.

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 THE APPLICATION

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 a wide array of accessories that 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

Next, you must determine what type of quality you require to get the job done. Quality can vary greatly from press to press. There are light-duty presses that are quite 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: Inspect the 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 psi to 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.

BUYER BEWARE

There are many potential pitfalls to take into account when selecting your hydraulic press. Specific 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 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.

The nearby schematic illustrates a typical hydraulic press decompression control. When the directional control valve (1) is activated to extend the cylinder, fluid enters the cylinder (5) via the check valve (2). Pressurization of the cylinder during extension closes the pressure reducing valve (3), so that when the directional control valve (1) is activated to retract the cylinder (5) the compressed volume of fluid is metered across the orifice (4).

When pressure upstream of the orifice (4) falls below the setting of the pressure reducing valve (3) the remaining fluid in the cylinder flows back to tank across the pressure reducing valve (3). The sequence valve (6) prevents pressurization of the rod side of the cylinder before the piston side has decompressed and the pressure reducing valve (3) has opened.

DO THE 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.

Furthermore, strongly consider selecting a vendor with which you can easily partner. Key things to look for regarding a vendor include excellent communication and support, engineering expertise and custom-build capability, all of which inevitably provide press success.

Greenerd Press and Machine Co., Inc., 41 Crown Street, P.O. Box 886, Nashua, NH , 603-889-, Fax: 603-889-, www.greenerd.com.

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