October 11, 2005
Choosing the right hydraulic press for your application is crucial. Identifying the right press capability up front can eliminate additional costs and startup delays. Four primary hydraulic press return capabilities are return on position, return on pressure, return on pressure with dwell capability, or combinations of the three. Shown is a press with return on pressure capabilities.
|Choosing the right hydraulic press for your application is critical. Four primary hydraulic press return capabilities are return on position, return on pressure, return on pressure with dwell, and combinations of the three. Shown is a press with a return-on-pressure feature. One of hydraulic press's greatest advantages is its flexibility. It can be configured to return at a given position or a given force, and you can adjust the retract position of the ram to suit your specification. This flexibility enables you to configure the press stroke to meet your application needs.|
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.
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 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.
Advances in control technology allow you to adjust the return pressure with a press interface. This simplifies setup.
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.
Stop blocks, which can be stationary or removable, can be used with dies that have a predetermined target shut height to ensure that the die is closed to this position every cycle. In this example, the stop blocks are sectioned so that shims can be added or removed to produce a repeatable precision shut height.
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.
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.
Dwell systems can range from a simple pressure lock valve and small accumulator for maintaining pressure over short periods of time to more complex systems that include other circuit components, such as a variable-volume pump system and a variable-volume pump system with an accumulator. These systems can be configured to vary the dwell force throughout the process.
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.
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.
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