How to Calculate Tonnage of Mechanical Press？
There is a threshold of deformability for every material. For stamping to be successful, the press must overcome this resistance in addition to any other forces acting against the motion of the press. For this reason, a press’s capacity for applying and withstanding force must exceed that required by the forming process. Stampers need to grasp these distinct but interconnected ideas in order to appropriately size a press.
Tonnage Capacity of a Press
Even while elastic deflections in a press may not be immediately apparent, they occur throughout any forming process. The term “tonnage capacity” is used to describe the greatest force that can be applied to a press without breaking it. However, the forward tonnage necessary to form the part is what matters most in terms of the press’s structural restrictions, hence the tonnage capacity is only applicable for that purpose (when connections between the shaft and the drive are in compression and uprights are in tension). Although the press’s tonnage capacity is often the first sign of its size or strength, there are other factors to take into account.
Combination of Snap-through and Inverse Tonnage
- Tonnage in both directions, including forwarding and backward “snap-through” and “reverse,” must be considered when calculating the press tonnage. The two are frequently confused, but they are not the same.
- Snap-through, on the one hand, is the quick acceleration of the press slide and the linked components following the sudden release of tensions caused by material failure.
- Forces that cause stress in the connections, however, can be used to determine the inverse tonnage. Although they frequently co-occur, either one can live independently.
- Snap-through is associated with vibration concerns, while reverse tonnage is linked to troubles with the elastic limitations of the press components.
- The capacity of a press in the reverse direction is often less than the capacity in the forward direction. Ten percent of a vessel’s tonnage capacity is typically considered an acceptable reverse tonnage. The reverse tonnage capacity of a press may be greater than 50% of the tonnage capacity, depending on the manufacturer and type. Although the forward press capacity may not always be reached, it is crucial to keep in mind that the reverse tonnage may be the limiting factor in some circumstances.
- The thickness and strength of the material, as well as the press speed, are typically the determining factors in snap-through and reverse tonnage. Last but not least, while a tonnage monitor system can detect reverse tonnage, snap-through can only be detected with the help of vibration analysis techniques.
The tonnage of the Stamping Press
When working with centered loads, all of the press’s tonnage is dispersed throughout the press’s connections.
Inadequate Distribution of Force
Press capacities are often stated for centered loads, where both connections are subjected to the same force. Using a 400-ton press with two connections as an example, they could absorb up to 200 tons per connection.
It is not the same force that is absorbed by the connections, however, when the loading is not centered. Therefore, when the resulting load moves away from the press’s center line, the tonnage capacity diminishes. Taking this into account is crucial since it is possible for the connections to be overloaded even if the forming tonnage is below the tonnage capacity, putting the press structure at risk. Using the press’s off-center load capability, determine the amount and position of the resulting load.
The press’s architecture, number of connections, and guiding mechanisms all contribute to how the press responds to off-center stresses. Because of the uneven distribution of weight, presses with only one connection are the most susceptible to tipping over. The left-to-right resistance of a press increases with the number of connections; the front-to-back support of a press increases with the number of connections.
Capacity in Tons
A press may be able to endure its entire tonnage at any point in the stroke, but it may not be able to apply it continuously. This is true of slider-crank mechanical presses, which constitute the vast majority of the industry. Because of how this mechanism works, the available tonnage maximizes when the rotation angle approaches zero (top dead center, or TDC) and 180 degrees (bottom dead center, or BDC).
At both BDC and TDC, theoretically speaking, the number of accessible tons would be infinite. This is obviously impossible, as the pressing infrastructure would collapse under the strain.
The tonnage of the Stamping Press
The thickness and strength of the material, as well as the press speed, are typically the determining factors in snap-through and reverse tonnage.
The rating point of a slider-crank press is the height above BDC at which the press can be fully extended. Ratings are also taken into account as a major factor when choosing which news outlets to support. Its value, however, is conditional on the specifics of the forming procedure being used. Therefore, picking a press only on the basis of its rating or tonnage capacity may not be the ideal strategy. It is recommended to use the existing tonnage curve instead.
Stampers should know that two presses with the same rating point and tonnage capacity may have various tonnage curves available. The press’s torque capability and stroke length are primary factors in this variation. The shorter the overall press stroke, the steeper the potential tonnage curve, assuming constant torque (same motor, clutch, and other transmission components).
A press with a longer stroke allows for deeper draws but at the expense of available tonnage. The press’s tonnage capacity is independent of the stroke length, as it is determined by the press’s structural capacity and the components.
Stampers typically determine whether or not a job is feasible by comparing the anticipated peak load to the press’s tonnage capacity. When time is of the essence, this may be a reasonable approximation, but it is not the right approach. Always be certain that your desired tonnage is lower than the press’s available tonnage curve. As a result, it is crucial to examine the full trajectory of the tonnage rather than just its maximum.
Two methods exist for going over press tonnage, each with its own unique repercussions.
The driving mechanism may fail and the press may stop if the needed tonnage exceeds the available tonnage curve before the rating point. The motor could overheat, or the clutch could slip (also known as the “press sticking at the bottom”). On the retail floor, this is a commonly held belief. Components subject to torsion stresses are more vulnerable to press overload above the rating point.
However, if the press is overloaded, the entire press structure could fail.
Volume Of a Press in Tones
Avoid making your press choice only on the basis of its tonnage. Instead, you should consult the available tonnage curve, which displays the maximum force that can be applied by a press at any given point in the stroke.
How to Pick the Right Press
As was previously indicated, a press’s capacity for force or tonnage should exceed the requirements of the forming process. The following are suggested measures to take while selecting a press:
- Obtain a tonnage curve for the press. The real available tonnage curve, which can be discussed with the press maker, can be found by doing the following: At a DOB of one-third the press’s stroke length, which is one-third of the press’s tonnage capacity. At a depth of blank (DOB) of one-tenth of the press stroke, the press’s tonnage capacity is cut in half. In other words, the press can handle its full tonnage at the specified DOB (rated point) and below.
- Second, determine the process’s tonnage needs for the entire stroke and evaluate them against the obtainable tonnage curve. Estimating forming (forward) tonnages is made more challenging by complex geometries, despite the availability of online calculators and relatively simple calculations. Now is the time to run a finite element analysis simulation.
- Third, determine how much off-center loading the process will have, and check that it is within the off-center load capacity of the press to achieve the required parallelism based on the die specifications. The press is more forgiving of off-center loading than the die is in most cases. In order to achieve harmony in the press, it may be necessary to rethink the process.
- Determine if the press can handle the potential concentrated load (large tonnages and small die sets) by checking the manufacturer’s specifications.
- Take snap-through and reversing tonnage into account. In most cases, the reverse tonnage is between 10 and 25 percent of the forward tonnage, but there is no easy method to predict it in advance. Even still, readings as high as half the ship’s forward tonnage have been recorded. In blanking or piercing operations, the press may need to be derated to 80% of its tonnage capacity if it is not sufficiently resistant. If there is too much-reversing weight, slowing down can assist.
- Evaluate press capabilities in relation to the energy needed to create the part.
In conclusion, selecting and maintaining a press requires an in-depth familiarity with the forces involved in formation. Misuse of a press can have long-term consequences, such as increased frequency of press breakdowns and higher maintenance expenses, which aren’t usually obvious at first. It is recommended that stampers collaborate with the press maker to improve the operation, even if the stampers themselves have sufficient knowledge of tonnage difficulties.GUANGDUAN
Guangdong Metal Forming Machine Works Co., Ltd.
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