Effect of Stroke Length on Productivity in Mechanical Presses

The impact of stroke length on press productivity is often not well understood in factories. Press operators frequently state reasons without explaining, "This task can only be done on this mechanical press" or "We cannot run this task as fast as it should be." Although there are many reasons, most of the time the cause lies in the integrity of the pressure and the stroke length.

Many stamping operations are carried out under less than ideal conditions, with the impact of stroke length often being ignored. There are four reasons why stamping operation should be the shortest possible stroke:

Figure 1

1. Minimize wasted time—Unnecessary ram movement leads to time and energy wastage. Shorter strokes reduce friction losses and slide wear.

   
   

2. Minimize the dynamic forces in the press structure - Moving the ram and upper die over a shorter distance within a specific period reduces the dynamic forces and fatigue effects within the press body and the drive system.

   
   

3. A more precise shaping - The shaping time is inversely proportional to the stroke length (see Figure 1). The time allocated for sheet metal forming is of utmost importance, and allowing more time for shaping results in better outcomes and a more stable process. As seen in Figure 1, for a 300 mm stroke (gray), the ram position curve intersects the material thickness line (assuming a 1mm material) at 173˚ of the crank angle, while for a 100 mm stroke (blue), the curve intersects this line at 165˚. This means that a press with a 100 mm stroke has an 8˚ longer stroke, equivalent to 38 milliseconds at 35 strokes per minute; representing a 114% increase in shaping time.

   
   

4. Longer tool life—Shorter strokes reduce the speed at which the die punches make contact with the workpiece (Figure 2). As a result, the frequency of the fracture or wear of the cutting punches is reduced. For example, in the graph below (mechanical press with a 300 mm stroke), when the ram is 1.26mm above bottom dead center, the ram speed is 130 mm/s, while in the 3. figure (mechanical press with a 100 mm stroke), the ram at the same position has a speed of 80 mm/s.

Figure 2

Figure 3

As bonus, you can make the acceleration comparison at the graphs above. Acceleration means dynamic forces and increased reverse tonnage problem. So that a short stroke stamping press will be again beneficial from the part fatigue aspect.

In mechanical presses, the optimal stroke is the shortest possible stroke achievable.

The analysis of the part to be shaped and the feeding system in mechanical presses will provide insight into the shortest possible stroke length. During the crank rotation, one of the following may occur at any point:

- Cutting, forming, etc.

- Material feeding

- Opening/closing gaps to provide clearance for feeding.

- Centering...

Other operations such as control or welding can also be included in the cycle. This should only be considered if the added value of such additional operations exceeds the efficiency decrease in the mechanical press. When adding or removing such additional operations, all factors discussed above must be taken into account.

The Impact of Stroke Length in Mechanical Presses on Productivity Includes the Following:

1. Less stamp breakage in the mold

2. Reduced need for tool sharpening

3. Improved shaping

4. Extended press life

5. Less vibration

6. Reduced noise

In equipment selection or die design, identifying and modifying conventional methods and assumptions can be challenging.

The most prominent reasons for suboptimal pressing operations are:

Improper use of equipment for its intended purpose;

• Running a job with the available equipment

• Purchasing equipment that is not suitable by management decision

• Purchasing new equipment at a discounted price

• Modifying existing equipment for a specific task

Significant changes in working conditions;

• Significant changes in production volumes/batch sizes

• Narrowing of product specifications/part tolerances

• Changes in production requirements related to quality, certifications, packaging, and delivery

Outdated equipment;

• Lack of newer technologies

• Lack of automation

• Lack of spare parts

• Decreased service convenience

Loss of precision in new equipment;

• Damages resulting from operational errors

• Material fatigue, improper repairs

• Machine wear

• Past modifications

How to Improve Metal Forming Processes?

The efforts to optimize begin with an open mindset and a willingness to be the implementer of technical advancements. It is necessary to have an environment where we change assumptions with logic and reevaluate our previous decisions based on changes in working conditions. Consulting with an external expert who provides unbiased analysis and technological insights can be beneficial. Sometimes, simple solutions can make significant differences.

Emrah Demirezen

Metal Forming Specialist - Press Designer

emrahdmrzn@gmail.com