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How to calculate stroke length in press machine?

The stroke length of a press machine is defined as the total distance the ram (slide) travels from its highest point (Top Dead Center, TDC) to its lowest point (Bottom Dead Center, BDC).

The calculation and determination of stroke length depend on whether you are calculating the mechanical capability of the machine or the required stroke for a specific manufacturing process.

1. Mechanical Calculation (Crank Press)

In a standard mechanical crank press, the stroke length is determined by the dimensions of the internal drive mechanism:

• The Basic Formula: The slide stroke length (S) is equal to two times the crank radius (or crank arm length).
• Variable Stroke Presses: Some machines allow for stroke adjustment using an eccentric shaft and bush mechanism. In these cases:
  • Maximum Stroke (Hmax​): 2×(r1​+r2​)
  • Minimum Stroke (Hmin​): 2×(r1​−r2​) where r1​ is the eccentricity of the main shaft and r2​ is the eccentricity of the adjustable bush.

2. Calculating the Required Stroke for a Process

When selecting a press for a specific part, the stroke must be long enough to allow for material feeding and part removal.

• General Rule for Deep Drawing: The stroke length must be significantly longer than the part depth to allow the finished part to be ejected and the next blank to be fed.
  • Standard guideline: Stroke ≥2 to 2.5× the part depth.
  • Specific formula for equipment selection: S≥2.5×hw​, where hw​ is the height of the workpiece.
• Progressive Dies (連續模): Because the metal strip must be lifted over the die components (like pilots or lifters) before being fed forward, a longer stroke is required.
  • Requirement: Stroke ≥3× the part depth.

3. Key Factors Influencing Stroke Selection

• Production Speed (SPM): Stroke length and speed are inversely related. High-speed automatic presses (reaching 600–2,000 SPM) typically utilize short strokes (e.g., 13mm to 38mm) to minimize travel time and vibration.
• Automation: If using automatic feeders or transfer mechanisms, the stroke must provide enough vertical clearance for the gripper fingers or transfer arms to move the part without hitting the upper or lower die.
• Die Height (Shut Height): While stroke length is the distance traveled, you must also ensure the die’s closed height fits within the press’s shut height (the distance from the bolster to the slide at BDC).

In summary, for a mechanical press, the stroke is 2× the crank radius. For practical application, you must choose a machine with a stroke at least 2 to 3 times the part’s depth to ensure stable production and automated part handling.