One-Step Ejector Pin Sizing Guide: Head and Shaft Selection
Key Takeaway: Select head diameter (D1) to cover the ejection area, shaft diameter (D2) to resist buckling. The minimum D1:D2 ratio is 1.5:1; optimal is 2:1 to 2.5:1 for the best balance of ejection coverage and shaft rigidity.
Step 1: Determine Shaft Diameter from Buckling Resistance
The shaft (D2) must resist buckling under ejection force. Use Euler's buckling formula adapted for ejector pins per ASME mechanical design standards:
- Critical buckling force: F_cr = π²EI / (KL)², where E = Young's modulus, I = moment of inertia, K = end condition factor (0.7 for fixed-guided), L = unsupported shaft length
- Safety factor: Design ejection force ≤ F_cr / 3 (factor of safety = 3 for dynamic loading)
- Practical rule: L/D2 ≤ 20 for unsupported shafts, L/D2 ≤ 30 with guide bushing
Step 2: Select Head Diameter for Ejection Coverage
| Part Feature | Recommended D1:D2 Ratio | Reasoning |
|---|---|---|
| Flat surface ejection | 2:1 | Standard coverage, even force distribution |
| Boss/rib ejection | 1.5:1 | Limited space around feature |
| Large flat area | 2.5:1 to 3:1 | Maximum coverage to reduce ejection marks |
| Deep pocket ejection | 1.5:1 (with guide bushing) | Limited bore space, need maximum shaft rigidity |
Step 3: Verify Step Transition Stress
The step transition (where D1 meets D2) is the highest stress concentration point. MISUMI recommends a fillet radius of R0.3–R0.5mm at the transition for D1:D2 ratios above 3:1. Without a fillet, the stress concentration factor is approximately 2.5×, which can lead to fatigue cracking after 200,000–500,000 cycles.