Case study: Cold forging of starter pinions for automotive starters

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Case study Cold forging of starter pinionsfor
automotive starters
Cold forged starter pinions
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Typical failure mode in the lower die insert
Vertical and horizontalfatigue cracks in the
lower die insert
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Tooling system for improved radial and axial
prestressing(STRECON E Axi-Fit)
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FE analysis of the tooling systemwith regard to
the vertical crack initiation
Assumed process pressure distribution
2D FE model of the cross-section with the gear
profile
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Stress response in the prestressed condition
? Because of its higher stiffness, prestressing
by the STRECON E leads - despite the lower
nominal interference - to a similar prestress
level
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Stress response in the condition at max. process
load
? Prestressing by the STRECON E reduces
thetensile stress in the fillets at maximum
process load
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FE analysis of the tooling systemwith regard to
the horizontal crack initiation
Contact area
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Contact stresses between upper and lower die
insertin the prestressed condition
? By reducing the plane contact area, the
maximum contact stress becomes higher and gets
closer to the inner bore of the die
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Axial stresses in the upper and lower die
insertin the prestressed condition
? By reducing the plane contact area, the axial
prestress in the shoulder (where the cracks
actually occur) is increased
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Axial prestress during assembly
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Recommendations for the designof dies for the
cold forging of tripods and cross-joints

• Recommended tooling system
• STRECON E Axi-Fitprovides high stiffnessand
  combines improved radial and axial prestressing
• Result
• Die life for starter pinion dies improved by
  factor 2-3(depending on the type of starter
  pinion)