Case study: Cold forging of tripods and cross-joints

1
Case study Cold forging of tripods and
cross-joints

• Die material ASP 23, 60 HRc
• Prestressing system ID 85 mm, OD 280
  mm

Cold forged tripods and cross-joints
2
Typical failure mode
Fatigue cracks in a tripod die
Fatigue cracks in a cross-joint die
3
FE analysis of the tooling system
A) conventional container with a double ring0.6
interference
B) STRECON Basic, 0.8 interference
windingsand casing
C) STRECON E, 0.6 interference
winding core
die
4
A) Prestressing by a double stress ring at 0.6
interference
Circumferential stress distribution at max.
process load
Tangential stress-strain response in the critical
area (5 load cycles)
400 200 0 -200 -400 -600 -800 -1000 -1200 -1400 -
1600
MPa
330 MPa
? risk of early crack initiation due to tensile
stresses and large plastic strains
5
B) Prestressing by STRECON Basic at 0.8
interference
Circumferential stress distribution at max.
process load
Tangential stress-strain response in the critical
area (5 load cycles)
400 200 0 -200 -400 -600 -800 -1000 -1200 -1400 -
1600
MPa
Tangential stress in MPa
60 MPa
? Reduced max. tensile stress, but still
considerable plastic strains
6
C) Prestressing by a STRECON E at 0.6
interference
Circumferential stress distribution at max.
process load
Tangential stress-strain response in the critical
area (5 load cycles)
400 200 0 -200 -400 -600 -800 -1000 -1200 -1400 -
1600
MPa
Tangential stress in MPa
-320 MPa
? Improved fatigue life no tensile stresses,
reduced stress range and cyclic plastic strains
7
Recommendations for the designof dies for the
cold forging of tripods and cross-joints

• Recommended die material
• Power-metallurgical tool steel (instead of
  carbide),provides an excellent fatigue strength
• Recommended prestressing system
• STRECON Ereduces the stresses and cyclic
  plastic strains,leads to improved fatigue life-
  the outer diameter of the die insert should be
  close to the inner contour (better effectivity of
  the carbide winding core)
• STRECON Basicin case of long-legged parts
  (especially cross-joints), where a carbide
  winding core would not be that effective