Automotive Leaf Spring Suspension

Automotive leaf spring suspension is a traditional suspension system widely used in commercial vehicles, pickups, trucks, and some off-road vehicles due to its simple structure, high load-carrying capacity, low cost, and durability. The manufacturing of leaf springs involves processes such as material selection, forming, heat treatment, assembly, and testing.

1. Composition and Function of Leaf Spring Suspension

• Composition:
  • Leaf Spring: Composed of multiple (or single) spring steel leaves stacked together and fixed with U-bolts.
  • Connecting Parts: Such as shackles, center bolts, and clamps.
  • Shock Absorber: Assists in damping vibrations, working in conjunction with the leaf spring.
  • Suspension Accessories: Bushings, brackets, mounting bases, etc.
• Function:
  • Supports the vehicles weight and absorbs road shocks.
  • Provides rigid support, suitable for heavy loads (e.g., trucks carrying 10-20 tons).
  • Guides wheel movement and maintains axle positioning.
• Applications: Commonly found in trucks (e.g., Dongfeng, Foton), pickups (e.g., Ford F-150), and off-road vehicles (e.g., Jeep Wrangler).

2. Manufacturing Process of Leaf Springs

The production of leaf springs mainly employs stamping, heat treatment, and assembly processes. Casting is rarely used for leaf springs because they require high toughness and elasticity, while casting materials (such as cast iron) have sufficient rigidity but lack toughness. The following is a detailed process:

Material Selection

• Spring Steel:
  • Common Materials: 60Si2Mn, 55CrMnA, SUP9 (Japanese standard), or 5160 (American standard).
  • Characteristics: High strength (tensile strength 1200-1800 MPa), high elastic limit, and fatigue resistance.
  • Carbon Content: 0.5-0.6%, containing alloy elements such as silicon, manganese, and chromium to enhance toughness and durability.
• Thickness and Dimensions:
  • Single Leaf Thickness: 6-20mm (depending on load requirements).
  • Length: 800-2000mm, Width 50-100mm.
  • Number of Leaves: 3-10 leaves (multi-leaf) or single leaf (single-leaf spring, such as in some light trucks).
• Other Materials: Rubber or polyurethane bushings are used in shackles to reduce friction and noise.

Design and Planning

• Functional Requirements:
  • Load Capacity: Designed according to the vehicles gross vehicle weight (GVW), such as 1-3 tons for light trucks and 10-20 tons for heavy trucks.
  • Stiffness (k-value): Generally 200-1000 N/mm, affecting suspension hardness and comfort.
  • Fatigue Life: Must withstand millions of cyclic loads (approximately 10 years of service life).
• CAD Design:
  • Use SolidWorks or CATIA to design the curvature, length, and number of leaves of the spring.
  • Finite element analysis (FEA) simulates stress distribution to ensure no deformation under maximum load.
• Shape: Parabolic or multi-leaf stacked type. Parabolic type is lighter and has more uniform stress distribution.

Forming Process

• Cutting:
  • Use a shearing machine or laser cutting machine to cut the spring steel plate to the specified dimensions.
  • Accuracy is controlled within ±0.5mm to ensure consistent stacking.
• Hot Forming:
  • Heating: Heat the steel plate to 850-900°C (austenitizing temperature) to increase plasticity.
  • Stamping or Roll Forming: Form the curved structure with a hydraulic press (1000-3000 tons) or roll forming machine. Parabolic springs require precise forming of each leaf.
  • Advantages: Hot forming ensures grain refinement within the steel plate, enhancing strength and elasticity.
• End Processing:
  • Eye Forming: Roll the ends of the steel plate into circular shackles for connecting to the axle or frame.
  • Drilling: The center hole is used to install the center bolt, connecting multiple leaves.

Heat Treatment

• Quenching:
  • Heat the formed steel plate to 850-900°C and rapidly cool it (oil quenching or water quenching).
  • Purpose: To form a martensitic structure, increasing hardness (HRC 40-50) and strength.
• Tempering:
  • Temper at 400-600°C to relieve internal stress and enhance toughness and fatigue resistance.
  • Hardness after tempering: HRC 30-40, balancing rigidity and elasticity.
• Shot Peening:
  • Bombard the surface with high-speed steel shots to introduce compressive stress, improving fatigue life (can extend by 50-100%).

Surface Treatment

• Rust Prevention Coating:
  • Spray epoxy resin or powder coating, thickness 50-100 μm.
  • Hot-dip galvanizing or electroplating, salt spray test >500 hours.
• Lubrication: Apply graphite lubricant or add plastic shims between multiple leaves to reduce friction and noise.

Assembly

• Stacking and Fixing:
  • Stack multiple leaves according to their curvature, and fix them with a center bolt (M12-M20).
  • Clamps (U-bolts) fix the spring to the axle.
• Accessory Installation:
  • Shackle bushings (rubber or polyurethane) connect the frame and the spring.
  • Shock absorbers are connected in parallel with the spring to control vibration.
• Alignment: Ensure the spring curvature and axle position are aligned, with a deviation <1mm.

Testing and Quality Control

• Stiffness Test:
  • Use a hydraulic testing machine to apply load (1-20 tons), measure the amount of deformation, and verify stiffness (k-value).
• Fatigue Test:
  • Simulate millions of cyclic loads (frequency 1-3 Hz) to ensure no fracture.
• Dimensional Inspection:
  • Use a coordinate measuring machine (CMM) to check curvature, length, and hole positions, with an accuracy of ±0.5mm.
• Corrosion Resistance Test:
  • Salt spray test (500-1000 hours) to verify coating durability.
• Load Test:
  • Simulate maximum load (e.g., 20 tons) to ensure no deformation or fracture.