What Are Extension Springs?
Extension springs are helical springs designed to resist tensile forces, absorbing and storing energy when stretched. These springs are characterized by initial tension (typically 15-25% of their maximum load capacity), which keeps coils closed until external force is applied. Key technical specifications include:
Spring rate: Ranges from 0.1 N/mm to 50 N/mm depending on wire diameter (0.2-10mm) and coil count
Material: Commonly made from music wire (ASTM A228) with tensile strength of 1900-2300 MPa or stainless steel (302/304) with 1700-1900 MPa
Maximum elongation: Typically 150-200% of free length before permanent deformation occurs
Temperature range: Standard springs operate between -40°C to 120°C; special alloys extend this to 300°C
Advanced extension springs may incorporate features like double hooks (withstand 20-30% higher loads than standard hooks) or threaded inserts for applications requiring over 10,000 cycles at 80% of maximum load.
Key Characteristics with Technical Data
1. Load Capacity: Industrial extension springs can handle static loads up to 5,000 N, with dynamic load ratings (for 10^6 cycles) typically 30-40% lower. For example, a 10mm diameter spring with 20 active coils made from 3mm wire can sustain 1,200 N at 100mm extension.
2. Fatigue Life: Properly designed extension springs achieve 500,000-1,000,000 cycles at 50% of ultimate tensile strength. Shot peening improves fatigue life by 30-50% by inducing compressive stresses of 400-600 MPa on the surface.
3. Hysteresis: Energy loss per cycle ranges from 5-15% depending on material and surface treatment. Silicon-chrome alloys show only 3-5% hysteresis compared to 8-12% for standard carbon steel.
4. Corrosion Resistance: 316 Stainless Steel Springs exhibit less than 0.1mm/year corrosion in salt spray tests (ASTM B117), compared to 0.5-1mm/year for uncoated carbon steel.
Industrial Applications
1. Automotive Systems:
Throttle Return Springs with 10-20N force and 50mm travel
Seat belt retractors using springs with 0.5-2 N/mm rate
Hood latch mechanisms requiring 150-300N pull force
2. Manufacturing Equipment:
CNC tool changers with springs rated for 5,000+ cycles/day
Conveyor tensioning systems using springs with 50-200mm travel
Die-cutting presses requiring 1,000-5,000N preload
3. Aerospace Applications:
Landing gear door mechanisms with springs meeting AMS 5112 specifications
Cargo door counterbalance systems using corrosion-resistant alloys
Flight control surface returns with ±0.5% force consistency over temperature
Medical devices represent a growing application sector, where extension springs with electropolished surfaces (Ra ≤ 0.4μm) are used in surgical robots and imaging equipment, requiring forces as low as 0.1N with micron-level precision.
Maintenance Procedures
1. Inspection Protocol:
Measure free length monthly (±0.5mm tolerance indicates wear)
Check for coil separation exceeding 10% of pitch
Monitor hook deformation (max 5° angular deviation from original)
2. Lubrication:
Apply dry film lubricants (e.g., PTFE-based) every 50,000 cycles
For high-temperature applications, use molybdenum disulfide paste (effective to 400°C)
Avoid petroleum-based lubricants in food processing equipment
3. Load Testing:
Quarterly verification of spring rate (should not deviate >5% from nominal)
Perform proof testing at 125% of working load annually
Use strain gauges for critical applications to monitor stress distribution
4. Replacement Criteria:
Permanent set >3% of free length
Visible cracks or pitting covering >10% of surface area
Force output variation >8% from specification
Installation Best Practices
Proper installation significantly impacts spring performance and longevity:
Preload should not exceed 30% of maximum allowable stress (typically 45-50% of material's tensile strength)
Alignment tolerance: ≤1° angular misalignment per 100mm of spring length
For parallel spring arrangements, load distribution should be within ±5% between springs
Use spherical seats or swivel joints when lateral movement exceeds 2% of compressed length
In corrosive environments, specify springs with zinc-nickel plating (5-15μm thickness) or Xylan® coatings, which reduce friction coefficients by 40-60% compared to uncoated springs while providing chemical resistance.