Constant Force Spring

Constant Force Spring is a specially designed spring that delivers near-constant output force during extension or retraction, unlike the variable force characteristics of traditional springs. Below is a comprehensive analysis of constant force springs:

Core Principle & Structure

Unique Design:
Consists of a pre-stressed spiral steel strip (typically high-carbon stainless steel) tightly wound into a coil. When extended, the strip unwinds from the spool and recoils when retracted.
Constant Force Mechanism:
The change in bending radius cancels out the material’s elasticity, maintaining stable force output throughout (Hooke’s Law does not apply).

Key Characteristics

Property	Description
Force Stability	Fluctuation within ±10% across the entire range (superior to traditional springs).
Cycle Life	Over 10,000 cycles (material- and load-dependent).
Motion Direction	Configurable as tension (pull force) or compression (push force) types.

Typical Applications

Office Equipment:
✦ Printer paper auto-rewind mechanisms
✦ Projector screen（lift systems)
Industrial Use:
✦ Automatic cable/hose retractors (e.g., CNC machine cable management)
✦ Constant-pressure control in safety valves
Medical Devices:
✦ Electric hospital bed height adjustment
✦ Force feedback systems in surgical instruments
Automotive Industry:
✦ Seatbelt pretensioners
✦ Anti-pinch mechanisms for power windows

Selection & Design Guidelines

Force Calculation:
F = \frac{E \cdot t^3 \cdot w}{6R^2}F=6R2E⋅t3⋅w
(E: Elastic modulus, t: Material thickness, w: Strip width, R: Coil radius)
Material Options:
301/304 stainless steel (corrosion-resistant applications)
High-carbon steel (high-load requirements)
Titanium alloy (aerospace/defense applications)
Failure Prevention:
Avoid exceeding max. displacement (typically labeled as Dmax)
Minimum bend radius ≥ 5× material thickness

Comparison with Traditional Springs

Feature	Constant Force Spring	Traditional Coil Spring
Force Curve	Near-horizontal line	Slope (follows Hooke’s Law)
Space Efficiency	Flat profile saves space	Requires axial installation space
Dynamic Response	No resonance risk	Prone to harmonic vibration

Troubleshooting Common Issues

Issue 1: End Vibration
➤ Solution: Add guide tracks or dual-spring parallel configuration
Issue 2: Premature Fatigue
➤ Solution: Switch to nanocrystallized materials
Issue 3: Force Decay
➤ Solution: Regular lubrication (molybdenum disulfide-based grease recommended)