Spiral Torsion Springs: Mechanical Principles, Design & Manufacturing, and Cordless Roller Shade

Spiral Torsion Springs: Mechanical Principles, Design & Manufacturing, and Cordless Roller Shade

I. What is a Scroll Spring?

1.1 Basic Characteristics

1.2 Materials Used

1.3 Historical Background

II. Types of Scroll Springs

2.1 Dynamic Scroll Springs (Clock Springs)

2.2 Constant-Force Scroll Springs (Balance Springs)

2.3 Bi-Directional Scroll Springs (Bidirectional Springs)

III. Mechanical Principles of Scroll Springs

3.1 Core Mechanical Laws: Bending Deformation and Elastic Recovery

3.2 Energy Storage and Release Mechanisms

3.3 Key Factors Affecting Scroll Spring Performance

IV. Application Scenarios of Scroll Springs

4.1 Core Application: Cordless Roller Blind Systems

4.2 Automotive Applications

4.3 Precision Instrument Applications

4.4 Industrial Machinery Applications

V. Design and Manufacturing of Scroll Springs

5.1 Scroll Spring Design Process

5.2 Scroll Spring Manufacturing Process

5.3 Innovative Trends in Scroll Spring Manufacturing

VI. FAQ: Applications of Scroll Springs in Cordless Roller Blinds

VII. Conclusion

Spiral Torsion Springs: Mechanical Principles, Design & Manufacturing, and Cordless Roller Shade 

Introduction

In modern mechanical systems and consumer product design, spiral torsion springs (also known as clock springs or coil springs) serve as the "invisible power source" for numerous core components, thanks to their compact structure, stable force output, and efficient energy storage/release capabilities. From the mainsprings that ensure the accuracy of mechanical watches to the balancing mechanisms that retract automotive seatbelts, and the widely adopted lifting systems of cordless roller shades in recent years, spiral torsion springs are ubiquitous across precision instruments, industrial equipment, and household consumer goods (cited from Spiral Torsion Springs.docx, the same below).

I. What Are Spiral Torsion Springs?

Spiral torsion springs are mechanical components that store and release energy through bending deformation. Their core structure consists of a continuous metal strip (or wire) coiled into a spiral shape. Under external torque, the spring contracts or expands, and outputs stable torque or linear force via elastic restoring force. Compared with coil springs, they offer higher space utilization, a larger stroke range, and smaller fluctuations in force output—making them ideal for scenarios with limited installation space and high demands for force stability (e.g., cordless roller shades, small precision instruments).

1.1 Key Characteristics

From an engineering design perspective, the core characteristics of spiral torsion springs directly determine their application suitability, which can be summarized in three aspects:

• Spiral Structure & Force Output Traits: The metal strip is coiled around a fixed center (or without a central axis), forming a spiral that expands "from inside to outside" or "from outside to inside." When deformed under force, the metal strip bends (rather than stretches or compresses), and the force output follows a linear or near-linear pattern as the coiling radius changes. This is critical for cordless roller shades, as it ensures uniform force during lifting and lowering, preventing issues like "excessive downward force" or "jamming during upward movement."
• Compatibility with Compact Space & Long Stroke: The coiled structure allows a long deformation stroke despite minimal axial dimensions. For example, a spiral torsion spring used in a 120cm-wide cordless roller shade typically has an outer diameter of only 50–80mm but can support a lifting stroke of 1.5–2m, perfectly fitting the narrow installation space above windows.
• Elastic Recovery & Fatigue Resistance: High-quality spiral torsion springs can undergo tens of thousands or even hundreds of thousands of "deformation-recovery" cycles within the rated deformation range without plastic permanent deformation. This is a core requirement for high-frequency use scenarios such as cordless roller shades and automotive seatbelts, directly influencing product lifespan.

1.2 Materials Used

Material selection for spiral torsion springs depends on the load requirements, environmental conditions (temperature, humidity, corrosion) and lifespan expectations of the application. Engineers typically evaluate materials based on three dimensions: "strength-elasticity-weather resistance." Taking cordless roller shades as an example:

For kitchen roller shades (exposed to oil and moisture), 304 stainless steel strips are preferred; for bedroom roller shades (cost-sensitive and dry environment), 65Mn spring steel is a more economical choice.

The thickness and width of the strip are calculated based on the shade’s weight: a 5kg roller shade usually uses a spring steel strip with a thickness of 0.3–0.4mm and a width of 15–20mm to balance force output and structural strength.

1.3 Historical Background

The development of spiral torsion springs is closely tied to humanity’s demand for "precision energy storage," with its technological evolution spanning centuries:

Early Prototypes (Late 15th–16th Centuries): During the Renaissance, the invention of mechanical clocks drove the first large-scale application of spiral torsion springs. Clockmakers coiled metal sheets into spirals as "mainsprings" to provide continuous power for gear trains, replacing the earlier weight-driven mechanism and achieving the core function of "energy storage-stable release."

Industrial Era (19th–20th Centuries): Advances in metallurgy (e.g., the invention of spring steel) and mechanical processing precision expanded their applications from clocks to the automotive and machine tool industries. In the early 20th century, they were integrated into automotive seatbelt systems to enable "emergency retraction and normal recovery"; they were also adopted in valve control and tension adjustment mechanisms for industrial equipment to enhance operational stability.

Modern Applications (21st Century–Present): The trend toward lightweight and miniaturized consumer goods has driven spiral torsion springs to develop in the direction of "high precision, low noise, and long lifespan." The popularization of cordless roller shades has become a key consumer scenario—by optimizing the spring force curve and installation structure, cordless roller shades eliminate the safety hazards of traditional pull cords and enable easy operation, with innovative spiral torsion spring design being the core of this breakthrough.

II. Types of Spiral Torsion Springs

Based on structural differences, force-bearing methods, and application scenarios, engineers classify spiral torsion springs into three categories. Each category has unique design logic and applicable scenarios, with constant-force springs being the core choice for cordless roller shades.

2.1 Power-Type Spiral Torsion Springs (Clock Springs)

Designed to "maximize energy storage," these springs feature uniformly thick metal strips with coiling radii that increase from the inside out. When under force, they store large amounts of elastic potential energy through bending deformation and drive components via torque during release.

Design Features

Typically equipped with a central axis: one end of the metal strip is fixed to the central axis, and the other end to the spring housing (or support structure).

Energy storage capacity is proportional to the cube of the strip thickness and the strip width, and inversely proportional to the coiling radius. Engineers often increase the strip thickness to improve energy density while avoiding excessive radial dimensions.

Torque output varies with the coiling angle (higher torque when coiled tightly, lower during release), making them suitable for scenarios with low torque stability requirements but high long-term energy storage needs.

Application Scenarios

• Precision Instruments: Mainsprings in mechanical watches and timers store energy to drive balance wheels and gear trains for accurate timekeeping.
• Toys & Small Devices: Wind-up toy cars and hand-cranked flashlights—manual winding tightens the spring to store energy, which drives wheels or generators when released.
• Emergency Equipment: Emergency lighting and manual alarms—spring energy storage provides short-term power when power is cut off or unavailable.

2.2 Constant-Force Spiral Torsion Springs (Balance Springs)

The core type for cordless roller shades, theiKr design focuses on "force output stability." By optimizing the strip cross-section (or coiling method), force fluctuations during deformation (expansion/contraction) are controlled within ±5%, achieving "linear force output" that perfectly matches the gravity balance needs of roller shade lifting.

Design Features

Uniformly thick strips with "equal stress design," ensuring consistent bending stress across all strip cross-sections to avoid fatigue failure due to local stress concentration.

No central axis or with a slidable central axis: one end of the metal strip is fixed to the housing, and the other end connected to moving components (e.g., cordless roller shade reels). The coiling radius is "linearly controllable" during deformation to ensure stable force output.

Force output is determined by the strip thickness, width, elastic modulus, and number of effective coils. Engineers can adjust these parameters to precisely match the load (e.g., roller shade weight).

Application Scenarios

• Cordless Roller Shades: Act as the core lifting component to balance the shade’s gravity, enabling "light pulling for lowering and releasing for rising."
• Automotive Industry: Seat adjustment mechanisms balance seat weight for easy height or position adjustment by passengers.
• Medical Equipment: Lifting systems for operating tables and support mechanisms for rehabilitation equipment provide stable balancing force to ensure operational safety.
• Industrial Equipment: Tension adjustment devices for conveyors maintain constant tension to prevent material deviation or slippage.

2.3 Double-Direction Spiral Torsion Springs (Bidirectional Springs)

Composed of two sets of spiral torsion springs with opposite winding directions, these springs can output torque or force in both clockwise and counterclockwise directions. They are suitable for "bidirectional reset" scenarios, with the key design challenge being force matching and synchronous deformation control of the two spring sets.

Design Features

Two spring sets share a central axis or housing, with opposite winding directions (one clockwise, one counterclockwise), enabling energy storage in both directions when under force.

Finite Element Analysis (FEA) is required to optimize the number of effective coils and strip dimensions, ensuring symmetric bidirectional force to avoid "one-way jamming."

Damping elements (e.g., friction plates) are usually installed to control spring release speed and improve operational smoothness.

Application Scenarios

Typical applications include bidirectional reset mechanisms for door hinges, valve control systems requiring reversible operation, and precision tools with bidirectional positioning needs.

III. Mechanical Principles of Spiral Torsion Springs

The working mechanism of spiral torsion springs is based on the "bending deformation theory" of elastic mechanics, fundamentally different from the "tension/compression deformation" of coil springs. Understanding these mechanical principles is key to precise design (e.g., force matching for cordless roller shades), requiring focus on force calculation, energy storage rules, and performance-influencing factors.

3.1 Core Mechanical Law: Bending Deformation & Elastic Recovery

When external torque acts on a spiral torsion spring, the metal strip undergoes pure bending deformation (the cross-section remains planar without stretching/compression), generating internal elastic force to resist deformation and store elastic potential energy. When the external force is removed, the elastic force drives the metal strip to return to its original shape, converting potential energy into kinetic energy or mechanical energy to drive associated components. This process follows the "generalized Hooke’s Law" for bending deformation.

For constant-force spiral torsion springs commonly used in cordless roller shades, the linear force (F, unit: N) is calculated using the core engineering formula:

F = (E × b × t³) / (4 × R³ × n)

Where:

E: Elastic modulus of the material (unit: MPa; E ≈ 206,000 MPa for 65Mn spring steel, E ≈ 193,000 MPa for 304 stainless steel);

b: Width of the metal strip (unit: mm; 15–25mm for cordless roller shades);

t: Thickness of the metal strip (unit: mm; 0.2–0.5mm for cordless roller shades);

R: Average coiling radius of the spring (unit: mm; the average of the central axis radius and the spring outer diameter);

n: Number of effective coils (coils involved in deformation; 5–10 coils for cordless roller shades).

From the formula, the strip thickness (t) has the most significant impact on force (F) (cubic relationship). For example, if the roller shade weight increases from 3kg to 5kg, adjusting the strip thickness from 0.3mm to 0.4mm can increase the force by approximately 2 times ((0.4/0.3)³ ≈ 2.37) without significantly changing the spring’s radial dimensions.

3.2 Energy Storage & Release Mechanism

Energy storage capacity is a core advantage of spiral torsion springs. Cordless roller shades require sufficient stored energy to overcome gravity for smooth rising. The stored elastic potential energy (U, unit: J) is calculated as:

U = (1/2) × F × s

Where s is the spring deformation stroke (unit: m; i.e., the lifting height of the cordless roller shade, typically 1.5–2.5m).

Key Factors Influencing Energy Storage

Material Properties: Higher elastic modulus (E) and elastic limit result in higher energy storage density (more energy stored per unit volume). For example, spring steel (≈25 J/cm³) has a higher energy storage density than stainless steel (≈20 J/cm³), allowing smaller spring volumes for cordless roller shades in dry environments.

Structural Parameters: Increasing the strip thickness (t) and width (b) significantly improves energy storage capacity (proportional to t³×b), but radial dimensions must be balanced—installation boxes for cordless roller shades are usually only 100–150mm wide, so engineers must find an optimal balance between "energy storage needs" and "space constraints."

Deformation Degree: Within the elastic limit, greater deformation stroke (s) increases energy storage. However, excessive deformation (exceeding the elastic limit) causes plastic permanent deformation. Thus, spiral torsion springs for cordless roller shades are designed with a 10–15% deformation margin.

Engineering Control of Energy Release

In cordless roller shades, "stability" of energy release is more important than "total energy":

Excessively fast release causes the shade to hit the top, generating noise and impact;

Excessively slow release leads to "jamming during rising."

Engineers typically control release through two methods:

Damping Mechanism Matching: A damping plate (e.g., rubber damping, metal friction damping) is added between the spring and the shade reel. The damping coefficient (usually 0.5–2 N·s/m) is adjusted to convert the spring’s released energy into the shade’s upward kinetic energy at a constant speed.

Force Curve Optimization: Changing the strip cross-section (e.g., from rectangular to trapezoidal) creates a "slightly decreasing force trend" when the spring expands, offsetting air resistance and frictional resistance during rising to ensure uniform lifting speed.

3.3 Key Factors Influencing Spiral Torsion Spring Performance

In engineering practice, spiral torsion spring performance (force stability, lifespan, weather resistance) is affected by multiple factors, requiring comprehensive consideration—especially for cordless roller shades, which face "high-frequency use and diverse environments":

1. Material Properties

• Elastic Limit: The maximum bending stress the material can withstand; exceeding this limit causes plastic deformation. For cordless roller shade springs, the maximum stress within the rated stroke must be ≤70% of the elastic limit (safety margin). For example, 65Mn spring steel has an elastic limit of approximately 1200 MPa, so the design maximum stress must be ≤840 MPa.
• Fatigue Strength: The material’s resistance to failure under repeated bending deformation, directly determining lifespan. Cordless roller shade springs are typically designed for 100,000 cycles (≈27 years of use with 10 daily uses), requiring materials with fatigue strength ≥500 MPa (e.g., 65Mn, 304 stainless steel).
• Corrosion Resistance: Moisture and oil can cause spring rust, reducing elasticity and strength. Cordless roller shades in bathrooms require 316 stainless steel (salt spray corrosion-resistant) or 65Mn spring steel with Dacromet surface treatment (salt spray test ≥500 hours).

2. Structural Parameters

• Strip Dimensional Tolerance: Thickness tolerance must be ±0.01mm, and width tolerance ±0.05mm—excessive thickness deviation causes force fluctuations exceeding ±10%, leading to "uneven weight" (one side heavier) in cordless roller shades.
• Coiling Precision: Concentricity error ≤0.1mm, coil spacing error ≤0.05mm. Poor concentricity causes eccentric spring operation, generating noise (≥50dB) and accelerating wear with long-term use.
• Number of Effective Coils: Too many coils increase volume; too few cause large force fluctuations. Cordless roller shade springs typically have 5–10 effective coils, calculated using the formula: n = (E×b×t³)/(4×R³×F) (F is the shade weight, with a 10% safety factor added).

3. Manufacturing Processes

• Heat Treatment: Quenching temperature (850–900℃) and tempering temperature (350–400℃) must be precisely controlled—insufficient quenching results in low hardness (<HRC40) and easy deformation; insufficient tempering causes excessive internal stress and shortened lifespan. Cordless roller shade springs require hardness control at HRC42–45 to balance elasticity and toughness.
• Surface Treatment: In addition to corrosion protection, surface treatment reduces friction. Cordless roller shade springs commonly use galvanization (friction coefficient 0.15–0.2) or plastic coating (friction coefficient 0.2–0.25) to avoid "abnormal noise" from dry friction with the housing.
• Finishing: Burrs must be removed from spring ends (chamfer radius ≥0.05mm) to prevent scratches on the housing or strip, which would cause local stress concentration and shorten lifespan.

4. Operating Environment

• Temperature: High temperatures reduce the material’s elastic modulus (e.g., E of 65Mn decreases by approximately 10% at 150℃), leading to force attenuation; low temperatures make the material brittle and prone to fracture. Cordless roller shade springs must operate within -20℃~60℃ (meeting most household environments).
• Humidity & Contaminants: High humidity in bathrooms and oil in kitchens accelerate spring rust, requiring targeted material and surface treatment selection.
• Installation Deviation: If the coaxiality deviation between the spring and reel exceeds 0.2mm during installation, the spring will bear uneven force, increasing force fluctuations and causing "jamming during lifting" with long-term use.

IV. Application Scenarios of Spiral Torsion Springs

Spiral torsion springs have penetrated industries such as industrial machinery, automotive, and consumer goods. Their "compact structure + stable force output" makes them core components for many products. This section focuses on cordless roller shade applications and briefly introduces other typical scenarios.

4.1 Core Application: Cordless Roller Shade Systems

Cordless roller shades are a typical consumer application of spiral torsion springs, with core requirements of "safety, convenience, and low noise." Traditional corded roller shades pose a risk of accidental entanglement for children; spiral torsion spring-driven cordless designs eliminate this hazard entirely. Meanwhile, precise force matching enables "easy operation and long lifespan." Their application covers three core links: "demand analysis, design matching, and installation debugging."

1. Working Principle of Cordless Roller Shades & Spring Matching Logic

A cordless roller shade lifting system consists of a "spiral torsion spring, reel, damping mechanism, and bottom bar." The working process is as follows:

• Lowering Process: The user pulls down the bottom bar, which drives the reel to rotate via a connector. The reel tightens the spiral torsion spring (the metal strip bends and stores elastic potential energy), and the shade unfolds as the reel rotates.
• Rising Process: When the user releases the bottom bar, the spiral torsion spring releases elastic potential energy, driving the reel to rotate in the opposite direction and retracting the shade to the top.
• Balance Control: A damping mechanism (coaxially installed with the spring) controls the reel’s rotation speed via frictional resistance, preventing the shade from hitting the top due to overly fast spring release and ensuring smooth, low-noise rising (noise ≤35dB).

The core of spring matching logic is "force balance"—the output force must be slightly greater than the shade’s gravity (usually 5–10% greater):

Insufficient Spring Force (< Shade Gravity): The shade cannot rise automatically or jams midway.

• Excessive Spring Force (> Shade Gravity +15%): The user needs to apply excessive force (>50N) to pull down the shade (poor user experience), and the shade may rise rapidly to cause impact when released.
• Ideal State: Downward pulling force ≤30N (easily operable by children), rising speed 0.1–0.2m/s (8–15 seconds from bottom to top), force fluctuation ≤±5%.

2. Design Key Points for Spiral Torsion Springs in Cordless Roller Shades

Design must be optimized based on the shade’s size, weight, and usage environment. Specific key points are as follows:

(1) Load & Force Calculation

First, determine the total load (G, unit: N) of the shade, including the weight of the fabric and bottom bar:

Fabric Weight: G_fabric = Fabric density (kg/m²) × Shade width (m) × Shade height (m) × 9.8 (gravitational acceleration);

Bottom Bar Weight: Typically 0.5–1.5kg (≈0.8kg for aluminum alloy bars, ≈0.5kg for plastic bars);

Total Load: G_total = (G_fabric + G_bottom bar) × 1.1 (safety factor to offset frictional resistance).

Example: A PVC cordless roller shade with width 1m and height 2m:

Fabric density of PVC: ≈1.2 kg/m²;

G_fabric = 1.2 × 1 × 2 × 9.8 ≈ 23.52 N;

G_bottom bar (aluminum alloy): ≈0.8 × 9.8 ≈ 7.84 N;

G_total = (23.52 + 7.84) × 1.1 ≈ 34.59 N (spring force needs to be 34.59 N).

(2) Structure & Space Adaptation

Installation boxes (above windows) for cordless roller shades have limited space (usually 100–150mm wide, 50–80mm high), so spring radial dimensions must be strictly controlled:

Spring Outer Diameter: ≤80% of the installation box height (e.g., ≤48mm for a 60mm-high box) to avoid friction with the box during operation.

Spring Length: Determined by the reel length, usually 5–10mm shorter than the reel to ensure installation margin at both ends.

Fixing Method: "Snap + welding" dual fixing—one end is snap-fixed to the reel, and the other end is welded to the installation box support to prevent loosening during long-term rotation.

(3) Durability & Reliability Design

Spiral torsion springs for cordless roller shades must withstand long-term repeated deformation, making durability design critical:

Fatigue Life Verification: A fatigue testing machine simulates 100,000 lifting cycles (≈27 years of use); a force attenuation ≤10% after testing is considered qualified.

Corrosion Protection: Springs for bathroom shades require "galvanization + passivation" treatment (coating thickness ≥10μm, passivation film thickness ≥2μm) to pass a salt spray test of ≥500 hours without rust.

Safety Redundancy Design: An "anti-slip key" is added between the spring and reel—even if the spring breaks, the anti-slip key can lock the reel to prevent the shade from falling rapidly (falling speed ≤0.3m/s).

(4) Noise Reduction & Smoothness Optimization

"Low noise" is a core user experience requirement, requiring noise control from three aspects:

Friction Optimization: The spring surface is coated with PTFE (friction coefficient 0.12–0.15) to reduce dry friction with the housing; silent bearings (noise ≤25dB) are used between the reel and bearing.

Force Fluctuation Control: Precision coiling technology (coil spacing error ≤0.05mm) keeps force fluctuation ≤±3%, avoiding "jamming noise" caused by sudden force changes.

Damping Matching: The damping coefficient of the damping mechanism is adjusted based on the spring force (usually 0.8–1.5 N·s/m) to stabilize the rising speed at approximately 0.15m/s, preventing "impact noise" when hitting the top.

3. Installation & Debugging Process for Spiral Torsion Springs in Cordless Roller Shades

Even a well-designed spring will fail to perform if installed or debugged improperly, requiring adherence to a standardized process:

1. Pre-Installation Inspection

• Dimensional Inspection: Use calipers to measure the spring’s outer diameter, strip thickness, and number of effective coils, ensuring consistency with design drawings.
• Visual Inspection: Check for rust, burrs, or coating peeling; ensure no cracks at welding points.
• Pre-Force Testing: Use a dynamometer to measure the initial force in the free state; a deviation ≤±5% is qualified.

2. Installation Positioning

• Coaxiality Adjustment: Use a dial indicator to measure the coaxiality of the reel and spring, ensuring an error ≤0.1mm.
• Preload Adjustment: Pre-tighten the spring by 3–5 coils during installation (preload ≈30% of the design value) to ensure smooth initial rising.
• Fixation Confirmation: Manually rotate the reel 3 times to check for spring loosening or deviation.

3. Debugging & Optimization

• Lifting Speed Test: Use a stopwatch to measure the time for the shade to rise from bottom to top (8–15 seconds is qualified); increase the damping coefficient if too fast, or decrease it if too slow.
• Force Testing: Use a dynamometer to measure the downward pulling force (≤30N is qualified); reduce the number of effective coils if too large, or increase it if too small.
• Noise Testing: Measure the maximum noise during lifting in a quiet environment (background noise ≤30dB); ≤35dB is qualified. If exceeding the limit, check and optimize friction points.

4.2 Automotive Industry Applications

Automotive industry demand for spiral torsion springs focuses on "safety, reliability, and resistance to harsh environments." Key application scenarios include:

1. Seatbelt Systems

Spiral torsion springs enable "normal retraction and emergency locking," providing continuous retraction force (3–5N) to keep the seatbelt close to the human body. In the event of a collision, the locking mechanism is triggered, and the spring stops retracting to protect the occupant.

2. Seat Adjustment Mechanisms

Constant-force spiral torsion springs balance the seat weight (15–30kg), allowing passengers to adjust the seat height or position with minimal force (<20N). Linear force output ensures smooth adjustment without jamming.

3. Door Checkers

Spiral torsion springs control the door opening angle and closing force:

Function: The spring provides resistance that changes with the opening angle—the door can stay at any position between 0–60°, and automatically closes when opened beyond 60° (closing force 5–10N).

Design Key Point: Small-sized spiral torsion springs (outer diameter ≤30mm) are used to fit the narrow space inside the door; a strip thickness of 0.2–0.3mm ensures precise force output.

4.3 Precision Instrument Applications

Spiral torsion springs for precision instruments must meet "high precision and low error" requirements. A typical application is mechanical watches:

Mainsprings (Power-Type Spiral Torsion Springs) store energy to drive gear trains and balance wheels, requiring:

Energy Storage Capacity: Supporting 36–48 hours of timekeeping (no winding needed);

Torque Stability: Torque fluctuation ≤±8% during release to ensure accurate timekeeping (error ≤±5 seconds/day);

Miniaturization: Outer diameter typically 8–15mm, strip thickness 0.1–0.15mm, fitting inside the watch case.

4.4 Industrial Machinery Applications

Industrial machinery demand for spiral torsion springs focuses on "high load and wear resistance." Typical applications include:

1. Valve Control Mechanisms

In industrial pipeline valves, spiral torsion springs provide "automatic reset force" (10–50N). When the control signal is interrupted, the spring drives the valve to close or open, preventing fluid leakage.

2. Tension Control in Textile Machinery

Constant-force spiral torsion springs maintain constant tension (5–20N) for textile threads. Linear force output ensures uniform thread tension, preventing thread breakage or slack.

V. Design & Manufacturing of Spiral Torsion Springs

The performance of spiral torsion springs depends on design and precision manufacturing processes, which are "complementary"—design must consider manufacturing feasibility, and manufacturing must accurately implement design intentions. This section outlines the design process, manufacturing techniques, and quality control standards, with a focus on cordless roller shade requirements.

5.1 Design Process of Spiral Torsion Springs

The design follows a standardized process of "demand analysis-parameter calculation-simulation verification-prototype testing" to meet application requirements (e.g., force, space, and lifespan for cordless roller shades):

1. Demand Analysis & Indicator Definition

Clarify core application requirements and form quantitative indicators:

Functional Requirements: Energy storage capacity, force range, stroke length (e.g., 30J energy storage, 30–35N force, 2m stroke for cordless roller shades);

Environmental Requirements: Temperature range (-20℃~60℃), humidity (≤95%RH), corrosion (oil, moisture presence);

Performance Requirements: Fatigue life (≥100,000 cycles), force fluctuation (≤±5%), noise (≤35dB);

Space Requirements: Maximum outer diameter, length, and installation interface dimensions (e.g., ≤48mm outer diameter, ≤950mm length for cordless roller shade springs).

2. Material Selection

Create a "material selection matrix" based on requirements to comprehensively evaluate performance and cost:

Prioritize elastic limit, fatigue strength, and corrosion resistance;

Balance cost: Spring steel is cheaper than stainless steel, suitable for dry environments; stainless steel is more expensive but suitable for humid environments;

Cordless Roller Shade Example: 65Mn spring steel (low cost, good elasticity) for living rooms; 304 stainless steel (strong corrosion resistance) for bathrooms.

3. Structural Parameter Calculation

Calculate core structural parameters using material properties and requirement indicators:

Strip Dimensions (Thickness t, Width b): Calculated using the force formula F=(E×b×t³)/(4×R³×n), with iterative optimization to match space constraints;

Coiling Parameters (Average Radius R, Effective Coils n): R is determined by installation space; n is calculated based on stroke length (n=Stroke length/(π×2R));

Fixing Structure Parameters: Snap size, welding point position, and anti-slip key size to ensure strength and installation convenience.

4. Simulation Verification (FEA Analysis)

Use finite element analysis software (e.g., ANSYS, ABAQUS) for simulation to identify design defects in advance:

Stress Analysis: Simulate stress distribution under maximum deformation to ensure the maximum stress ≤70% of the elastic limit;

Force Curve Simulation: Output the force change curve over the full stroke to verify fluctuation ≤±5%;

Fatigue Life Prediction: Predict lifespan based on stress distribution to ensure it meets design requirements (e.g., 100,000 cycles);

Cordless Roller Shade Simulation Focus: Simulate the coordinated operation of the spring and damping mechanism to verify uniform lifting speed and qualified noise levels.

5. Prototype Production & Testing

Produce prototypes and conduct physical testing to verify the design:

• Prototype Production: Use CNC spiral spring coiling machines to produce 10–20 samples, ensuring process stability;
• Performance Testing:
• Force Testing: Use a dynamo-meter to measure force over the stroke and plot the force curve;
• Life Testing: Use a fatigue testing machine to simulate 100,000 cycles and measure force attenuation;
• Environmental Testing: Use a high-low temperature chamber (-20℃~60℃) and a humidity chamber (40℃, 95%RH) to test performance stability;
• Iterative Optimization: Adjust parameters (e.g., strip thickness, damping coefficient) based on test results until all indicators are met.

5.2 Manufacturing Processes of Spiral Torsion Springs

Manufacturing involves six links: "material pretreatment-forming-heat treatment-surface treatment-finishing-inspection." Precision control in each link determines the final performance:

1. Material Pre treatment

Strip Cutting: Precision cutting machines cut strips to the design width, with a tolerance of ±0.05mm to avoid edge burrs.

Surface Cleaning: Ultrasonic cleaning (50–60℃, 10–15 minutes) removes oil and oxide scale from the strip surface.

Straightening: Multi-roller straightening machines (≥12 rollers) straighten the strip, with a straightness error ≤0.1mm/m to avoid coiling distortion.

2. Coiling Forming (Core Process)

Specialized CNC spiral spring coiling machines are required, which are key to ensuring spring precision:

• Parameter Setting: Input coiling radius (R), number of effective coils (n), and coil spacing into the control system, with a precision ≤0.01mm.
• Coiling Process: The strip is fed into the coiling die at a constant speed via a feeding mechanism; the die rotates at a constant speed to coil the strip into a spiral shape.
• Real-Time Monitoring: A visual inspection system (resolution ≥0.001mm) monitors coiling precision in real time and stops operation if deviations occur.
• Coiling Requirements for Cordless Roller Shade Springs: Coil spacing error ≤0.05mm, concentricity error ≤0.1mm to ensure small force fluctuations.

3. Heat Treatment

Heat treatment improves material elasticity and hardness, requiring strict temperature and time control:

• Quenching: Coiled springs are placed in a quenching furnace, heated to 850–900℃ (for spring steel) or 1050–1100℃ (for stainless steel), held for 1–2 hours, then water-quenched (spring steel) or oil-quenched (stainless steel) to form martensite structure.
• Tempering: Quenched springs are placed in a tempering furnace, heated to 350–400℃ (spring steel) or 450–500℃ (stainless steel), held for 2–3 hours, then air-cooled to eliminate internal stress and adjust hardness to HRC42–45.
• Quality Control: A hardness tester measures surface hardness (10% sampling per batch); a deviation ≤±2HRC is qualified. A metallographic microscope observes the structure to ensure no overheating or burning.

4. Surface Treatment

Surface treatment is selected based on the application environment, with core goals of "corrosion protection and friction reduction":

Galvanization: Suitable for dry environments (e.g., living room cordless roller shades). Hot-dip galvanization (coating 10–15μm) followed by passivation (passivation film 2–3μm) improves corrosion resistance.

Plastic Coating: Suitable for springs requiring aesthetics. Electrostatic plastic coating (coating 20–30μm) offers customizable colors and a friction coefficient of 0.2–0.25.

Dacromet Treatment: Suitable for humid/corrosive environments (e.g., bathroom cordless roller shades). The coating (5–8μm) passes a salt spray test of ≥500 hours without rust.

PTFE Coating: Suitable for springs requiring low friction. The coating (5–10μm) has a friction coefficient of 0.12–0.15, reducing operational noise.

5. Finishing

Finishing removes defects and optimizes installation adaptability:

Burr Removal: Grinding machines chamfer spring ends (chamfer radius 0.05–0.1mm) to avoid scratching installation components.

Fixing Structure Processing: Laser welding of snaps or supports (weld diameter 2–3mm, strength ≥50N) ensures secure fixing.

Dimensional Correction: Fine-tune the spring’s outer diameter and length to match the installation space, with a correction amount ≤0.1mm.

6. Quality Inspection (Full-Process Control)

Key inspection items include:

• Dimensional Inspection: Outer diameter, strip thickness, and coil spacing (100% inspection via automated calipers);
• Force Testing: Force curve measurement over the full stroke (sampling rate 5% per batch);
• Fatigue Testing: 100,000-cycle fatigue test for prototype and batch sampling (1% sampling);
• Environmental Testing: Salt spray test (≥500 hours for corrosion-resistant requirements) and high-low temperature test (-20℃~60℃);
• Appearance Inspection: 100% visual inspection for rust, burrs, or coating defects.

5.3 Innovation Trends in Spiral Torsion Spring Manufacturing

Advances in materials science and manufacturing technology are driving spiral torsion spring manufacturing toward "higher precision, longer lifespan, and intelligence," providing better solutions for applications like cordless roller shades:

Material Innovation: Development of "high-strength spring steel" (elastic limit ≥1500 MPa) and "corrosion-resistant alloys" (e.g., Hastelloy) to improve energy storage density and weather resistance.

Process Innovation: Adoption of "integrated continuous coiling-heat treatment equipment" reduces transfer errors between processes, improving coiling precision to ±0.005mm.

Intelligent Manufacturing: Integration of industrial robots and AI visual inspection systems enables "unmanned production" and "100% full inspection," increasing inspection efficiency by 3 times.

Customized Services: 3D printing technology enables customization of miniaturized, high-force spiral torsion springs for special-sized cordless roller shades (e.g., extra-wide 1.8m, extra-high 3m), breaking the size limits of traditional manufacturing.

VI. FAQ: Spiral Torsion Springs in Cordless Roller Shades

Q1: Why are constant-force spiral torsion springs preferred for cordless roller shades over other types?

A1: Through optimized design, constant-force spiral torsion springs control force fluctuations within ±5% during deformation, achieving linear force output that perfectly balances the shade’s gravity. This ensures uniform force during lifting and lowering (no "heavy downward pulling" or "jamming upward movement"). In contrast, power-type spiral torsion springs have large torque fluctuations, making them unsuitable for stable balancing scenarios; double-direction spiral torsion springs are designed for bidirectional reset and unnecessary for the one-way lifting of roller shades, also increasing costs (cited from "Design Features and Application Scenarios of Constant-Force Spiral Torsion Springs" in Spiral Torsion Springs.docx).

Q2: What problems occur if the spring force of a cordless roller shade does not match the shade’s weight?

A2: Force mismatch directly affects user experience and may pose safety hazards:

Spring Force < Shade Gravity: The shade cannot rise automatically or jams midway.

Spring Force > Shade Gravity +15%: The user needs to apply excessive force (>50N) to pull down the shade (difficult for children and the elderly), and the shade may hit the top rapidly when released, generating noise and impact.

The ideal state is spring force 5–10% greater than the shade’s gravity, ensuring downward pulling force ≤30N (cited from "Spring Matching Logic for Cordless Roller Shades" in Spiral Torsion Springs.docx).

Q3: How to select spring materials for cordless roller shades in different environments (kitchen, bathroom, bedroom)?

A3: Selection depends on environmental humidity and contaminants, focusing on balancing corrosion resistance and cost:

Bedroom (Dry, No Oil): 65Mn spring steel is preferred for its low cost and high elastic modulus (≈206,000 MPa), meeting usage needs.

Kitchen (Humid, Oily): 304 stainless steel is suitable for its resistance to oil corrosion, passing a salt spray test of ≥480 hours.

Bathroom (High Humidity, Steam): 316 stainless steel or 65Mn spring steel with Dacromet treatment is recommended—the former resists salt spray corrosion, while the latter passes a salt spray test of ≥500 hours to prevent rust from steam (cited from "Material Selection for Spiral Torsion Springs" and "Material Matching for Cordless Roller Shades" in Spiral Torsion Springs.docx).

Q4: How to determine if the spiral torsion spring of a cordless roller shade needs replacement?

A4: Replace the spring if any of the following occurs:

• Lifting Failure: The shade cannot rise automatically or jams frequently during rising, indicating force attenuation exceeding 10%.
• Abnormal Noise: Noise during lifting exceeds 40dB, mostly caused by spring wear or coating peeling increasing friction.
• Visible Damage: Obvious rust (reducing elasticity), cracks (risk of breakage) on the spring surface, or loose fixing parts (may cause the shade to fall).
• Exceeding Lifespan: Springs used for over 27 years (based on the design lifespan of 100,000 cycles, 10 uses per day) will likely have substandard performance (cited from "Fatigue Life of Spiral Torsion Springs" and "Reliability Design for Cordless Roller Shades" in Spiral Torsion Springs.docx).

Q5: Does coaxiality deviation between the spring and reel during installation affect cordless roller shade performance?

A5: Yes, the impact is significant. If the coaxiality error exceeds 0.2mm, the spring will operate eccentrically, causing:

Force fluctuations exceeding ±10%, leading to "uneven weight" (one side heavier) and shade tilting.

Long-term eccentric operation accelerates wear between the spring and housing, generating noise ≥50dB and shortening the spring lifespan by 30–50%.

Thus, a dial indicator must be used during installation to ensure coaxiality ≤0.1mm (cited from "Precision Requirements for Spiral Torsion Spring Structural Parameters" and "Installation Positioning for Cordless Roller Shades" in Spiral Torsion Springs.docx).

VII. Conclusion

As elastic components based on bending deformation, spiral torsion springs have become core components of modern mechanical systems and consumer goods, thanks to their compact structure, stable force output, and efficient energy storage capabilities. From mechanical watch mainsprings to automotive seatbelt mechanisms and cordless roller shades, their application scenarios continue to expand. The precision of their design and manufacturing directly determines the performance, lifespan, and user experience of end products.

In the core scenario of cordless roller shades, spiral torsion springs deliver significant value—through precise force matching (balancing shade gravity), compact space adaptation (fitting narrow installation boxes), long-term durability design (100,000-cycle lifespan), and noise reduction optimization (≤35dB), they completely eliminate the safety hazards and operational pain points of traditional corded roller shades, driving the popularization of cordless roller shades. This process requires comprehensive consideration of material properties, structural parameters, manufacturing processes, and application environments, achieving perfect alignment between spring performance and product requirements through a closed loop of "design-simulation-testing-iteration."

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