An "ultimate guide" to flat springs requires covering their definition, various types, advantages, disadvantages, applications, materials, design considerations, manufacturing processes, and how to source them.
The Ultimate Guide to Flat Springs
1. What is a Flat Spring? (Definition & Basic Principle)
A flat spring is a type of mechanical spring made from flat or rectangular strip metal, as opposed to round wire (which is common for helical compression, extension, or torsion springs). Flat springs store and release mechanical energy through flexion rather than twisting or coiling.
Their fundamental characteristic is their axial space efficiency. They excel in applications where radial space is plentiful but axial (along the axis of motion) space is highly constrained.
Basic Principle: When a force is applied to a flat spring, the strip material elastically deforms (bends). When the force is removed, the spring returns to its original shape, releasing the stored energy. The amount of force, deflection, and energy storage depends heavily on the material, thickness, width, shape, and length of the flat strip.
2. Why Choose Flat Springs? (Advantages)
- Exceptional Axial Space Savings: This is the primary advantage. They can provide significant force in a very small axial height, making them ideal for compact designs where traditional coil springs would be too bulky (Par exemple, bearing preload in electric drives, compact mechanisms).
- High Force in Small Deflection: Many flat spring types (like Belleville washers or wave springs) can generate substantial force over relatively short deflections.
- Unique Geometric Flexibility: Flat springs can be stamped, formed, or wound into intricate and custom shapes that are impossible with round wire. This allows them to integrate multiple functions (Par exemple, spring, electrical contact, retainer) into a single component.
- Self-Retaining: Many flat springs are designed with features that allow them to snap into place or be easily affixed, reducing assembly time and part count.
- Excellent for Static or Low Dynamic Applications: While some can handle dynamic loads, they often excel as static preload or clamping elements.
- Precise Loading: Can be engineered to provide very accurate and consistent force/deflection characteristics.
- Cost-Effective for High Volume: For stamped or formed flat springs, once the tooling is made, production can be very rapid and cost-effective, especially for high quantities.
- Reduced Vibration and Noise: By taking up play or maintaining preload, they contribute to quieter and smoother operation in mechanical assemblies.
3. Limitations of Flat Springs (Disadvantages)
- Lower Deflection Range: Generally provide less deflection than helical springs for a given mass/volume of material.
- Higher Stress Concentration: Sharp bends or complex forms can create stress concentration points, potentially reducing fatigue life if not properly designed.
- Complex Design & Tooling: Designing flat springs, especially complex wire forms or stampings, often requires more engineering expertise and can involve higher initial tooling costs compared to a simple coil spring.
- Sensitivity to Material Properties: The performance is highly dependent on the material's elastic modulus and yield strength.
- Prone to Buckling (in some forms): Long, thin flat strips under compressive load can buckle if not properly guided.
4. Types of Flat Springs
Flat springs encompass a diverse range of designs, each optimized for specific applications:
A. Wave Springs
- Description: Manufactured from flat wire, edgewound into a wavy or serpentine pattern.
- Fonction: Primarily used for bearing preload and to absorb shock or vibration in axially restricted applications.
- Advantages: Up to 50% axial space reduction compared to conventional round wire coil springs, precise loading.
- Subtypes: Single-turn, multi-turn, nested (for higher force), gap/overlap ends.
- Applications: Bearing preload in electric motors/drives, valve actuation, connectors, medical devices, aérospatial.
B. Belleville Washers (Disc Springs)
- Description: Conical-shaped washers, typically made from high-strength steel.
- Fonction: Store energy when flattened. They can be stacked in various configurations (parallel, series, or mixed) to achieve different force/deflection characteristics.
- Advantages: High force in a very small axial space, consistent force.
- Applications: Heavy machinery, bolted joints, clutch and brake mechanisms, pipe supports, check valves, tool holders.
C. Constant Force Springs (Motor Springs, Brush Springs)
- Description: Made from pre-stressed flat strip material coiled into a tight spiral.
- Fonction: Provides a virtually constant force over its entire extension length.
- Advantages: Constant force output, smooth operation, long extension capability for their size.
- Applications: Counterbalancing, retracting mechanisms (Par exemple, tape measures, window sashes, fire doors), brush springs for motors, medical devices (IV pumps).
D. Flat Wire Compression/Extension/Torsion Springs
- Description: While often called "coil springs," these are made from rectangular or square wire, coiled helically.
- Fonction: Similar to round wire coil springs but benefit from the flat wire geometry.
- Advantages: Increased spring rate in a given diameter, reduced solid height (for compression), sometimes more favorable stress distribution.
- Applications: Where a traditional coil form is desired but with performance benefits of flat wire.
E. Leaf Springs
- Description: Simple rectangular strips of metal, often stacked (multi-leaf) or used individually (mono-leaf).
- Fonction: Designed to deflect under transverse (perpendicular) loads.
- Advantages: Simplicity, often used for suspension.
- Applications: Vehicle suspensions, simple clamping mechanisms, electrical switches (as a cantilever leaf).
F. Clips, Clamps, Retainers (Fourslide/Multi-slide Components)
- Description: Often complex, custom-formed flat metal components made in high volumes using "fourslide" or "multi-slide" stamping and forming machines.
- Fonction: Can combine spring functionality with fastening, connecting, or retaining functions.
- Advantages: Highly integrated function, cost-effective for high volumes, precise and repeatable.
- Applications: Automotive clips, battery contacts, electrical contacts, medical device fasteners, appliance components.
G. Cantilever Springs
- Description: A simple beam fixed at one end and loaded at the other.
- Fonction: Provides force and deflection based on the beam's material, dimensions, and length.
- Advantages: Simplistic design, often integrated into other components.
- Applications: Electrical contacts, switches, small mechanisms.
H. Power Springs (Spiral Springs)
- Description: A flat strip of spring material wound into a spiral and mounted in a case; one end attaches to an arbor, the other to the case.
- Fonction: Stores energy as it is wound up and releases it as it unwinds. Produces high torque over a limited number of turns. Often referred to as clock springs.
- Advantages: Concentrated power, smooth torque.
- Applications: Wind-up toys, retracting cords/hoses, brush motors, cable reels, engine starters. Note: Distinct from constant force springs.
5. Common Applications of Flat Springs
Flat springs are ubiquitous across nearly every industry:
- Electric Drives / Motors: Bearing preload (wave springs!), brush contacts, brake mechanisms.
- Automotive: Suspension systems (leaf springs), clips, fasteners, seat mechanisms, sensor contacts, clutch/brake components (Belleville washers).
- Electronics: Battery contacts, switches, connectors, EMI/RFI shielding, circuit board retainers.
- Dispositifs médicaux: Miniaturized mechanisms, surgical tools, drug delivery systems, guides, pinces, precision contacts (often requires specialized materials like Nitinol).
- Appliances: Door latches, timer mechanisms, switchgear, oven door springs, dishwasher components.
- Aerospace & Defense: Actuators, sensor mechanisms, lightweight components, control systems.
- Industrial Machinery: Clutches, brakes, valves, heavy load support, damping.
- Consumer Goods: Toys, watches, locks, dispensers, cameras.
- Pumps & Vannes: Mechanical seal preload, valve return mechanisms.
6. Materials for Flat Springs
The choice of material is critical and depends on load, environment, fatigue requirements, and cost.
- Carbon Spring Steels:
- High Carbon Steel (Par exemple, 1074, 1095): Excellent strength and fatigue life for general-purpose use, but prone to corrosion. Requires plating or coating.
- Hard Drawn (Par exemple, ASTM A227): Economical, good strength, but lower fatigue life.
- Music Wire (Par exemple, ASTM A228): Highest tensile strength and excellent fatigue life for round wire, less common for flat strip.
- Stainless Steels:
- Type 301 (UNS S30100): Good strength and corrosion resistance, suitable for general-purpose applications where some corrosion resistance is needed. Often specified in temper rolled condition.
- Type 302/304 (UNS S30200/S30400): General corrosion resistance, good formability.
- Type 316 (UNS S31600): Superior corrosion resistance, especially to chlorides, good for medical and marine environments. Lower strength than 301.
- 17-7 PH (UNS S17700): Precipitation-hardening stainless steel, excellent strength after heat treatment, good corrosion resistance. Ideal for high-stress applications.
- Copper Base Alloys:
- Beryllium Copper (Par exemple, C17200, C17300): Excellent electrical conductivity, good strength, résistance à la corrosion, and good fatigue life. Non-magnetic. Commonly used for electrical contacts.
- Phosphor Bronze (Par exemple, C51000): Good electrical conductivity, résistance à la corrosion, and reasonable strength. More economical than beryllium copper.
- Nickel Base Alloys:
- Décevoir (Par exemple, 600, 718, X-750): Excellent strength at high temperatures, good corrosion resistance, and non-magnetic. Used in aerospace, oil & gas, high-temperature applications.
- Monel (Par exemple, 400, K-500): Extremely good corrosion resistance, especially in sea water, good strength for cryogenic to high temperatures.
- Elgiloy® / Conel (Haynes® 263 / MP35N): Cobalt-nickel based alloys. Excellent corrosion resistance, high strength, non-magnetic, and good for high temperatures. Used in medical and aerospace.
- Other Specialty Alloys:
- Nitinol: Shape memory alloy and superelastic properties. Used in medical devices, orthodontics.
- Titanium: High strength-to-weight ratio, excellent corrosion resistance.
7. Design Considerations for Flat Springs
Effective flat spring design requires careful attention to several factors:
- Load & Deflection Requirements:
- What is the target force at a given deflection?
- What is the maximum allowed deflection?
- Is the load static or dynamic (fatigue)?
- Available Space: Axial, radial, and tangential constraints are paramount.
- Material Selection: Based on stress, temperature, corrosion, conductivity, magnetic properties, and cost.
- Operating Environment: Temperature range, exposure to chemicals, humidity, vacuum.
- Fatigue Life: If dynamic, how many cycles are required? This heavily influences material choice, stress levels, and surface finishes.
- Stress Analysis: Bending stress is primary. Ensure stresses remain below the material's yield strength, with an appropriate safety factor. Finite Element Analysis (FEA) is often used for complex designs.
- End Conditions / Mounting: How will the spring be held in place? Integrated features for retention are common.
- Tolerances: What is the acceptable variation in dimensions and load? Tighter tolerances increase cost.
- Manufacturing Process: The chosen manufacturing method (stamping, forming, edgewinding) will influence design possibilities and costs. Consider manufacturability early in the design phase.
- Secondary Operations: Heat treatment (stress relieving, hardening), plating (résistance à la corrosion, conductivity), passivation, deburring, polishing.
8. Manufacturing Processes for Flat Springs
- Stamping & Forming:
- Process: Flat strip metal is fed into presses where dies shear, bend, and form the material into the desired shape.
- Used for: Clips, contacts, retainers, intricate 2D and 3D wire forms (fourslide/multi-slide machines).
- Advantages: High speed, high volume, cost-effective for mass production once tooling is made.
- Limitations: High initial tooling cost, can be less suited for very thick material or extremely complex 3D forms.
- Edgewinding:
- Process: Flat wire is wound on its edge into a coil. This is the primary method for manufacturing wave springs and spiral retaining rings.
- Used for: Wave springs, spiral retaining rings, some constant force springs.
- Advantages: Burr-free, no waste material, precise control over dimensions and spring rate.
- Bending & Press Forming:
- Process: For simpler leaf springs or custom shapes where stamping dies aren't economical or appropriate, individual bending operations may be used.
- Used for: Prototypes, low volume production, large leaf springs.
- Coiling (for Flat Wire Helical Springs):
- Process: Similar to round wire coiling, but using specialized machinery to coil flat wire.
- Used for: Flat wire compression, extension, and torsion springs.
- Heat Treatment: Essential for stress relieving newly formed springs and for hardening certain materials (Par exemple, 17-7 PH stainless steel) to achieve final material properties.
9. Sourcing and Suppliers for Flat Springs
Finding the right manufacturer is key, as flat springs often require specialized engineering and manufacturing capabilities.
- Prepare a Detailed Specification: Include all dimensions, matériel, load/deflection, operating environment, quantity, and tolerances. A drawing is essential.
- Identify Manufacturer Types: Look for manufacturers specializing in:
- Wave springs: Smalley Steel Ring Company, Lee Spring, Associated Spring Raymond.
- Belleville washers: Belleville Inc., Key Bellevilles, Solon Manufacturing.
- Constant force springs: Lee Spring, Stanley Engineered Solutions (Associated Spring Raymond).
- Custom metal stampings/wire forms (fourslide/multi-slide): Many specialized companies, also larger general spring makers like MW Industries' subsidiaries, Newcomb Spring, Boker's Inc.
- Precision coil springs (from flat wire): Companies listed above, check their specific capabilities.
- Utilize Industry Resources:
- Spring Manufacturers Institute (SMI): Member directory in North America.
- Thomasnet.com: Filter by spring type, matériel, manufacturing capabilities.
- Evaluate Suppliers:
- Capability: Do they have the technology and expertise for your specific type of flat spring and material?
- Engineering Support: Do they offer design assistance and DFM (design for manufacturability)?
- Quality & Certifications: Look for ISO 9001, AS9100 (aérospatial), IATF 16949 (automotive), ISO 13485 (medical).
- Volume & Lead Time: Can they handle your quantities and meet your schedule?
- Cost: Get multiple quotes, but prioritize value (quality + service) over just the lowest price.
Flat springs are an indispensable category of mechanical components, offering elegant solutions to complex engineering challenges, especially where space is at a premium. Their versatility in form and function makes them a designer's powerful tool.