Compression coil springs resist compressive forces and return to their original length after being compressed. The changing of spring ends, direction of the helix, material, and finish allows a compression spring to meet a wide variety of special industrial needs. You'll find them in ballpoint pens, valve assemblies, and countless industrial applications. These springs may be cylindrical, conical, tapered, concave, or convex in shape, and are typically placed over a rod or inside a hole.
Coil Spring Guide: Types, Materials & Terminology
Understand how coil springs work, what they’re made of, and how to select the right one based on shape, load and deflection.
Types of Coil Springs
Coil springs, also known as helical springs, are mechanical devices that store and release energy when compressed, extended, or twisted. Made of hardened steel, they can be close-wound or open-wound and are used in thousands of industrial and everyday applications. The three main types of coil springs are compression springs, extension springs, and torsion springs.
COMPRESSION SPRINGS
Compression Spring End Types
The end type impacts solid height, free length, and other characteristics of the spring:
- Open Ends, Not Ground: Coils have a consistent pitch; ends are not modified.
- Open and Ground Ends: Ends are open but ground flat for stability.
- Square or Closed Ends (SQ): End coils touch to close the spring.
- Square (Closed) and Ground Ends (SG): End coils touch and are ground flat for load-bearing surfaces.
EXTENSION SPRINGS
Extension coil springs are found in garage door assemblies, vise-grip pliers, and carburetors, as well as thousands of other places. These springs resist stretching by design. When attached at both ends, they try to return to their resting position when pulled apart.
These springs are wound with initial tension, known as stored energy, to hold coils together and provide resistance. End styles vary by application, with the most common being hooks and loops:
Extension Spring End Types
- Machine Loop: Wire extends from the spring body and circles across the center.
- Crossover Loop: Wire crosses the center line of the spring.
- Double Loop: Two consecutive coils with no gap between the end and the spring body.
TORSION SPRINGS
Torsion springs store energy through twisting rather than compression or extension. When the ends rotate around the center, the spring resists the torque and returns to its original position. These are commonly used in clipboards, door hinges, and locks.
Torsion springs are typically close-wound and must be used in the direction that winds the coil (left- or right-hand wound). An arbor or mandrel supports these springs. Users select them based on the Degree of Deflection required: usually 90, 180, 270, or 360° from the spring body.
Key Torsion Spring Specs
- Leg Length: Measured from the center of the spring body to the end of the leg or tip.
- Wind Direction: Torsion springs should be used in the direction that winds the coil.
Coil Spring Materials, Finishes & Processes
Choosing the right coil spring material and finish ensures optimal performance for your application, especially under stress, heat, or corrosive environments.
Common Coil Spring Materials
MBHD Carbon Steel: Medium basic carbon cold-drawn spring wire per ASTM 227, suitable for parts subjected to average stress and applications; also suited to static or very low cycles
Music Wire: High-strength spring wire (ASTM A228) suitable for high-stress environments with good thermal properties. Recommended for use up to 250°F.
Type 302/304 Stainless Steel: The most widely used stainless steel spring wire (ASTM A313). Offers excellent corrosion resistance and good tensile strength. Slightly magnetic and rated for use up to 550°F.
Protective Finishes for Coil Springs
Black Oxide Coating: A basic finish that enhances corrosion resistance on MBHD Carbon Steel and Music Wire.
Zinc Plating: A better corrosion-resistant option than black oxide, often used in harsh environments.
Processes
Passivation: An acid-based treatment that removes surface contaminants from stainless steel springs to improve corrosion resistance.
Nickel-Coated Stainless Steel: Made with proprietary wire such as Suzuki™ Wire®, Nippon-Seisen, and Sumiden® Type 302. These coil springs offer superior resistance in marine and corrosive environments. Nickel plating is used in electronics because it provides a good surface for soldering, prevents tarnishing, and conducts electricity well.
Coil Spring Terminology
Use this glossary to better understand how to select and specify coil springs based on common mechanical terms:
- Body Length: Distance from coil to coil, excluding any ends like hooks or loops.
- Deflection at Load: The amount a spring compresses or extends when a load is applied.
- Free Length (FL): Total length of the spring when it is not under load.
- Initial Tension (IT): The force required to begin separating the coils of an extension spring.
- Inside Diameter (ID): Inner width of the spring, important when fitting over a rod or mandrel with clearance.
- Load: The external force that causes the spring to deflect.
- Maximum Deflection: The difference between the overall length and maximum extension, measured on one side of the spring.
- Maximum Extended Length: Full length of an extension spring at maximum rated stretch.
- Outside Diameter (OD): specified when the spring must fit into a circular hole ("Work in Hole Diameter) with enough clearance to operate freely or when there are other housing issues.
- Pitch: Distance from the center of one coil to the center of the next in active coils.
- Solid Height: Compressed height of a spring when all coils are touching.
- Spring Rate (Stiffness): The change in load per unit deflection.
- Stress Relief: A heat treatment process that reduces internal stresses in the spring wire.
- Wire Diameter (WD): The diameter of the spring wire.