When discussing the types of extension springs, it’s important to note that unlike compression springs, which come in a wide variety of body shapes, most extension springs share the same fundamental design: tightly wound coils designed to resist stretching. However, the most important variables in extension spring function and performance lie in the end configurations, optional features like initial tension, and specialty variations such as drawbar springs.
At Wermke Spring, we specialize in manufacturing custom extension springs with a variety of hook styles, materials, and mechanical properties to suit a wide range of applications, from consumer products and industrial machinery to aerospace and medical devices. This page provides an overview of the various options and types of extension springs, helping engineers and buyers better understand what defines each spring’s capabilities. To learn more about extensions springs or request a spring manufacturing quote, contact us today.
What Are Extension Springs?
Extension springs are helical springs that work under tension. When pulled apart, they store mechanical energy and resist the applied force, returning to their original shape when released. The coils are typically wound tightly together, with initial tension holding them closed until an external force is applied.
Where:
- F = Force required to extend the spring (lbs or N)
- k = Spring rate or stiffness (lbs/in or N/mm)
- x = Distance of extension beyond the free length (in or mm)
The initial tension must be overcome before any extension occurs.
Learn more about: What Are Extension Springs?
Extension Spring End Types
The true diversity in extension spring designs lies in the variety of hook and loop configurations, which allow the spring to connect to other components. These end types influence strength, fatigue resistance, space requirements, and attachment reliability.
1. Machine Loops
The most common end configuration. Formed into a full circular loop, machine loops provide a reliable attachment and balanced load distribution.
Applications:
- Garage door systems
- Appliances
- Light industrial machinery
2. Crossover Center Hooks
Formed by extending the last coil over the spring’s center. These offer a compact profile and moderate strength.
Benefits:
- Reduced overall length
- Lower stress concentrations
Common Uses:
- Automotive linkages
- Tools and mechanical devices
3. Side Hooks
Hooks extend out to one side rather than over the center, making them suitable for limited clearance or angular attachment points.
Advantages:
- Easy to manufacture
- Useful when alignment is not critical
4. Extended Hooks
A longer, straight piece of wire extending from the last coil to form the hook. These provide added flexibility for custom mounting.
Applications:
- Equipment assemblies with custom spacing or unusual geometries
5. Double Full Loops
Two full loops are formed on each end, increasing strength and redundancy for high-load applications.
Ideal For:
- Heavy-duty machinery
- Critical fail-safe systems
6. No Hooks (Plain Ends)
Used when springs are mounted in housings or over rods, with alternative means of anchoring such as caps or threaded fasteners.
Best Suited For:
- Specialized medical and aerospace designs
Drawbar Springs: A Unique Extension Spring Variant
One of the most notable types of extension springs is the drawbar spring. Unlike typical extension springs that extend linearly, drawbar springs are a combination of a compression spring and tension system.
How They Work:
A drawbar spring consists of a compression spring with rods (drawbars) running through its center. When force is applied, the drawbars stretch while compressing the spring itself, creating a fail-safe assembly.
Advantages:
- Provides tensile load with compression spring mechanics
- Safe failure mode – spring collapse won’t lead to uncontrolled movement
- Used in applications where catastrophic failure must be avoided
Applications:
- Trailer ramp springs
- Agricultural equipment
- Heavy industrial linkages
Design Considerations
When designing or selecting an extension spring, engineers must evaluate:
- Material – Stainless steel, music wire, chrome silicon, or phosphor bronze
- Wire diameter and body size – Affects spring rate and fatigue life
- Free and maximum extended length – Based on required motion
- Initial tension – Preloaded force before deflection begins
- Hook design – Impacts load distribution and durability
At Wermke Spring, our engineers work closely with clients to optimize spring geometry, material selection, and end type for performance, manufacturability, and cost-effectiveness.
Frequently Asked Questions About Types of Extension Springs
Are there different types of extension springs like there are with compression springs?
Not exactly. While extension springs generally share a uniform body shape, the hook type, material, and optional features create distinctions in how they function and what applications they serve.
What is a drawbar spring, and when should I use one?
Drawbar springs are a safe and durable way to generate extension force using a compression spring. They’re ideal when failure prevention or force redundancy is critical.
Can I get a custom extension spring with non-standard hooks?
Yes. We offer fully custom end configurations, including bent hooks, swivels, threaded ends, or mounting features, to suit your unique requirements.
What if my spring is failing prematurely?
This could be due to improper hook design, poor material selection, or overload. Our team can help evaluate and redesign the spring for improved fatigue resistance.
Industry Applications for Extension Springs
Wermke Spring supplies high-performance extension springs for a wide range of markets:
- Automotive – Throttle return springs, door mechanisms
- Aerospace – Retention and latching systems
- Medical Devices – Cord retraction and spring-loaded assemblies
- Industrial Machinery – Braking and locking systems
- Consumer Products – Retractable cords, lid closures
- HVAC – Damper systems and return actuators
Each of these industries may benefit from different end styles, materials, and design features to meet function, durability, and compliance needs.
Trust Wermke Spring for Custom Extension Spring Solutions
At Wermke Spring, we understand that while the core design of extension springs may be consistent, performance is heavily influenced by the right combination of material, hook style, and geometry. That’s why we offer full-service support for design, prototyping, and production, whether you need 10 custom springs or a million parts for a production run.
With more than 70 years of manufacturing excellence, we serve clients ranging from local businesses to Fortune 500 manufacturers. Our commitment to quality, technical expertise, and responsive service makes us the partner of choice for extension spring solutions.
Contact Us to Get Started
Ready to explore the best types of extension springs for your project? Call Wermke Spring at (636) 677-5500 or contact us online to request a quote, discuss design options, or consult with our experienced engineering team. With our custom extension spring manufacturing, we’re here to help you create the perfect extension spring for your application.
