What Is a Pull Spring and How Does It Work?
You need a "pull spring" but find different names online. This confusion makes it hard to buy the right part, and using the wrong one could cause your project to fail.
EN "pull spring[^1]" is the common name for an extension spring[^2]. It is a tightly wound coil spring designed to resist a pulling force. When you stretch it, it stores energy and constantly tries to pull its ends back together to its original, shorter length.
As a spring manufacturer, I work with these parts every day. While "pull spring[^1]" is a great description of what it does, the official industry name is an extension spring. The real magic is not just that it pulls, but hvordan it is engineered to pull with a consistent and reliable force. Let's look at the key details that make this simple part so effective.
How Does a Pull Spring Store Its Energy?
You see a spring stretch and then snap back into place. It seems simple, but there's a hidden force inside holding it together, even when it's just sitting there.
This built-in force is called initial tension. It is a specific amount of force pre-loaded into the spring during manufacturing that holds the coils tightly together. You must overcome this initial tension[^3] before the spring even begins to stretch, which allows it to store energy immediately.
When we make an extension spring[^2] on our CNC machines, we intentionally twist the wire as it's being coiled. This twist creates a force that presses each coil tightly against the next one. This is the initial tension. Think of it as a starting line for the spring's work. A spring without initial tension would be loose and floppy. With initial tension[^3], the spring is a solid, compact unit until you apply enough force to separate the coils. This feature is critical in applications like screen doors, where you need the spring to hold the door firmly shut, not just let it hang loosely. We can control the amount of initial tension to match exactly what a customer needs.
The Two Forces Inside
An extension spring[^2]'s pull comes from two distinct forces working together.
- Indledende spænding: This is the constant, internal force that holds the spring's coils together at rest. It is the amount of pull needed just to get the spring to start extending. It does not change as the spring stretches.
- Spring Rate (or Stiffness): This is the additional force needed to stretch the spring further once the initial tension[^3] is overcome. It is typically measured in pounds per inch (lbs/in) or Newtons per millimeter (N/mm). For every inch you stretch the spring, you have to add this much more force.
| Force Type | Beskrivelse | When It Applies |
|---|---|---|
| Indledende spænding | A fixed, built-in force holding coils together. | Must be overcome before any stretching occurs. |
| Spring Rate | The extra force needed for each unit of distance. | Applies after initial tension is overcome. |
What Are the Most Important Parts of a Pull Spring?
You look at a pull spring[^1], and it seems like just a coil of wire. But some parts are much more critical than others, and they are the most common points of failure.
The most important parts of a pull spring[^1] are its hooks or loops. These ends are responsible for transferring all the force from the spring to your product. A poorly designed hook is the number one reason why extension springs fail prematurely.
In our factory, we spend more time engineering the hooks than any other part of the spring. The body of the spring is strong because the force is distributed evenly across many coils. But at the hook, all that force is concentrated on a single, small bend in the wire. This area of high stress is where fatigue cracks are most likely to start. A simple crossover hook is common and easy to make, but a machine hook with a full loop provides much more strength and durability. We also have to consider the angle of the hook and its opening size to make sure it attaches correctly and doesn't create extra stress points. For heavy-duty applications, we often recommend swivel hooks that can rotate to prevent the spring from being twisted and damaged.
A Look at the Ends
The body creates the force, but the hooks deliver it.
- Spring Body: This is the tightly wound section of coils. Its length, diameter, and wire size determine the spring's initial tension and rate.
- Hooks or Loops: These are the end attachments that connect the spring to other components. Their design is critical for the spring's overall lifespan and safety.
- The Transition Point: The point where the last coil of the body bends up to form the hook is the area of highest stress. A sharp bend here creates a weak spot that can easily break under repeated use. En glat, gradual bend is the mark of a well-designed spring.
| Hook Type | Beskrivelse | Common Use |
|---|---|---|
| Crossover Hook | The simplest type, where the last coil is bent up across the center. | Generelle formål, light-duty applications. |
| Machine Hook | The last coil forms a full loop before the end is bent outwards. | Stronger and more durable for industrial use. |
| Extended Hook | The hook has a long, straight section to reach a distant anchor point. | Custom machinery and unique assemblies. |
Where Can You Find Pull Springs in Everyday Life?
You understand the mechanics, but you wonder where these springs are actually used. Are they just for industrial machines, or are they hiding in plain sight?
Pull springs, or extension springs, are everywhere. You can find them holding your screen door[^4] shut, providing the bounce in a trampoline, counterbalancing your garage door, and inside farm equipment, automotive mechanisms, and countless other consumer and industrial products.
One of the most classic examples I can think of is the old-fashioned screen door. That single, long spring that runs diagonally across the door is a perfect example of a pull spring at work. It's stretched when the door is open, and its stored energy is what pulls the door shut. Another great example is a traditional trampoline[^5]. Dozens of extension spring[^2]s connect the jumping mat to the metal frame. When you jump, you stretch all those springs at once, and their collective pulling force is what launches you back into the air. In a car, you might find smaller extension springs in the carburetor to return the throttle or in the drum brake assembly to pull the brake shoes back into place. They are truly one of the most fundamental mechanical components.
Fælles applikationer
These springs are essential in any device that needs a return or tensioning force.
- Counterbalancing: In heavy garage doors or industrial lids, a pair of large extension springs holds the weight, making the object feel light and easy to move.
- Returning: In levers, pedals, og screen door[^4]s, the spring's job is to return the component to its original position after it has been moved.
- Tensioning: On trampoline[^5]s or in belt-driven machinery, extension spring[^2]s provide a constant tension to keep everything tight and responsive.
| Application Area | Specific Example | Spring's Function |
|---|---|---|
| Household | Screen Door Closer | Pulls the door shut automatically. |
| Recreation | Trampoline Mat | Provides the bounce by pulling the mat taut. |
| Automotive | Drum Brakes | Pulls the brake shoes away from the drum. |
| Industrial | Conveyor Belt Tensioner | Keeps the belt tight to prevent slipping. |
Konklusion
A "pull spring," or extension spring, is a coil designed to resist a pulling force using initial tension[^3] and its spring rate. Its hooks are the most critical part.
[^1]: Understanding pull springs is essential for various applications, ensuring you choose the right type for your needs.
[^2]: Explore the versatility of extension springs and their applications in everyday products and machinery.
[^3]: Learn about initial tension and its importance in the performance and reliability of springs.
[^4]: Learn how extension springs ensure screen doors close automatically and securely.
[^5]: Learn how extension springs provide the bounce in trampolines and their importance.