Torsion Spring vs. Forlengelsesfjær: Which One Do You Really Need?

Choosing the wrong spring for your design is a common mistake. It leads to products that feel flimsy, wear out quickly, or fail completely, forcing expensive redesigns and delays.

The choice is simple once you understand their function. Torsion springs provide a rotational force (dreiemoment) for twisting applications, while extension springs provide a linear pulling force for tensioning applications. Your design's motion dictates which one you need.

Over min 14 år i denne bransjen, I've seen countless drawings where an engineer tried to make one type of spring do the job of the other. They'll try to use an extension spring to force a lever to rotate, resulting in a clumsy and inefficient mechanism. Understanding the fundamental difference between a twisting force and a pulling force is the first and most important step in good mechanical design. Getting this right from the start saves time, penger, og mye frustrasjon.

When Do You Need the Rotational Force of a Torsion Spring?

You need a door, lid, or lever to snap back into place, but your current design is bulky and complicated. It feels weak and unreliable, and you know there has to be a simpler way.

A torsion spring offers a compact and elegant solution for storing and releasing rotational energy. It uses torque to provide a consistent return force, perfect for applications that pivot around a central point.

I once worked with a team designing a high-end medical waste bin. De trengte fotpedallokket for å føles glatt og lukkes sikkert hver gang. Deres første prototype brukte en klumpete forlengelsesfjærmekanisme skjult i basen. It was noisy and the force wasn't consistent. Jeg viste dem hvordan en enkel dobbel torsjonsfjær, montert rett ved hengselpunktet, kunne gjøre jobben bedre. Det var stille, ga en jevn lukkehandling, og var helt skjult. Ved å bytte til en torsjonsfjær, they not only improved the product's function but also its perceived quality.

Forstå rotasjonskraft (Dreiemoment)

A torsion spring doesn't stretch; det vrir seg.

• Hvordan det fungerer: The spring's body, spolene, vrir seg rundt en sentral aksel eller pinne. Denne vridningshandlingen belaster fjæren. Kraften den utøver er ikke et trekk, men en rotasjon dreiemoment[^1] that tries to push the spring's arms (eller ben) tilbake til sin opprinnelige vinkel. Think of a clothespin—you squeeze the legs together, loading the spring, and when you let go, the spring's torque provides the clamping force.
• The Importance of the Arms: The arms are the levers that transfer the dreiemoment[^1] til produktet ditt. Lengden deres, form, and angle are critical. A longer arm will travel a greater distance but exert force with less leverage.
• Direction of Wind: Torsion springs are wound in either a right-hand or left-hand direction. They should always be loaded in a way that tightens the coils, not unwinds them. Applying force in the wrong direction can cause the spring to deform and fail.

When is a Linear Pulling Force from an Extension Spring the Answer?

You need to pull two components together, but your mechanism feels loose. Without a reliable return action, your product simply doesn't function correctly or feels cheap and poorly made.

An extension spring is designed specifically for this job. It provides a consistent and reliable linear pulling force, making it the perfect solution for tensioning belts, returning levers, and holding assemblies together.

Think about the classic screen door. The spring that pulls it shut is a perfect example of an extension spring at work. A client once came to us while developing an exercise machine. They needed to provide variable resistance for a cable pulley system. Their initial design used a complex stack of weights, which was heavy and expensive. We helped them replace the weight stack with a series of long extension springs. This new design was lighter, cheaper to manufacture, and provided a much smoother resistance profile for the user. It showed how a simple extension spring can be the most effective solution for a linear force problem.

Understanding Linear Force and Tension

An extension spring's job is to pull.

• Hvordan det fungerer: Extension springs are made with their coils pressed tightly together. This creates a built-in force called initial tension. You must first apply enough force to overcome this innledende spenning[^2] før våren i det hele tatt begynner å strekke seg. Once it starts stretching, it stores energy and pulls back with a consistent, linear force.
• The Critical Hooks: The spring is useless without its ends, which are typically formed into hooks or loops. This is where all the pulling force is transferred to your product. The design of the hook is often the most critical part of the spring, as it is the most common point of failure.
• Safety Considerations: Because an extension spring is always under tension when in use, a failure can be dangerous. If a spring breaks, it can release its stored energy violently. In applications like garage doors or playground equipment, a safety cable is often run through the center of the spring to contain it if it breaks.

Torsion or Extension: How Do You Make the Right Choice?

You're looking at your design, and you're not sure which spring to use. The wrong choice will make your product more complex, more expensive, and less reliable in the long run.

The choice is determined by one simple question: does your part need to rotate around a pivot[^3], or does it need to pull in a straight line? Your answer directly points to the correct spring.

I've found that the best way to solve this is to physically act out the motion with your hands. Does your hand need to twist, like turning a doorknob? That's a job for a torsion spring. Does your hand need to pull back, like closing a drawer? That's a job for an extension spring. This simple test cuts through all the complexity. An engineer for a toy company was struggling with the launch mechanism for a toy car. He was trying to use an extension spring to make a launch arm pivot[^3]. I had him act out the motion. He immediately saw that the arm was rotating. We sketched out a simple torsion spring design, and it solved his problem.

A Simple Decision Framework

Focus on the function, not just the space available.

• Motion Type: This is the most important factor. If the primary motion is angular or rotational around a fixed point (like a hinge), you need a torsion spring. If the motion is linear between two points, you need an extension spring.
• Mounting Points: A torsion spring requires a shaft, pin, or rod for its coils to mount on. It cannot function without this central pivot[^3]. An extension spring requires two separate anchor points, one for each hook, to pull between.
• Force Delivery: A torsion spring delivers dreiemoment[^1], measured in inch-pounds or Newton-meters. An extension spring delivers a linear force, measured in pounds or Newtons. You must calculate the correct type of force for your application.

Konklusjon

Choose a torsion spring for rotational, twisting motion around a pivot[^3]. Choose an extension spring for linear, straight-line pulling force. Matching the spring to the motion is the key to a reliable design.

[^1]: Explore the definition and calculation of torque, essential for understanding torsion springs.
[^2]: Learn about initial tension and its role in the functionality of extension springs.
[^3]: Discover how pivots function in mechanical systems and their importance in spring applications.