Punawai Torsion vs. Exithins: ʻO wai ka mea āu e makemake ai?

ʻO ke koho ʻana i ka punawai hewa no kāu hoʻolālā he hewa maʻamau. Ke alakaʻi nei ia i nā huahana e manaʻo palupalu, pau koke, a hāʻule loa paha, ke koi ʻana i nā hoʻolālā hou a me nā lohi.

He mea maʻalahi ke koho ke hoʻomaopopo ʻoe i kā lākou hana. Hāʻawi nā pūnāwai torsion i ka ikaika rotational (torque) no ka wiliwili ana, ʻoiai nā pūnāwai hoʻolōʻihi e hāʻawi i ka ikaika huki laina no nā noi tensioning. Your design's motion dictates which one you need.

Ma luna o koʻu 14 makahiki ma keia oihana, 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, ka hopena i kahi ʻano hana ʻino a maikaʻi ʻole. ʻO ka hoʻomaopopo ʻana i ka ʻokoʻa koʻikoʻi ma waena o ka ikaika wili a me ka ikaika huki ka hana mua a koʻikoʻi loa i ka hoʻolālā mechanical maikaʻi. ʻO ka loaʻa ʻana o kēia pono mai ka hoʻomaka ʻana e mālama i ka manawa, kālā, a me ka nui o ka pilikia.

I ka manawa hea ʻoe e makemake ai i ka ikaika hoʻololi o kahi pūnāwai Torsion?

Pono ʻoe i kahi puka, poʻi, a i ʻole lever e hoʻihoʻi i kahi, akā ʻo kāu hoʻolālā o kēia manawa he nui a paʻakikī. He nāwaliwali a hilinaʻi ʻole, a ʻike ʻoe he ala maʻalahi.

Hāʻawi kahi pūnāwai torsion i kahi hopena paʻa a nani no ka mālama ʻana a me ka hoʻokuʻu ʻana i ka ikehu rotational. Hoʻohana ia i ka torque e hāʻawi i ka ikaika hoʻihoʻi mau, kūpono no nā noi e pivo a puni kahi kiko waena.

Ua hana au i kekahi manawa me kahi hui e hoʻolālā ana i kahi pahu ʻōpala lapaʻau kiʻekiʻe. They needed the foot-pedal lid to feel smooth and close securely every time. Their first prototype used a clunky extension spring mechanism hidden in the base. It was noisy and the force wasn't consistent. I showed them how a simple double torsion spring, mounted right at the hinge point, could do the job better. It was silent, provided a smooth closing action, and was completely hidden. By switching to a torsion spring, they not only improved the product's function but also its perceived quality.

Understanding Rotational Force (Torque)

A torsion spring doesn't stretch; it twists.

• How it Works: The spring's body, the coils, twists around a central shaft or pin. This twisting action loads the spring. The force it exerts is not a pull, but a rotational torque[^1] that tries to push the spring's arms (or legs) back to their original angle. 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 torque[^1] to your product. Their length, shape, 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.

• How it Works: 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 initial tension[^ 2] before the spring even begins to stretch. 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 torque[^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.

Hopena

E koho i kahi pūnāwai torsion no ka hoʻololi ʻana, wiliwili a puni a pivot[^ 3]. E koho i kahi puna hoʻonui no ka laina laina, kaha huki laina pololei. ʻO ka hoʻohālikelike ʻana i ka pūnāwai i ka neʻe ke kī i kahi hoʻolālā hilinaʻi.

[^1]: E ʻimi i ka wehewehe a me ka helu ʻana o ka torque, mea nui no ka hoomaopopo ana i na punawai torsion.
[^ 2]: E aʻo e pili ana i ka hoʻopaʻapaʻa mua a me kāna kuleana i ka hana o nā puna hoʻonui.
[^ 3]: E ʻike i ka hana ʻana o nā pivots i nā ʻōnaehana mechanical a me ko lākou koʻikoʻi i nā noi puna.