The Simple Reason I Changed My Spring Rate (and Why You Might Need To Too)

Table of content:

• What is spring rate?
• How do you calculate spring rate and use it to find load or deflection?
• Why did I change my spring rate (and why might you)?
• How does spring rate work for extension springs (and what is initial tension)?
• How does spring rate work for torsion springs?
• What do you have to remember about spring rate?

I’ll admit it: I once struggled with a spring in one of my projects because it just wasn’t doing what I needed. Every time I applied force, it didn't compress enough. After a bit of head-scratching, I realized the culprit was simple: the spring’s rate was too high for my application. In other words, the spring was too “stiff,” so it compressed less than I wanted under the load I was applying. The fix? I switched to a spring with a lower spring rate (a softer spring), and voilà – problem solved.

If you’ve ever had a spring that’s either too stiff (hardly compresses) or too soft (compresses too much) for your needs, you might have the same issue. The simple reason I changed my spring rate was to get the right amount of force and travel in my design. In this article, we’ll walk you through what spring rate means, how to calculate it, and why picking the right spring rate is so important. By the end, you’ll understand spring rate like a pro – and know when you might need to change it, too. Let’s dive in!

What is spring rate?

How do you calculate spring rate and use it to find load or deflection?

Calculating spring rate and using it in practice is straightforward with Hooke’s Law. As mentioned, the core formula is F = k × x (force = spring rate times deflection). From this, you can derive two very handy equations depending on what you want to find:

• To calculate the force (load) a spring will exert or require for a given deflection: multiply the spring rate by the deflection. (F = k × x)

• To calculate the deflection (travel) of a spring under a given load: divide the load by the spring rate. (x = F / k)
  
  

Using these formulas, you can solve most basic spring problems. For instance, suppose you have a small spring with a rate of 15 lbs/in (a relatively stiff small spring). How much load will it take to compress that spring by 0.5 inches? Using F = k × x, we get: F = 15 lbs/in × 0.5 in = 7.5 pounds. This means if you put 7.5 lbs of force on the spring, it will compress about half an inch.

We can flip that scenario: imagine you need a spring to compress 0.5 inches under a load of 7.5 lbs. What spring rate should you look for? Using the rearranged formula k = F / x, we get: k = 7.5 lbs / 0.5 in = 15 lbs/in. This simple calculation shows you’d need a spring rate of 15 lbs/in for those requirements. In fact, this is exactly what I encountered in my project: I had a target load and deflection and realized the current spring’s k was off, so I recalculated the required spring rate and switched to a spring that met those specs.

To ensure everyone’s clear, let’s do one more quick example: Say you have a spring that is only 0.25 inches long (free length), and you want it to compress by 0.05 inches under a certain load. If the spring rate is 20 lbs/in, the force needed for 0.05” deflection would be F = 20 × 0.05 = 1 lb of force. The spring rate formula ties force and deflection together linearly, which is a powerful tool for design. It allows you to predict how a spring will behave: if you know any two of the three variables (F, k, x), you can find the third.

Most spring manufacturers (including Acxess Spring) provide spring rate information for their products, and they often have online spring rate calculators where you can input your desired load or deflection to see what spring rate is needed. If you prefer formulas, remember that for compression or extension springs the equation is linear as above, and for torsion springs (which we’ll cover shortly), a similar concept applies using torque and angle. Don’t worry, we’ll explain that too.

Why did I change my spring rate (and why might you)?

You might be wondering why anyone would bother “changing” a spring’s rate. In my case, the reason was simple: the original spring in my design was too stiff for the job. The device I was working on needed a certain flexibility – a certain amount of movement under load – but the spring I had installed barely compressed at all because its spring rate was too high. The solution? I switched to a spring with a lower spring rate (a softer spring). Suddenly, the device could move as intended when force was applied. Problem solved.

So, why might you need to change your spring rate? Here are a few common reasons, which might sound familiar if you’ve ever tinkered with spring-loaded mechanisms:

• Your spring is too stiff or too soft: If a spring is too stiff (high spring rate), it won’t compress or extend enough under the forces in your application – your system might feel “jammed” or unresponsive. If a spring is too soft (low spring rate), it might compress too much or too easily, failing to provide the needed force or support. In either case, choosing a spring with a more appropriate rate can fix the issue.
  
  

• Achieving a specific deflection or load: You might have a target deflection (say, a button must travel 1/2 inch when pressed) or a required force (say, a spring must push back with 5 lbs). If the current spring’s rate doesn’t match that requirement, you’ll need to find one that does. In other words, you “change the spring rate” by selecting a different spring (or redesigning the spring) so that F = kx works out in your favor.
  
  

• Tuning performance: In applications like vehicle suspensions, machinery, or even a simple pogo stick, adjusting the spring rate can drastically change performance. A car with an adjusted spring rate might handle better or give a smoother ride. A machine’s spring-loaded valve might operate more reliably after choosing a spring with the right stiffness. Often, engineers will iterate on spring rate during prototyping to fine-tune how something feels or functions.
  
  

• Preventing failure: Using a spring with an inappropriate rate can cause other problems, for example, an overly stiff spring might not compress as needed and could cause parts to bend or the spring to bottom out (fully compress) and take damage. On the flip side, a spring that’s too soft might compress fully under load (coil bind) and also get damaged or allow too much movement. Changing to the correct spring rate ensures the spring operates within safe limits and lasts longer.
  
  

In essence, changing a spring’s rate usually means swapping in a different spring or altering the design (wire diameter, coil count, etc.) to achieve a new rate. The simple reason is always to better meet the design requirements. This is a normal part of engineering: you define what you need (force and travel), calculate the required spring constant, and then pick or make a spring to match.

From personal experience, I’ll say this: Don’t be afraid to adjust your spring choice if things aren’t working. The spring rate is a key parameter you can play with. And if you’re not sure how to choose the right spring rate, remember that tools like spring calculators or experts at spring companies (like our team at Acxess Spring) can help guide you. We’ve helped many customers tweak their designs by selecting springs with the proper rate, be it for a delicate medical device or a heavy industrial machine. The right spring rate can make all the difference, moving you from a rough prototype to a smooth-running final product.

How does spring rate work for extension springs (and what is initial tension)?

Extension springs (the kind that have hooks or loops on the ends and get pulled apart) also have a spring rate, but there’s an extra factor to consider: initial tension. Initial tension is the built-in tension that keeps the coils of an extension spring snug together. Essentially, when an extension spring is at rest (no external load), it’s actually already under a bit of tension internally, the coils want to stay touching. You have to apply a certain minimum force just to start separating those coils. Only after that initial force is overcome will the spring start to extend following its spring rate.

What does this mean in practice? Let’s break it down:

• Just like a compression spring, an extension spring has a rate k (say, in lbs/in or N/mm). This tells you how much additional force is needed per unit of extension.

• However, unlike a compression spring, at zero extension an extension spring might already require some force to start moving. That’s the initial tension (often measured in the same units of force, like lbs or N).
  
  

For example, suppose you have an extension spring with a spring rate of 5 lbs/in, and it has an initial tension of 10 lbs. This means:

• You need to apply 10 lbs of force just to begin to pull the spring at all (to overcome the initial tension and get the coils to start separating).

• After that point, for each inch you pull the spring further, it will require an additional 5 lbs per inch of force.

So, if you wanted to extend this spring by 1 inch, you’d first spend 10 lbs overcoming initial tension, and then need another 5 lbs to stretch that 1 inch. In total, about 15 lbs to get 1 inch of extension. If you want 2 inches of extension, it would be the 10 lb initial tension + (5 lbs/in × 2 in) = 20 lbs total.

Mathematically, you can think of it this way:

• Total Force required for a given extension = Initial Tension + (spring rate × deflection).

• Conversely, if you know the total load on an extension spring and want to find how far it extends, you first subtract the initial tension, then divide by the spring rate.
  
  

Using the example above: say we have 15 lbs hanging on that spring. Subtract the 10 lb initial tension, we get 5 lbs “effective” load for extension. Divide that by the 5 lbs/in rate, and we get 1 inch of extension (which matches our forward calculation). If we only had, say, 8 lbs on the spring, subtract the 10 lb initial tension = negative 2 lbs (which just means the spring won’t even start to pull apart because 8 lbs is not enough to overcome the 10 lb initial tension). So the spring would stay at its full, tight coil length until you exceed 10 lbs load.

If this is sounding a bit complex, don’t worry. The main point is: extension spring rate works like compression spring rate, but you must add initial tension to your calculations. So always check if your extension spring has an initial tension value specified. And if you’re designing or ordering a custom extension spring, you can often request a certain initial tension depending on your application. For our purposes, just remember that initial tension means extra force needed at the beginning.

One more thing: initial tension can usually be adjusted by the spring manufacturer by how they coil the spring. If you find that your extension spring’s initial tension is too high (you don’t want to need that much force just to start movement) or too low (maybe you want the spring to stay snug until a higher force is applied), that’s something to discuss with a spring engineer. At Acxess Spring, for example, our design team can tweak initial tension on custom extension springs to match your requirements. If you’re unsure about how initial tension affects your project, feel free to contact our spring experts – we’ll help you sort it out and find the right extension spring formula for your needs.

How does spring rate work for torsion springs?

Torsion springs work a bit differently from compression or extension springs. Instead of being pushed or pulled in a line, a torsion spring (think of a mousetrap spring or a clothespin spring) works by twisting. It exerts a torque (a rotational force) when you rotate it. But torsion springs do have a spring rate as well – it’s just expressed in rotational terms.

For torsion springs, spring rate is usually given as “inch-pounds per degree” (in·lb/deg) in Imperial units or Newton-millimeters per degree (N·mm/deg) in metric. This tells you how much torque (twisting force) the spring will exert per degree of angular deflection. Sometimes you might see it given per radian, but per degree is common for practical use (since a torsion spring often only twists a few dozen degrees in use).

Here’s how to understand torsion spring rate:

• Imagine you have a torsion spring with a rate of 0.5 inch-pounds per degree. This means if you twist the spring by 1 degree, it will exert 0.5 inch-pounds of torque. If you twist it by 10 degrees, it will exert about 5 in·lb of torque. 20 degrees would be 10 in·lb, and so on, linearly (again, up to its safe limit).

• Conversely, if you need a certain torque, you can divide by the spring’s rate to see how many degrees of twist are required. For example, if you need about 8 in·lb of torque from that spring: 8 / 0.5 = 16 degrees of deflection would be needed.
  
  

Another way torsion spring rate is described: sometimes manufacturers specify it per full revolution (360 degrees). For instance, a spec might say “rate = 180 in·lb per 360°”. That means if you twist it one full turn, it provides 180 in·lb, which equates to 0.5 in·lb per degree (since 180/360 = 0.5) – same thing, just presented differently. In metric, they might say “X N·mm per degree” or sometimes even “per 360°” as well. The concept is the same: force per unit angle.

It’s worth noting that torsion spring calculations use a similar Hooke’s Law idea but in angular form: T = k · θ, where T is torque, k is the torsional spring rate, and θ is the angle of twist (in radians or degrees, just be consistent). If using degrees, ensure k is per degree; if using radians, k would be per radian. For simplicity, we’ll stick to degrees here.

One thing to consider for torsion springs: they often have physical limits in terms of how far you should twist them (just as compression springs have a maximum compression and extension springs a maximum stretch). Torsion springs might be rated, for example, for a maximum safe deflection of, say, 180° or so. Beyond that, they might permanently deform or break. So, when using the spring rate, ensure you’re staying within the spring’s safe working range of rotation.

If you’re ever unsure about a torsion spring’s calculations, don’t hesitate to use online torsion spring calculators or reach out to experts. At Acxess Spring, we have tools and engineers who can help calculate torsion spring requirements for you, just provide a few details (like how much torque you need and the space you have) and we can guide you to the right spring or custom design.

What do you have to remember about spring rate?

We’ve covered a lot of ground on spring rate and why it matters. To wrap things up, here are five key takeaways to remember about spring rate and its applications:

• Spring rate = force per unit length (or torque per angle): It’s the constant that tells you how stiff a spring is, typically in lbs/in or N/mm for linear springs (and in-lb/deg for torsion springs). This is essentially the spring’s stiffness or spring constant.
  
  

• Hooke’s Law is your friend: Use F = kx (and T = kθ for torsion) to calculate what you need. If you know any two of force, deflection, and spring rate, you can find the third. This helps in figuring out how much load a spring can handle or how far it will move under load.
  
  

• Choosing the right spring rate is crucial: A spring that’s too stiff or too soft can make your design fail or perform poorly. Changing the spring rate (by selecting a different spring or design) can solve issues with insufficient deflection or force. Always match the spring rate to your application’s needs for the best results.
  
  

• Extension springs include initial tension: Unlike compression springs, extension springs require an initial force to start extending. Remember to add initial tension to your calculations for extension springs. The total force for a given extension = initial tension + (spring rate × extension).
  
  

• Torsion springs use angular units: Torsion spring rates are given in terms of torque per degree (or per radian). The concept is similar to linear springs, but make sure to work in degrees or radians as specified. This tells you how much twisting force you get per unit of angle wound up.
  
  

Next Steps? Need help with springs? Understanding spring rate will help you make more informed decisions when selecting a spring. If you’re designing a project and aren’t sure which spring rate you need, try using our online tools like Spring Creator 5.0 to calculate and experiment with different spring parameters. And as always, you can contact the spring experts at Acxess Spring for personalized assistance. We’re here to help you find or design the perfect spring that meets your requirements. With the right spring rate and the right spring, your project will spring to life (pun intended)!