Progressive vs. Linear Springs: What’s the Difference?
The difference between linear and progressive springs is not immediately evident to the untrained observer. It’s readily apparent that helical coil springs come in a wide range of sizes, wire diameters, and configurations. Still, the key distinction between the two is the spring rate, or rate of spring deflection, which determines crucial aspects of spring performance.
The spring rate is a function of observable features, such as mean diameter and the number of coils, and it measures the resistance of the spring to applied forces. For most spring applications, you can calculate the approximate spring rate your application needs by dividing the load your spring must absorb by the deformation your mechanism can accommodate. In such a simplified application where the design displacement and loads are uniform and quantifiable, a linear spring rate is the best choice.
However, the spring rate calculation can sometimes be more complex. Depending on your application, consider a progressive spring that can provide a manageable load response over a wide range of variable displacements.
This article will dive deeper into the performance characteristics of both constant-rate, aka linear springs, and progressive, aka variable, spring rates. When you understand the force-deflection relationship relative to the loading and displacement of each spring type, you will be better equipped to determine whether you need a linear or progressive spring.
What Is a Linear Spring? (Constant-Rate Spring)
In simplest terms, a constant-rate spring has a constant spring rate. The “linear” terminology simply means that the graph of force-deflection of the spring is a straight line, or a linear mathematical relationship with a constant slope denoted by the spring rate stiffness.
For example, a compression spring with a linear spring rate (k) of 10 lb/in requires 10 lb to compress the spring by 1 in. In such a spring, this value holds, meaning that it requires 10 lb of force to compress the spring the first 1.0 in. and 10 lb to compress the spring the last 1.0 in.
Now that you know the technical definition, it’s helpful to discuss the physical and geometric properties of a constant spring rate spring that creates a constant spring rate. For example, the spring’s outer diameter is uniform over the entire length of the spring, giving it a cylindrical shape. The spring pitch, as defined as the spacing between the helical spring coils, is constant across the length of the spring.
Constant spring rate springs are the most common type of springs, and they are excellent options for small, precision instruments that require predictable force and displacement control.
What Is a Progressive Spring? (Variable-Rate Spring)
In contrast to constant-rate springs, progressive springs do not have a constant spring rate over the entire deformation profile of the spring. Using our previous example of a spring with a notional linear spring rate (k) of 10 lbin, a comparable progressive spring might require 10 lb of force to compress the spring the first 1.0 inch, while requiring 20 lb to compress the spring the last inch.
Now that you know the technical distinction between constant vs. variable rate springs, it’s helpful to detail the physical and geometric properties of a variable-rate spring that allows for variable spring rates. In direct contrast to constant-rate springs, the outer diameter of a progressive spring may change over the free length of the spring. This means that progressive springs might have a conical, taper, or barrel-shaped profile. Additionally, the spring pitch is variable across the length of the spring. Progressive rate springs typically have greater spacing between coils towards the base of the spring, which serves to increase the spring rate as it is compressed. These are the primary features that mark the difference between linear springs vs progressive springs.
In contrast, variable-rate springs are a specialized type of helical coil compression spring designed for more specialized applications, like dynamic load applications.
Progressive vs. Linear Springs: Key Differences
When you are trying to decide between a progressive vs linear spring for your application, it’s helpful to keep in mind the following key differences.
| Linear Spring Rate Springs | Progressive Spring Rate Springs | |
|---|---|---|
| Common Name: | “Constant-Rate” Springs | “Variable-Rate” Springs |
| Resists Applied Loads at: | Constant Stiffness | Variable Stiffness |
| Coil Spacing: | Equal Spacing between Coils | Variable Spacing between Coils |
| Outer Diameter: | Uniform Diameter, straight, cylindrical coil shape | Commonly varies in Diameter, tapered, cone-shaped |
| Typical Applications: | Precision components where predictable loads and displacements occur | Performance components that experience dynamic loads and displacements |
It’s also worth exploring the exact definition of spring rate in relation to spring geometry and material properties, to visualize how the spring rate changes as a function of spring geometry and material.
Where:
G = Spring Material Modulus of Rigidity (spring material property)
d = Wire Diameter
D = Mean Diameter
Na=Number of Active Spring Coils
The important point here to illustrate is that progressive springs achieve larger (or smaller) spring rates by adjusting the Number of Active Spring Coils (Na) by modifying the spacing between the spring coils, leading to additional or fewer active spring coils. The spring rate can be tuned by modifying the mean diameter to wider coils at the base of the spring.
Since both the mean diameter and number of active coils are in the denominator of the equation, the relationship between them at the spring rate is an inverse relationship, meaning that an increase in either will lead to a decrease in the spring rate. In contrast, a decrease in either will increase the spring rate.
Choosing the Right Spring Type for Your Application
The following factors are where you should start when choosing the spring for your application:
- Displacement and Load Profile of Mechanism: If your application must be designed to accommodate dynamic loads and displacements of varying magnitudes, a progressive spring is likely the best choice because the progressive spring provides additional stiffness the more it is compressed.
- Available Design Space: Some types of progressive springs, like conical springs, offer a more compact compressed height compared to the standard helical coil shape of linear springs because the smaller spring coils nest into the larger ones during compression.
- Durability: Both progressive and linear springs offer similar durability if the spring size, material, and surface coatings are correctly specified for a particular load and use environment.
- Cost: In general, linear springs are a more cost-effective option because they are less expensive to manufacture. Verify that you need a progressive spring before selecting such a high-performance spring configuration.
In summary, constant-spring rate springs are used in applications that have a uniform design load or use case and require a predictable, uniform displacement profile under the design load or displacement conditions for which it is specified.
In contrast, a progressive rate spring is desirable to efficiently absorb loads that are not consistent across a single design profile. For example, a progressive spring is an excellent option for shock absorption elements in an automotive suspension system because the spring must be rated to accommodate a wide range of deformations from the suspension. Progressive springs are great options for vibration isolation equipment, providing damping at large displacement magnitudes.
Finally, it’s important to note that when you shop for progressive springs, the catalogue specification values shown for spring rates are denoted as an average value across the entire compression profile of the spring.
In conclusion, linear springs are the best choice when a single design load case is quantifiable. In contrast, a progressive spring is the obvious choice for applications that experience varying load or displacement magnitudes.
If you are unsure of the load or displacement requirements of your application, a brief consultation with Century Spring’s spring design experts will yield valuable insights and customized design recommendations.
Century Spring’s Expertise in Custom Spring Solutions
Century Spring is a quality-first manufacturer with decades of experience designing and manufacturing reliable linear and progressive springs for the most demanding applications and industries. As the most trusted name in spring and wire form products, our dedicated customer support teams can answer all your questions about progressive and linear spring performance, precision load deflection optimization, corrosion protection, and fatigue life.
We are ISO 9001-certified and produce high-quality springs that deliver unrivaled performance, engineered to resist common spring failure modes in any application or environment. Our state-of-the-art manufacturing capabilities have positioned us to offer unmatched service to industries that require large volumes of customized spring designs in accelerated development programs.
We offer rapid turnaround, shipping, and delivery on over 40,000+ stock springs available to ship today. In custom spring development programs, we are committed to minimizing the total turnaround time, passing on time savings to you through reduced procurement lead times.
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