Successful machine and product design require making the right choices about what components to install in your application. Many varied spring designs are manufactured to fulfil specific roles, but similar designs can be used uniquely by carefully choosing their material, finish and spring rate. For example, progressive and linear spring rates are used very differently.

What are Linear Springs?

Linear springs are the ones that people are the most familiar with, as they have a uniform diameter and even spacing between their coils. The term linear relates to the spring rate. Linear rate (or constant rate) springs are based on Hooke’s Law which states that these designs have a linear (or constant) relationship between load and deflection.

What is Hooke’s Law?

Hooke’s Law is a specific law of physics that is essential to spring design. Below is the equation for linear rate springs.

The force (Fs) required to extend or compress a spring (x) will scale linearly concerning the distance in question. (K) is the constant factor of the characteristics of the spring, for example, its stiffness.

Tension spring manufacturers know that when a tension spring is installed into a fixed object, and the other end is pulled by an external force (Fs), then the (x) dictates the amount that the free end of the spring is displaced from its relaxed state. The spring constant is stated with (K) and provides a baseline for the displacements created by (x).

Above is the equation related to energy storage in springs. The energy storage of the spring will be vital data for many applications; fortunately, Hooke’s Law helps us determine this. When a spring is compressed, the external force comes from the same direction as the displacement, making the energy stored either negative (stretched) or positive (compressed), depending on how it is manipulated.

Hooke’s Law provides vital data for designers working with spring manufacturers to predict the relation between strain and stress for complex objects based on their materials.

The essential consideration for linear rate spring designs is that they provide a constant rate that is easily predictable and reliable for machine design. These designs will have a single defined spring rate per inch of deflection throughout their range. For example, a 300lb/in (pound/inch) linear rate spring 12 inches long will take approximately 300lb to defect in 1 inch.

What Are Progressive Springs?

Progressive rate (or variable rate) springs have uneven spaces between their coils; they lack the uniformity that linear rate springs are known for. As a result, progressive-rate springs will have a different deflection rate along their length; the rate increases as the spring is compressed further.

A compression spring manufacturer can provide a spring with superior adaptability. Still, it does come with the disadvantage that you will need a longer spring to have the same deflection as a linear rate spring.

Progressive rates are popular for vehicle suspensions as they will stiffen quickly. For example, the adaptable nature of the rate lets these suspension springs readily absorb the varying amounts of force from an uneven road. The varied applications progressive springs are needed for have created two subcategories of progressive rates.

Constantly Increasing Spring Rates

A constantly increasing rate is used chiefly in load-compensating applications. These have been chosen for the rear of vehicles to compensate for a load area, such as the back of a pickup truck. For example, stock replacement suspension springs are designed at a constantly increasing rate. These springs would not be used as main suspension springs but supplemental springs.

Dual Spring Rates

Dual-rate springs have two linear rates connected with a rate transition range. This spring design is far more specific to road vehicles and would be rare in other applications. Coils will identify dual-rate springs wound closely together but increasing in separation along the length of the spring.

Because of their dual nature, their rates will be labelled differently. For example, a dual-rate spring may be labelled 200/425lb/in. To elaborate, this would state that the spring has a starting rate of 200lb/in through some deflection range. Then, the rate would transition to 425lb/in through a deflection range of 1’’-1.5’’. Increased roll control is provided with these springs compared to constantly increasing or linear rates, and they are popular for high-speed, off-road racing.

Spring Rate Designs

Despite the different results of spring rates, there is only one visual characteristic that separates linear and progressive rates, coil separation. Therefore, the design of both spring rates can be applied to almost all spring designs effectively. Unevenly spaced coils signify a progressive spring, and evenly spaced coils will be part of a linear spring.

Spring rates are an essential characteristic that can be applied to any spring design and size. Spring manufacturers will use various methods, materials and treatments to imbue your chosen spring designs with the required spring rate.

Choosing which spring rate is best for your application is vital to getting the most out of your components. Airedale Springs is a custom spring manufacturer with decades of experience supplying high-quality springs to various companies in various industries.