Dealing with torsion springs--which are designed to withstand rotational stress--involves understanding their elastic limit. Past this limit, a spring can experience plastic deformation, meaning it won't return to its original state, similar to an overused garage door spring. This article delves into the details of plastic deformation in torsion springs, including its causes and timing. It also provides guidance for engineers on how to account for this deformation when choosing a torsion spring, promoting safer and appropriate designs.

Plastic Deformation vs Elastic Deformation

The reaction of torsion springs to various forces is a part of spring design. Elastic deformation is where the spring returns to its starting shape once the external force is gone. For example, in a vehicle's suspension system, the springs undergo elastic deformation. They absorb forces from road irregularities and return to their unchanged shape.

Plastic deformation is a lasting change in the spring's structure, persisting even after the stress is gone. This change, which impairs the function of the spring, can happen when the stress goes beyond the spring's tolerance level. For instance, if a vehicle's load surpasses its maximum limit, the excess stress could trigger plastic deformation of the suspension springs. The vehicle may not recover its normal height following a road shock.

The choice of material can have a notable effect on a spring's likelihood of experiencing plastic deformation. High-carbon steel, for instance, can endure more stress before undergoing plastic deformation when compared to common stainless steel. Thus, understanding the stress limits of the chosen material type can reduce the possibility of plastic deformation. This preventive action contributes to the function and lifespan of the spring.

Why Plastic Deformation Occurs

Plastic deformation in torsion springs happens when the spring endures stress exceeding its specified range. Such stress typically originates from an excessive load or high operating temperatures. To clarify, consider a common vehicle's suspension system. If a car carries a load heavier than its capacity or frequently operates in high temperatures, the overburdened torsion spring may undergo more stress than it was intended to bear. This unseen stress may change the spring's molecular structure, resulting in permanent alterations in its shape and function.

Nevertheless, not all increases in load or operating temperature result in plastic deformation. The relationship between the applied load, the environmental conditions, and the spring's design limits must be carefully examined. Plastic deformation generally happens when the application conditions surpass the spring's stress or temperature tolerances.

The best method to prevent plastic deformation is to ensure that the spring operates within its design parameters. For example, a spring designed for regular temperature conditions should not be used in an environment subject to high heat. Even if the load on the spring is within the allowable range, the excessive heat may exceed the spring's tolerance levels. This scenario shows the importance of understanding a spring's operational environment. Knowing which environmental factors and temperature conditions a spring will face can influence the decision on material selection and spring design, thus benefiting the spring's service life and performance.

Proportionality Limit vs Elastic Limit

The proportionality limit denotes the maximum stress level at which a torsion spring's performance remains regular and predictable. If this limit is breached, the performance of the spring will not match predictably with the applied load. For instance, in a mechanical watch escapement, the torsion spring must maintain stress within the proportionality limit to avoid irregularities in time-keeping.

On the other hand, the stress level from which a torsion spring can return to its initial state after the stress is dispatched is termed as an elastic limit. Stress levels exceeding this limit leads to permanent alterations to the spring, which is referred to as plastic deformation. For instance, in a garage door torsion spring, if the weight or force exceeds the elastic limit, the spring may fail to fully lift the door after it's been shut, displaying an occurrence of plastic deformation. The values of both limits are useful in predicting plastic deformation in torsion springs, given the stress level and purpose of the application.


Plastic deformation in a torsion spring's structure doesn't mean that the spring is no longer functional. Many materials can still work well after deformation, though they have changed properties. Understanding the effects of plastic deformation is essential for engineering effective torsion springs.

Grasping elasticity limits, proportionality, and plastic deformation triggers can contribute to the creation of lasting, able torsion springs. For instance, recognizing that the limits of proportionality are exceeded when the load surpasses the elastic limit of the spring material can lead to designing springs capable of managing higher stress levels.

This awareness can guide decisions in spring design and choice, contributing to improved design quality. This can result in longer-lasting springs, reducing the need for replacements and maintenance. Therefore, a clear understanding of plastic deformation in torsion springs is advantageous for both product longevity and cost savings.