Static Load vs Dynamic Load: Implications for Springs
It’s important to understand the difference between static and dynamic loads in spring design and manufacturing across every industry where mechanical springs are employed. Extension, compression, and torsion springs are all designed to absorb mechanical loads in either tension, compression, or torsion directions, respectively.
However, the magnitude of loading over time is a critical factor that is sometimes overlooked at the design stage. Springs encounter two kinds of time-dependent application loads: dynamic and static loads. It is crucial to understand the differences and how they impact the performance of your spring in application.
Static Load vs Dynamic Load: Unraveling the Differences
In simplest terms, the distinction between static vs dynamic load is as follows.
A static load is generally a fixed, unchanging load or force. It is applied very gradually and does not change in magnitude. A good way to visualize this load is to think of an automobile suspension when parked and not moving. The compression springs in the automobile’s suspension are compressed to absorb the car's weight, which is static and unchanging while the vehicle is parked.
Conversely, adynamic load is a load that changes in magnitude over time. Sometimes, a dynamic load changes many times per second depending on the intended application. Using an automobile suspension as an example, when the car begins to drive, the dynamic load on the spring is the portion of the load that the spring sees when traversing uneven surfaces. The suspension must absorb the displacement of the tire or link arm. From the spring design and selection perspective, the dynamic load the suspension springs absorb during driving differs significantly from the static loads on the springs while the car is parked.
Effects on Spring Design and Performance
Impact of Static Load on Spring Design
For static applications, the material's yield strength or stress relaxation resistance limits a spring's load-carrying ability. In static applications, the spring is required to operate for a limited number of cycles with a low end-coil velocity, which precludes high stresses in the spring due to surging or impact conditions.
For springs that do not contain beneficial residual stresses induced by set removal, maximum allowable torsional stress values are from 35 to 50% of the tensile strength. You should use a stress correction factor to calculate spring stress before set removal. If the calculated stress at solid is greater than the indicated percentage of tensile strength, the spring will take a permanent set when deflected to solid. The amount of set is a function of the calculated stress at the spring’s solid height that exceeds the indicated percent of tensile strength.
To increase the load-carrying ability of springs in static applications, it is common practice to make the spring longer than its required free length. A secondary operation compresses the spring to its solid height. This operation causes the spring to “set” to the desired final length and induces favorable residual stresses.
This process is called removing set, presetting, or set removal and can be conducted at room or elevated temperatures. The loss of deflection from the free position to solid height by cold set removal should be at least 10%. If the set is less than this, controlling the spring’s free length is difficult. Ratios of stress greater than 1.3 lead to distortion and do not appreciably increase the load-carrying capability of the spring For torsion springs, the allowable torsion stresses with set removed are significantly higher than those that have not been preset.
It is important to note that because yielding has occurred during this operation, the stress is relatively uniform around the wire cross-section. As stated previously, it can be calculated using the stress correction factor.
In closing, set removal is a relatively costly secondary process that significantly increases the spring's energy storage capacity.
| Materials | Maximum % of tensile strength before set removal | Maximum % of tensile strength after set removal |
|---|---|---|
| Cold Drawn Carbon Steel | 45 | 60-70 |
| Hardened and tempered Carbon and low alloy steel | 50 | 65-75 |
| Austenitic Stainless Steel | 35 | 55-65 |
| Nonferrous Alloys | 35 | 55-65 |
Dynamics of Dynamic Load on Spring Performance
Fatigue Resistance (Fatigue Strength) is a material property that determines how well a material endures cyclic loading conditions.
Fatigue Life is the number of dynamic loading cycles a material can sustain before fracturing. The velocity of the end coils is low compared with the natural frequency. To select the optimum stress level, it is necessary to balance spring cost vs. reliability. Reducing operational stresses increases spring reliability as well as cost.
A full grasp of the spring’s operational environment, frequency of operation, speed of operation, and permissible levels of stress relaxation is required to make the best choice between cost and reliability.
Additionally, because the maximum stress occurs at the spring wire's surface, any surface defects, such as pits or seams, severely reduce fatigue life. Shot Peening improves fatigue life and minimizes the harmful effects of surface defects, but it does not remove them.
The maximum allowable spring design stresses for fatigue applications should be calculated using the correction factors shown for typical spring materials in the table below:
% of tensile strength
| Fatigue Life (Cycles) | ASTM A228, Austenitic Stainless Steel and Nonferrous (Not Shot-Peened) | ASTM A228, Austenitic Stainless Steel and Nonferrous (Shot-Peened) | ASTM A230 and A232 (Not Shot-Peened) | ASTM A230 and A232 (Shot-Peened) |
|---|---|---|---|---|
| 100,000 | 36 | 42 | 42 | 49 |
| 1 Million | 33 | 39 | 40 | 47 |
| 10 Million | 30 | 36 | 38 | 46 |
Notes:
These values are guidelines and should only be used if explicit material specification data is unavailable.
These values are for a stress ratio (R) of zero at ambient temperatures and pressures with no surging.
“Hot” shot-peening increases the fatigue strength by as much as 20% for design lives of 10 million cycles.
In conclusion, most springs designed to recommended stress levels will exceed the indicated lives. However, detailed information on material, manufacturing method, and operating conditions is required to fully quantify a spring’s reliability in application.
Strategies for Optimal Spring Selection in Dynamic Load Environments
Admittedly, spring selection becomes more complex in dynamic loading environments. However, once you understand your system’s dynamic load profile and frequency, you can use these parameters to shop for springs.
System Resonance
The spring's natural frequency is another important consideration in dynamic loading environments. First, you must evaluate your system to determine its vibrational frequency. Then, you must compare this value to the spring's natural frequency. You may have resonance in the system if the two values are similar. Suppose a spring’s natural frequency is near the system's vibrational frequency. In such cases, your application may be vulnerable to premature failure due to amplified displacement and loading due to vibrational resonance. Add damping or isolation features to your application to mitigate this concern.
Spring Geometry
Pay careful attention to stress concentrations and select a spring that minimizes these as much as possible. You should also carefully evaluate the end type needed for your spring and choose one that does not feature sharp corners or bends if they can be avoided.
Century Spring: Your Partner in Precision Spring Solutions
Century Spring is a quality-first manufacturer with years of experience and innovation in manufacturing stock and custom springs for the most challenging static and dynamic load environments.
We are an ISO 9001-certified spring manufacturer, proudly producing the highest-quality springs that deliver unrivaled system performance even in the most demanding applications and environments. Our state-of-the-art manufacturing capabilities have positioned us to offer unmatched service to industries that need large volumes of customized spring designs in accelerated development programs.
We proudly boast an in-house team of spring design experts ready to partner with you today to help you choose the best spring for your application under static and dynamic load profiles. We are committed to minimizing total development time and passing the time savings to you as reduced procurement lead times.
We offer rapid turnaround, shipping, and delivery on over 40,000+ in-stock designs available to ship today, and all of our springs are always made in the USA.