Reliability is of great importance to many spring users. Without the spring many applications will either cease to work or work less efficiently. Therefore much study has been made of the behaviour of springs under fluctuating loads.
If a spring is operated less than 10,000 cycles during its operating life then it is deemed to be working statically and fatigue does not play a part. Over this, fatigue will affect the performance of the spring and should be taken into account during the design process.
The factors that affect the fatigue performance of springs in the main are:
- Working Stress
- Material Surface Quality
In operation springs generally work between two fixed positions. The working stress at these positions can simply be calculated, the results can then be used to predict the working life of the spring. To do this, Goodman diagrams need to be used, these are based on data that has been obtained through many years of experimentation at centres such as The Institute of Spring Technology. Goodman diagrams are available for the many different grades and types of material used.
An example of a Goodman diagram is shown below (Fig.1). To calculate the expected life of the spring the working stresses are plotted against the relevant axis. If the intersection of the plotted stresses falls within the shaded area, the spring can be expected to work for the number of cycles the graph represents. Generally the graphs represent 95% surety, i.e. 95% of the springs can be expected to achieve the number of cycles.
The majority of Goodman diagrams only apply to compression springs.
Fig.1: Example of a Goodman Diagram
The initial stress and final stress are plotted. From the intersection it can be seen that this spring will exceed 100,000 cycles
Extension springs suffer a number of problems when operating in a dynamic environment, they are:
- Breakage near the loop. The most common cause of failure in extension springs is when the loop of the spring breaks off in the area where the hook meets the body of the spring. This point of transition between the spring body and the loop is generally the point of highest stress. The loops are subjected to a bending stress and torsion stress and the majority of Goodman diagrams for spring materials are for materials stressed in torsion.
- Tooling marks creating stress. When loops are formed in extension springs small tooling marks are unavoidably created. Such marks are stress raisers which increase the likelihood of a failure at this point.
- Loop bends too small. Another reason is that sometimes loops are formed using bends that are too small. A small radius is a stress raiser.
- Different types of end loop will lead to different fatigue performances.
- There are two available solutions:
- Use a loop with a transition radius between the spring body and the end loop of approximately the body radius.
- Use coned ends with swivel loops. This is very successful in reducing fatigue providing the swivel loops only contact the coned section (see Fig.2).
Fig.2: Example of a Swivel Loop Extension Spring
If an extension spring is required to work dynamically, it must be remembered that extension springs have approximately 20% lower performance with regard to fatigue than compression springs.
The lower the working stresses the greater the expected life of the spring.
Material Surface Quality
Material surface quality is important when seeking to avoid risk of spring failure. Fatigue cracks generally propagate from the surface of the material, therefore the better the surface quality, the greater the fatigue performance. It is possible to improve the surface quality by a number of methods. Most popular is Shot-peening.
Shot-peening involves firing small rounded beads of material at the surface of the spring. This will lead to a small residual compressive stress on the material surface which lowers the chance of a fatigue crack propagating and increases the working stresses possible. Shot-peening is generally carried out only on compression springs and large leaf springs as the shot would get trapped in the coils of close wound torsion and extension springs. Also, the inside face of the coils would not be peened and this would eliminate the benefits of the process.
The better the surface quality of the material, the better the fatigue performance.
Wear can be caused in a number of ways. When a spring is operating dynamically it is important that the maximum deflection should not exceed 85% of the available deflection. The reason for this is that when a spring is working close to its solid (coil bound) length, the number of active coils will reduce due to coils coming into contact with each other. When this happens there is a chance that the contacting faces of the coils will wear. This can lead to a reduction in material cross section, increasing the stress at this position.
Another cause of wear is when a spring works over a shaft or in a bore. If a spring is allowed to contact either the shaft or the wall of the bore when operating, the wear can lead to premature failure especially if the inside diameter of the spring is worn as this is where the working stresses are the greatest. Wear, therefore, should be avoided at all costs.
Torsion springs have a lower fatigue performance than compression and extension springs.
This is mainly due to the friction and wear between the spring and the shaft that it is working over and the leg fixings. This can be reduced by good design, but is unlikely to be eliminated.
Other factors that affect the fatigue performance include corrosion, material cleanliness and speed of operation. Any questions regarding these or any of the above should be directed to the author.
Removing the possibility of wear in a spring application will improve the spring’s fatigue performance.
What is Prestressing and what Benefits can it provide?
Prestressing is used to improve a Spring’s ability to withstand stress, therefore increasing its load-carrying capability and improving its fatigue life. For instance, compression springs manufactured from BS5216 cold drawn carbon steel without prestressing can be stressed to 49% of the material’s ultimate tensile strength. If the spring is prestressed, this can then increase to 70% of its ultimate tensile strength.
When a spring is prestressed there are dimensional changes. This means that the springmaker must allow for this during manufacture.
The prestressing operation for compression springs is relatively simple. Once the spring has been coiled, stress relieved and ground, the spring is placed on a press or similar and compressed to a solid or fixed position which is greater than its maximum working position. This is then repeated a number of times, generally no less than three. The spring will then be shorter than the coiled spring but with the correct initial set up, it will be possible to achieve the required final length.
Prestressing can also be carried out for tension and torsion springs. Unfortunately, when an extension spring is prestressed the amount of initial tension is reduced and is therefore not often carried out.
Torsion springs require special jigs to successfully prestress them. When prestressing is carried out the leg relationship changes. ie the number of coils slightly increases.
As prestressing is an additional operation in the manufacture of a spring, this will increase its unit cost. The benefits, however, generally outweigh the additional cost.
Article written by David Banks-Fear and published on MDF by kind permission of Southern Springs & Pressings Limited.
David Banks-Fear is a Mechanical Design Forum Group member. He is a technical author and consultant design engineer with nearly 40 years of experience. He and his design team are available to assist with any technical design issues with springs, pressings and precision engineered parts. Email: [email protected]