Understanding the Process of Deflection Using a Tin Foil
Consider a tin foil that you’d cover a roast with. When you first use that tin foil, it is supple and conforms to the container you are trying to wrap. However, as you unwrap the foil to slice off a piece of the meat and then try to re-wrap the container, the foil is a little less supple than before. The portion of the foil that hardens is the portion of the tin foil that is being used or “deflected” the most. Eventually after repeated use, the tin foil will actually become harder and harder until it rips or breaks.
Steel is very similar to tin foil, especially if it is designed in such a way that the deflection taking place is in a highly stressed portion of the clip. As the clip is deflected more and more, the steel becomes harder and harder until it breaks. Clips are most stressed in areas close to the bend. It is therefore preferable that the portion of the spring or clip that is deflecting is not near any bend that was introduced during the forming of the clip. If this cannot be accomplished, it is preferable that the bend be radiused rather than remain a sharp bend. The sharper the bend, the more significant the stress. Hence, if a bend is quite sharp, rather than round, the spring becomes more susceptible to fail.
There is really only one way to mitigate the higher stresses near a bend and that is to form the spring or clip while the steel is soft or “annealed” and then harden and temper the steel after the forming is complete.
Work Hardening of Steel
Spring temper is a steel temper characterized by an increased upper limit of elasticity that is obtained by hardening and tempering in the usual manner, and then reheating until the steel turns blue. However, the process of hardening oil tempered wire or flat stock to become spring temper takes several steps.
First, the material is heated up to over 1,000 degrees Fahrenheit, and then it is usually dropped in oil to cool be cooled. At this point the material is so hard that it is frequently referred to as “glass hard”. The reason that term is used is because the material is so hard that if you were to drop it on a hard surface it would react much like glass and shatter into a thousand pieces. In order to prevent that from happening, the material is usually heated a second time in a “tempering oven” which is kept at about 700 degrees Fahrenheit.
This gives the material “memory” and afterwards it can be formed into a spring or a spring clip. Once formed, we can deflect the spring or the clip in a desired way and the memory will bring the spring or clip back to its original state. This is why springs are often referred to as stored energy. Unfortunately, this process is not permanent. When we deflect the spring or clip it puts stress on the wire. If the stress is greater than the material can take, the material may actually deform and not come back all the way. In addition to that, cycling the spring or clip repeatedly—depending on how high the stress is—will result in the phenomenon of work hardening of steel. This means that the more we use a clip, or any other steel device, the harder the steel becomes until it ultimately breaks.
At Hardware Products, our aim is to make your spring ordering process as easy as possible. Contact one of our spring geeks today to help you find answers to your spring-related questions. To read more about similar spring terminology, check out our comprehensive guide to spring terminology here.