Compression Spring: Stress and Spring Set
Compression Spring Stress
The dimensions, along with the load and deflection requirements, determine the stresses in the spring. When a compression spring is loaded, the coiled wire is stressed in torsion. The stress is greatest at the surface of the wire; as the spring is deflected, the load varies, producing a range of operating stress. Stress and stress range govern the life of the spring. The higher the stress range, the lower the maximum stress must be to attain comparable life. Relatively high stresses may be used when the stress range is low or if the spring is subjected solely to static loads. The stress at solid height must be high enough to permit presetting, yet low enough to avoid permanent damage since springs are often compressed solid during installation.
In designing compression springs the space allotted governs the dimensional limits of a spring with regard to allowable solid height and outside and inside diameters. These dimensional limits, together with the load and deflection requirements, determine the stress level. It is extremely important to consider carefully the space allotted to insure that the spring will function properly to begin with, thereby avoiding costly design changes.
Compression Spring Set
When a custom spring is supplied longer than specified to compensate for length loss when fully compressed in assembly by customer, this is referred to as “Allow for Set”. This is usually recommended for large quantity orders to reduce cost. When a compression spring is compressed and released, it is supposed to return to its original height and, on further compressions, the load at any given point should remain constant at least within the load limits specified. When a spring is made and then compressed the first time, if the stress in the wire is high enough at the point the spring is compressed to, the spring will not return to its original height (i.e., it will get shorter). This is referred to as "taking a set", or "setting". Once the spring is compressed the first time and takes this set, the spring will generally not take any significant additional set on subsequent compressions.
One way to deal with this problem is to make the spring initially a little bit too long and then compress the spring all the way to solid so that after the spring takes the initial set, it is now at the correct height to meet the load requirement. This is referred to as "presetting", "removing the set" or sometimes "scragging". Presetting is a labor intensive and relatively costly operation due to the amount of handling of the springs involved. In most cases, the customer will also handle each spring as the springs are assembled into the product. As part of this handling, the customer could press each spring and "remove the set" so that the spring will be stable and perform satisfactorily. Another alternative would be to assemble the spring as is and allow the first operation of the mechanism into which the spring has been assembled to "remove the set". In either case, in order for the spring to be correct after removing the set, the spring would have to be received by the customer in a condition longer than the final height.
Compression Spring Weight
For cost and manufacturing purposes, it is useful to calculate the weight of springs .For manufacturing purposes, it is easier to work with a unit quantity of 1000 springs, so the weight per 1000 springs is used instead. For round wire compression springs, the following formula can be used: