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Top tips for compression spring specification

We asked the experts at Lee Spring for their top tips and some useful guidance for anyone designing a compression spring into their application.

Springs are mechanical objects that deform when acted upon by an external force, and return to their original shape when the external force is removed. Compression springs are used primarily for 'push' applications. Hooke's Law, the principle of spring design that relates to load and deflection, states that a restoring force due to a spring is proportional to the distance the spring is deflected, and acts in the opposite direction. 

Another important principle relating th spring design is spring rate - the ratio of force per unit of deflection. Spring rate is the change of force or load per unit deflection as a spring is compressed. As an example, spring rate may be specified as the amount of force to move a spring 25.4mm or 1in. A standard helical compression spring has a spring rate that is essentially linear over most of its operating range. The first and last few percent of a spring's deflection has a non-linear rate. When a spring reaches solid height, the spring coils are at a stop against each other. When assessing spring rates, generally the spring rate between 20% and 80% of available deflection is taken into consideration.

A primary design element is the direction of wind. A coil spring can be wound in either left hand or right hand direction, similar to a screw thread. A left hand wound spring will spiral in the same direction as a left hand threaded screw. A right hand wound spring will spiral in the same direction as a right hand threaded screw. This direction of spring wind can be important depending on how the spring is used. To determine the best wind direction, consider the application. For example, coil wind is important when you have one spring working inside another. To keep the springs from binding against each other, design so the inner spring and outer spring are installed in opposite directions. If a spring screws onto a threaded component, match the direction of wind to the direction of screw threads.

Another characteristic is spring squareness. This is defined as the angular difference between the outermost limit of a spring diameter when compared with a straight edge at a right angle to a horizontal flat plate on which the spring is standing. Consider also spring parallelism, which relates to the ends of a spring and how parallel they are to one another.

Spring diameter
Spring diameter is often referenced in different ways. One method is the outside diameter or OD. This dimension is important when the spring is used within a cavity. Another dimension is the inside diameter or ID, which is important for springs that work over a rod or shaft. Another important measurement is the mean diameter which is most often used for calculations for stress and deflection.

Free length is the overall spring length in a free or unloaded position. If definite loads are specified, the free length should be an approximate dimension, which may be varied to meet the load requirements.

Spring index is defined as the ratio of the mean diameter to the wire diameter. A high index spring would have a smaller wire diameter and a larger spring diameter. A low index spring would have a larger wire diameter and a lower spring diameter.
Load is the force required to compress a spring to a specific height, rather than the amount of force to move the spring a specific unit. Load differs from spring rate in that spring rate is the amount of force required to move the spring in increments.

Another important design element is solid height. This is the dimension of the spring when all the coils are closed. If critical to an application, this dimension should be specified as the maximum dimension. The number of coils, on the other hand, should be specified as a reference figure and stated whether it refers to active coils (coils which are free to deflect under load) or the total coils (which include all coils as well as those which are used to form the ends and may not deflect under load). Pitch is a dimension related to the distance between the centres of adjacent coils.

Prior to its first compression, a spring's free height may be longer than the specified height. This is common during the manufacture of springs and can be compensated for in two ways. A spring can be built with free length which has an allowance for set. This involves compensating for the length loss when a spring is fully compressed for the very first time. Another method is known as removing the set - also known as presetting or a set spring. This is an additional manufacturing step that may be done to ensure that the spring is at the correct free height for use. A set spring will have a new free length that is stable and consistent through future compression cycles.

Types of compression springs include closed end - where the end coil pitch is reduced so the end coils touch - or open end, where the coils are consistent with no pitch change through the end of the spring. Other ends for compression springs are 'not ground' and 'ground' types. The ground' end type spring is ground flat, where the 'not ground' type as a less parallel end.

Compression springs can come in any variety of shapes and custom designs may have any number of shapes depending on the application. Some common shapes include the cone shape, where the spring radius decreases. This common, for example, for a battery spring. An hour glass shape tapers tighter towards the centre and the outer coils have a larger diameter. The barrel shape is reduced at the ends and wider in the centre. The reduced ends spring is straight across the centre coils and tapers only towards the end coils.
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