It is fundamental to the optimal operation and lifespan of a spring that it fits and is able to work freely within the space available. While this may sound obvious, a common cause of spring failure is when insufficient space is allowed for the spring.
Compression springs often fit over a rod, within a bore, or sometimes both. It is imperative that the spring does not rub on either of these as this will prevent the spring from operating properly and will cause increased stress and eventual breakages.
The inside diameter (ID) of a compression spring, including any tolerances, should always be greater than the maximum rod size (again allowing for any tolerances). Care needs to be taken to ensure that the end coils, especially if ground, do not affect this. The video below details what is meant by ‘tolerances’ and how they affect the functionality of a spring.
When considering what outside diameter (OD) should be used in a specified bore, the tolerances for this dimension again need to be considered. In addition to this, the fact that a compression spring expands in diameter when compressed needs to be factored into the design of the spring. This second video (see below) demonstrates the importance of allowing for expansion and tolerance in relation to the OD of a compression spring.
Because tension springs are usually closed coiled, it is unusual (although not unheard of) for there to be an object inside the spring. Also, as the spring extends, the OD reduces, so if the spring – including tolerance for the OD – fits in the bore available it will work within it.
Extra consideration needs to be given to the type, form and orientation of the ends of a tension spring to ensure the spring fits and operates correctly (see diagram below). A simple example would be that if an end needs to fit through a hole in a mating component, a sufficiently large gap must be specified to ensure this can be achieved. Similarly, if the fittings for a spring are at 90 degrees, this should be specified in the design for the spring.
As a tension spring extends, the diameter tries to reduce and, if the ends are fixed, this puts a torsional stress on the ends of the spring – which are its weakest points. This potential problem can be overcome in a number of ways to allow the ends to rotate: one option would be to have separate, coned-in ends – other available solutions include joining two spring sections by a central rod, or attaching centre hooks to allow for some rotation when load is applied.
The leg position and form for torsion springs need to be designed to suit the application in which the spring is operating. There are three main types of ends on torsion springs: axial, tangential and radial (or a combination).
The form of the end can vary from simple, straight legs to very complex shapes – depending on the intended application.
A torsion spring should always work by closing, so the direction of coiling is very important to get correct; should it operate the ‘wrong way’, the spring is likely to fatigue very quickly and ultimately fail. Discover more about spring coiling direction in our Springtelligence video below.
As a torsion spring deflects, the ID reduces and the body length of the spring increases. It is imperative that these factors are taken into consideration when designing the spring, as if the spring binds on the shaft over which is it operating, or on the inside of the lateral space available, the operating stresses will be transferred to the spring’s legs and they may break.
As with the expansion of the OD of a compression spring, it is possible to accurately calculate how much the ID of a torsion spring will reduce and the body length will increase for any given deflection.
The working requirements of any spring design need to be considered, along with the space available for it to work in. The laws of physics can unfortunately be fairly unforgiving if too little space is left for the spring to operate within! It is, therefore, always a good idea to take spring design into consideration at an early stage in any new project.
As we touched upon in our previous article, different materials and grades can allow for greater stresses to be accommodated in a design, but the working stresses for any type of spring always need to be evaluated in order to establish the optimal design and fit.
In summary, each type of spring is affected differently during the respective uses. However, it is important for any form of spring to ensure that there is sufficient space for the spring to work effectively within and to consider how the intended application will affect the spring’s function and performance.
To find out more about springs and their functions, visit our other website pages and blogs, or to request a quote please get in touch with us through filling out the contact form at the bottom of the page.