A common practice for mounting Constant Force Springs is to utilize two or more springs to gain more force in a small space. This might be required if the diameter or width of a single spring exceeds the allotted space but there is room to add multiple smaller springs. A much smaller spring design may be used when engaging multiple springs. The total force is the sum of the individual spring forces. When combining springs it is recommended that the same spring is utilized for multiple mounting. It is more economical and the design will be much more simplified using this method as compared to using two or more totally different springs to achieve a load. Since the force is the sum of the springs, the total required force should be divided equally between the number of springs used. We will discuss three different mounting methods used with multiple springs.
Back to Back Method
The back to back method utilizes two springs that unwind in opposite directions and create a very stable extension. The springs should be spaced far enough away from each other to allow for the natural bowing of the spring material so the parts do not bind when retracting. As shown in the photo below, Vulcan can even spot-weld the spring ends together to create an assembly for easy installation into a product. This method allows for one mounting point and is preferred with longer extensions.
Mounting the springs in tandem is also a very common practice when utilizing two or more springs. While this method does not have the stability of the Back to Back method, space availability will usually determine that this design be used.
The springs in the photo below were manufactured with different “pickup” or flat lengths so that they can fit together in tandem mounting. More than two springs can be put together in this fashion in order to increase the force and keeping the design as small as possible.
In the picture below, the actual mounting of the spring would separate the two coils. These coils should not rub in the design since it would cause binding during retraction and could shorten cycle life.
Laminating the springs together is the third option in this series. We use the term laminating to mean that the springs are interwound together, usually welded together on the outer end, the inner end or both. This method is used for relatively short extensions as the springs will bind if extension is too long. This is used when there are many springs required to meet the force requirement. The outside diameter will grow as more springs are added but the overall size is much smaller than other options, including a single spring of the same force.
The above photo shows a Laminar Spring using 9 springs interwound together to create 9 times the force of a single spring. While this is not common, it does show the possibilities of using this method.
When available space in a design is limited, it is best to consider these mounting methods. Please contact a Vulcan Spring Expert at 215-721-1721 or at www.vulcanspring.com to discuss possible design alternatives.