Spring Mounting Methods for Constant Force Springs

What Is Spring Mounting?

Spring mounting is when a constant force spring is placed onto a spool or shaft to reduce friction, making repeatable operations much smoother and decreasing wear on the spring.

Mounting a Constant Force Spring on a Spool

One of the most common questions about Constant Force Springs concerns the mounting of the spring on a spool. For starters, the spring doesn’t have to be mounted on a spool; it can be placed in a cavity or on an undersized bushing or pin.

There are differences in the performance of each spool design so let’s look at the advantages and disadvantages:

Advantages of Spool Mounting

The main advantage of spool mounting is smooth, repeatable operation due to the reduction of friction. Material selection of the spool/shaft interface should be based on an effort to create a bearing surface that will minimize friction throughout the life of the spring.

When a spring is mounted in a cavity, the designer must manage friction and the resulting reduction in usable spring force. The advantage of a cavity mount is the ease of assembly. Likewise, when mounted on an undersized pin, there will be an audible “click” and drop of load as the inner end of the spring passes over the pin. The thicker the steel, the more pronounced this will be.

Again, the ease of assembly is the standout advantage of spool mounting.

Disadvantages of Spool Mounting

Since assembly is more difficult when placed on a spool, we need to discuss this operation next. The spool diameter should be 15%–20% greater than the spring’s nominal inside diameter due to the 10% tolerance for the inner diameter of a spring. The spring should fit tightly on the spool, so this tension must also be considered.

One misconception about mounting a spring on a spool is that the spring needs to be attached or fastened to the spool, but this is not the case. When fully extended, the spring must be long enough to have a minimum of 1.5 wraps on the spool, allowing the spring to stay tight and retract when released. Therefore, the spring cannot be used as a stop.

The spring’s inner diameter can be opened up by securing the inner end and pulling back on the outer end. The spool can then be slid into place. For longer and heavier springs it’s sometimes best to reverse the spring, making the outer end the inner end. Once the inner loop is exposed, it can be placed over the spool and re-wound onto the spool in the correct orientation. When performing this operation, the spring mustn’t be back bent (forcing the spring against its natural curvature). In some cases, the spring can be produced inside-out, so the initial reversing is unnecessary. When wound onto the spool, the inner end of the spring as received will be the outer end.

Common Practice for Mounting Constant Force Springs

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’s room to add multiple smaller springs. A much smaller spring design may be used when engaging numerous springs. The total force is the sum of the individual spring forces. When combining springs, it’s recommended that the same spring is utilized for multiple mounting, which is more economical. The design will be much more simplified using this method than 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.


Different Spring Mounting Methods

Back-to-Back Method

back to back method

The back-to-back method utilizes two springs that unwind in opposite directions and create a 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, 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.

Tandem Method

tandem methodMounting the springs in tandem is another 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 when this design be used.

The springs in the photo were manufactured with different “pickup” or flat lengths to fit together in tandem mounting. More than two springs can be combined in this fashion to increase the force and keep the design as small as possible.

In the picture, the actual mounting of the spring would separate the two coils. These coils should not rub and create friction in the design since it would cause binding during retraction and could shorten cycle life.

Laminar Method

laminar methodLaminating 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 the extension is too long. The laminar method is used when many springs are 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 photo shows a Laminar Spring using nine springs interwound together to create nine times the force of a single spring. While this is not common, it does show the capabilities of this method.

Finding Your Custom Mounted Spring

Here at Vulcan Spring, we are pleased to custom-mount springs in whatever way best suits our customers’ needs. No need to worry about mounting springs yourselves when Vulcan has you covered.

You’ll receive the spring mounted in the manner of your choosing and then assemble the spring where intended, saving considerable time and money.

For more information about custom springs and mounting solutions for your next design, please contact us or buy springs online.

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