Design Study: Spool-Mounted Constant Force Springs Used for Dampened Linear Motion

Extended sliding tray assembly vs. retracted sliding tray assembly


Design Principles 

There are several critical principles that influence the design of a linear motion application when powered by a spool-mounted, dampened constant force spring. These principles support the robust and consistent performance of products in a variety of applications such as automotive and aircraft interiors, industrial machine componentsand many other user-interface applications.  

Spool-mounted constant force springs used for dampened linear motion assembly example

The sample assembly shown above will be referenced as an example of various design principles detailed below. This example assembly features a sliding drawer (shown in red) powered by a spool-mounted Conforce® spring (spool shown in green; spring shown in yellow), operating within a mounting base (shown in gray), such as the center console of an automobile. A ratcheting or latch mechanism (not pictured) can be implemented to allow the tray to lock in place and release with a single press from the user. This example is typical of a group of applications used in the automotive industry.  

1. Pre-loading the Spring 

When a constant force spring is resting in its free state, it exerts no force. As the spring is extended, the spring force quickly increases from zero to the nominal load. However, to avoid including this ramp-up portion in the working range, a design may make use of a pre-load. An example of a pre-load can be seen in the image below. Note how the depicted spring is already extended despite the mechanism being in its retracted state. 

To include a pre-load in a design, ensure that the spring is extended during assembly by a distance equal to the outside diameter of the coil in order to engage the mounting feature. Thus, at the start of the working range, the spring will already have been extended far enough to reach the rated load. Please note that the pickup does not contribute towards the pre-loading length. For more information on the pickup feature, please see Spring End Details below. 

Spring extended while mechanism is in a retracted state

2. Spool-Mounting Conforce® Spring 

The ideal fitment for a Conforce® spring on a spool is tight enough to prevent the spring coil from slipping or spinning on the spool diameter. To ensure a good fit, Vulcan Spring typically recommends over-sizing the spool diameter by 12%. For example, if the nominal spring inside diameter is 1.00, a spool diameter of 1.12 is recommended. This raises the issue of assembly concerns – how do you fit a 1.12 spool into a 1.00 hole? Fortunately, Vulcan Spring offers assembly services. Our in-house machine shop and experienced technicians enable us to provide comprehensive assembly services. The images below represent a Conforce® spring mounted tightly on a drum. 

Conforce® spring mounted tightly on a drum

Spool design will also have an impact on the ease of assembly. The simplest design, for assembly purposes, is a plain cylindrical spool with a hole through the axis for an axle. The exclusion of flanges, while sometimes infeasible, allows quicker, more simple assembly. Though the spool oversize described above creates a snug fit for spring to spool, flanges are sometimes desired for the secure alignment of components. In these cases, including one snap-on flange is recommended. Thus, the spring would be assembled to the spool featuring a single (fixed) flange, and then a second flange with snap-on features would be assembled to the spool. 

Finally, when addressing the physical envelope of parts within the system, ensure enough room is left for the natural motion of the spring. As the spring is extended off the spool, it will raise off the coil as shown in the image below. Restricting this motion by not allowing enough space may cause noise and excessive friction within the system. In the formula for offset, shown in the image below (0.8 X I.D. MIN), ensure to substitute spool diameter for I.D. (inside diameter) if a spring will be spool mounted. In cases where thickness and length combine to form a large O.D. (outside diameter), the O.D. can be substituted for I.D. to ensure sufficient space.  

Spring raising off coil as it’s extended off the spool

3. Spring End Details 

Vulcan Spring designs, builds, and maintains the dies used to create the end details of our constant force springs. We offer both standard and custom options to suit the full variety of our customers needs. The end details in the images below show two of the many options we offer.  

Standard and custom spring end details

pickup is the straightened portion sometimes included on the outer end of a Conforce® spring. Its basic purpose is as an assembly aid, allowing secure grip on the spring for pre-loading and fixing the spring to its mounting feature in the assembly. As depicted in the images above, the pickup typically has a gentle curvature, allowing a smooth transition to the main coil. This natural pickup is the recommended shape, as opposed to a truly linear straightened section. The standard tolerance on pickup length is ±.100. Closer tolerances may be feasible, depending on the design. As experts in Conforce® spring design, Vulcan Technical Sales staff are available to consult on the best pickup specifications for your application. 4. Effects and Cautions for Adding a Viscous Fluid Damper 

Adding a motion-dampening component to a Conforce® springpowered assembly is a valuable, and sometimes necessary, part of a design. A common solution, viscous fluid dampers, use the viscosity of contained fluid to delay the motion of a gear pinion. This pinion can interface with a rack gear (shown in the image below), slowing the motion of the whole assembly. Vulcan Spring also offers dampers that engage unidirectionally, allowing for easy reset of the mechanism.  

Benefits of this component are two-fold: First, the spring is protected from damage (and early fatigue) caused by free release. Second, and perhaps equally valuable, the user enjoys a smoother, more elegant, and quiet experience when operating the assembly. 

pinion interfacing with a rack gear

Design Variants 

Several design features can be applied to meet space constraints, refine assembly movement, and efficiently reduce part quantity for lowered material and labor costs. These can be applied in tandem with other principles to achieve an assembly design that meets requirements and exceeds expectations. Vulcan Spring regularly assists industry-leading companies in the development of refined, cutting-edge designs to ensure the success of their products.  

1. Embedding the Viscous Fluid Damper in the Spool 

By embedding viscous fluid damper inside the spool (where space allows), the overall mechanism can be simplified, leading to reduced mold complexity. In this arrangement, the damper would resist the free-spinning motion of the spool on the axle, delaying the motion of the assembly.  

2. Implementing Multiple Springs in the Same Assembly 

Including two or more constant force springs operating in parallel presents several benefits for various applications, including automotive consoles. First, the two springs can each have a smaller outside diameter while exerting the same net force as a single spring design. This allows critical space constraints to be honored without sacrificing force requirements.  

The second benefit of including multiple springs is to improve alignment and tracking of the assembly while in motion. When combined with a rackandpinion gear set and a damper, a dual spring arrangement allows for symmetrical force vectors in the design.  

3. Incorporating Alignment Features 

In addition to the above techniquerails or other engagement features can be added to ensure smooth operation and prevent racking or poor alignment. These should be designed to minimize noise and friction without adding an excessive number of parts.  

4. Cavity-Mounting of Springs 

Another design variant exists, which immediately reduces part count and assembly time. This technique is cavitymounting of the spring (shown below). This mounting may increase fiction and sound but leads to a fast-assembling, lowpartcount product. The cavity may be molded into whichever component the spool wouldve been mounted to, had the spool been included.  

Cavity-mounting of springs

At Vulcan Spring, we pride ourselves in advancing our customers products with technical excellence, responsive service, and extensive industry experience. If you’re working on a spring-powered design, we want to help! Our Technical Sales staff can be reached at (215) 721-1721 or We look forward to speaking with you soon! 

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