5 Problems to Avoid When Designing a Power Spring

The 5 Biggest Problems in Power and Conpower® Spring Design and How to Avoid Them.

Our customers are creative and often come up with unique problems they want to solve using springs.  We enjoy working on these situations as it fully utilizes our team’s experience and skill to push our limits and keep us charging forward.   However, even with all of our experience and design capability, we must live with some fixed rules of physics and mechanical design regarding our springs.  We will discuss here several traps that our customers sometimes fall into, specifically in the power spring segment.

Before we get to the list, let’s review the basics.  Traditional power springs and Conpower® springs are manufactured with strips of spring steel to provide torque.  Spring steel is a low-alloy, medium-carbon steel or high-carbon steel with a very high yield strength. Objects made of spring steel can return to their original shape despite significant bending or twisting.

A “power” spring is wound with flat (non-stressed) steel, while a “Conpower®” spring uses pre-stressed steel to generate a larger usable torque range than a traditional power spring. Vulcan primarily manufactures the Conpower® version due to its advantages overpower springs.  Conpower® springs ramp up faster to usable torque, are smaller, lighter, have longer range (turns), don’t require as much lubrication, and are significantly smoother in operation.

During manufacturing, the spring is wound into either a retaining ring or customer’s housing (retaining ring shown to the right). When ready for final installation, the spring is carefully inserted into its case with the outside end secured to the outer edge and the inside end attached to an arbor (see illustration below).   During operation, either the case or the arbor will be fixed while the other component is left free to rotate.  Depending on the requirements of your specific design, it may be beneficial to use a fixed arbor/free case or free case/fixed arbor arrangement.

These springs are often used to retract a cord or cable around a spool.  You might find one in your lawnmower, a retractable lanyard or dog leash.  This is also the type of spring used to drive a number of mechanical clocks and timers, which is why they are sometimes referred to as “clock springs.”

The load that the springs provide grows with the cross-sectional area of the material.  The number of turns the spring can provide is determined from the space available between the arbor and the case ID along with the total length of the spring.  All  Conpower® springs require pre-winding from their free state in order to reach their effective working range.  More general background information can be found on our Conpower® and power spring page or our previous blog.

Now that we have the basics covered, let’s get to the list.  Here are the 5 most commonly seen problems in Conpower® spring designs:

1. “Suffocating” the spring.

A common problem we see coming from customers’ requests are designs with undersized cases and oversized arbors.  A small case and a large arbor doesn’t lead to a healthy Conpower® spring design.  This can cause a variety of problems including binding and jerky motion.  Your spring needs space to do its job!

It’s important to understand that every time your arbor and case rotate relative to each other, spring material will be moving.  As the spring winds, the outer coils migrate towards the middle, engaging more of the spring to provide torque in the opposite direction.  As the spring unwinds, the outer coils make their way back to their natural state.  The stroke of the spring material from outside to inside is crucial to providing torque.  If this space is limited, performance will suffer.

As a starting point, it may be useful to take a look at some of the Conpower® spring designs Vulcan keeps in its express order store to get a feel for what sort of outputs can be expected and what sort of case and arbor sizes are effective.  Additionally, a Vulcan Spring applications engineer can help with this specification.

2. Underestimating the weight.

Steel is heavy, and these springs use a lot of it!  If you plan on using your spring to push, pull, or otherwise propel an assembly that contains your spring, you will need to account for the weight of that spring.  We have seen concepts for spring-powered vehicles: A power spring may be an effective way to store energy for a toy car, but we have yet to see a successful bicycle, car, or yacht spring motor make it to production.

As you increase the torque and number of turns required from a Conpower® spring, you will inevitably be increasing the size of the spring.  By increasing the spring’s size, you will also be designing a heavier spring.  A heavier spring will require more torque to overcome the weight added.  You can see where this is going;  It’s a continuing cycle which leads quickly to a bulky, heavy Conpower® spring or a very strong spring with limited range of motion due to a small number of available turns.

3. Expecting too much from a Conpower® spring.

We love making springs, but we also know that a spring isn’t always the answer.   Springs have limited capacity and can only function as long as they have sufficient space and material to operate.  Designs that require large numbers of turns will contain a lengthy strip of steel which requires a lot of space to operate.  Stronger springs will require additional space to allow for thickness and width to increase as well.  You can see from our stock Conpower® listings, we can offer springs with many more turns if the torque requirements are low.  Generally, as spring requirements (torque and number of turns) grow, so do the springs and their cases.  If you are designing for hundreds of turns in a small space, it may be time to consider a motor for your application. One thing to consider with cord retracting applications: If you are retracting a cord or cable on to a spool, a larger OD spool will allow for more cord to be retracted with less total turns.

4. Expecting maximum constant torque from a Conpower® spring.

A common misconception is that the maximum torque which may specify a spring is seen throughout its rotational range (turns).  This is simply not true.  When you receive specifications from Vulcan for a Conpower® spring, we will list a maximum torque value.  This is the expected torque output only when your spring is fully wound.

Additionally, Conpower® springs need several turns, called a ramp-up period, to reach the useful range,  what we call “useful turns.”   The spring will provide a torque lower than the maximum value when wound to a lesser number of winds.  When discussing your spring design with Vulcan, please be sure to note the required torque values over your working range so that we can design towards an appropriate torque profile to fit your needs.  It is likely that the final maximum torque specification will exceed the torque required within the application.

For example, if you required 6 working turns from your spring and you were looking for approximately 3 in-lbs of torque, the SCP18D162VS spring (torque curve shown above) might be a good fit for the application.  The highlighted working range in orange indicates the retracting torque curve from 12-6 turns.  In order to operate in this range, the spring must be pre-wound to 6 turns.  After an additional 6 turns of extension, the torque provided while unwinding will follow the highlighted profile, starting around 3.75 in-lbs and finishing with approximately 2.75 in-lbs.  It is important to keep the general spring profile in mind when developing your Conpower® spring designs.  Conpower® springs will be stronger as they extend, but you may notice that longer Conpower® springs with more total available turns have a more gradual slope to their torque profiles.  This can be used to your advantage if you wish to operate in a relatively flat section of the profile.  Our stock site offers approximate torque curves for all of our stock springs which can be very useful in development.

5. Forgetting to consider installation concerns – end details are crucial.

It is important to give your spring something to hold on to but also to keep the spring ends simple.  There are a number of ways to install springs in to their cases, but the design of the end detail can affect the operation of the spring, the ease with which the spring is installed, and the ability to manufacture the part.  We ask that you consider the standard end details that Vulcan provides in our stock springs as a good place to start.  We typically offer a U-bend on the outside of the spring designed to wrap around the outside edge of the case with a 90° bend on the inside for engagement with a slotted arbor. Other variations are possible, but please consult with us early in your design process to discuss feasibility and fit to your design.

Our stock Conpower® springs are shipped in retaining rings to keep them in place until they are pushed directly into their housing at the customers’ facility.  However, most of our Conpower® spring customers utilize Vulcan’s assembly services to install the springs into their housings, greatly reducing handling issues at the customers’ site.  In designing the spring case and arbor, avoiding a situation requiring “blind” assembly or a complex fit between the spring and the assembly components can be crucial. This has huge potential to slow down the final assembly of the product.

I hope that our list was informative and that our guide will be useful through your design process.  When you need help with a project, contact us!

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