Constant Force Spring
Control your power
Our know-how makes us the best partner to provide the perfect constant force spring for your project
At RPK Group we team up with product engineers to develop custom solutions to meet your requirements. We design specific constant force spring for every necessity, considering the load conditions, materials, life cycle and mounting methods (drum, couplings, ends, etc.) It is ideal when smooth return and recovery are required, including counterbalance, tension, and load applications.
They are generally prestressed springs that are wound tightly around a drum or shaft at the center and the hole at the free end is attached to a load-bearing force as a counterweight. As the spool unwinds, it exerts a nearly constant force because the geometry of the spool remains nearly constant throughout the drum. The full rated load of the spring is reached after being stretched to an extended length of 1.25 times its diameter. There is no friction between the turns (smooth action).
The load is determined by the width and thickness of the material and the diameter of the coil. These springs can be extended partly or completely with no limit to speed or acceleration.
Advantages and benefits of constant force spring
- Small space requirements
- Smooth extension and retraction
- Counterbalance effect
Types of constant force spring
Measures
- Strip thickness: 0.05-1 mm (0.002-0.04 in)
- Strip width: 1.5-100 mm (0.06-3.9 in)
- Other dimensions as required
Materials
The choice of the right material, surface treatment and coating are critical for the final quality and safety of the product. From stainless steels to the most specific materials bought in from the best suppliers.
- Carbon steel
- Stainless steel
- Inconel
- Elgiloy
- Hastelloy
- Chrome silicon
- Other materials according to your special requirements
Industries / Applications
Medical
Industrial
E-Mobility
Power tools
Consumer Goods
Other industries
Applications for Bike and eBike
Frequently Asked Questions
What is a constant force spring?
A constant force spring is a tightly wound strip spring that delivers a nearly constant force throughout its extension. Unlike a helical compression or tension spring, whose force increases with deflection, a constant-force spring maintains a steady pull as it unrolls. This makes it ideal for applications requiring consistent tension over long travel, such as retractable cables, window shade mechanisms, badge reels, and medical device deployment systems.
What are typical applications for constant force springs in automotive and e-mobility?
Automotive and EV applications include retractable charging cables, vehicle window regulator balance mechanisms, sliding door counterbalance systems, and retractable power outlets. The constant pull characteristic eliminates the variable-force problem of conventional spring systems over long strokes.
Are constant force springs used in medical devices?
Yes. Constant force springs are used in infusion pump mechanisms (to maintain constant pressure on the syringe plunger), retractable IV tubing management systems, and powered surgical tool drive mechanisms. A constant, predictable force is essential for accurate dosing in drug delivery applications.
What determines the force output of a constant force spring?
Force depends on strip material, strip thickness, strip width, and the mandrel diameter around which the spring is wound. Increasing strip thickness or reducing mandrel diameter increases the output force. RPK Group's engineering team designs each constant force spring to match the required force level and extension travel.
Can constant force springs be used in power tools?
Yes. Power tools use constant-force springs in retractable cord reels, blade-guard return mechanisms, and depth-stop adjustment systems. The consistent force prevents cable tangling in cord reels and ensures reliable guard engagement.
Get the most out of our expertise
Our engineers work alongside our customers to find the best solution for them, applying scientific methods and knowledge accumulated over almost 50 years.
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