Applications of Constant-Force Springs

I.   Definition

As the name suggests, a constant‑force spring is a spring that maintains a constant force throughout its stroke; it is also known as a constant‑load spring. It is typically used in linear motion, with its restoring force being the primary function. Its English term is “constant spring.” Common graphical representations are shown below:

Advantages: Compared with conventional compression or tension springs, this spring delivers highly consistent force and produces nearly linear spring characteristics, ensuring that the product’s performance remains stable despite variations in spring force. In narrow, elongated spaces, it can minimize the required footprint, thereby enhancing design flexibility and aesthetic appeal. Its force‑displacement curve is shown below:

Disadvantages: Because it operates in a straight-line fashion, it requires a certain minimum length of space to function effectively. Compared with coil springs, its service life is significantly shorter, typically lasting only about 2,000 to 5,000 cycles.

II.    Applications of Constant-Force Springs

1. Lift-up fan

Compared with the older design that relied on screws to secure the fan head, this new approach uses a constant‑force spring, allowing for effortless height adjustment and easy locking at any intermediate position—resulting in a more aesthetically pleasing and user‑friendly fan. This type of design has been available overseas for many years and is now well‑established.

2. Liquid Crystal Display

As monitors grow larger and demands become more stringent, older models that only allow front‑to‑back tilt adjustment can no longer meet today’s needs. Modern displays now offer 360° rotation, adjustable forward/backward tilt, and vertical height adjustment—practically everything you could ask for. Notably, the ability to slide the screen up and down and lock it at any position is made possible by a precision‑tuned spring mechanism. Thanks to its balanced spring force, the monitor can securely hold the panel in place at any height, preventing it from slipping or sagging. Today, leading PC manufacturers worldwide have adopted this innovative design, continually enhancing its functionality.

3. Sliding windows and doors

In the European and American markets, top‑and‑bottom sliding doors and windows have remained consistently popular, with their upward‑sliding motion powered by constant‑force springs. The rebound force of a pair of such springs on each side enables the glass panels to glide smoothly upward, significantly reducing the heavy resistance they would otherwise encounter.

4. Supermarket Automated Shelf

To ensure that merchandise on the shelves looks neat and visually appealing, sales associates often have to rearrange them. Nowadays, some upscale department stores are adopting automated push‑out shelving: when items at the front are removed, the ones behind automatically slide forward, maintaining a tidy appearance while saving time. This functionality is made possible by using constant‑force springs in the automated shelving system.

5. Motor carbon brush compression device

Because conventional carbon brush assemblies rely on compression springs, the spring force gradually diminishes as the spring is worn down, causing the carbon brush to fail to press against the shaft with a consistent force and thereby degrading motor performance. In contrast, a constant‑force spring with a double‑coil design maintains a steady spring force throughout its entire service life, ensuring that the motor’s performance remains consistently stable.

III. Material Selection for Constant-Force Springs

The SUS301 series is a premium material for constant‑force springs. It boasts high hardness, excellent tensile strength, a smooth surface, superior corrosion resistance, and easy side‑machining—making it virtually the material of choice for such applications. Its hardness can reach approximately Hv600 after low‑temperature heat treatment, with tensile strengths exceeding 1665 MPa. With a nickel content ranging from 6.0% to 8.0%, it ensures outstanding rust‑proof performance.

Of course, in more severe service environments—such as extremely high or extremely low temperatures—the SJ101 series of wide‑temperature, high‑strength, constant‑elasticity alloys can be selected; these materials maintain high strength, constant elasticity, and a low coefficient of thermal expansion even at elevated temperatures. However, due to their relatively high material cost, they are generally not used in standard applications.

IV.   Constant-force spring design

The design of constant-force springs is typically considered from two perspectives:

1. Performance

The two most important performance characteristics are elasticity and service life. Generally, for a given material, the thicker it is and the wider its width, the greater its elasticity. Among materials of the same dimensions, the smaller the inner diameter of the coil, the higher the elasticity; conversely, the lower the elasticity. By contrast, with respect to service life, under identical manufacturing conditions, thinner materials tend to have longer lifespans. For materials of the same size, the smaller the inner diameter of the coil, the shorter the service life; conversely, the longer the service life.

2. Installation Space

Typically, the outer diameter of a constant‑force spring is determined based on the available installation space; when using a shaft, the inner diameter must also be taken into account. In general, the outer diameter of a constant‑force spring should not exceed the dimensions of the mounting space; otherwise, friction may occur and compromise performance.