Constant Force Spring Calculator

What this calculator does

A constant-force spring delivers an almost constant pull over its working travel. The strip is pre-stressed and coiled on a drum, so force depends mainly on strip width, thickness and natural curvature rather than on extension. The calculator reports the constant force, available travel, number of layers and bending stress.

Inputs and outputs

Inputs

Strip width and thickness
Natural coil diameter
Drum diameter
Strip length
Working travel
Material

Results

Constant force
Available travel
Number of coils
Bending stress
Weight and mounting space

Typical applications

Cable reels
Counterbalances
Lift-assist mechanisms
Window shades
Medical devices and robotics

Design notes

Constant force depends on a stable natural diameter; heat treatment and strip quality matter more than nominal dimensions. Prototype before committing to a long travel requirement, and specify the extension direction because the force differs slightly between the two sides of the strip.

The interactive calculator interface is currently in Chinese. Enter your values there, or send us the load, travel and space constraints and our engineers will return a checked calculation sheet in English.

Frequently asked questions

How constant is the force?

Typically within a few percent over the middle of the travel, with a small rise near the ends as the strip leaves the drum. The calculator shows the working band.

Can it replace a counterweight?

Often yes, especially where space is limited. The spring must still be sized for the peak load at the end of travel, not only the nominal force.

What is the maximum practical travel?

Travel is limited by strip length and by the number of layers that can coil on the drum. Longer travel requires a longer strip and a larger drum.