Power Spring Design Calculator

Calculator Inputs

Unit System

Strip Width

Strip Thickness

Total Strip Length

Arbor Diameter

Case Inside Diameter

Elastic Modulus, GPa

Yield Strength, MPa

Preload Turns

Working Turns

Target Average Torque

Friction Loss, %

Cycle Derating, %

Packing Factor, %

Inactive Length, %

Springs in Parallel

Formula Used

Active length: L = total strip length × (1 - inactive percentage)

Angular travel: θ = 2π × turns

Torque constant: k = Ebt³ / 12L

Torque: T = kθ × efficiency × number of springs

Bending stress: σ = 6T / bt²

Stored energy: U = 0.5 × k × (θ₂² - θ₁²)

Design factor: yield strength × cycle derating / calculated stress

These formulas give a preliminary design estimate. Final spring approval needs testing and supplier review.

How to Use This Calculator

Select the unit system for geometry and target torque.
Enter strip width, thickness, active length, arbor size, and case size.
Add material modulus and yield strength from the supplier data sheet.
Enter preload turns and working turns for the expected motion range.
Set friction loss, cycle derating, packing factor, and inactive length.
Press Calculate to view the result above the form.
Use CSV or PDF buttons to export a report.

Example Data Table

Design Case	Width	Thickness	Length	Working Turns	Material	Purpose
Small timer	10 mm	0.30 mm	500 mm	3	Spring steel	Light torque
Retractor drive	18 mm	0.50 mm	850 mm	4	High carbon steel	Moderate energy
Instrument return	25 mm	0.70 mm	1200 mm	5	Stainless spring steel	Corrosion resistance
Heavy rewind	32 mm	0.90 mm	1600 mm	6	Tempered strip	Higher torque

Understanding Power Spring Design

Power springs store work in a wound strip. They are also called clock springs or constant torque springs. A good design balances torque, travel, stress, space, and life. This calculator helps you compare those needs before detailed testing.

Why Inputs Matter

A power spring is usually made from thin spring steel. The strip is wound on an arbor inside a case. When the arbor turns, the strip bends tighter or relaxes. That bending creates torque. The usable torque depends on elastic modulus, strip width, thickness, active length, and bend radius. Small changes in thickness can change torque a lot. That is because thickness is cubed in the stiffness term.

Checking the Load

Start with the load you need to drive. Then enter strip width, thickness, length, and material values. Use conservative values for yield strength and modulus. Add friction when the spring runs inside a tight housing. The design factor gives a quick warning. A larger factor means more stress reserve. A low factor means the strip may set, crack, or fail early.

Reading the Energy Result

Energy is the work stored through rotation. For a simple estimate, average torque is multiplied by angular travel. The calculator reports energy in joules and inch pounds. It also estimates active turns from travel degrees. This helps you compare the result with package limits. A spring that fits on paper may still rub, buckle, or overfill the case.

Practical Design Notes

Use the example table to check typical input ranges. Do not treat the result as final manufacturing approval. Real power springs need prototype tests. Edges, heat treatment, lubrication, case fit, and fatigue history matter. Sharp edges raise local stress. Poor lubrication wastes torque and adds heat. High cycling can require much lower working stress.

Improvement Workflow

A helpful workflow is simple. Run a first calculation. Adjust thickness for torque. Adjust width for stress reserve. Adjust length for travel and energy. Then repeat with safety margins. Export the result for review notes. Share the file with your designer, machinist, or supplier. The final design should follow material data, shop limits, and tested performance. This tool gives a structured starting point. Keep units consistent during every run. Review rounded values before ordering stock. When results look close to limits, choose a safer section. Or reduce required travel slightly today.

FAQs

What is a power spring?

A power spring is a wound strip spring that stores rotational energy. It is often used in timers, retractors, reels, instruments, and return mechanisms.

Can this calculator approve a final design?

No. It gives a preliminary estimate only. Final approval needs material data, manufacturing limits, fatigue testing, lubrication review, and prototype validation.

Why does thickness strongly affect torque?

Thickness is cubed in the stiffness formula. A small thickness increase can greatly raise torque and stress, so adjust it carefully.

What does design factor mean?

Design factor compares derated yield strength with calculated bending stress. Higher values show more reserve. Low values warn against overload or permanent set.

What is packing factor?

Packing factor estimates how efficiently the strip fills the case space. It allows room for clearances, friction, shape changes, and imperfect winding.

Why include preload turns?

Preload keeps torque available at the start of motion. It also raises maximum stress, so it should be included in design checks.

How can I increase torque?

You can increase strip thickness, width, modulus, preload, or spring count. Check stress and package limits after every change.

Why is friction loss included?

Power springs rub during winding and unwinding. Friction reduces delivered torque and energy, especially in compact cases or poorly lubricated designs.