Constant-Torque Springs

Constant TorqueContinuousCascadeCustom PartsAdvanced

A strip spring transferred between two drums delivers nearly constant torque. Wrapping the lift string on a driven drum of radius r converts torque T into linear force F = T/r, sharing the lift's existing spool/stringing architecture.

Best for

Lifts whose stringing already runs through a spool or drum, where the counterspring can act at the winch instead of alongside the slides.

Poor fit for

Teams without a supported shaft and drum arrangement, or lifts needing stage-specific force levels from a single unit.

Complexityest.Elevated

Two-drum spring pack, output drum sizing, winding direction, and turn limits all have to be engineered together.

Force accuracyest.High (single level)

Torque is nearly constant over rated turns, so line force is nearly constant — but it is one force level unless multiple units are used.

Packagingest.Concentrated

Demand is concentrated at the drum/shaft area rather than distributed along the slides.

Tuning effortest.Design-time

Force is set by spring rating and drum radius; changing it means changing hardware, not adjusting tension.

In-season riskest.Elevated

Less community documentation than the other methods; mistakes in turn budget or retention show up as sudden failures.

Click a metric to see why it’s rated that way. Ratings marked est. are editorial estimates awaiting test data.

Quick recommendation

Use this when

Your lift is driven by a string spool and you can add a spring drum on or near that shaft
Packaging alongside the slides is impossible but there is room at the winch
You want steel-spring consistency without a spring coil at every stage
You can respect the spring's rated number of turns over your full travel

Avoid this when

Total drum rotations for full travel would exceed the spring's rated turns
You need different counterspring force per stage from a single unit (a single drum gives one force)
Shaft support and retention are marginal — the spring pack loads the shaft continuously
You are not prepared to research winding direction and output-drum sizing carefully

Mechanism overview

A constant-torque spring looks like a constant-force spring's strip steel, but it works between two drums: the strip is stored on one drum and back-wound onto a second (the output drum). As the output drum rotates, the strip transfers between drums, and the strip's resistance to being re-curved delivers a nearly constant torque on the output drum over its rated number of turns.

Annotated diagram — to be addedConstant-torque spring assembly: storage drum and output drum with the steel strip transferring between them, torque arrow on the output drum, and a string line leaving a lift drum on the same shaft.

The trick for a lift: put the output drum on (or gear it to) the shaft your lift string wraps around. Constant torque on a drum of fixed radius becomes constant linear force on the string.

Terminology used on this page

Storage drum — where the strip is coiled at rest.
Output drum — the driven drum the strip back-winds onto; torque appears here.
Rated turns — how many output-drum rotations the spring supports; exceeding this damages it.
Winding direction — which way the strip curves onto the output drum; the torque only acts one way.

Continuous and cascade implementations

A single constant-torque unit delivers one force level at the winch — but a continuous lift's gravity load steps upward as stages engage. The single level must therefore be a compromise: sized for the average load, it over-assists early stages and under-assists late ones (see the force graph below).

Where it attaches: output drum on or geared to the lift's string spool shaft, winding in the direction that assists extension.
Force profile: constant at the string, versus a stepped target.
Options for stepping: multiple smaller units engaged at different extensions is theoretically possible but compounds the turn-budget and packaging problems — at that point, per-stage constant-force springs are the simpler path to the same profile.
Verdict: workable for partial assist; poorly matched for float-anywhere behavior on multi-stage continuous lifts.

Physics and force matching

Torque to line force

F_{\text{line}} \;=\; \frac{T}{r}

In plain terms: The force the spring contributes to the lift string equals the spring's torque divided by the drum radius the string wraps on. Half the radius, twice the force — and twice the rotations for the same travel.

Assumes: String stays at constant wrap radius (single-layer wrap) · No gearing between spring and string drum

Turn budget

N_{\text{turns}} \;=\; \frac{L_{\text{travel}}}{2\pi r} \;\le\; N_{\text{rated}}

In plain terms: Full lift travel divided by drum circumference is how many times the drum rotates. That number must stay inside the spring's rated turns, with margin — exceeding it damages the spring.

Variable definitions
Symbol	Quantity	Unit	Notes
$T$	Spring torque	N·m	From the manufacturer datasheet; nearly constant over rated turns
$r$	String drum radius	m	Effective radius including string wrap
$L_{\text{travel}}$	String travel at the drum	m	Cascade rigging multiplies this — count it at the drum, not at the carriage
$N_{\text{rated}}$	Rated turns	turns	Hard limit from the datasheet

Why drum radius couples force and turn budget

Force wants a small radius (F = T/r) while the turn budget wants a large one (N = L/2πr). Substituting, the two constraints collapse into one: F·L = T·2πr·N/2πr·... more usefully, the spring's total stored work T·2πN must exceed the counterspringing work F·L. A spring can only offset a load over your travel if its torque × rated-turns budget covers the job — no drum radius can cheat that energy balance. Check the work budget first when shortlisting springs; then pick r to hit the force.

Worked example: Drum sizing for a cascade lift

Constant 25 N needed at the winch, 90 cm of string travel at the drum, candidate spring rated 0.45 N·m over 12 turns.

Required radius: r = T / F = 0.45 / 25 = 18 mm. Rotations: N = 0.9 / (2π × 0.018) ≈ 8.0 turns — inside the 12-turn rating with ~30% margin. Workable on paper; verify the datasheet's torque-vs-turn curve is actually flat over the first 8 turns.

Interactive force graph

Note how the single constant force level behaves against each architecture: flat-on-flat for cascade, flat-versus-staircase for continuous.

Force vs. extension — Constant torque

Illustrative model, not measured data

RMS error 0.0 NNormalized RMS 0%Max |error| 0.0 N

Stringing architecture

Carriage / outtake mass1.20 kg

Mass per stage0.45 kg

Stages3

Travel per stage30 cm

Lift angle from horizontal90 °

Spring torque0.48 N·m

Drum radius19.0 mm

View chart data as a table

Extension (cm)	Gravity (N)	Counterspring (N)
0.0	25.0	25.0
7.5	25.0	25.0
15.0	25.0	25.0
22.5	25.0	25.0
30.0	25.0	25.0
37.5	25.0	25.0
45.0	25.0	25.0
52.5	25.0	25.0
60.0	25.0	25.0
67.5	25.0	25.0
75.0	25.0	25.0
82.5	25.0	25.0
90.0	25.0	25.0

Design and sizing

Constant-torque drum calculator

Required line force

Drum radius

Total lift travel

Spring rated turns

turns

Required torque

0.475 N·m

Drum rotations for full travel

7.5 turns

Turn budget

OK — within rated turns

Check the spring's rated number of turns against your drum rotations with margin — exceeding it damages the spring. Ratings and life-cycle data come from the manufacturer's datasheet.

Selection checklist

Work budget: spring torque × 2π × rated turns must exceed load × travel (see derivation above).
Torque and radius: pick the catalog torque nearest your target with the drum radius that satisfies both F = T/r and the turn budget.
Winding direction: the torque assists only one rotation direction — confirm it assists extension given your string wrap direction. Getting this wrong doubles the motor's load instead of halving it.
Rotation limits: leave ≥25% turn margin; include any extension beyond normal travel (hardstop overshoot, hang mechanisms).
Mounting: both drums need supported shafts with controlled center distance — strip tracking between drums is sensitive to misalignment.

Packaging

The whole mechanism concentrates at the winch: two drums, their shafts, and the strip path between them. That frees the slide envelope entirely — the method's main packaging advantage — but the winch area must absorb roughly a drum-plus-spring of volume, continuously loaded.

CAD, manufacturing, and assembly

CAD embed — to be addedConstant-torque counterspring reference build — No verified reference CAD is available yet for this method. A documented implementation belongs here.

Design considerations while modeling:

Drum bores and retention: the output drum transmits full spring torque to the shaft — key it, pin it, or clamp it; friction fit will slip.
Shaft support: fully supported (both ends) shafts for both drums; the spring loads them constantly, even parked.
Strip guarding: enclose the strip path — it's a moving steel edge.
Assembly pre-wind: the spring must be installed pre-wound to its working window; design the drums so pre-winding is possible with the assembly on the robot, or make the unit a removable cassette.

Testing and validation

The same protocol as the other methods, plus torque-specific checks:

Bench-test the unit first: measure line force with a luggage scale across full travel off the robot. Verify flatness and the datasheet torque before integrating.
Unpowered lift test: with the motor disconnected, pull the lift through travel and record force up and down; the spread is friction + strip hysteresis.
Release test: confirm which regions float, climb, or settle — on continuous, expect climbing early and settling late with a single unit.
Turn-limit check: mark the drum; count actual rotations over full travel plus overshoot; confirm margin against rated turns.
Powered testing: current, cycle time, thermal, and cycle-count testing as described in the constant-force testing section.

Real-world examples

Your team here

Constant Torque

— · architecture unrecorded

No verified constant-torque counterspring is on file. A documented build — CAD, spring datasheet, measured force flatness, lessons learned — would make this page.

Placeholder card — a real, verified team example belongs here. Contributions welcome.

Common failure modes

Spring wound past rated turnsHigh severity

Cause: Turn budget miscounted, or overshoot travel not included.
Symptoms: Torque drops or becomes erratic; visible strip deformation.
Prevention: Count turns with the calculator including overshoot; leave ≥25% margin; add a rotation hardstop.
Fix: Replace the spring — over-wound strip steel doesn't recover.

Output drum slips on the shaftMedium severity

Cause: Friction-fit or set-screw-only retention against continuous torque.
Symptoms: Counterspring force fades or disappears; witness marks on the shaft.
Prevention: Positive drive: key, pin, or clamping hub.
Fix: Re-machine or reprint with positive retention.

Strip tracks off the drumsMedium severity

Cause: Drum misalignment or center distance out of tolerance.
Symptoms: Strip walks to a flange, rubs, or folds an edge.
Prevention: Rigid, fully supported shafts; check parallelism in CAD and at assembly.
Fix: Realign; replace the strip if creased.

Wrong winding directionLow severity

Cause: Assist direction not checked against string wrap direction.
Symptoms: Lift is dramatically heavier, not lighter; motor current doubles.
Prevention: Trace torque direction through to string tension on paper before building.
Fix: Flip the spring orientation (or re-route the string).

How this compares

Bungee

Bungee is vastly simpler and its linear profile is often no worse than a single constant level on a continuous lift. Constant torque earns its complexity only when steel-spring consistency at the winch matters.

Guide

Constant Force

Same steel, different application point: constant force can step per stage on continuous lifts and is far better documented. Prefer constant torque mainly when packaging forces everything to the winch — typically on cascade.

Guide

Sprung Linkage

Both are advanced, but a linkage shapes its profile while constant torque is locked to one level. Linkage for custom profiles; constant torque for a clean constant force with COTS springs.

Guide

For the full matrix, force-curve overlay, and decision summary, see Compare Methods.

References

Constant-torque spring manufacturer datasheets (to be added)Manufacturer data
Needed: torque-vs-turn curves, rated turns, and life-cycle data for candidate springs in robot-scale torque ranges.
counterspringing.com planning notesEngineering source
Framing source: methods list and stringing-architecture trade-off.
Verified implementations (to be added)Robot example
None on file — this page's biggest gap.