Traction & Roping Arrangements · TRACTION
How Traction Drive Works
A lift is held by friction — engineered, measured friction.
A traction lift is not bolted to its ropes: the machine turns a grooved sheave, and friction between rope and groove carries the car. The grip depends on three things — the tension difference across the sheave (T1/T2), the angle of wrap, and the groove’s profile — and every one of them changes as ropes and grooves wear.
The explainer
Watch how it works.
Schematic · loops automatically
1Two tensions, one sheave
Car side and counterweight side pull with different tensions — T1 and T2 — across the driving sheave.
2Friction carries the load
The machine turns the sheave; grip between groove and rope drags the ropes with it. No clamps — just engineered friction.
3Wrap angle buys grip
The longer the arc of contact, the more tension difference the sheave can hold before ropes slip.
4Groove profile sets the pinch
U-grooves cradle, undercuts and V-grooves pinch — more grip at the price of higher wire pressure.
5Wear moves the margins
Groove wear, rope wear and imbalance all shift T1/T2 or the grip that holds it — the traction margin is a living number.
How it works, plainly
Hang the car on one side of the sheave and the counterweight on the other and you get two tensions: T1 on the loaded side, T2 on the other. The sheave can drive the ropes only while the ratio T1/T2 stays inside what friction can hold — the classic capstan relationship, where available grip rises with the coefficient of friction and the arc of rope actually in contact.
Designers buy grip with geometry. Wrap angle sets how much rope touches iron; groove shape sets how hard the rope is pinched. A round-seat U-groove cradles the rope gently; adding an undercut narrows the seat so the rope bears on two flanks and grips harder; a V-groove pinches harder still. More grip, though, means more pressure on the wires — traction design is always a trade between holding the load and consuming the rope.
This is why traction problems are system problems. Worn grooves change the pinch; mixed old and new ropes change the load share; loss of counterweight balance changes T1/T2. The remedy is rarely "more tension" — it is restoring the geometry and balance the design assumed, which is exactly what a proper roping survey measures.
Where this matters
On the job
- Understanding re-roping recommendations and groove machining
- Diagnosing slip, ride and stopping-accuracy complaints
- Specifying rope and groove combinations at modernisation
Standards & references
| Reference | Covers |
|---|---|
| EN 81 series context | Traction requirements the design margins serve |
| ISO 4344:2022 | The suspension ropes the traction system drives |
Re-Ropes surveys measure the traction system as a whole — groove profiles, rope condition and tension balance — so recommendations restore designed margins rather than chase symptoms.
Why does a lift slip its ropes?
Because the required tension ratio momentarily exceeds what friction can hold — from worn groove profiles, glazed or over-lubricated ropes, loss of wrap, or load imbalance. Slip is a symptom of eroded margin; the survey finds which ingredient eroded it.
Can you increase traction by tightening the ropes?
No — overall tension does not change the T1/T2 ratio the sheave must hold, and over-tensioning simply loads bearings and shortens component life. Traction is restored by fixing geometry: grooves, rope condition, wrap and balance.
What is the undercut in a sheave groove for?
It narrows the seat of a U-groove so the rope bears on two flanks instead of the bottom, increasing effective grip without going to a full V. As the undercut wears away, grip falls — one reason groove assessment belongs in every rope survey.
Lift Industry Specialists
Questions about traction on your installation?
Describe the lift and the symptom — an engineer who has seen it before will tell you what it means and what it costs to fix.