The friction-based driving principle in which the tension difference between the car side and counterweight side of the suspension ropes is transmitted to the car through the grip between ropes and the grooved drive sheave.
Full Definition
Traction is the fundamental operating principle of a rope-traction elevator. The drive sheave grips the suspension ropes by friction — the ropes do not wrap around the sheave but pass over it in shaped grooves (U-groove, V-groove, or undercut groove). The Euler-Eytelwein capstan equation governs the traction: T₁/T₂ ≤ e^(μα), where T₁ is the tension on the heavier side, T₂ on the lighter side, μ is the coefficient of friction between rope and groove material (typically 0.09–0.14 for steel rope on cast iron in oil), and α is the angle of wrap. In practice, the groove geometry is designed to achieve the required friction without excessive rope wear. Undercut grooves provide higher effective friction than plain U-grooves by increasing contact stress. Traction check is a statutory commissioning test under EN 81-20: the drive sheave must be capable of holding the car at rest with 125% of rated load without the ropes slipping, and must also not be able to move the car and counterweight both at maximum imbalance without rope slip (to prevent rope breakaway).
