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Pulleys & Sheaves — Frequently Asked Questions
What traction sheave groove profiles are used in elevator systems?
Semi-circular (round) grooves offer low rope contact stress and long rope life but low traction coefficient, requiring large sheave diameter or high wrap angle to generate adequate grip. Undercut-V (U-groove with undercut) increases contact stress via the wedge effect, improving traction at the cost of faster rope and groove wear — the most common profile for geared traction machines. Trapezoidal grooves used on flat steel-belt (Otis Gen2 / KONE UltraRope style) drives offer long belt life and low noise but require matched belt cross-sections.
What is the minimum sheave-to-rope diameter ratio required by EN 81-20?
EN 81-20 Clause 9.2.1 requires that traction sheaves and diverter pulleys have a diameter at least 40 times the nominal rope diameter for running ropes (D/d ≥ 40). For compensation ropes and chains the ratio is D/d ≥ 25. These ratios limit rope bending fatigue — smaller ratios increase the stress range per cycle and reduce wire rope fatigue life. Gearless machines with larger motor shaft diameters typically achieve D/d ratios of 50–80× providing extended rope service life of 10–15 years under normal operating conditions.
How is traction sheave wear detected and measured?
Traction sheave groove wear is assessed by measuring groove depth with a purpose-made gauge or digital calliper. EN 81-50 specifies maximum groove depth wear limits beyond which the sheave must be re-grooved or replaced — excessive groove depth causes rope to bottom-out, eliminating the wedge traction effect and causing uncontrolled slippage. Groove wear monitoring is required at annual inspection visits; re-grooving is typically possible twice before sheave replacement is necessary.
Can traction sheaves be re-grooved, and when is replacement preferred?
Re-grooving (machining worn grooves back to specified profile) is economical for large cast-iron sheaves where the material thickness permits removal of 2–4 mm. Re-grooving requires matching rope replacement to maintain D/d ratio compliance as the effective sheave diameter is reduced. Replacement is preferred when sheave diameter is below minimum D/d limits, when cracks are detected in cast iron material, or during gearless machine modernisation where the sheave is integral with the motor rotor assembly.