Real-time tension, safety factor, elastic stretch and sag for the suspension ropes of a high-rise traction elevator. Adjust travel height, rope count, diameter and counterweight ratio and immediately see how standard steel, high-tensile and CF UltraRope compare against the EN 81-50 minimum safety factor of 12.
Parameters
Elevator type
Traction, MRL, hydraulic or ropeless magnetic
Rope material
Sets tensile strength σ_break and mass per unit length ρ
Travel height H
m
Car weight W_car
kg
Rated load W_load
kg
Rope count n
ropes
Rope diameter d
mm
Counterweight ratio
%
CW = W_car + W_load × (this value / 100)
Results
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Rope area (mm²)
—
Break load per rope (kN)
—
Tension per rope (kN)
—
Safety factor SF
—
Rope elastic stretch (mm)
—
Max sag (mm)
—
Elevator shaft — ropes, car and counterweight
Ropes hang from the top sheave to lift the car and the counterweight; the load balance shifts as the car position changes. Colour indicates the safety factor (green = ample, red = unsafe).
Tension vs travel height
Rope material comparison — safety factor
Theory & Key Formulas
$$T = (W_{car}+W_{load})g + w_{rope}gL,\qquad SF = \frac{F_{break}}{T_{rope}}$$
T: total rope tension at the top sheave, SF: safety factor (EN 81-50 requires SF≥12, ASME A17.1 SF≥7.6-11.9). CF UltraRope offers roughly twice the strength and one-fifth the mass of steel for the same diameter, enabling 1 km of single-run travel.
$$\mathrm{sag} \approx \frac{w\,g\,s^{2}}{8\,T},\qquad \Delta L = \frac{T\,L}{A\,E}$$
Catenary sag approximation and elastic rope stretch. w: rope mass per unit length, s: span, A: metallic area, E: Young's modulus.
Elevator Rope Pretension & Sag — High-Rise Buildings
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I always thought elevators were pushed up from below — they actually hang from ropes? Can six wires really hold a 1.5-tonne car?
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Most passenger elevators are traction units: the friction of a sheave (pulley) up in the machine room pulls the ropes up and down. A single 13 mm standard steel rope already has about 100 kN of break load. Bundle six of them and you have 600 kN — sixty tonnes. Meanwhile a fully loaded car pulls down with only about 2.5 tonnes, leaving a safety factor above 20. EN 81-50 demands at least 12, so we are routinely twice the legal minimum.
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If there is so much margin, why fix the requirement at SF≥12?
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The reason is fatigue. An elevator passes over the sheave hundreds of times a day, so the rope is bent millions of times. Tiny notches and abrasion accumulate on each wire strand, and the new-rope break load steadily drops. Designing to one twelfth of new strength means there is still margin after 20-30 years of service. Smaller sheaves bend the rope more sharply, so separate D/d limits apply too — typically D/d ≥ 40 for steel rope and D/d ≥ 30 for UltraRope.
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When I move the "travel height" slider from 50 m up to 500 m the safety factor slowly drops. What is happening?
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Rope self-weight starts to dominate as the building gets taller. Six 13 mm steel ropes weigh about 420 kg per 100 m; at 500 m that is 2.1 tonnes — the same order as the car itself. The tension at the top sheave becomes car + load + rope self-weight, so the rope is effectively lifting itself. That is why 500 m was the practical limit for steel-rope elevators. KONE broke through it in 2013 with CF UltraRope: the carbon-fiber core is one-fifth the mass and twice the strength of steel, opening up super-tall buildings such as Jeddah Tower (660 m) and Burj Khalifa (828 m) with a single-run cab.
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When I set the "counterweight ratio" to 0% the tension jumps up. The default is 50% — how is that chosen?
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"Car weight + half of rated load" is the industry standard for the counterweight. It makes the motor torque demand roughly symmetric whether the car is empty (light) or fully loaded (heavy). Any other ratio helps one direction but penalises the other. Modern energy-saving installations with regenerative drives push it to 60-70% to harvest more energy on loaded descent. By the way, ThyssenKrupp's MULTI (2017) is ropeless: linear motors drive multiple cabs through one shaft horizontally and vertically.
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One last thing — what does "sag" mean? The chart shows just a few millimetres, so why worry about ropes drooping?
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Sag is how much a near-horizontal rope between sheaves droops under its own weight, given by the catenary approximation sag ≈ w·s²/(8T). The tool uses "travel/3" as a notional span; on a real machine the spans are a few metres and sag is sub-millimetre. The real concern is not the sag itself, but the fact that it changes with car position. Once travel exceeds 50 m, a compensating rope is run under the car and counterweight to keep the rope mass on both sides equal at every position, preventing the drive sheave from losing traction.
Frequently Asked Questions
European standard EN 81-50 requires a minimum safety factor of 12 on the suspension ropes of a traction lift, while US ASME A17.1 demands between 7.6 and 11.9 depending on the number of ropes. Japan's JIS A 4302 follows EN 81 closely, and design practice typically aims for 10-13. The high margin exists because the ropes pass over the sheave millions of times: wire-strand fatigue gradually erodes the new-rope breaking load. This tool flags SF below 10 as red and below 12 as a warning.
The counterweight balances the car mass plus a fraction of the rated load to cut motor torque and energy consumption. The classic design is "car + 50% of rated load", which makes the motor load roughly symmetric whether the car is empty or fully loaded. Pushing the ratio above 50% lowers the energy needed to lift a near-empty car but increases regenerative energy during loaded descent, so energy-saving installations with regenerative drives sometimes use 60-70%.
Launched by KONE in 2013, UltraRope uses a carbon-fiber core with a high-friction coating that weighs 1/5 of an equivalent steel rope and carries roughly double the tensile strength. This pushes the travel limit of single-run elevators from about 500 m for steel up to 1000 m, enabling deployment at Jeddah Tower (660 m) and similar projects. Open issues include sheave diameter requirements (D/d ≥ 30), cost, UV durability for outdoor use, vibration damping characteristics, and standardised maintenance procedures.
Once the travel exceeds about 50 m, the ratio of rope self-weight on the car side and counterweight side changes with each trip, and traction (friction-driven force transfer) on the drive sheave becomes unstable. A compensating rope (or compensating chain) connects the underside of the car and counterweight in the opposite direction, keeping the total rope mass equal on both sides at any position. Sag is estimated by the catenary formula sag ≈ w·s²/(8T), where w is mass per unit length, s the span and T the tension.
Real-World Applications
Super-tall building express elevators: Burj Khalifa (828 m), Shanghai Tower (632 m) and Jeddah Tower (660 m, under construction) all exceed the 500 m single-run limit of conventional steel rope, which historically forced a two-stage layout with a sky-lobby transfer. KONE UltraRope (CF composite) enables more than 1000 m of single-run travel, removing the sky lobby and reducing the elevator footprint by 15-20%. Switching the rope material in this tool makes the gap above 500 m immediately visible.
Machine-room-less (MRL) elevator design: Otis Gen2, Schindler 3300 and KONE MonoSpace mount a thin gearless drive directly to the shaft wall at the top. The architectural saving and the freedom on the roof are attractive, but the smaller sheave diameter means a smaller D/d and faster bending fatigue. Varying diameter and rope count in this tool shows why MRL designs lean on many thin ropes or flex-coated belts.
Ropeless maglev elevators (MULTI): Announced by ThyssenKrupp in 2017, MULTI replaces ropes with linear motors and runs multiple cabs through one shaft both horizontally and vertically. The number of shafts drops by roughly a third and useful floor area rises sharply. Selecting "ropeless magnetic" in this tool keeps the rope-related parameters in a conceptual model.
Hoist and sheave life prediction: The tension and SF computed here also feed rope-life prediction (number of sheave passages). The Feyrer equation (DIN 15020 / ISO 4309) expresses life as a log-hyperbolic function of D/d and T/F_break, so raising SF from 12 to 14 roughly doubles the service life. For elevators in service for 10 years, an annual magnetic rope test (MRT) quantifies broken wires and wear and is checked against the original design SF to decide when to replace ropes.
Common Misconceptions and Pitfalls
The biggest pitfall is confusing the breaking load with the design allowable. The minimum breaking load (MBL) listed in a rope catalogue is a new-rope, static-tensile value. In service it is eroded by (1) bending fatigue on the sheave, (2) abrasion and corrosion, (3) tension imbalance between ropes, (4) termination efficiency (80-90%), and (5) dynamic shock (1.5-2x during an emergency stop). Even an EN 81 SF of 12 can leave an effective margin of only 4-5x. The SF computed here is a static design value and does not account for dynamic and ageing effects.
Next, "more ropes always means proportionally more safety". In theory doubling the rope count n halves the tension per rope and doubles SF. In reality there is always some tension imbalance between ropes, and the most heavily loaded rope fatigues first. EN 81-1 requires inter-rope tension differences within ±5%; beyond that, equaliser bolts must be re-adjusted. Maintaining uniform tension becomes harder as n grows, and at some point "more ropes ≠ higher safety". This is why most real machines use 4-8 ropes.
Finally, "sag ≈ 0 means nothing to worry about". The tool may show only a few millimetres, but the absolute sag is not the issue — what matters is that it changes with car position. Above about 50 m of travel, the car-side rope grows longer and the counterweight-side rope grows shorter as the car descends, shifting self-weight asymmetrically across the drive sheave. The two-sided tension ratio T1/T2 drifts and can leave the friction-transfer limit (Euler equation T1/T2 ≤ e^(μθ)), at which point the rope slips. The compensating rope or chain is the key design element that prevents this in high-rise elevators.
How to Use
Enter travel height in meters (typical range 20–400 m for high-rise elevators)
Input empty car weight in kg and rated load capacity in kg
Specify number of ropes (commonly 4, 6, or 8 for redundancy)
Simulator calculates rope cross-sectional area, break load per rope (typical steel wire rope: 1770–1960 MPa tensile strength), and maximum tension at full load
Review safety factor (target SF ≥ 12 per EN 81-1), elastic stretch during acceleration, and maximum sag between suspension points
Worked Example
A 20-story office building elevator: travel height 65 m, empty car 1200 kg, rated load 1000 kg, 8 ropes. Total suspended mass = 2200 kg. At full load with acceleration 1.0 m/s², tension per rope ≈ 3.2 kN. Assuming 8×10 mm diameter steel rope (area ≈ 50 mm² each, break load ≈ 88 kN), safety factor = 88/3.2 ≈ 27.5 (well above minimum 12). Elastic stretch over 65 m travel ≈ 8 mm; maximum sag at mid-span ≈ 45 mm under design load.
Practical Notes
Rope diameter selection: 10–13 mm typical for passenger elevators; verify with rope supplier datasheets (e.g., Fasten, Bridon)
Safety factor must exceed 12; higher SF (16+) preferred for 24/7 duty cycles to account for wear, corrosion, and fatigue in humid/coastal environments
Sag increases with travel height squared; 300+ m systems require intermediate guide sheaves to limit deflection below 30 mm and prevent lateral vibration
Rope stretch during peak load reduces ride comfort; pre-tension 5–10% of break load minimizes elongation perception