A belt-driven elevator is a vertical lift system that uses flat polyurethane-coated steel-cord belts — instead of round wire ropes — wrapped around a small traction sheave driven by a gearless permanent-magnet motor. Modern systems like the Otis Gen2 run belts as thin as 3 mm over sheaves around 88 mm in diameter, achieving travel speeds up to 2.5 m/s in mid-rise buildings. The design eliminates the dedicated machine room, cuts hoistway energy use by roughly 50% versus geared traction, and now moves people in buildings from the Burj Khalifa annex towers to standard 8-storey residential blocks.
Belt-driven Elevator Interactive Calculator
Vary belt thickness, sheave size, and reference drum size to see bend ratio, minimum sheave diameter, compactness, and safety margin.
Equation Used
The calculator checks the sheave-to-belt bending geometry from the worked example: an 88 mm sheave carrying a 3 mm flat belt is approximately a 30:1 bend ratio when measured at the belt centerline. It also compares that compact sheave against a 600 mm drum.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Uses belt centerline bend diameter for the 30:1 comparison.
- Applies to flat polyurethane steel-cord elevator belts.
- Checks geometry only, not traction, motor torque, or code compliance.
Inside the Belt-driven Elevator
A belt-driven elevator works on the same traction principle as a rope elevator — friction between a belt and a grooved sheave lifts the car on one side while a counterweight drops on the other — but the rope is replaced with a flat composite belt typically 30 mm wide and 3 mm thick. Inside that belt, 12 to 16 thin steel cords carry the actual tensile load, while a polyurethane jacket grips the sheave and protects the cords from corrosion. Because the belt is flat and flexible, you can wrap it around a tiny sheave the size of a coffee mug rather than a 600 mm cast-iron drum, which is the single change that lets the entire drive unit fit inside the hoistway at the top of the shaft. No machine room. No oil-bath gearbox. Just a permanent-magnet synchronous motor, the sheave, the belts, and a controller.
The geometry matters more than people think. Sheave-to-belt diameter ratio sits around 30:1 minimum on a polyurethane steel-cord belt versus 40:1 for traditional wire rope, and that smaller wrap is what unlocks the compact gearless layout. If you spec a sheave too small the cords flex past their fatigue limit and you start seeing belt elongation creep beyond the 0.2% threshold the controller monitors — at which point the elevator throws a fault and parks itself. Tension imbalance across the parallel belts is the other quiet killer. Most installations use 4 to 6 belts running side by side, and tension between any two must stay within roughly 5% of each other, otherwise one belt carries an outsized share of the load and starts shedding polyurethane onto the sheave grooves. You'll see black dust at the bottom of the hoistway long before the belt actually fails.
Why design it this way at all? Round steel ropes need lubrication, they're heavy, and they demand a separate machine room above the shaft to house the bull gear and DC motor. The flat belt gets rid of all three problems at once. Coated steel cords don't need oil. The belt assembly weighs around 80% less than equivalent wire rope. And the small sheave lets you bolt the whole drive to the hoistway rail at the top of the shaft. The most common failure modes you'll actually see in the field are jacket cracking from UV exposure during long construction storage, cord fatigue at the termination wedges if the belts were over-tensioned during commissioning, and sheave groove wear if the building settled and threw the belt run out of vertical alignment by more than 2 mm over the full hoistway height.
Key Components
- Coated Steel Belt: The flat tensile member, typically 30 mm wide and 3 mm thick, with 12-16 steel cords of 1.65 mm diameter encased in polyurethane. Each belt carries roughly 25 kN working load. Belts must be matched within 0.1% length tolerance across a set of 4-6 to keep tension balanced.
- Traction Sheave: A small grooved drive wheel — often around 88-100 mm in diameter on a Gen2-class system — driven directly by the motor shaft. Groove profile is V-shaped to grip the polyurethane jacket. Surface hardness sits at 55-60 HRC to resist groove wear over a 20-year service life.
- Permanent Magnet Synchronous Motor: A gearless flat pancake motor mounted directly to the sheave shaft, typically rated 5-15 kW for residential service. Eliminates the worm gearbox and its 70% efficiency penalty, pushing total drive efficiency above 90%.
- Counterweight: A stack of cast-iron filler weights sized at car weight plus 50% of rated load, riding its own guide rails on the opposite side of the hoistway. Balances static load so the motor only fights the unbalanced portion plus friction.
- Belt Termination Wedges: Self-locking tapered wedge sockets that anchor each belt end to the car frame and counterweight. Wedge angle is critical — typically 14° — and must clamp without crushing the steel cords. Over-torque here is the leading cause of premature cord fracture.
- Belt Monitoring System: Continuous resistance check across the steel cords. A 5% rise in cord resistance flags impending failure and the controller takes the elevator out of service before any cord actually breaks. Nothing equivalent exists on traditional rope systems — you inspect ropes visually on a schedule.
- Governor and Safety Gear: Independent of the belts. An overspeed governor mounted on a separate rope triggers wedge-type safety gear on the car rails if descent speed exceeds 115% of rated speed. This is unchanged from rope elevator design and handles the worst-case 'all belts fail' scenario.
Real-World Applications of the Belt-driven Elevator
Belt-driven elevators dominate the machine-room-less segment of mid-rise construction, where shaft real estate and energy efficiency drive the spec. You'll find them anywhere a building owner cares about usable floor area on the top floor — which is essentially every commercial development built since 2005. They're not used at the extreme high end of skyscraper service yet, where rope still wins on travel height, but the operating envelope keeps expanding as belt manufacturers push tensile ratings higher.
- Residential Construction: Otis Gen2 systems installed in 6-12 storey condominium towers across North America, replacing the older Otis HydroFit hydraulic units.
- Commercial Office: KONE EcoSpace MRL elevators in mid-rise office buildings up to 30 m travel, where the eliminated machine room frees rentable rooftop floor area.
- Hospitality: ThyssenKrupp Synergy belt-drive elevators in mid-scale hotels like the Marriott AC and Courtyard product lines, where 1.6 m/s service speed handles peak check-in flow.
- Retail and Mixed-Use: Schindler 3300 polyurethane-belt MRL elevators in shopping centres and mixed-use developments where a basement plant room is being repurposed for retail use.
- Healthcare: Mitsubishi NexWay-S belt elevators in regional hospitals up to 8 storeys, chosen for smoother ride quality during patient transport on stretchers.
- Construction Hoists: Temporary belt-driven personnel and material hoists on high-rise construction sites, where the lighter belt assembly speeds up jump-up cycles as the building grows.
The Formula Behind the Belt-driven Elevator
The number that matters most when sizing a belt-driven elevator is the working tension per belt, because that determines how many belts you run in parallel and how big the sheave has to be. The formula below gives you tension in a single belt as a function of the moving mass and the system's mechanical advantage from the roping ratio. At the low end of typical residential service — say a 630 kg car at 1.0 m/s — you're well inside the comfort zone of a 4-belt set. At the nominal mid-rise spec around 1000 kg at 1.6 m/s, you're using the full rated tension margin and the belt count drives the sheave width. Push to the high end of belt-drive territory — 1600 kg cars at 2.5 m/s — and you either add belts, go to 2:1 roping, or accept that you've hit the practical ceiling and rope is the better answer.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tbelt | Working tension carried by a single belt | N | lbf |
| mcar | Empty car mass including frame and platform | kg | lb |
| mload | Rated payload mass | kg | lb |
| mcwt | Counterweight mass (typically mcar + 0.5 × mload) | kg | lb |
| mtotal | Total moving mass (car + load + counterweight) | kg | lb |
| g | Gravitational acceleration, 9.81 | m/s² | ft/s² |
| a | Peak car acceleration during start/stop | m/s² | ft/s² |
| nbelts | Number of parallel belts | — | — |
| rroping | Roping ratio (1 for 1:1, 2 for 2:1) | — | — |
Worked Example: Belt-driven Elevator in an 8-storey residential MRL retrofit
You're sizing the belt set for an 8-storey residential MRL elevator retrofit in a Vancouver mid-rise condo conversion, replacing an existing 1980s geared hydraulic unit. The car frame masses 750 kg, rated payload is 1000 kg (13 passengers), and the counterweight is balanced at 1250 kg (car + 50% load). The drive runs 1:1 roping on a 100 mm sheave, peak acceleration is 0.8 m/s², and you're choosing between a 4-belt and 6-belt configuration. Each belt is rated for 25 kN working tension with a safety factor of 12 against ultimate.
Given
- mcar = 750 kg
- mload = 1000 kg
- mcwt = 1250 kg
- a = 0.8 m/s²
- rroping = 1 —
- Trated = 25000 N per belt
Solution
Step 1 — at the nominal full-load condition, calculate the unbalanced static force the belts must carry:
Step 2 — add the dynamic force from peak acceleration acting on the total moving mass:
Step 3 — total belt-side force at nominal full load with 4 belts in parallel:
That sits at roughly 7.3% of the 25 kN rated tension — well inside the comfort zone, with a working safety factor around 13.7 against ultimate. The drive feels effortless and the controller never sees current spikes during start.
Step 4 — at the low end of the operating envelope (empty car, 0 kg payload), the unbalanced force flips direction because the counterweight is now heavier than the car:
The negative sign just means the counterweight side is pulling — magnitude is 826 N, well below nominal. This is the regenerative condition where a properly specced VFD recovers energy back to the building bus.
Step 5 — at the high end, imagine an overload condition at 125% rated load (1250 kg) plus emergency-stop deceleration of 1.5 m/s²:
Still only 12.2% of rated tension. A 4-belt set has all the margin you need. Going to 6 belts here would be over-engineering — you'd cut tension to around 1217 N nominal but you'd also be paying for two belts that never see meaningful work, plus a wider sheave and wider hoistway clearances.
Result
Run 4 belts. Nominal working tension lands at 1826 N per belt — about 7.3% of rated capacity, comfortably below the 30% threshold where polyurethane fatigue starts to dominate belt life. Across the operating envelope you're seeing 826 N empty (regenerative pull on the counterweight side) up to 3058 N at 125% overload with emergency stop, so the design sweet spot is squarely in the middle and the 4-belt config never approaches its limits. If your installed system measures higher than 1826 N at full load — say 2400 N — check first for counterweight mass error (filler plates miscounted is the most common commissioning mistake), then for guide-rail friction above 200 N from misaligned brackets, and finally for belt tension imbalance where one belt is carrying 30%+ of the total load because its termination wedge wasn't seated properly during install.
When to Use a Belt-driven Elevator and When Not To
Belt drive isn't always the right answer. It owns the mid-rise MRL space but loses ground at the extremes — very low-rise where hydraulic still wins on cost, and very high-rise where steel rope still wins on travel height. Here's how the three real-world options compare on the dimensions that matter to a spec writer.
| Property | Belt-Driven (MRL) | Steel-Rope Traction | Hydraulic |
|---|---|---|---|
| Max travel speed | Up to 2.5 m/s | Up to 10 m/s+ | 0.6-1.0 m/s |
| Max travel height | ~75 m practical | 500 m+ | ~20 m |
| Energy efficiency vs geared baseline | 50% better | Baseline | 30-40% worse |
| Machine room required | No | Yes (geared) or No (gearless MRL with rope) | Yes (pump room) |
| Tensile element service life | 20 years / 1.5M cycles | 10-15 years / 1M cycles | N/A (cylinder seals 10-15 yr) |
| Sheave diameter ratio | ~30:1 | ~40:1 | N/A |
| Installed cost (8-storey residential) | Medium | Medium-High | Low |
| Hoistway footprint | Smallest | Medium | Largest (cylinder bore) |
| Ride quality (vibration) | Smoothest at mid-speed | Smoothest at high speed | Good at low speed only |
Frequently Asked Questions About Belt-driven Elevator
The controller compares motor encoder count to a separate sheave-side encoder, so any relative motion between belt and sheave registers as slip — even when the belts themselves are fine. The two most overlooked causes are condensation in unconditioned hoistways glazing the polyurethane surface, and silicone contamination from nearby caulking work transferring onto the sheave during construction. Wipe the sheave with isopropyl alcohol and run the diagnostic again before you start tensioning belts.
If slip persists after cleaning, check the sheave runout with a dial indicator. Anything over 0.05 mm TIR at the groove will cause repeatable slip events at the same shaft angle, and the fault log will show evenly spaced timestamps that match one sheave revolution.
Use 1:1 below about 1000 kg rated load and 2:1 above. The decision is driven by motor torque, not belt count. At 2:1 roping the car moves at half the sheave surface speed, which doubles the motor's available lifting force from the same torque output and lets you use a smaller, cheaper PM motor. The penalty is that the belt runs through twice as many bend cycles per metre of travel, cutting belt life by roughly 30-40%.
For a typical 1600 kg hospital service elevator, 2:1 is almost always the right answer. For a 630 kg residential unit, 1:1 keeps the install simpler and the hoistway diverter sheaves out of the spec entirely.
Belts transmit polygon effect more than ropes do because the flat belt sits flush on the sheave instead of nesting in a U-groove. If the sheave has even minor groove wear creating depth variation across the belt set, you'll feel a periodic vertical pulse at sheave rotation frequency. On a 100 mm sheave at 1.6 m/s, that's about 5 Hz — right in the range humans perceive as 'soft jerk'.
Check belt tension equalisation first. Imbalance over 5% between belts shifts the load and starts walking the car laterally against the guide shoes, which the passenger feels as a side-to-side rocking. A proper belt tensioning rig measures each belt individually under static load — don't trust the visual deflection method on a multi-belt set.
UV exposure from open hoistway shafts is the number-one storage killer. Polyurethane jackets surface-craze after roughly 800 hours of direct sunlight, creating fine cracks that don't reduce tensile capacity but do let moisture reach the steel cords. Six months of uncovered storage on the top floor of a partially-clad building is enough to reject the belt set on installation inspection.
Cover the belts with opaque material from delivery to install, and don't unspool them until the hoistway is enclosed. If you inherit a jobsite where belts have been exposed, a borescope check at the cord-jacket interface is cheap insurance — surface haze you can polish off, but visible cord oxidation through the jacket means the belt set is scrap.
Differential elongation between belts in a set is the leading indicator of a single belt approaching end-of-life, even when cord resistance still reads within tolerance. Polyurethane creep is normal and uniform — what you're seeing is permanent set in one belt because its termination wedge slipped slightly during a hard stop event, transferring tension to the other three temporarily and then never recovering when the wedge re-seated.
The elevator isn't immediately unsafe — the safety gear and governor are independent of the belts — but you're outside the 5% tension-balance window and the worn belt is now carrying 25-35% of total load instead of 25%. Replace the full set, not just one belt. Mixing new and aged belts creates a worse imbalance than what you started with because new belts are stiffer and grab a disproportionate share of dynamic load.
Not in any current production system. Polyurethane steel-cord belts top out around 5500 kg car-plus-load on a 2:1 roped configuration with 8 belts in parallel, and even there the duty cycle is rated for passenger service (around 180 starts/hour, 60% load average), not freight (50 starts/hour, 100% load average). Heavy freight pounds the polyurethane jacket flat in the sheave grooves within months under that duty profile.
For 5000 kg freight, geared traction with steel rope is still the right answer → or a hydraulic ram if travel is under 15 m. The belt-drive sweet spot is passenger and light freight up to about 2000 kg.
References & Further Reading
- Wikipedia contributors. Elevator. Wikipedia
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