How Elevators Work: Traction, Hydraulic & MRL Systems

Introduction

Elevators are one of the most common machines in modern life, yet remarkably few people understand what happens between the moment they press a call button and the moment the doors open at their floor. Across the world, an estimated 18 billion elevator trips take place every day. Whether in a four-storey office building or a 100-storey supertall, the fundamental engineering challenge is the same: move a heavy load vertically, stop it precisely at each landing, and do it all safely, thousands of times a day.

Three dominant elevator technologies handle the vast majority of the world's vertical transportation: traction elevators, hydraulic elevators, and machine-room-less (MRL) systems. Each has a distinct set of mechanical principles, installation requirements, and ideal use cases. This guide covers all three in depth, explaining the core components, how they work together, and why an engineer or building owner might choose one over another.

Traction Elevators

Traction elevators are the most widely installed type in mid-rise and high-rise buildings. The name comes from the traction — or grip — between steel wire ropes and the grooved surface of a drive sheave. Rather than pushing or pulling the car directly, the motor rotates the sheave, and friction between the ropes and the sheave grooves transmits motion to the car.

Core Components

• Machine (motor and gearbox): In geared traction systems, an electric motor drives a worm gear reducer that turns the sheave at a lower speed but higher torque. Gearless traction systems eliminate the gearbox entirely — the sheave is mounted directly on the motor shaft, allowing higher speeds and smoother rides.
• Drive sheave: A grooved wheel, typically 60–120 cm in diameter, around which the hoist ropes wrap. The groove profile (U-groove, V-groove, or undercut) determines how much traction is generated and how quickly the ropes wear.
• Hoist ropes: Usually 4–8 steel wire ropes, each 9–12 mm in diameter, running from the car hitch plate, over the sheave, and down to the counterweight. Modern installations may use coated steel belts for quieter operation and longer service life.
• Counterweight: A stack of cast-iron or concrete blocks sized to equal the car weight plus roughly 40–50 percent of the rated load capacity. The counterweight balances the system so the motor only needs to move the difference between car-side and counterweight-side loads.
• Controller: The electronic brain of the elevator. Modern controllers use variable-voltage, variable-frequency (VVVF) drives to precisely control motor speed and torque, enabling smooth acceleration, deceleration, and floor-level accuracy within ±3 mm.
• Governor and safeties: The governor is a speed-sensing device mounted at the top of the hoistway. If the car exceeds a preset overspeed threshold (typically 115% of rated speed), the governor trips the safety device — a pair of wedge-shaped clamps mounted on the car frame that grip the guide rails and bring the car to a controlled stop.

How It All Works Together

When a passenger presses a hall call button, the controller evaluates the request against the car's current position and direction. The VVVF drive ramps the motor up, rotating the sheave. The ropes, held in the sheave grooves by traction, pull the car upward while the counterweight descends on the opposite side of the hoistway. As the car approaches the target floor, the controller references an encoder or position sensor and begins decelerating. The car levels precisely at the landing, the controller engages the brake, and the door operator opens the doors.

The counterweight is the key to the system's efficiency. When the car is at 40–50 percent load — the most common scenario in normal traffic — the car side and counterweight side are nearly balanced. The motor barely has to work, which dramatically reduces energy consumption compared to a system that lifts the full weight every trip.

Geared vs. Gearless Traction

Geared traction machines use a worm gear to reduce motor speed and increase torque. They are less expensive upfront and work well for buildings up to about 75 metres (roughly 25 storeys) at speeds up to about 2.5 m/s. However, the gear introduces friction, noise, and efficiency losses, and requires periodic oil changes.

Gearless traction machines connect the sheave directly to a low-speed, high-torque permanent-magnet synchronous motor. They support speeds from 2.5 m/s up to 10 m/s or more, ride quality is noticeably smoother, and energy efficiency is higher. The trade-off is a significantly higher purchase price, which is why gearless machines are typically reserved for buildings above 15–20 storeys or where ride quality is a premium requirement.

Hydraulic Elevators

Hydraulic elevators take a fundamentally different approach: instead of pulling the car with ropes, they push it from below (or from the side) using a hydraulic piston driven by pressurised oil. This makes them mechanically simpler and less expensive for low-rise applications, but limits their practical height to about 6–8 storeys.

Core Components

• Hydraulic power unit (HPU): A motor, pump, reservoir, and control valve assembly, typically located in a machine room adjacent to the hoistway at the lowest landing. The pump pressurises hydraulic oil to around 200–350 bar and pushes it through piping to the jack.
• Jack (cylinder and piston): In a direct-acting (in-ground) configuration, a steel cylinder is sunk into a borehole beneath the pit, and the piston rises up through the pit floor to push the car platform. In a holeless configuration, the jack is mounted above ground beside or behind the car, using a roping arrangement to multiply the stroke length.
• Control valve: Regulates the flow of oil to and from the jack. To go up, the pump runs and the valve opens to the jack. To go down, the pump shuts off and the valve opens to the reservoir, allowing gravity and the car's weight to push oil back through a metering orifice that controls descent speed.
• Hydraulic oil: Typically a fire-resistant mineral oil or biodegradable vegetable-based fluid, maintained at a controlled temperature to ensure consistent viscosity and ride quality.

How It Works

A hydraulic elevator ride begins when the controller energises the pump motor and opens the up-valve. Oil flows into the cylinder, extending the piston and lifting the car. Speed is controlled by regulating pump output and valve position. When the car reaches the target floor, the pump stops and the valve closes, holding the car in position by trapping oil in the cylinder.

For the down direction, the pump remains off. The controller opens the down-valve, and the weight of the car pushes the piston back into the cylinder, forcing oil through a flow-control orifice back to the reservoir. The orifice size determines descent speed. Since the car descends under gravity rather than motor power, hydraulic elevators use energy only when going up — a meaningful efficiency advantage in buildings with balanced up/down traffic.

Hydraulic systems do not use counterweights. This means the motor must lift the full weight of the car plus passengers on every up trip, which requires a substantial motor and consumes more energy per trip than a counterweighted traction system. It also means the hoistway does not need to accommodate counterweight rails, simplifying construction.

Advantages and Limitations

Hydraulic elevators shine in low-rise buildings where initial cost matters more than operating cost. The machine room can be located up to 15 metres away from the hoistway, offering flexible placement. Installation is faster and requires less overhead clearance. However, speeds are limited to about 0.5–1.0 m/s, rise is limited by piston stroke length, and the oil system requires temperature management to maintain ride quality in extreme climates.

Machine-Room-Less (MRL) Elevators

MRL elevators are a modern evolution of the traction concept, first introduced commercially in the mid-1990s. The defining feature is the elimination of the traditional overhead machine room. The motor, controller, and drive are compact enough to fit inside the hoistway itself — typically mounted on a bracket at the top of the shaft or on the guide rail structure.

How MRL Differs from Traditional Traction

The enabling technology behind MRL systems is the permanent-magnet gearless motor (PMSM). These motors are flat, disc-shaped, and produce high torque at low speed without a gearbox. A typical MRL motor might be 40–60 cm in diameter and only 15–25 cm thick, compared to a traditional geared machine that could fill a 2 × 3 metre room.

The controller and VVVF drive are housed in a cabinet mounted on the landing wall adjacent to the top floor, accessible through a locked panel. This means technicians can service the electronics without entering the hoistway, satisfying code requirements for safe access to the controller.

Roping Configurations

Because the sheave is inside the hoistway rather than directly above it, MRL systems often use a 2:1 roping arrangement. In a 2:1 system, the ropes travel from a fixed point at the top of the shaft, down to a sheave on the car, back up to the drive sheave, down to a sheave on the counterweight, and back up to another fixed point. This halves the rope speed relative to the car speed, allowing a smaller, slower-spinning motor to achieve the same car speed. The trade-off is that twice as much rope is required, and the load on each rope is halved (so fewer ropes are needed).

Why MRL Has Become Dominant

MRL elevators now account for the majority of new traction installations worldwide for buildings up to about 30 storeys. The reasons are straightforward: eliminating the machine room saves the building owner valuable floor space (a typical machine room is 10–15 square metres), reduces construction cost, and shortens the build schedule. Energy efficiency is comparable to or better than geared traction, and ride quality meets the expectations of most commercial and residential buildings. For very high-rise or high-speed applications, a traditional machine room is still preferred because it allows for larger, more powerful machines and more accessible maintenance.

Comparison at a Glance

Feature	Geared Traction	Gearless Traction	Hydraulic	MRL
Typical rise	Up to ~75 m	Up to 500+ m	Up to ~20 m	Up to ~100 m
Speed range	0.5–2.5 m/s	2.5–10+ m/s	0.3–1.0 m/s	1.0–4.0 m/s
Machine room	Required (overhead)	Required (overhead)	Required (adjacent)	Not required
Counterweight	Yes	Yes	No	Yes
Energy efficiency	Good	Excellent	Fair	Very good
Relative cost	Moderate	High	Low	Moderate
Best for	Mid-rise commercial	High-rise, premium	Low-rise, budget	Low- to mid-rise new builds

Safety Systems Common to All Types

Regardless of the drive type, every elevator shares a set of safety devices mandated by codes such as ASME A17.1 (North America) or EN 81 (Europe). The car safety device, activated by the governor, clamps the guide rails to stop a free-falling car. Buffers at the pit floor absorb energy if the car or counterweight over-travels. An unintended car movement (UCM) detection system prevents the car from leaving a landing with the doors open. Door interlocks ensure the hoistway doors cannot be opened unless a car is present and stopped at that landing. Together, these systems make elevators one of the safest forms of transportation — statistically far safer than escalators, automobiles, or stairs.

Conclusion

Understanding the differences between traction, hydraulic, and MRL elevator systems is essential for anyone involved in building design, elevator maintenance, or vertical transportation consulting. Traction systems — both geared and gearless — dominate mid-rise and high-rise applications through their efficient counterweight design and scalable speed range. Hydraulic systems remain a practical choice for low-rise buildings where simplicity and low upfront cost are priorities. And MRL technology has reshaped the market by eliminating the machine room while delivering traction-grade performance, making it the default choice for a wide range of new installations.

Each technology continues to evolve. Regenerative drives are reducing traction energy consumption by feeding braking energy back to the building's power grid. IoT-connected controllers are enabling predictive maintenance that catches faults before they cause callbacks. Rope-free linear motor systems, still in the pilot stage, promise to reshape what is architecturally possible. But the fundamentals covered in this guide — motors, ropes, sheaves, pistons, counterweights, and controllers — remain the foundation of nearly every elevator in service today.