How an Elevator Traction Machine Works
author: Cloe
2026-09-29
Most explanations of how an elevator traction machine works say the same thing: the motor turns a wheel, the wheel moves the ropes, the car goes up. That is true, but it skips the part that actually explains the machine.
A traction machine does not wind the ropes onto a drum. It grips them.
It Grips the Rope, It Does Not Wind It
In a winch, rope is stored on a drum and the load hangs from a single line. In an elevator, the rope passes over a grooved wheel called the traction sheave and never accumulates. Movement depends entirely on friction between rope and groove. TK Elevator defines the term directly: traction is "the adhesive friction of a body on a surface on which it moves."
That one difference explains most of the machine's design. Because nothing is wound, rope length is not limited by the size of a drum, so the same principle serves a two-storey building and a tower. Because the drive depends on friction, the way the rope is wrapped around the sheave becomes an engineering decision rather than a detail. And because friction can be lost, a traction machine has to be judged on grip, not only on torque.
The Load Path
To see how an elevator traction machine works, follow the load. Four things carry it: the car, the counterweight, the ropes, and the sheave.
One end of the hoisting ropes is attached to the car, the other end to the counterweight. The ropes pass over the traction sheave, and the weight hanging on each side presses them into the grooves. That pressure is what produces the grip.
When the sheave turns, the two sides move in opposite directions. The car rises as the counterweight descends, and the counterweight rises as the car descends. The counterweight is not there to lift the car — it is there to balance it, so the motor never has to move the full weight of the car and its passengers.
Deflector sheaves are added when the car and counterweight need more space between them, or when the layout needs more rope wrapped on the drive sheave.
Inside the Machine: Five Parts
| Part | What it does |
|---|---|
| Motor | Produces the torque. Gearless machines use a permanent-magnet synchronous motor, with the sheave mounted directly on the rotor and no gearbox between them. |
| Traction sheave | The grooved wheel. Groove shape and undercutting determine how firmly the rope is pinched. |
| Brake | Holds the machine at rest. It is spring-applied and electrically released, mounted on the machine, so losing power applies the brake instead of releasing it. |
| Encoder | Reports rotor position and speed to the drive, so the motor is controlled in a closed loop rather than open loop. |
| Drive | Converts the supply into the current the motor needs, and runs the start, stop and levelling profile. |
A geared machine adds a gearbox between motor and sheave. A gearless machine removes it, which makes the assembly more compact and eliminates gear oil.
The Start Sequence
This is the part most explanations leave out, and it is where the machine earns its reputation.
Starting an elevator is not simply a matter of releasing the brake and going. The drive establishes torque before the brake releases. If the brake opens first, the car rolls toward the heavier side and the passenger feels it.
The sequence runs roughly like this:
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The drive builds torque against the applied brake, holding the car still.
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The brake releases. Torque is already present, so there is no rollback.
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The car accelerates, then runs at speed.
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Before stopping, speed is brought back near zero while the brake is still open.
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The brake applies, and torque is removed.
Because the encoder closes the loop, the drive knows the car's position continuously. That is what makes accurate levelling possible — the car arrives at floor level rather than near it.
When a machine does roll back, the cause is usually in that timing rather than in the motor. A drive engineer will compare brake command, brake feedback, torque current and speed feedback on one timeline to find where the order broke down.
Why the Roping Arrangement Matters
How the ropes are wrapped around the sheave changes the load the motor has to move. The two common arrangements are 1:1 and 2:1.
In 1:1 roping, the rope travels over the sheave once. The car, the counterweight and the rope all move at the same speed. One end is tied to the car, the other to the counterweight. This is the usual layout for low-rise and low-speed installations, although a gearless machine allows higher speeds with the same arrangement.
In 2:1 roping, the rope passes over a secondary sheave and back over the drive sheave before reaching the car and counterweight through compounding sheaves. Two rope falls support the load, so the machine lifts about twice the weight for the same rope tension — but the car moves at half the rope speed, so the motor must turn twice as fast. The sheaves are also wider, because twice as many grooves are needed.
A second wrap of rope around the drive sheave raises the friction available and reduces rope wear, which is why high-speed gearless installations often use a double wrap. Arrangements go as far as 5:1 on large freight traction elevators.
The geometry is doing real work. In 2:1 single-wrap layouts the grooves pinch the rope with pressure that varies with groove shape and undercutting, and the arrangement is designed to give roughly 180 degrees of rope contact on the sheave.
What Makes a Machine Slip
Traction fails when the friction between rope and groove cannot carry the load. Several things erode that margin:
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Worn or polished grooves, which reduce the pinch on the rope.
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Contamination, particularly oil or grease reaching the groove.
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A counterweight that is out of balance, which widens the tension difference between the two sides.
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Rope stretch, or a rope specification that does not suit the groove profile.
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On long travel, the weight of the ropes themselves changes as the car moves. A compensation device is used to keep the two sides balanced through the full travel.
Slip rarely announces itself as a failure. It appears first as inaccurate levelling, unusual starting behaviour, or a car that settles after it stops. All three are reasons to inspect the machine rather than adjust the drive.
FAQ
1.In one sentence, how an elevator traction machine works?
The motor turns a grooved sheave, and friction between the sheave and the ropes lifts the car while the counterweight descends. Nothing winds onto a drum.
2.Does the rope wind around the traction sheave?
No. The rope passes over the sheave and is held by friction in the groove. Nothing is stored on a drum.
3.Why does a traction elevator need a counterweight?
It balances the car so the motor moves a smaller mass. Without it, the machine would have to lift the entire car and its load, and there would be far less pressure holding the ropes in the grooves.
4.What is the difference between a geared and a gearless traction machine?
A geared machine uses a gearbox between the motor and the sheave. A gearless machine drives the sheave directly, which makes the assembly smaller, quieter and free of gear oil.
5.Can a traction machine run on belts instead of ropes?
Yes. Flat steel belts with a high-friction coating are used as an alternative to round steel ropes, and they do not need lubrication.
The Short Version
How an elevator traction machine works comes down to one idea. The car and the counterweight hang on opposite ends of the same rope, and the weight on each side presses that rope into the grooves of a rotating sheave. The motor supplies torque, the sheave turns torque into friction, the counterweight keeps the load balanced, and the encoder keeps the drive informed.
Everything else — the roping arrangement, the groove profile, the brake sequence — exists to keep that friction dependable.
POTENSI supplies elevator traction machines and traction system components, including gearless machines, sheaves, brakes and encoders, for repair and modernization projects. Browse the traction system range or send us the nameplate details and we will match the machine.
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