Traction machine influence factors

Balance Factor

The traction force in an elevator system is generated by the gravitational forces of the car and the counterweight acting on the rope groove of the traction sheave through the hoisting rope. The counterweight plays a crucial role in creating the necessary friction between the rope and the groove. It helps balance the weight of the car along with its load, which significantly reduces the power required for the elevator’s operation. Therefore, the counterweight is suspended from the opposite side of the traction sheave to offset the car's weight.

When the weight on both sides of the system is equal (T1 = T2), the traction machine can operate smoothly by overcoming only the various frictional resistances. However, the actual weight of the car changes depending on the load it carries—whether it’s passengers or cargo. A fixed counterweight cannot fully compensate for these variations, so the design of the counterweight directly influences the traction performance and energy consumption of the system.

To ensure that the torque experienced during full load and no-load conditions remains balanced, the national standard specifies a balance coefficient K ranging from 0.4 to 0.5. This means the counterweight should balance 40% to 50% of the rated load. As a result, the total weight on the counterweight side must be equal to the car’s weight plus 0.4 to 0.5 times the rated load. This ratio, known as the balance factor, is essential for optimal elevator performance.

When the balance factor K is 0.5, the elevator experiences zero load torque at half load. At this point, the car is perfectly balanced with the counterweight, ensuring the best possible working condition. As the load increases from no load to full load, the torque on the traction sheave changes by only 50%, which helps reduce energy use and eases the burden on the traction motor.

Equivalent Friction Coefficient and Rope Groove Shape

The type of rope groove on the traction sheave affects the friction between the rope and the groove, which in turn impacts the traction force. Common groove shapes include semi-circular grooves, V-shaped grooves, and semi-circular grooves with notches. Semi-circular grooves have the lowest friction coefficient, making them ideal for certain types of traction systems. V-shaped grooves provide higher friction but wear more quickly, especially as the opening angle decreases. Over time, this wear tends to make the groove resemble a semi-circular shape. The semi-circular notch groove offers a middle ground, with stable friction properties even as it wears, making it widely used today.

Lubrication of the steel wire rope within the groove also plays a key role in maintaining the friction coefficient. Only the internal oil core of the rope should be lubricated; applying oil externally can reduce friction, leading to slippage and reduced traction force.

Wrap Angle of the Traction Rope on the Traction Sheave

The wrap angle refers to the arc length of the rope that comes into contact with the traction sheave. A larger wrap angle increases the frictional force, thereby enhancing the traction force. To improve safety and performance, modern elevators often increase the wrap angle using two main methods: a 2:1 rope ratio, which achieves a 180° wrap, or a rewinding system (α1 + α2).

The way the rope is wound around the traction sheave depends on factors such as the traction requirements, the rated load capacity, and the speed of the elevator. There are several winding methods, each representing a different transmission mode. These methods affect the traction ratio—the ratio of the linear speed of the traction sheave’s pitch circle to the car’s movement speed. Depending on the number of wraps, the rope may be wound once (single winding) or multiple times (rewinding). Single winding results in a wrap angle of 180° or less, while rewinding allows the rope to pass over the sheave twice, increasing the wrap angle beyond 180°.

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