Traction Elevators Size Counterweights to Tare Plus 45 Percent Capacity to Halve Peak Motor Torque

Before a traction elevator ever carries a passenger, its energy consumption, braking requirements, and motor size are permanently fixed during the engineering specification phase. This is the step where technicians calculate and stack the counterweight mass, determining the system's mechanical balance.[4]

Because the electric motor's only job is to overcome the difference in weight between the car and the counterweight, getting this mathematical calculation right dictates whether the building requires a massive rooftop machine room or a compact gearless drive.[4]

If the counterweight is too light, the motor draws excessive current to lift a full car. If it is too heavy, the brakes struggle to hold the empty cabin at a landing. The precise ratio is the foundation of modern vertical transportation.[4]

The Mechanics of the Seesaw

A traction elevator does not simply hoist a cabin up a vertical shaft. It operates as a precisely balanced seesaw, with the passenger car on one end of the steel suspension ropes and a stack of heavy plates on the other.[1]

These ropes pass over a deeply grooved drive sheave, which is turned by an electric motor located at the top of the hoistway. The friction between the steel ropes and the sheave provides the traction that moves the entire system.[1]

The motor does not bear the full weight of the elevator; it only pushes or pulls the out-of-balance mass. If the two sides were perfectly equal in weight, the motor would only need to overcome mechanical friction to move the car.[1]

The 45 Percent Overbalance Rule

However, an elevator's total weight constantly fluctuates as passengers enter and exit at different floors. Engineers cannot balance the system for every individual trip. Instead, they size the counterweight to equal the tare weight—the mass of the completely empty car frame and cabin.[5]

To that tare weight, they add a specific percentage of the elevator's maximum rated passenger capacity. Industry standards, including the American Society of Mechanical Engineers (ASME) A17.1 code, typically dictate an overbalance ratio between 40 and 50 percent of the rated load.[2][6]

A 45 percent overbalance is the most common mathematical compromise used in modern installations. For a standard commercial elevator with a 3,000-pound (1,360 kg) capacity and a 4,000-pound (1,814 kg) empty car, the counterweight is set to exactly 5,350 pounds (2,426 kg).[5]

Halving Peak Motor Torque

This specific ratio minimizes the maximum out-of-balance load the motor will ever face during its service life. When the car is completely empty, the counterweight is heavier by exactly 45 percent of the rated capacity, or 1,350 pounds.[4]

When the car is fully loaded to its 3,000-pound limit, the car side is heavier by 55 percent of the capacity, or 1,650 pounds. By splitting the difference, the motor never has to lift more than roughly half of the elevator's rated capacity.[4]

Because the maximum out-of-balance mass is effectively halved, the peak torque required from the electric motor is also halved. Torque is the rotational force the motor applies to the drive sheave to accelerate the load, and reducing it allows for significantly smaller motors.[1]

Steady-State Versus Dynamic Loads

"The steady-state method safeguards so that the motor can move the out of balance masses at the desired steady-state speed," explains a technical guide from engineering manufacturer Zetwerk. This ensures the motor does not overheat during continuous operation.[1]

Meanwhile, the dynamic calculation ensures the motor can generate enough torque to accelerate that out-of-balance mass up to the rated travel speed within the required time frame. This acceleration phase is when the motor draws the most electrical current.[1]

If the system lacked a counterweight entirely, the motor would have to lift the full 7,000-pound combined weight of the car and passengers from a dead stop. This would require massive amounts of electrical power and a motor three times larger.[1]

Enabling Machine Room-Less Designs

This mathematical reduction in torque is the foundational principle behind Machine Room-Less (MRL) elevators. By capping the torque requirement, manufacturers can use compact permanent-magnet synchronous motors that fit entirely inside the elevator shaft, eliminating the need for a dedicated rooftop penthouse.[4]

The chosen overbalance percentage often depends on the type of traction machine installed. Older geared traction elevators, which use a worm gear to convert high-speed motor rotation into low-speed sheave rotation, typically use a 40 to 45 percent overbalance.[2]

Modern gearless traction machines connect the drive sheave directly to the motor shaft. Because they lack the mechanical advantage of a gearbox, gearless systems are highly sensitive to load imbalances and are frequently balanced closer to a strict 50 percent ratio.[2]

Maintaining Traction and Rope Tension

Beyond motor sizing, the counterweight ensures the system maintains enough friction to actually move. The combined mass of the car and the counterweight pulls down on the suspension ropes, pressing them firmly into the grooves of the drive sheave.[5]

This downward normal force prevents the steel cables from slipping when the motor turns. If the counterweight were too light, an empty car might not provide enough tension on the ropes, leading to dangerous slippage during acceleration or braking.[5]

As the elevator travels up and down a tall building, the weight of the steel suspension ropes themselves shifts from one side of the sheave to the other. To counteract this, engineers install a heavy compensating chain beneath the car and counterweight.[5]

Optimizing the Braking System

The 45 percent overbalance also optimizes the elevator's mechanical braking system. Brakes in a traction elevator are not designed to stop a free-falling car, but they must hold the maximum out-of-balance load safely at a floor while passengers board.[4]

Because the load difference never exceeds 55 percent of the rated capacity, the mechanical brakes can be significantly smaller. During a routine stop, the motor's regenerative drive slows the car, and the mechanical brakes drop to hold it in place.[4]

By capping the imbalance, the brakes dissipate far less heat, extending the lifespan of the brake pads and ensuring pinpoint leveling accuracy at every floor. This precise leveling is critical for accessibility and preventing trip hazards.[4]

Code Requirements and Data Plates

Because the overbalance ratio is critical to passenger safety, regulatory codes strictly govern how elevator weights are managed and documented. The ASME A17.1-2019/CSA B44-19 safety code introduced Section 2.24.2.3.5, which mandates permanent documentation of these metrics.[2][6]

This section requires that the designed maximum and minimum percent counterweight overbalance must be permanently documented on a data plate. This data plate is typically attached to the elevator's crosshead, providing a permanent record for future technicians.[6]

This documentation is vital when buildings upgrade their elevators. Adding heavier stone flooring, mirrored walls, or new control panels to an existing cab increases its tare weight, which throws off the carefully calculated 45 percent balance.[2]

The Five Percent Alteration Rule

Under the ASME A17.1 code's strict alteration guidelines, any modification that increases or decreases the deadweight of the car by more than 5 percent triggers a mandatory recertification. This is commonly known in the industry as the 5 percent rule.[2]

"Where an alteration results in an increase or decrease in the deadweight of the car... by more than 5%, the installation shall conform to [strict testing requirements]," the code states. Exceeding this limit can force the replacement of major mechanical components.[2]

To avoid these massive regulatory costs, elevator consultants advise building owners to weigh their cars and counterweights with certified dynamometers before beginning any modernization project. This ensures the final cabin weight remains within the safe operating envelope.[2]

Rebalancing the System in the Field

When an elevator is modernized and its tare weight changes, technicians must physically rebalance the counterweight to restore the 45 percent ratio. They typically add or remove filler plates from the counterweight frame to match the new cabin mass.[3]

Standard steel plates are often used, but if the frame lacks physical vertical space, technicians substitute lead plates. According to Mars Metal, a manufacturer of test weights, lead offers approximately 45 percent more mass for the same volume, making it ideal for tight hoistways.[3]

The rebalancing process is verified using precise electrical current measurements. Technicians run the empty car up and down the hoistway, measuring the amperage drawn by the motor. When the current is identical in both directions, the car and counterweight are perfectly equal.[5]