Sheaves, Roping Configurations, and Counterweighting

Sheaves, Roping Configurations, and Counterweighting

What Is a Traction Elevator?

A traction elevator moves the car by gripping wire rope between a grooved drive sheave (also called the driving machine sheave) and the ropes themselves. Friction — traction — between the rope and the sheave groove transmits force from the motor to the car and counterweight. Understanding how ropes are arranged and how the counterweight is sized is fundamental to installing, inspecting, and troubleshooting any traction elevator.

Sheaves and Their Roles

The drive sheave is mounted on the motor/gearbox output shaft and is the primary powered sheave. Deflector sheaves redirect the rope path when the drive sheave is not positioned directly above the car or counterweight. Secondary or idler sheaves are unpowered sheaves used in 2:1 roping to create a mechanical advantage. All sheaves must be inspected regularly for groove wear, tread diameter, and alignment. A worn groove can reduce traction and accelerate rope wear — two safety-critical conditions.

Roping Configurations

1:1 Roping is the simplest arrangement. The rope goes from the car, over the drive sheave, and straight down to the counterweight. One sheave revolution moves the car and counterweight by the same distance. The rope pull (tension on each side of the drive sheave) equals the actual load being supported — car weight on one side, counterweight on the other.

2:1 Roping introduces a secondary (idler) sheave on the car or counterweight. The rope's dead end is anchored to the overhead structure; the rope then loops under the car or counterweight sheave, back up over the drive sheave, and down to the opposite side. This halves the rope pull compared to 1:1 but requires the drive sheave to rotate twice as far to move the car the same distance. The mechanical advantage reduces the motor torque requirement, which is why 2:1 roping is used for heavier-duty or higher-capacity installations.

A useful rule of thumb: in 1:1 roping, rope speed equals car speed; in 2:1 roping, rope speed equals twice the car speed.

Counterweight Principles

The counterweight (CWT) balances most of the car's weight so the motor only needs to overcome the net load difference plus friction and acceleration forces. Industry practice and NEIEP references establish that the counterweight is set equal to the empty car weight plus a percentage of the rated load — typically 40 to 50 percent, with 40 percent being the most commonly specified value. Always use the percentage given in the job specification or the manufacturer's data sheet; the worksheet problems in this course use 40 percent unless stated otherwise.

Counterweight formula:

> CWT = Car Weight + (Percentage × Rated Load)

This balance point means the motor lifts or lowers only the unbalanced portion of the load during normal operation, reducing energy consumption and motor size.

Rope Tension and Traction Ratio

Wire rope in a traction system has two critical tensions at the drive sheave:

• T1 — the tight side (heavier side, normally the car side when loaded)
• T2 — the slack side (lighter side, normally the counterweight side when loaded)

For basic calculations in pre-apprentice work, approximate T1 and T2 by dividing the total side load by the number of rope falls supporting that side (important in 2:1 roping).

Traction ratio is T1 ÷ T2. The drive sheave must be capable of sustaining traction without slipping under any operating condition. The Elevator Safety Code (ASME A17.1) sets minimum traction requirements; the machine manufacturer's engineering ensures the groove geometry meets them. As a technician, you must never substitute sheaves or change groove profiles without engineering approval.

Safety factor for wire rope is calculated as:

> SF = Rope Breaking Strength ÷ Maximum Load on One Rope

ASME A17.1 specifies minimum safety factors for suspension ropes; confirm the applicable requirement with your inspector or NEIEP reference tables.

Safety Points

• Lockout/Tagout (LOTO): Before inspecting, measuring, or replacing any sheave or rope component, de-energize the elevator controller, apply your personal lock to the disconnect, and verify zero energy with a test instrument. Sheaves can rotate unexpectedly if the brake releases.
• PPE: Wear safety glasses when near moving rope or when inspecting sheave grooves. Hard hats are required in the hoistway at all times.
• Rope inspection: Never operate an elevator with broken wires, corrosion, or kinking beyond the limits in ASME A17.1 and the manufacturer's data. Tag the equipment out of service immediately.
• Overhead work: Use fall protection when working at heights in the machine room or pit; follow your employer's and the IUEC's fall protection procedures.
• Never bypass the governor or safeties to test traction manually. Any slip test must be performed according to the approved testing procedure with all safety devices active.

Summary

Sheaves direct rope path and create mechanical advantage. Roping configuration (1:1 or 2:1) determines the relationship between motor output and car movement. Counterweight mass is calculated as car weight plus approximately 40 percent of rated load. Rope tension and traction ratio confirm the machine can safely move the car. Every calculation you do in the field has a safety implication — set up your formulas correctly, label your units, and double-check your arithmetic before any equipment is commissioned or returned to service.