Safeties, Governors, and Overspeed Protection

Safeties, Governors, and Overspeed Protection

Every traction elevator car must have a mechanical system that stops it if the car travels too fast in the downward direction. This system has three main parts that work together: the governor, the safety device (often called the safeties or safety jaws), and the governor rope. Understanding how these parts interact, in the right order, is essential knowledge for any elevator mechanic.

Why Overspeed Protection Exists

A traction elevator is suspended by wire ropes attached to the hoist machine. If those ropes break, a brake fails, or the machine runs away, the car can accelerate beyond its rated speed. Without a backup system, the car would strike the pit with catastrophic force. The governor-and-safety system is that backup — a purely mechanical final line of defense that does not depend on electrical power to stop the car.

The Governor

The governor is a centrifugal speed-sensing device mounted in the machine room or overhead space. A continuous loop of governor rope runs from the governor sheave down the hoistway, connects to the car's safety linkage, and returns to the governor. As the car moves, the governor sheave rotates at a speed directly proportional to car speed.

Inside the governor, flyweights or balls are held inward by spring tension. As rotational speed increases, centrifugal force pushes the weights outward. At a preset overspeed switch setting, the outward movement first actuates an electrical switch, cutting power to the machine and applying the electromagnetic brake. This is the first intervention — electrical, not mechanical.

If the car continues to accelerate to the governor trip speed (a higher threshold than the overspeed switch setting), the flyweights move far enough to engage a mechanical trip mechanism — typically a jaw or latch that grips the governor rope and stops it from moving.

The Sequence from Overspeed Trip to Car Arrest

Learning this sequence in order is required for the oral check at the end of this lesson. The five steps are:

1. Car exceeds governor trip speed. The car descends faster than its rated speed by a margin defined in ASME A17.1. The governor's flyweights move outward far enough to engage the trip mechanism.

2. Governor rope is gripped and stopped. The trip jaw locks around the governor rope, preventing it from moving. Because the rope loop connects the car to the governor, the rope that was moving with the car is now held stationary relative to the building.

3. Safety linkage is pulled upward. With the governor rope stopped and the car still descending, the rope pulls upward on the car's safety linkage (a lever or pick arm beneath the car). This mechanical connection transmits force to the safety wedges or jaws mounted in the car frame's guide rails.

4. Safety jaws engage the guide rails. The wedge-type or roller-type safety jaws are forced against the guide rails. The harder the car tries to move downward, the more tightly the wedges grip. The jaws apply enormous friction force to the rails.

5. Car is arrested and held stationary. The friction from the engaged safeties stops the car and holds it against the guide rail until qualified personnel reset the system.

Types of Safety Devices

ASME A17.1 recognizes several safety designs. The two most common in modern installations are the instantaneous safety (which arrests the car very abruptly and is limited to slower-speed elevators) and the progressive safety (which uses a graduated braking action and is required above certain rated speeds). The governor trip speed and the type of safety must be matched — installing the wrong combination is a code violation.

Governor Trip Speed Settings

The governor trip speed is set at the factory and stamped on the governor data plate. ASME A17.1 specifies that the trip speed must not exceed a defined percentage above the elevator's rated speed — always confirm the actual values from the current edition of the code and the manufacturer's documentation for each job. A governor set too high may not trip until the car is traveling dangerously fast; one set too low will cause nuisance trips during normal operation.

Conditions Requiring a Full-Load Safety Test

ASME A17.1 requires periodic safety tests. A full-load safety test — in which the car is loaded to rated capacity, the governor is tripped, and the safety jaws must hold the car — is required in specific situations, including: initial acceptance of a new installation, after any repair or replacement of the safeties, governor, or governor rope, and after certain alterations. Routine periodic inspections may require a no-load or partial test, but the full-load test verifies that the jaws can hold the car under realistic operating conditions.

Safety Precautions During Demonstrations and Tests

Governor and safety work involves significant hazards. Always apply lockout/tagout (LOTO) procedures before entering a hoistway or pit, following the site-specific energy control program. Wear appropriate PPE — at minimum, hard hat, safety glasses, and steel-toed footwear. Never stand under a car whose safeties are being tested. Keep all unauthorized personnel out of the hoistway. After a safety test, safeties must be reset by a qualified elevator mechanic, and the car must be inspected for rail damage before returning the elevator to service.