How overspeed governors work

A speed detector with a mechanical trigger.

An overspeed governor responds when a lift travels too fast. In a conventional rope-operated system, it can trigger the safety gear on the car frame.

Despite its name, it does not regulate the speed of each journey. The drive and control system manage normal travel. The governor provides a protective response if its set operating conditions are reached.

Governor: Detects overspeed and initiates a protective action.
Governor rope and linkage: Transfer the mechanical activating force to the safety gear.
Safety gear and guide rails: Provide the braking contact that arrests and holds the car.

This lesson follows a descending car in a conventional governor-rope arrangement. Other layouts and methods of activation exist.

How does it know the car’s speed?

A separate governor rope runs around the governor wheel and a tensioning pulley. It connects to the safety-gear operating linkage on the car frame. As the car travels, it moves the rope, which turns the wheel.

With the intended rope grip and geometry, faster car travel produces faster wheel rotation. The governor therefore gets its speed information mechanically, through the rope. It does not have to rely on a speed command from the lift controller.

Two ropes with different jobs.

The suspension ropes or belts support the car. The governor rope monitors movement and transfers an activating force. It is not a spare suspension rope, and the suspension does not have to break for overspeed protection to operate.

A closer look: travel speed and wheel speed

For a rope moving without slip at the wheel’s effective running diameter, v = πDn ÷ 60, where v is rope speed in metres per second, D is the effective diameter in metres and n is revolutions per minute.

For example, a 0.20 m effective diameter and a rope speed of 1 m/s give approximately 95 rpm. Doubling the rope speed doubles the rotation rate. This is an illustrative geometry calculation, not a trip-speed setting. Rope routing and the particular installation determine how rope speed relates to car speed.

Turning rotation into a trip.

In a centrifugal governor, rotating masses called flyweights are held by springs and a linkage. Increasing speed changes the balance of forces and moves the weights outwards. At a designed position, the mechanism operates a trip.

It is rather like a mechanical speed switch: the rotating parts respond to speed, and their movement releases or operates another mechanism. The precise pivots, springs and latch arrangement depend on the product.

Why does speed have such a strong effect?

A mass following a circular path needs an inward force. For a fixed mass and radius, its magnitude is F = mrω², where ω is angular speed in radians per second. Twice the rotational speed requires four times that force.

Viewed from the rotating mechanism, this is described as an outward centrifugal effect. Springs and linkage geometry oppose the flyweights’ outward movement. Real mechanisms change geometry as they move, so the equation explains the sensitivity to speed; it is not a governor calibration method.

Rated speed is the lift’s intended normal travel speed. Mechanical tripping speed is the specified threshold at which the governor’s mechanical protective action operates. They are different values. The governor must suit the installation’s rated speed, the safety gear and the permitted tripping conditions; there is no single trip-speed figure for all lifts. Dynatech’s product data lists rated and tripping speeds separately.

Not every governor uses flyweights. A pendulum governor can use a cam on the wheel to drive a spring-loaded oscillating mechanism. At sufficient speed, its movement causes a catch to engage. PFB’s LKT120 manual describes this alternative.

Follow the trigger to the safety gear.

Choose a stage to see what changes. This example uses a centrifugal detector and a separate rope brake. It follows the mechanical protection through to engagement if movement continues after the electrical stopping response begins.

From speed to stopping forceDownward travel · simplified

At the governor

At the car frame

1. Normal travel

The car moves the governor rope and turns the wheel. The flyweights remain within their normal operating position. The rope brake is clear and the safety gear is not gripping the rail.

Overspeed contact: Closed
Governor rope: Free to travel
Safety gear: Clear of rail

Grey rope loop: carries the speed information and activating force. The lower pulley maintains tension.
Orange connection: the rope attachment, operating linkage and engaging element. Compare their position with the grey frame at stages 3 and 4.
Fixed rail: the braking surface. The orange upward arrow in the last stage shows the braking reaction against the descending car.

Conceptual stages, not a real-time simulation or a product drawing. Flyweight travel, car movement and clearances are enlarged. One rail and one engaging element are shown; suspension, guards, the second gear and detailed latch/force-control parts are omitted. Electrical and mechanical trip timing is equipment-specific.

Normal travelThe wheel follows rope movement. The gear runs clear of the rails.
The electrical contact opensThis initiates the installation’s electrical stopping response. A successful stop here can prevent the mechanical trip being reached.
If overspeed continues, the governor mechanically tripsThe rope is restrained. The illustration uses a rope brake; other governors lock the wheel and rely on the specified rope/groove grip.
Relative movement pulls the gear onContinued car movement against the restrained rope pulls the operating linkage. The gear engages the rails, providing the braking reaction and then holding the car.

See Wittur’s OL100 for a flyweight and separate rope-brake example, and the Dynatech VEGA instructions for a centrifugal locking arrangement. The diagram explains the common chain of action, rather than reproducing either product.

Opening a contact is not the same as gripping a rail.

The governor’s electrical contact forms part of the protective electrical system. Opening it initiates a stopping response through the relevant drive and brake controls. The contact itself does not produce a rail-clamping force.

The mechanical action restrains the rope and operates the safety-gear linkage. It is this connection that can bring the gear into contact with the rail if movement continues.

Our diagram separates the events so they are easier to follow. Their speed thresholds and timing are not universal. For example, PFB describes an electrical contact operating before mechanical engagement for its illustrated model; other arrangements must be understood from their own documentation.

The governor starts the action; the gear does the braking.

The rope transfers an activating pull. It does not need to support the car’s full weight. Once engaged, the gear, rails and supporting structure carry the stopping forces.

Why the tensioning pulley matters.

The rope needs the correct tension and grip to transmit motion and the required activating force. A tensioning assembly may use a weight or a spring, depending on its design. It also accommodates the rope loop rather than leaving it loose in the shaft.

A rope-slack contact can respond if the tensioning assembly moves outside its permitted range. This is a different condition from overspeed: a slack-rope contact operating does not mean the car was travelling too fast.

The rope, governor and tensioning assembly must be compatible. Rope size and type, pulley groove, tension and the safety gear’s required activating force all matter. PFB’s tensioner explanation describes the purpose of this assembly and why it must match its governor.

What varies between installations?

Direction: governors and safety gears may act in one direction or both. A bidirectional governor does not, by itself, prove that the complete lift has the corresponding protection.
Arrangement: the governor may be at the top of the shaft, in a machine room or incorporated into a different rope layout. The illustration is one common arrangement.
Activation: some systems use electronic detection and electromechanically activated gear instead of a conventional mechanical governor rope.
Purpose: overspeed protection is not automatically Unintended Car Movement Protection (UCMP). Hazardous movement away from a landing can occur below an overspeed trip threshold.

Dynatech’s VEGA range distinguishes directional variants and optional anti-creep equipment. Wittur’s ESG system illustrates an alternative to rope-operated triggering. Always consider the complete detection, activation and stopping arrangement.

After a governor or safety-gear operation, the cause and the protective system need assessment before the lift returns to service. Trip settings, release and testing are manufacturer-specific work for competent lift specialists; this lesson explains the mechanism, not those procedures.

Check your understanding

If the overspeed contact opens, has the safety gear necessarily engaged?

Reveal the explanation

No. An electrical stopping response and mechanical rail engagement are different events. The lift may stop after the contact opens without reaching the mechanical trip. A contact indication alone does not tell you that the gear is gripping the rails.

Apply the idea

What changes when the rope is held?

Imagine the governor rope is restrained but the car still moves a short distance downwards. What happens at their connection?

Show the reasoning

The frame moves down relative to the restrained rope. The connection therefore pulls upwards relative to the frame, operating the linkage. That relative movement brings the engaging parts towards the rail. The rope provides the trigger force; rail contact supplies the braking reaction.